Fabrication and Characterization of Magnetometer for Space Applications

Fabrication and Characterization of magnetometer for space applications

FATJON QEJVANAJ

Doctoral Thesis in Physics School of Information and Communication Technology KTH Royal Institute of Technology Stockholm, Sweden 2016 ii

ISBN 978-91-7595-982-5 KTH School of Information and TRITA-ICT 2016:15 Communication Technology SE-164 40 Stockholm SWEDEN

Akademisk avhandling som med tillstånd av Kungliga Tekniska Hög- skolan framlägges till oﬀentlig granskning för avläggande av teknologie doktorsexamen i Fysik fredagen den 10 juni 2016 klockan 13:00 i Sal C, Electrum, Kungl Tekniska Högskolan, Isafjordsgatan 26, Kista.

©Fatjon Qejvanaj, June 2016

Tryck: Universitetsservice US-AB, Stockholm, 2016 Abstract

The present rapid increase in the number of space missions demands a decrease in the cost of satellite equipment, but also requires the development of instruments that have low power consumption, low weight, and small size. Anisotropic magnetoresistance (AMR) sensors can answer these needs on account of their small size, weight, and power consumption. AMR sensors also produce lower noise than either giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) devices and are thus more suitable for space applications. The type of AMR sensor developed in this study was a Planar Hall Effect Bridge (PHEB) sensor. The FM layer was also coupled with an AFM layer in order to fix the internal magnetization of the FM layer. One technique that was employed in order to meet the low-noise require- ment was to make the FM layer thicker than has previously been attempted. In doing so, the exchange bias field between the AFM layer and the FM layer is no longer high enough to bias the thicker FM layer, so in order to correct this unwanted effect, the material stack was upgraded to two AFM– FM interfaces. With this configuration, it became possible to increase the exchange field by up to 60%. Stronger exchange bias leads to a thicker FM layer and so to lower noise in the device performance. Another strategy that was used to lower the resistance of the device was to implement an NiFeX alloy instead of the standard NiFe. NiFeX consists of an alloy of NiFe and Cu, Ag, or Au; the last of these is known to have very low resistivity. This solution leads to a significant lowering of the device’s resistance. A re- cent technological advance used to fabricate devices with lower resistance is to deposit a multilayer of AFM–FM. Keyword: AMR, Magnetic sensor, Ferromagnetic, Antiferromagnetic, NiFe, IrMn, exchange bias.

iii

Sammanfattning

Då antalet rymduppdrag snabbt ökar fordras lägre kostnader för satelli- tutrustning och det blir nödvändigt att utveckla instrument med låg ener- giförbrukning, låg vikt och liten storlek. Framtagandet av en magnetisk sensor baserad på AMR överensstämmer väl med dessa krav genom sin ringa storlek, vikt och strömförbrukning. AMR- sensorer uppvisar också lägre brus än sensorer baserade på GMR och TMR vilket gör dem mer användbara för rymdtillämpningar. AMR-sensorn som utvecklats i denna studie är en s.k. Planar Hall Effect Bridge-sensor (PHEB). Det ferromagnetiska skiktet (FM) har dessutom kom- binerats med ett AFM skikt för att fixera den interna magnetiseringen av FM-skiktet. För att kunna uppfylla kravet på lågt brus behövdes FM skiktet göras tjockare än vad som någonsin gjorts tidigare. Därigenom blir dock det magnetiska utbytesfältet mellan AFM skiktet och FM skiktet inte tillräckligt starkt för att förspänna ett så tjockt FM skikt, så för att korrigera denna oönskade effekt har materialstacken uppgraderats till två AFM-FM-gränssnitt. Med denna konfiguration var det möjligt att öka det magnetiska utbytesfältet upp till 60%. Ett starkare utbytesfält medger ett tjockare FM lager vilket ger sensorn bättre brusprestanda. En annan strategi som använts för att sänka resistansen hos sensorn var införandet av NiFeX-legeringar istället för standardlegeringen NiFe. NiFeX är en legering mellan NiFe och endera av Cu, Ag eller Au, varav den sista är känd för att ha en mycket låg resistiv- itet. Den här lösningen leder till en signifikant sänkning av sensorernas resistans. Ytterligare en tekniska förbättring som använts för att nå lägre resistans var att införa multipla skikt av AFM-FM. Med denna konfiguration var det möj- ligt att tillverka sensorer med ett totalt sett ännu tjockare FM-skikt med bibe- hållen uniform inre magnetisering.

v vi Contents

Contents vii

Symbols and acronyms ix

Publication list xii

Author´s Contribution to the Publications xiii

Acknowledgments xvi

1 Introduction 1

2 Magnetometers in space 3 2.1 Magnetism ...... 3 2.1.1 Earth’s magnetic ﬁeld ...... 4 2.1.2 Ferromagnetism ...... 5 2.1.3 Antiferromagnetism ...... 5 2.2 AMR ...... 6 2.3 Planar Hall eﬀect sensors ...... 6 2.3.1 Exchange bias ...... 7 2.3.2 Sensitivity ...... 8 2.3.3 Noise ...... 8 2.3.4 Detectivity ...... 9

I Fabrication and characterization methods 11

3 Methods and techniques 13

vii viii CONTENTS

3.1 Sputter deposition ...... 13 3.2 Lift-oﬀ lithography ...... 13 3.3 Gamma ray irradiation ...... 15 3.4 Probe station and project magnet ...... 15 3.5 Noise setup ...... 16

4 PHEB fabrication 19 4.1 Sensing layer fabrication ...... 19 4.2 Contact pad and corners ...... 19 4.3 PCB integration ...... 21 4.4 Sensitivity characterization ...... 22 4.5 Noise characterization of the integrated circuits ...... 23

II PHEB development 25

5 Double bias PHEB 27 5.1 Characterization of materials ...... 28 5.2 Double and single bias ...... 29 5.3 Double bias PHEB ...... 31

6 NiFeX PHEB 33 6.1 Characterization of NiFeX ...... 34 6.2 NiFeX and 100% NiFe ...... 35

7 IrMn/NiFe multilayer PHEB 39 7.1 Characterization of IrMn/NiFe multilayer ...... 39 7.2 IrMn/NiFe single and multilayer ...... 42

8 PHEB irradiation 47 8.1 Characterization of irradiated PHEB ...... 48

9 Conclusion 51

Bibliography 53 Symbols and acronyms

ix x CONTENTS

Table 1

Σ Sensitivity AFM Antiferromagnetic Ag Silver AGM Alternating Gradient Magnetometer AMR Anisotropic magnetoresistance Au Gold Cu Copper D Detectivity EB Exchange bias Fe Iron FM Ferromagnetic H Magnetic field Ir Iridium M Magnetization Mn Manganese MS Saturation magnetization Ni Nickel PHE Planar Hall effect PHEB Planar Hall effect bridge PSD Power spectral density SNR Signal-to-noise ratio TID Total irradiation dose RS Sheet resistance Publication list

List of papers included in the thesis

Paper 1: F. Qejvanaj, M. Zubair, A. Persson, S. M. Mohseni, V. Fallahi, S. R. Sani, S. Chung, T. Le, F. Magnusson, and J. Åkerman, “Thick Double-Biased IrMn/NiFe/IrMn Planar Hall Eﬀect Bridge Sensor,” IEEE Trans. Magn., vol. 50, no. 11 (2014)

Paper 2: F. Qejvanaj, H. Mazraati and S. Jiang A. Persson, S. R. Sani, S. Chung, F. Magnusson, and J. Åkerman, “Planar Hall Eﬀect Bridge Sensor with NiFeX,” IEEE Trans. Magn., vol. 51, no. 11 (2014)

