Chapter 2 MAGNETOTRANSPORT PROPERTIES OF ULTRATHIN METALLIC MULTILAYERS: MICROSTRUCTURAL MODIFICATIONS LEADING TO SENSOR APPLICATIONS C. Christides Department of Engineering Sciences, School of Engineering, University of Patras, 26 110 Patras, Greece Contents 1.Sensors,Materials,andDevices........................................ 65 1.1.SensorCharacteristics.......................................... 65 1.2.All-MetalThin-FilmMagnetoresistiveSensors............................ 66 1.3. Spin Engineering of Metallic Thin-Film Structures . 68 1.4.MagnetoelectronicMemories..................................... 73 2. Morphology-Induced Magnetic and Magnetotransport Changes in GMR Films . 73 2.1. Oscillatory Magnetic Anisotropy and Magnetooptical Response . 73 2.2. Comparison between Epitaxial and Polycrystalline GMR Structures . 75 3. Performance Parameters of Microfabricated GMR Multilayers in Sensors . 77 4. Magnetotransport Properties in Polycrystalline Co/NM Multilayers . 79 4.1. Planar Hall Effect in Co/Cu Multilayers . 79 4.2.GMRinCo/CuMLs.......................................... 83 4.3.Low-FieldGMRinCo/AuMLs.................................... 90 4.4. Spin-Echo 59Co NMR Used for Nondestructive Evaluation of Co Layering . 100 4.5. Structural, Magnetic, and Magnetotransport Properties of NiFe/Ag Multilayers . 109 5.ColossalMagnetoresistanceinManganesePerovskiteFilms........................114 5.1.MaterialsProperties...........................................114 5.2.Low-FieldMagnetoresistanceinManganites.............................116 5.3. Exchange Bias in La–Ca–Mn–O Multilayers . 117 5.4.Advantages,Drawbacks,andProspectsofCMRFilmsinApplications...............123 6. Outline . 124 References...................................................126 1. SENSORS, MATERIALS, AND DEVICES tion by itself, it is always a part of a data acquisition system. Thus, a sensor responds to the stimulus and converts it into 1.1. Sensor Characteristics an electrical signal which is compatible with electronic cir- cuits. The sensor’s output signal may be in a form of voltage, In man-made devices, where the information is transmitted current, or charge. An ideal or theoretical output-stimulus re- and processed in electrical form through the transport of elec- lationship exists for every sensor that is characterized by the trons, a sensor is defined as a device that receives a signal so-called transfer function. This function establishes depen- or a stimulus and responds with an electrical signal [1]. The dence between the electrical signal S produced by the sen- stimulus is the quantity, physical property, or condition that sor, and the stimulus s: S = f(s). The transfer function is sensed as an input signal. Since a sensor does not func- may be a simple linear connection or a nonlinear dependence, Handbook of Surfaces and Interfaces of Materials, edited by H.S. Nalwa Volume 4: Solid Thin Films and Layers Copyright © 2001 by Academic Press All rights of reproduction in any form reserved. 65 66 CHRISTIDES that determines the most important characteristics of a sen- the number of circuits that fits on a chip doubles while the price sor: remains the same every year and a half. That trend has been driven by the steady shrinking of the size of microcircuits, as (i) In the general case of a nonlinear function, the sensitiv- defined by the smallest feature size, or linewidth. However, be- ity is defined as the slope b = dS(s )/ds at any particular input 0 sides the computing power of the microprocessors, the perfor- value s , and is not a fixed number as for a linear relationship: 0 mance of a computer depends on faster random-access memo- S = α + bs. ries (RAM) and denser magnetic storage of information as well. (ii) Full scale output is the algebraic difference between the In 1993, the cost of one megabyte of storage capacity was about electrical output signals measured with maximum input stimu- one dollar, a dramatic decrease from 104 dollars per megabyte lus and the lowest input stimulus applied. in 1956 when IBM first introduced the disk-drive technology. (iii) Accuracy errors arising from hysteresis, that is a devia- In 1998, the cost of one megabyte had decreased further, to tion of the sensor’s output at a specified point of the input signal less than five cents. The rapid areal density increase and the when it is approached from the opposite direction, and nonlin- stunning price–performanceimprovement have transformed the earity, which is the maximum deviation of a real transfer func- disk drive into the ubiquitous storage workhorse for computers tion from the approximation straight line. of all sizes. (iv) Inherent noise, which arises within the sensor’s circuit IBM innovations in the technology of magnetic hard-disk no matter how well it was designed, produces systematic dis- drives have driven up storage density at a phenomenal rate, tortions of the output signal. Such distortions are related to the now approaching 60% per year [2]