Paper 3: F. Qejvanaj, H. Mazraati, S. M. Mohseni, V. Fallahi, S. Jiang, S. Chung, F. Magnusson, and J. Åkerman, “Order of magnitude increase in AMR sensor layer thickness throughmulti-interface exchange biasing”, Manuscript

Paper 4: F. Qejvanaj, H. Mazraati, S. M. Mohseni, S. Jiang, S. Chung, A. Isernia, F. Magnusson, and J. Åkerman, “A PHEB magnetometer with record thick multiexchange-biased sensor layer”, Manuscript

Paper 5: F. Qejvanaj, H. Mazraati, S. M. Mohseni, S. Jiang, S. Chung, F. Magnusson, and J. Åkerman, “Gamma radiation hardness of PHEB magnetometers”, Manuscript

xi xii CONTENTS

List of papers not included in the thesis

Paper 1: B. Bruhn, F. Qejvanaj, T. Gregorkiewicz, and J. Linnros, “Temporal correlation of blinking events in CdSe/ZnS and Si/SiO2 nanocrystals ”, "Physica B: Condensed Matter ", vol. 453, pages. 63 - 67, (2014)

Paper 2: B. Bruhn, F. Qejvanaj, I. Sychugov and J. Linnros, “Blinking Statistics and Excitation-Dependent Luminescence Yield in Si and CdSe Nanocrystals”, The Journal of Physical Chemistry C, vol. 118, no. 4, (2014) Author´s Contribution to the Publications

Paper 1: Most of the planning and experimental work, evaluation and writing.

Paper 2: Most of the planning and experimental work, evaluation and writing.

Paper 3: Most of the planning and experimental work, evaluation and writing.

Paper 4: Most of the planning and experimental work, evaluation and writing.

Paper 5: Most of the planning and experimental work, evaluation and writing.

xiii

Acknowledgments

During this time that I worked to my thesis I had the opportunity to be in- spired by many great people that made me grow in knowledge but also as a person. I´d like to thanks all the people that I meet and spend this time of my life. I´ll be always grateful to all of you. Fist of all, I´d like to express my gratitude to my supervisor Professor Johan Åkerman for this immense opportunity that he gave me. Thanks to Johan, I learned how to organize my work, from the beginning ideas to writing the ﬁnial report. I learned how to do scientiﬁc research and how to collaborated and work with other people. Thanks Johan for these years, I´ll be forever grateful. Special thanks to my industrial co-supervisor Fredrik Magnusson for his presence and motivation during all my PhD studies. Fredrik has been always encouraging and motivating during the time spent together. He teaching many valuable lesson on how to work in a company environments. Fre- drik, tack så mycket för dessa oförglömliga år. Many thanks to my co-supervisor Professor Majid Mohseni for his teaching and his support. His knowledge was very important for my growth from the start until the end of my PhD. Thanks to Dr Sunjea Chung for being always present when I needed help. Thanks for Dr. Sohrab Sanni for is teaching he has been valuable person to as for advice. I´d like to acknowledge all the members of the Applied Spintronics group as well. Many thanks to Hamid Mazraati, Shang Jiang, Tuan Le Quang, Amir Banuazizi, Alberto Isernia form KTH part of the group and Randy Dumas, Ezio Iacocca, Phil- ipp Dürrenfeld, Martina Ahlberg, Afshin Houshang, Mohammad Haidar, Ahmad Awad, Masoumeh Fazlali from the Göteborg part of the group. I realy enjoiyed to work in the Material and NanoPhysic department. Thank to Professor Jan Linnros that gave me the chance to learn and grown during the Master Thesis, thanks professor Ilya Sytjugov, Federico, Dennis, Mahdi

xv xvi CONTENTS and the other players of the football team Real KTH. Thanks to Mahtab, Rody, Elena, Karin, Luca, Simone, Ahmad, Yashar, Miao, Tobias, Viktor and all the other members of the corridor, all of you made the working environment very pleasant. Thanks to all the clean room members that helped me these years, specialy Reza Nikpars, Magnus Lindberg, Cecilia Aronsson and Gabriel Roupillard. Many thanks to the department administrators, in particular to Madeleine Printzsköld for her warm and kindness in helping me with all the paper work. Finally, I would like to thank my family for their support and encourage- ment through all my life. Without you on my side I could not have gone so far. Special thanks to my grandmother for her love, aﬀection and prayer. Ti je njeriu me i mir ne bote. Chapter 1

Introduction

Space has always fascinated the human race. Some of the very earliest written data that we have concern the sky and the universe beyond our planet. With the development of new scientific instruments, it became possible to understand more about our planet and our solar system. With the increas- ingly rapid improvement in technology, the conquest of space also acceler- ated. One central aspect of space technology that influences our life is the increased usage of communication satellites. Many cellphones today have global positioning systems, a technology that was used by few people just a decade ago. In satellites, magnetometers are crucial devices that map the Earth’s magnetic field and assist in attitude control. The work described in this thesis made up part of a study carried out for a PhD degree at NanOsc AB and KTH Electrum Laboratory. The work was part of two projects funded by the ESA: MagSense and MagSense B. The aim of these projects was to fabricate a miniaturized magnetic sensor to replace the currently used Flux Gate Magnetometer (FGM). Because of its weight, size, and power consumption, FGMs can no longer follow the trend of mini- aturization of satellite component. The primary focus of this work was on creating a magnetosensor compar- able with FGMs in terms of their characteristics. The approach was to use magnetosensors that have already shown improvement and potential for further progress as a result of magnetic material studies. This work is part of a series of five research papers in the fields of magnetic materials, magnetic sensors, nanofabrication, and electronics embedding.

Chapter 2

Magnetometers in space

2.1 Magnetism

The existence of natural magnetism was already known to the ancient Greeks (sixth–fifth centuries BC), but had probably been previously discovered in ancient China, where it was used in a rudimentary prototype of a magnetic compass[1, 2]. It seems that Archimedes (287–212 BC) attempted to magnetize the swords of the Syracusan army in order to disarm enemies more quickly [3]. Pliny the Elder (23–79 AD) attributes the etymology of the word “magnet” to a Cretan shepherd named Magnes, who accidentally discovered the properties of magnetite when leaning on his iron-tipped cane [3]. What is certain, however, is that the ancients had discovered the ability of some minerals (e.g., magnetite) to attract iron filings and small ferrous objects. This ability to exert a force at a distance gave a special meaning to the word “magnetism” over the centuries. Even in the twenty-first century, we still hear magnetic forces being talked about as if they were something mys- terious or mystical. The most important medieval study on the subject is certainly the Epistle de magnete of Peter Peregrinus of Maricourt (1296) [3], which introduces the concept and terminology of the two poles (north and south) of the magnet. He explained how to determine their precise pos- ition, described their reciprocal attractive and repulsive interactions, and proposed the experiment of the broken magnet. In 1600, William Gilbert’s De magnete appeared; this was long the reference

3 4 CHAPTER 2. MAGNETOMETERS IN SPACE text on the subject of magnetism. The first quantitative studies of magnetostatic phenomena can be traced back to the late eighteenth and early nineteenth centuries, with the work of the Frenchmen Biot and Savart and subsequently of Ampère, also in France [3][4]. Ampère suggested that magnetism is generated by current being driven in a loop. This first interpretation of the magnetic phenomenon was the only explanation that classical physics could give. With the rise of quantum mechanics, there also came a better understanding of magnetism. Two major contributions helped clarify the magnetic properties of materials: angular momentum and the spin of the electron. Angular momentum presents an explanation similar to that given by Ampère: the orbit of an electron around the nucleus generates a magnetic field. The second contribution, the spin of the electron, is not yet fully understood[5, 6]. The overall magnetic moments of almost all materials tend to be close to zero [3, 4]. However, in few metals—such as iron, cobalt, nickel, and some rare Earth metals—called ferromagnetic (FM) materials, the total magnetization is significant. This is due to the exchange interaction between the magnetic moment of the individual atoms which, in these materials, is parallel rather than antiparallel, as occurs in other materials [3][4].

2.1.1 Earth’s magnetic ﬁeld

The Earth’s magnetic field is generated by a magnetic dipole located at the center of the Earth and inclined at 11° 30’ to the axis of Earth’s rotation. It is also known as the geomagnetic field and is a frequent phenomenon in many space objects. The exit points of the magnetic dipole axis on the Earth’s surface are called the “geomagnetic poles”, of which there are two: one in the northern hemisphere (78° 30’ N, 69° W) and one in the southern hemisphere (78° 30’ S, 111° E). The discovery of the geomagnetic field is at- tributed to Peter Peregrinus of Maricourt[7, 8]. The magnetic axis and the geographical axis of our planet are two distinct aspects. Also, the Earth’s magnetic axis does not pass exactly through the geographic center of the Earth. The Earth’s magnetic field acts as an electromagnetic shield that deflects the charged particles of the solar wind. For this reason, the Earth’s magnetic field is compressed on the side of the Earth facing the sun, and extends further into space on the night side of the planet[9]. This electromagnetic shield is of great importance for life on Earth, as it pro- 2.1. MAGNETISM 5 tects living organisms from the most dangerous solar rays and energy loads. The meeting of the cosmic rays (solar wind) with the Earth’s magnetic field is what generates the polar aurora phenomena (called aurora borealis in the north and aurora australis in the south), which occur in the magnetosphere above the planet’s poles[7]. The Earth’s magnetic field has its origin in the fluid outer core of the Earth, which lies between about 2900 km and 5200 km below the surface. This field overlaps with the less intense fields due to ferromagnetic materials in the crust, which is magnetized by the nuclear field, and the Earth’s atmosphere, which contains strongly ionized electric currents [7]. The Earth’s magnetic field is about 50 µT at the surface. This field is quite weak and decreases at a rate of 5% per century. Furthermore, the Earth’s magnetic field shifts its axes at a rate of 2% per century [7, 10]. The altitude of an orbital space mission is up to 500 km from the Earth’s surface. At these altitudes, the Earth’s field is around 100 nT and field vari- ation occurs in the sub-Tesla range [11, 12].

2.1.2 Ferromagnetism

In ferromagnetic materials, the atoms have a magnetic moment that is not zero, but the orientation of these magnetic moments is also not completely random. In fact, between neighboring atoms of such a material, there is a strong interaction that leads to the formation of groups of atoms (the magnetic domains)[13], within which all the spins are aligned with each other, though the different domains are misaligned with each other[5]. Under the action of an external magnetic field B, a domain whose magnetic orientation is not parallel to B will tend to change its orientation and become aligned with the external magnetic field. In this way, the total field parallel to the external field considerably increases [4, 14].

2.1.3 Antiferromagnetism

Antiferromagnetism is a characteristic property of some materials, such as manganese, chromium, hematite, IrMn, the MnO2 oxides, FeO, CoO, and others[15, 16]. Unlike in ferromagnetic materials where the minimum energy configuration is for parallel spins, the interaction between atoms in 6 CHAPTER 2. MAGNETOMETERS IN SPACE antiferromagnetic materials is such that a minimum energy configuration is achieved when the spins are antiparallel [4, 17]. The magnetization of these materials, below a certain temperature (the Néel temperature) is, in the absence of an external magnetic field, practically zero. Even when there is an external magnetic field, the magnetic dipoles tend to maintain the antiferromagnetic arrangement [14, 15, 18].

2.2 AMR

The anisotropic magnetoresistance effect (AMR) was first discovered by Lord Kelvin in 1856, when he was studying the resistance of iron and nickel in a magnetic field[19, 5, 20]. He found that the resistance of the material de- pended on the relative direction of the current through the metal and the magnetization imposed by the field. The resistance showed a weak maximum when the two were parallel (i.e., aligned or antialigned), and a weak minimum when the two were perpendicular[21, 22]. Even though the effect was quite small (AMR rarely amounts to more than 4%), it was successfully used in the first magnetoresistive hard-drive read-heads, which were, however, long ago replaced by the first GMR[23] and later TMR[24] read-heads. More recently, it has been found that AMR also produces very low noise in bridge-based sensors, making it attractive for modern magnetic field sensor applications [14].

2.3 Planar Hall eﬀect sensors

Planar Hall effect (PHE) sensors are based on the AMR effect and have tra- ditionally been made by patterning magnetic thin films into bars or cross shapes (Fig. 2.1a,b)[25]. The PHE voltage—that is, the signal of the PHE sensor—is measured as the voltage perpendicular to the bias current. (It is the similarity between this measurement setup and that used for measuring the ordinary Hall voltage that has given the PHE its name.) [26, 27]

A PHE bar or cross of thickness tFM and width w, carrying a current I = JwtFM (J being the current density) and with a resistance diﬀerence ∆ρ between the parallel and perpendicular states will have a PHE voltage of: 2.3. PLANAR HALL EFFECT SENSORS 7

Figure 2.1: Geometry of a PHE resistor element (a); a PHE cross (b); a PHEB (c); and a meander PHEB (d)[26, 28].

I∆ρ sin 2θ Vy = (2.1) 2tFM where θ is the angle between the current and the magnetization. Improve- ments to the PHE cross geometry include both the simple PHE bridge (PHEB)— i.e., a Wheatstone bridge design (Fig. 2.1c)—and the meander PHEB, where each branch consists of a number n of segments of equal length l in a meander shape (Fig. 2.1d). It can be shown that the PHEB sensor will have a voltage that is “ampliﬁed” a factor l/w compared to the PHE cross. Simil- arly, the meander PHEB will have a voltage that is ampliﬁed a factor nl/w compared to the PHE cross [26].

2.3.1 Exchange bias The PHEB sensor relies on the exchange bias (EB) technique in order to force the direction of the magnetization to lie along θ = 0° in zero field [15, 29]. The magnetization direction is typically realized by depositing the FM film 8 CHAPTER 2. MAGNETOMETERS IN SPACE on an antiferromagnetic layer and setting the EB direction by cooling the FM/AFM bilayer in a magnetic field. A properly exchange-biased PHEB sensor will be linearly sensitive to a magnetic field along the y-axis (i.e., for fields along θ = 90°) [30, 31]. Exchange bias arises from a direct interaction between the FM and AFM layers at their interface, characterized by a particular value of the exchange anisotropy energy at this interface. The exchange bias field, Hex, is an inverse function of the FM thickness, since it arises from competition between the thickness independent exchange anisotropy energy and the Zeeman energy, which grows linearly with FM thickness [31].

2.3.2 Sensitivity The sensitivity Σ of the PHE, PHEB, or meander PHEB is defined as the de- rivative of the sensor voltage w.r.t. fields along the y-axis in units of V/T. For a PHE cross sensor, this sensitivity is given for small fields by:

∂Vy I∆ρ Σ = = (2.2) µ0∂Hy tFM (Hex + HK + Hd) where Hex, HK , and Hd are the EB field, the intrinsic anisotropy field, and the demagnetizing field, respectively. The formula assumes that these three sensor fields all lie along the x direction, which is possible to achieve in the PHE cross geometry, but no longer holds for PHEB sensors [26, 28]. How- ever, in the literature [26], this formula multiplied by the bridge amplifica- tion factor is still used for PHEB sensors:

∂Vy nlI∆ρ Σ = = (2.3) µ0∂Hy tFM (wHex + HK + Hd)

2.3.3 Noise AMR based sensors aimed at space applications are targeted at low-frequency, low-ﬁeld operation. At low frequencies, both 1/f noise and thermal noise may add to the relevant noise power spectral density (PSD) [32]. Thermal noise (also called Johnson or Nyquist noise) is caused by the random thermal motion of electrons in a conductor. It has a ﬂat spectrum and the PSD is given by:

ST = 4kBTR (2.4) 2.3. PLANAR HALL EFFECT SENSORS 9

where kB is the Boltzmann constant, T is the temperature, and R = ρnl/(wtFM ) is the resistance. Low-frequency 1/f noise can be of both electric and magnetic origin. Ex- amples of electric origins of 1/f noise include fluctuations in the number of charge carriers NC due to charge trapping and the hopping of de- fects within the conductors. Magnetic 1/f noise originates from coupling between charge transport properties and magnetic fluctuations (i.e., Barkhausen noise). While there is no unified theory of 1/f noise in magnetic devices, the PSD is typically described by a phenomenological Hooge expression:

2 S = V γH (2.5) 1/f NC f where V is the voltage, f the frequency, NC the number of charge carriers, and γH the dimensionless Hooge parameter characterizing the low- frequency performance of the material or device. The total number of charge carriers is simply given by the material-dependent charge carrier density times the sensor volume: NC = nC nlwtFM [32].

2.3.4 Detectivity

The detectivity, DT , is one of the ﬁnal metrics of any sensor, and is deﬁned as the ratio between the voltage noise and the sensitivity [31]: √ ST DT = Σ (2.6)

Part I

Fabrication and characterization methods

Chapter 3

Methods and techniques

3.1 Sputter deposition

Sputter deposition is a well-known technique for depositing magnetic materials. The method works by sputtering away from the target the material with a high energy inert gas such as argon. A high electric field is applied between the target and the substrate holder. The material sputtered from the target is then electrically attracted to the substrate. In this way, the target material are not damaged during sputtering since this is done at room temperature, thus also material alloy like permalloy are used for thin film deposition. In this work, the instrument used was an AJA ATC Orion-8 with seven guns. The instrument has a confocal sputter system. The seven guns are all direc- ted to a focal point toward the substrate holder. Figure 3.1 shows a model of the sputtering set-up used in this work. During deposition, the substrate holder is placed in the focal point of the beam flow. The substrate holder rotates during the deposition in order to deposit the film in a uniform thickness. In the AJA sputter, a uniformity of ±2% has been measured between the center and the border thickness deposited on the substrate.

3.2 Lift-oﬀ lithography

Lithographic techniques play a role of fundamental importance in the ﬁeld of microelectronics, as they allow reductions in the size of circuit paths and the integration of an ever-increasing number of devices on a single chip. A

13 14 CHAPTER 3. METHODS AND TECHNIQUES

Figure 3.1: This is a sputter lithographic process presupposes the presence of the following basic elements: the resist, the source for the exposure, and the mask. The resist is a liquid organic substance or polymer that, when exposed to radiation, varies its chemical nature, becoming more or less soluble. A positive resist becomes more soluble upon interaction with the radiation, while a negative resist undergoes a strengthening of the links between the molecules that compose it in the areas exposed to the radiation, as shown in Figure 3.2. The source of the exposure can be of diﬀerent types, depending on the type of emitted radiation (ultraviolet, electron, ion, and X-ray). The mask is made of a quartz plate covered by a chromium thin ﬁlm and shows the design of

Figure 3.2: Lift-oﬀ lithography schematic[33] 3.3. GAMMA RAY IRRADIATION 15 the circuit paths to be transferred to the substrate.

3.3 Gamma ray irradiation

A gamma ray is a photon emitted by an excited nucleus from radioactive atoms, possessing energy levels between few keV to several MeV [34, 35]. In this work Cobalt-60 gamma-ray irradiation has been used to study how the devices perform under irradiation. Cobalt-60 is a synthetic radioactive isotope of cobalt metal. Because of its short half-life of 5.27 years [6], cobalt- 60 is not found in nature, but is produced artiﬁcially by neutron activation. It decays by negative beta to the stable isotope nickel-60[35]. The resulting nickel-60 energized nucleus emits two gamma rays with energies of 1.17 and 1.33 MeV in order to become stable, and the equation of the core reaction is:

59 60 60 Co + n → Co → Ni + e− + γ (3.1) 27 27 28

3.4 Probe station and project magnet

The magnetoresistance measurements were performed using a four-probe setup (Fig. 3.3) in contact with the samples. The samples were chosen to be long and narrow in order to allow a linear current ﬂow through them. The probes were placed at each end of the samples, as shown in Figure 3.3. The DC and AC current source used was a Keithley 6221 and the voltmeter was a Keithley 2182A Nanovoltmeter. The magnetic ﬁeld was applied from a GMW Projected Field Electromagnet 5201. The magnet was placed under

Figure 3.3: Probe station setup 16 CHAPTER 3. METHODS AND TECHNIQUES the sample holder and could generate a constant in-plane magnetic field. The magnet was mounted on a rotation motor in order to vary the in-plane field angle, as shown in Figure 3.3[36]. All the instruments were connected to a computer and controlled using the LabVIEW program. With this program, it was possible to measure the resistance while varying the magnetic field, the magnetic angle, and the current intensity. The measurement requires one second for each point, regardless of whether the magnetic field, the magnetic angle, or the current is being varied. The duration of the measurement thus depends on the number of points measured. The data was processed using the program Origin, which also handled all the plots and fitting in the figures.

3.5 Noise setup

The noise setup was built to perform the measurements on the devices. This structure consists of two shielded boxes that help avoid interference with external magnetic and electric fields; see Figure 3.4. The measurements were carried out by applying a DC current from lead–acid batteries to minimize the noise from the current source. The output voltage was amplified with a low-noise amplifier (a Femto DLPVA-100-BLN-S) with a gain of 100 dB

Figure 3.4: Noise setup 3.5. NOISE SETUP 17

√ and an inherent Johnson noise level of 0.7 nV/ Hz at 100 Hz[37]. The frequency region investigated was between 1 to 100 Hz. The ﬁnal result was an average of 2000 measurements and took 5 minutes in total.

Chapter 4

PHEB fabrication

4.1 Sensing layer fabrication

All the materials used in this project were deposited by an AJA magnetron sputter with a base pressure below 1 × 10−8 Torr. The depositions were made at different temperatures in an Ar atmosphere with a pressure of 2.5 mTorr. The materials were deposited over a 4” Si wafer under an applied magnetic bias field of 100 Oe. The device layout was done by optical lithography and lift-off. A mask design was used with a large range of devices by varying the length l and the width w, and by repeating each line n times. Figure 4.1a shows the layout and Table 4.1 shows a list of all the parameters of the devices fabricated. The FM material of choice in the literature is permalloy—i.e., Ni80Fe20[38]. NiFe is cheap, easy to work with, has a sizeable AMR, and has excellent soft magnetic properties over a wide temperature range. While in this work the primarily used material of choice was NiFe, it also investigated NiFeX—i.e., alloys of permalloy and Cu, Ag, and Au—in order to reduce demagnetizing effects (Fig. 4.1b)[39].

4.2 Contact pad and corners

The second step was the fabrication of the contact pad; this one was done again with lift-oﬀ lithography. In this technique, a resist is coated and developed on the wafer, and then gold is deposited by sputtering. Then the parts that are not needed are removed, together with the resist, leaving only

19 20 CHAPTER 4. PHEB FABRICATION

Table 4.1: List of devices with diﬀerent layout parameters

Device number length (mm) width (µm) number of lines 1 1.5 75 17 2 1 125 17 3 1 75 13 4 1.25 75 17 5 1 75 15 6 1 25 17 7 1 50 17 8 1 75 17 9 1 100 17 10 1 75 19 11 0.75 75 17 12 1 75 21 13 0.5 75 17

the material that is necessary for the devices on the wafer. In some devices, gold was also deposited over the corners between the lines of the devices, as shown in Figure 4.2[40]. This was done in order to reduce the total resistance of the bridge and so to increase the sensitivity, as gold is a good conductor. Furthermore, the magnetic response of the corners is less linear than the straight lines and it is therefore interesting to test whether the sensors improve if these parts of the sensor are short-circuited [39].

Figure 4.1: a) Device layout; b) material stack of the sensing layer[40] 4.3. PCB INTEGRATION 21

Figure 4.2: Image of a PHEB with gold deposited over the corners between the sensor lines

4.3 PCB integration

The sensor integration was carried out on a custom-designed PCB board. The PCB layout was designed with a computer program and then printed

Figure 4.3: Image of the PCB electronic circuit and integrated PHEB 22 CHAPTER 4. PHEB FABRICATION on a transparence. The PCB is double-sided and has a photoresist layer on top of both copper layers. By placing the transparence over the PCB and exposing it to UV light to develop the photoresist, the desired layout is transferred to the copper layers on the PCB. Afterward, the copper layers are etched and the remaining photoresist is removed with a solvent. Fi- nally, the desired layout is obtained. The PCB board is glued to a thick alu- minum sheet in order to dissipate the heat generated by the device during the measurements. The sensor chip is fixed to the PCB with a thermocon- ductive paste and is connected to the PCB board by wire bonding (Fig. 4.3). Finally, the amplifiers and the other components are soldered to the PCB board. The amplifiers and the resistors are surface-mount components, but the electrolytic capacitors are through-hole components.

4.4 Sensitivity characterization

With patterned devices, the dependence of the sensitivity on the length, width, and thickness of the elements (and on the number of items, in the case of meander bridges) was measured in order to verify that the standard equation for the sensitivity applies (Eq. 2.2)[26]. The same instrument set-up was used for the AMR measurements. Each of the four probes was placed on the extremity of the bridge, with the two

Figure 4.4: Sensitivity slope of PHEB 4.5. NOISE CHARACTERIZATION OF THE INTEGRATED CIRCUITS 23 probes of current and the two probes of voltage placed opposite to each other in order to have uniform current going through the devices. Figure 4.4 shows the linear behavior of devices around zero field. The error of the linearity of the devices is calculated by a linear interpolation of devices sensitivity around zero field. The accuracy of the device linearity is around ± 5 Oe; field range is slightly above 0.5%.

4.5 Noise characterization of the integrated circuits

The noise characterization of the amplifiers is performed by short-circuiting the inputs of the amplifiers to ground. In Figure 4.5, it is possible to compare the noise level of an integrated circuit using one or two circuits. As√ expected from the datasheet[41], the noise level of two amplifiers is 1/ √2 of the noise of one amplifier. The noise at 1 Hz is slightly below 5 nV/ Hz√ for the circuit with one amplifier, while the noise at 1 Hz is around 3 nV/ Hz for the circuit with two amplifiers. For comparison, the noise of the instrumental amplifiers was compared with the noise of the FEMTO commercial amplifier used in the noise measurement setup. The result of this comparison shows that the amplifier noise of the integrated circuit is close to the optimal level for low-noise applications. The noise levels refer to device

Figure 4.5: Noise levels of the integrated amplifiers compared with the noise of the FEMTO instrumental amplifier 24 CHAPTER 4. PHEB FABRICATION resistances below 100 Ω. Devices with higher resistances can increase the ground noise of the amplifier slightly. The amplifier power consumption is about 0.12 W and the power consumption of the PHEB devices is 0.08 W, with a total power consumption of 0.2 W for the circuit. Part II

PHEB development

Chapter 5

Double bias PHEB

The PHEB sensors developed in this thesis use anisotropic magnetoresistance in a ferromagnetic layer to read out the direction of the magnetization in the FM. The PHEB sensor also uses an antiferromagnetic layer to deﬁne the direction of the FM magnetization at zero applied ﬁeld. Part of the material development in this work hence relates to these two materials [42, 43]. The AFM material of choice in the literature is IrMn[44]. Even though this is expensive, it is easy to work with and provides a strong and stable coupling for many FM materials within a broad temperature range. IrMn was therefore used throughout the project [45, 46].

Figure 5.1: Schematic of single and double bias material sensing layer

27 28 CHAPTER 5. DOUBLE BIAS PHEB

Single layer biasing (typically AFM–FM, so-called bottom pinning) has so far been the only choice in the PHEB literature [47, 28]. One of the core ideas in using the double-biasing scheme is to use the AFM–FM–AFM trilayers, which should provide a much stronger biasing field for the NiFe [48, 40]. With the double bias scheme, a significant (60%) increase in the exchange bias field has indeed been demonstrated in this thesis. After this demon- stration, all sensors were designed with double bias (Fig. 5.1).

5.1 Characterization of materials

The typical IrMn thickness in the literature is between 7 and 20 nm [30]. Thinner IrMn may reduce the coupling with the FM layer and decrease the temperature range over which the coupling is eﬃcient. Thicker IrMn shunts current away from the active FM layer [31]. The IrMn thickness was initially chosen to be 10 nm. Following an optimization study, we concluded that 12– 15 nm is a more optimal range, and 15 nm IrMn was used thereafter (Fig. 5.2a)[44]. The typical NiFe thickness in the literature is between 10 to 45 nm [47, 28]. The detectivity of a PHEB sensor scales very favorably with increasing NiFe thickness. However, in the literature, an upper limit of 45 nm has been found, above which there are both demagnetization and domain eﬀects that limit the operation of the sensor[28, 49]. This thesis has extended this thickness range dramatically, by over a factor of three, demonstrating good per-

Figure 5.2: a) Exchange bias vs. IrMn thickness; b) MR vs. NiFe thickness 5.2. DOUBLE AND SINGLE BIAS 29 formance in sensors with 150 nm NiFe (Fig. 5.2b) [32, 40].

5.2 Double and single bias

One of the core ideas behind this work is the idea of double biasing the NiFe layer. By sandwiching the NiFe between two IrMn layers, we believe that we can use much thicker NiFe before demagnetizing effects deteriorate the magnetic performance. The concept is shown schematically below. In Figure 5.3, we show hysteresis loops of both single and double-biased NiFe. The double-biased NiFe exhibits a 62% higher exchange bias field, clearly demonstrating that the double biasing scheme works. An unexpected consequence of the double biasing scheme is the markedly reduced coercivity, which indicates better switching properties of the NiFe. The coercivity improvement might arise as an averaging effect of uncorrelated imperfections at the two interfaces and may ultimately have positive con- sequences for any magnetic noise In the NiFe film, the improved switching characteristics persist in thicker double-biased NiFe films, as shown in Fig.5.3. From Figure 5.4b, we can extract the thickness dependence of the exchange bias field. The exchange bias field is clearly inversely proportional to the

Figure 5.3: Exchange bias ﬁeld of double and single bias 30 CHAPTER 5. DOUBLE BIAS PHEB

Figure 5.4: a)Dependence of the resistance vs. material stack thickness. In black are the results from samples with a single FM–AFM interface, and in red those from the double FM–AFM interfaces. b)Dependence of the exchange bias vs. material stack thickness.

NiFe thickness, as expected for coupling only at the two interfaces. The coupling energy is independent of the NiFe thickness and is hence the same in all samples, while the energy associated with the applied ﬁeld increases linearly with the NiFe thickness. In Figure 5.4a, we also show how well the resistance scales with the inverse of the NiFe thickness and how the res-

Figure 5.5: AMR vs. NiFe thickness values for a double bias IrMn/NiFe later 5.3. DOUBLE BIAS PHEB 31 istance values for both single and double-biased stacks fall on top of each other. This further corroborates the conclusion above that there is minimal current shunting through the IrMn layer(s). Figure 5.5 shows the magnetoresistive properties of all double-biased NiFe films. It is interesting to note that there exists an apparent maximum in the AMR versus NiFe thickness. While the rather strong reduction in AMR for the thinnest NiFe films is well known from the literature [14], the weaker decrease in the high thickness range is likely indicative of detrimental demagnetizing effects [40].

5.3 Double bias PHEB

The sensitivity measurements of patterned devices were performed in order to measure the dependence of the sensitivity on the length, width, and thickness of the elements (and on the number of items, in the case of meander bridges). The goal was to verify that the standard expression for the sensitivity applies—i.e., Σ(tFM , w, l). The measurements were carried out using an electromagnet and the four- probe setup. Each of the four probes was placed on the extremity of the bridge with the two probes of the current and the two of the voltage placed opposite to each

Figure 5.6: a) Sensitivity curves of a PHEB sensor. b) Sensitivity vs. line length. The two interpolation lines are related to devices with different line widths of 35 and 70 µm 32 CHAPTER 5. DOUBLE BIAS PHEB other so as to have uniform current flowing through the devices. Figure 5.6a shows a typical response of the device when a magnetic field is applied perpendicular to the exchange bias direction. There is a linearity section around zero field. The sensitivity is taken from the slope of the curve. In Figure 5.6b, the sensitivity of several devices of different lengths and widths is plotted. For comparison, the resistance is also plotted. Fig- ure 5.6b shows that there is a correlation between the sensitivity and the resistance of the material. From Figure 5.6b, we can see that the sensitivity intersection point is at 0 mm in length, and that it then increases linearly. The sensitivity of the 35 µm line-width device increases twice as fast as the one with 70 µm width, as expected from the theory [40]. Chapter 6

NiFeX PHEB

The PHEB sensors to be developed in this work use AMR in a ferromagnetic layer to read out the direction of the magnetization in the FM. The PHEB sensor also uses an antiferromagnetic layer to define the direction of the FM magnetization at zero applied field [26]. IrMn was still used as the AFM material, due to its high resistivity, which garanties that a large part of the current is forced to flow in the less resistive material NiFe[17]. In this work, the FM material was NiFe improved by alloying it with other low resistance materials. The final alloy combination was Ni77Fe18X5, where X is Cu, Ag, Au (for simplicity, this is referred to as NiFeX here). NiFe is

Figure 6.1: a) Material stack of double bias sensing layer; b) material stack of NiFeX sensing layer

33 34 CHAPTER 6. NIFEX PHEB cheap and easy to work with, has a sizable AMR, and has excellent soft magnetic properties over a wide temperature range. Cu, Ag, and Au were chosen because of their very high conductivity, still a thin layer of 100% NiFe was used in order to avoind difusion of the alloy into the AFM layer (Fig. 6.1b)[39].

6.1 Characterization of NiFeX

The FM materials were sputtered by using a single NiFeX alloy target; they exhibited good AMR, close to 3%, for NiFeAu and NiFeCu, but lower for NiFeAg. In Figure 6.2a, we show how the magnetoresistance and the resistance change for different NiFeX thicknesses. The AMR graph shows a significant difference between the NiFeX alloys: NiFeAu has an AMR close to 3%, which is similar to 100% NiFe. NiFeCu has lower AMR than NiFeAu, with values between 2.2% and 2.4%. NiFeAg has a significantly lower AMR, at less than 2%. Figure 6.2b shows the resistance behavior of NiFeX for different layer thicknesses. As expected, the resistance decreases with the increase in the material thickness. What is not expected is the high resistance of the NiFeAg alloy. Ag has a nominal resistivity lower than Au and Cu, but when alloyed with NiFe has a greater resistance. This behavior can be explained

Figure 6.2: a) AMR vs. thickness of NiFeX alloys; b) resistance vs. thickness of NiFeX alloys 6.2. NIFEX AND 100% NIFE 35 by considering the Ag as impurities in the NiFe, and not as part of the material alloy; this can aﬀect the electron current path in the material, which increases the resistance. The high resistance of NiFeAg might also be explained in part by the greater roughness of this alloy; see Figure 6.2. Also, other factors may aﬀect the resistance of NiFeAg, but investigating them would go beyond the scope of this work. In order to understand how Cu, Ag, and Au change the physical parameters of the NiFe, and to determine whether the cause of the poor magnetic properties and high resistance of NiFeAg is the roughness of Ag, the roughness of the materials has been studied using atomic force microscopy [39].

6.2 NiFeX and 100% NiFe

Figure 6.3a shows the hysteresis loop data for NiFeX. The exchange bias values are similar between the different alloys; only the coercivity is slightly different for the NiFeAg. Figure 6.3b shows the sensitivity curves of NiFeX and NiFe. As expected, 100% NiFe has higher sensitivity than NiFeX. The constant sensitivity and coercivity of all the devices with different thickness is not always confirmed. The sensitivity of devices with an FM layer thickness greater than 150 nm decreases rapidly. NiFeAu and NiFeCu show sensitivity values of around 3 V/T, with a 1 mA applied current. Thicker sensing layers than 150 nm of NiFeX show much lower sensitivities and

Figure 6.3: a) Exchange bias ﬁeld of NiFeX; b) sensitivity curves of 100% NiFe and NiFeX alloys 36 CHAPTER 6. NIFEX PHEB

Figure 6.4: Sensitivity vs. NiFeX thickness[39]

also higher coercivities, indicating a loss of film internal unidirectional anisotropy, which can be explained by the nonsingle domain behavior of the FM layer. The exchange field in an FM layer goes in inverse proportion to the FM film thickness, and this behavior helps to compensate for the loss of sensitivity due to the lower resistance of thicker devices; as a result, we get devices with sensitivities independent of thickness. This theoretical predic- tion is confirmed by actual measurements in Figure 6.4, where it is shown that the sensitivity does not follow the resistance versus thickness behavior for thicknesses up to 150 nm. For thicker films, both sensitivity and resistance decrease, due to the loss of film quality. In summary, the sensitivity is proportional to the resistance and, because the resistance is inversely proportional to the FM thickness, the sensitivity should also be inversely proportional to the FM film thickness. However, we also have to consider the exchange bias field, which is inversely proportional to the FM film thickness. As a result of this, the resistance behavior and the exchange bias field behavior cancel each other, leaving the sensitivity independent of the thickness. Characterization of the resistance values of devices with different NiFeX thicknesses is a target of this work. The data shows that, in general, the resistance of the devices decreases when the thickness of the NiFeX increases, as shown in Figure 6.5. The general behavior between resistance and the thickness follows 1/(a + bt), where t is the thickness of the NiFeX material and a is a constant describing the resistance of the other layers, IrMn, Ta, and NiFe. NiFeCu and NiFeAu, on the other hand, show expected behavior, 6.2. NIFEX AND 100% NIFE 37

Figure 6.5: Sensitivity vs. NiFeX width[39] with the values being similar to NiFeAu but slightly higher. The reason for this unexpected result is not understood yet. The sensitivity of the NiFeX alloy is, in general, above 3 V/T, with 1 mA current between the samples with different thickness—except for NiFeAg, which rapidly deteriorates at higher thicknesses (Fig. 6.4). Figure 6.6 shows how the sensitivity of NiFeX and 100% NiFe behave when the width of the device lines changes. In Figure 6.6, devices with 50 nm NiFe are compared with devices with a 50 nm NiFeX layer. The two layers of NiFe are necessary to avoid the nonmagnetic material (Cu, Ag, Au) diffusing into the IrMn and thus affecting the exchange performance. The other parameters of the devices—line length, the number of lines, and thickness—are similar between them. The resistance of the devices decreases with line width; this is expected, since wider lines have lower resistance than thinner lines. The sensitivity follows, in general, the same behavior as the resistance when it comes to line length, width, or number. The results show a higher resistance for NiFeX than for NiFe when the width of the devices is 25 µm, while for 50 µm and larger, the situation is the opposite. The sensitivity follows this behavior since it is proportional to the resistance [39]. The sensitivity values conferm the data given from the resistance and the MR of the NiFeX, where NiFeAg has the lowest values. NiFeAg is confermed to be an alloy 38 CHAPTER 6. NIFEX PHEB that lead to devices with poor performance[39]. The correlation between the sensitivity and the FM thickness was investigated as well. The thicknesses studied ranged from 50 nm to 200 nm for NiFeX. A correlated measurement between resistance and sensitivity was performed for the devices shown in Figure 6.5. Finally, we performed the first noise measurements on a double bias NiFe and NiFeX sensor (Fig. 6.6). The goal was to verify if the low resistance of NiFeX alloy compared to 100% NiFe translates into lower noise. Figure 6.6 shows the outcome. First, it is clear that the devices with the NiFeAg have the highest noise levels. This was expected from the resistance and sensitivity data.√ At 2 Hz, the noise of the device with the NiFeAg alloy is about 20 nV/ Hz with 1 mA applied current. An unexpected result is the relation between the noise levels of 100% NiFe compared to those of NiFeCu and NiFeAu. We use the same measurement point for this comparison: noise at 2 Hz with 1 mA applied current. Even though the NiFeCu and NiFeAu have lower resistance√ than 100% NiFe, they show higher√ noise levels: Ni- FeCu at about 10 nV/ Hz√ and NiFeAu at about 7 nV/ Hz, whereas NiFe shows only about 5 nV/ Hz [39].

Figure 6.6: Noise levels of 100% NiFe and NiFeX Chapter 7

IrMn/NiFe multilayer PHEB

The goal of increasing the number of exchange bias layers is to decrease the resistance without losing, according to theory [50, 51], device performance[52, 53]. A comprehensible explanation of what happens to the overall device performance with multilayer FM can be obtained by imagining a rep- lica of the ﬁrst device with a 150 nm sensing layer being placed on top of a second device with the same characteristics(Fig. 7.1). By doing this, we are using two identical devices in parallel, so we have, in theory, two similar devices working as a single device; however, the total resistance is half of that of the single device. Following this path, we can further increase the number of layers to further decrease the resistance of the devices. By hav- ing lower resistance, the device will also have less noise and, ﬁnally, a lower detectivity[54].

7.1 Characterization of IrMn/NiFe multilayer

Figure 7.2a shows a thickness study for the quadruple exchange-biased material stack resulting in uniform AMR values slightly under 3% for total FM thicknesses up to 300 nm. For 400 nm, the AMR is reduced and shows a nonuniform behavior. Figure 7.2b shows the exchange bias ﬁeld for a fourfold IrMn/NiFe interface. The exchange ﬁeld behaves similarly to the double-biased case. The internal magnetization decreases with increased FM thickness. The switching of the internal magnetization is uniform for FM thicknesses until 300 nm.

39 40 CHAPTER 7. IRMN/NIFE MULTILAYER PHEB

Figure 7.1: IrMn/NiFe multilayer

The hysteresis loop of 400 nm in NiFe shows a double switching of the internal magnetization, indicating that the exchange bias ﬁeld is not strong enough to keep the internal magnetization uniform, but the two layers for NiFe switch separately. Figure 7.3a shows AMR measurements for fourfold and sixfold exchange-

Figure 7.2: a) AMR results from NiFe double layer with different thickness. b)Hysteresis loops of Ta/2(IrMn/NiFe)/IrMn/Ta. The graph shows different curves for different NiFe thicknesses[54]. 7.1. CHARACTERIZATION OF IRMN/NIFE MULTILAYER 41

Figure 7.3: a) AMR graphs of diﬀerent NiFe layer with 150 nm thickness. b) Hysteresis loop of Ta X(IrMn NiFe) IrMn Ta, where the NiFe is 150 nm thick[54]. biased IrMn/NiFe material stacks. The AMR values are similar for all these layers and show a similar behavior for both the ﬁeld sweep directions. Here, the thickness is 50 nm for each NiFe layer. Thicker NiFe layers for the sixfold layer, and even higher numbers of IrMn/NiFe layers, are described later [51]. Figure 7.3b shows the hysteresis loop of the fourfold and sixfold bias material stacks. The exchange bias and the coercivity of these two samples are similar, which indicates that we can use even thicker FM [51].

Figure 7.4: Exchange bias feld and coercivity ﬁeld for IrMn/NiFe multilayer 42 CHAPTER 7. IRMN/NIFE MULTILAYER PHEB

In this work, we did not stop at a sixfold bias multilayer, but continued the study to even more layers. As shown in Figure 7.4, the study continued to fourteen bias interfaces, although the ﬁgure illustrates the number of NiFe layers instead of the number of FM/AFM interfaces. The exchange bias ﬁeld increases slightly with the number of AFM/FM interfaces but, on the other hand, the coercivity decreases. This behavior of coercivity and exchange bias can help us understand how devices with multilayers will behave. Hav- ing a coercivity that does not decrease, but which slightly increases with the number of layers, indicates that the sensitivity of the devices can still act in a linear manner. The stable coercivity between all the multilayers can lead to devices with lower noise [54].

7.2 IrMn/NiFe single and multilayer

Figure 7.5 shows the sensitivity of a fourteenfold exchange-biased device. The black curve indicates the sensitivity of the device before annealing. The red curve indicates the sensitivity after annealing at 200 °C for 8 hours under an applied magnetic ﬁeld of 0.2 T. It is quite clear that the sensitivity of the multilayer device is not linear around zero ﬁeld before annealing, and only after annealing is it possible to obtain a device that behaves linearly around

Figure 7.5: IrMn/NiFe multilayer before and after annealing 7.2. IRMN/NIFE SINGLE AND MULTILAYER 43

Figure 7.6: Noise levels of IrMn/NiFe multilayer vs. applied current zero ﬁeld. Although the device acts linearly after annealing, the sensitivity value is not similar to that of double bias devices. As shown above, the sensitivity for double-biased devices is around 25 V/T with 10 mA applied current, whereas the sensitivity of the fourteenfold-biased device after annealing is around 17.5 V/T with 50 mA applied current. The diﬀerence in the applied current is necessary in order to compare eventually, the devices with the best performance. The goal was that each of the seven FM layers

Figure 7.7: The different colors indicate the temperature gradient of the simulated device 44 CHAPTER 7. IRMN/NIFE MULTILAYER PHEB need to have the same current density as the single FM layer devices, but this was not possible. In theory, the device with double bias and the device with the fourteenfold bias should have the same sensitivity, since each of the FM layers carries the same current density. However, the experimental data do not follow the theoretical explanation [51]. These results can have many explanations—for example, when increasing the number of layers, the roughness of the materials increases too; this could affect the exchange bias interface between the AFM and FM. Another explanation might be that the internal anisotropy of the single layers is different from them, affecting total sensitivity. The noise and then the detectivity study can make the understanding clearer[54]. The noise levels of the fourteenfold-biased devices, with applied currents varying from 0 to 82 mA, are shown in Figure 7.6. This study is critical in understanding how the devices behave at different applied currents. The goal is to reach the threshold current where the noise starts to increase at a higher rate. In other words, currents higher than the threshold create heat in the device causing higher noise. In Figure 7.6, it can be seen that the noise rate increases faster in devices with applied current higher than 50 mA, which is due to the increased device temperature. For a better understanding of device performance at small fields, it is ne-

Figure 7.8: a) Detectivity values of the double-biased devices and fourteenfold-biased devices measured at 2 Hz vs. applied current density. b) Simulation data of the heat generated vs. current density from one and seven NiFe layers. 7.2. IRMN/NIFE SINGLE AND MULTILAYER 45 cessary to study the detectivity behavior. The detectivity is proportional to the noise over the sensitivity and applied current. A range of currents from 0 to 83 mA has been studied in order to understand what current gives the lowest detectivity. In order to better understand the reason behind the difference in performance between double-biased and fourteenfold-biased device, we run some simulation (by Comsol). As we it was written above 50 mA of applied current the noise was increasing at higher rate. We simulate a single line of the device by using the same lenght and widgh og the fabricated devices. The line was placed in a Si subatrate of thickness of 500µm in order to seimlutae the Si wafer substrate. Than it was used W/(m2k) as a heat transfer coe- ficient in order to simulte a stationary air dissipation, Fig.7.7. In Fig. 7.8a it was compared the detectivity values at 2 Hz of single and seven NiFe layer and in Fig. 7.8b, it is shown the simulated results. It si clear the the seven layer reaches higher temperature at the same current density of the double-biased. This results confirm that the fourteenfold-biased device has less efficient heat dissipation than the single Nife layer. In Figure 7.9, we show the detectivity versus applied current of a fourteenfold- biased device at both 100 Hz and at 2 Hz. As the figure shows, the detectivity at 100 Hz decreases with applied current up to a knee at 50 mA, but then increases, whereas at 2 Hz this knee is at 70 mA. This is due to 1/f noise playing a more important role at 2 Hz: below the knee, the detectivity at

Figure 7.9: Detectivity levels at 2 and 100 Hz of IrMn/NiFe multilayer vs. applied current 46 CHAPTER 7. IRMN/NIFE MULTILAYER PHEB

100 Hz is inversely proportional to the applied current, but at 2 Hz, the detectivity is inversely proportional to the square root of√ the applied current. The lowest detectivity at 2 Hz is slightly above 600√ pT/ Hz, while the lowest detectivity at 100 Hz is slightly above 100 pT/ Hz. Chapter 8

PHEB irradiation

Another important study was the investigation of the performance of the devices upon irradiation with Co-60 gamma rays[56]. For a better understanding of how radiation can aﬀect the physical propri- eties of the material, we have compared the resistance before and after irradiation. The samples exposed to radiation were from the same fabricated wafer with fourteen exchange bias interface layers. The samples have the same line length and the same line width, but a diﬀerent number of lines [55].

Figure 8.1: a) Devices resistance before and after irradiation together with the resistance of the reference devices; b) Diﬀerence of the resistance after the irradiation vs. TID[55]

47 48 CHAPTER 8. PHEB IRRADIATION

Figure 8.2: a) Sensitivity curves of IrMn/NiFe before and after irradiation at 300 krad. b) Noise performance of the devices powered with 5 mA and not powered[55]

8.1 Characterization of irradiated PHEB

In Figure 8.1a, we compared the total resistance of the devices before and after irradiation. One sample was used as a reference and was not irradiated. In Figure 8.1b, we compared the differential resistance (resistance before and after the irradiation) with the total irradiation dose (TID). The samples were irradiated with four different TID ( 10, 100 and 300 krad). In each samples was included four identical devices. A nonsignificant change in resistance was found for samples irradiated with TID up to 100 krad. In Figure 8.1b indicated the mean value of the difference in resistance of the samples irradiated with different TID, the error bars indicate the the standard deviation of the resistance mean value. There is a slight decrease in resistance in the samples irradiated at 300 krad[57]. One other aspect taken into account was the sensitivity curves. As shown in Fig- ure 8.2a, the linearity of the device around zero field has not changed, even at the highest TID of 300 krad. FIG. 8.2b shows the noise of tw The influence of applied current during the irradiation was investigated as well. During the irradiation, two of the four devices for each samples were powered with a constant current of 5 mA. The samples were irradiated with 100 krad and nonsignificative difference were seen in the noise performance. the devices seems not to be affected by the applied current during irradiation [57]. The noise of the irradiated 8.1. CHARACTERIZATION OF IRRADIATED PHEB 49

Figure 8.3: a) Noise performance of the devices after irradiation together with the noise of the reference devices. a) Detectivity of the devices after irradiation[55] samples at 2 Hz is compared with a reference sample in Figure 8.3a. It is clear that the noise levels of the devices irradiated with a TID of up to 100 krad are similar to that of the nonirradiated reference sample, while the noise level of the device irradiated with 300 krad is signiﬁcantly higher[57]. In Figure 8.3b, we show the detectivity vs. applied current of a fourteenfold- biased irradiated device at 2 Hz. As shown above in the noise ﬁgure, the detectivity data also show results for the samples irradiated up to 100 krad that are uniform with the nonirradiated reference sample. The low detectivity of the sample irradiated at 50 krad is likely a consequence of the accident described above [55].

Chapter 9

Conclusion

This work has completed the material property study as intended and has manufactured some promising sensor devices that were proven to be toler- ant to irradiation of up to 100 krad. Such sensors have been integrated with electronics into a magnetometer module to demonstrate its feasibility for use when combined with a communications module and a power supply module into a complete magnetometer unit. The material development part of the work investigated NiFeX and IrMn/NiFe multilayers and demonstrated significant progress. All fundamental material properties and their thickness dependencies have been determined. The NiFeX alloy showed lower resistivity than 100% NiFe, although this result did not translate into low noise. The IrMn/NiFe multilayers showed the most promising results. The multilayer structure lowered the resistance of the devices and also led to a significant decrease in noise. This resulted in the possibility of applying high currents of up to 50 mA and still have a performing device. However, the choice of a multilayer device leads to a slightly reduction in sensitivity, on the other side with the multilayer choice we reached also a significant reduction of resistivity which√ means that we improved the detectivity of the devices down to 600 pT/ Hz at 2 Hz.

Future work

This work has made substantial progress in the ﬁelds of magnetometers of space application. The main eﬀort of the study was about material de-

51 52 CHAPTER 9. CONCLUSION velopment. Although the important progress made by this work, there are several question still not answered. The study focused on the development of the material alloy used as a sensing layer and also of the multilayer material stack. The NiFeX alloy has been investigated only with a 5% concentration of the alloyed material with the Permalloy. An interesting study can be done by investigation also another percentage of this alloying material. A different parentage of the alloying material can affect not only the physical performance but also the magnetic one. Finally, the goal is the investigate how the detectivity is affected by the different alloy. The multilayer stack was the core part of this work. This work investigated IrMn–NiFe multilayers up to seven. From the devices side only devices with seven NiFe layer were fabricated, beside the single layer one. Investigating also the devices performance with different multilayer can be an interesting study. The goal is the have a better understanding of the devices performance vs. number of IrMn–NiFe multilayer. Bibliography

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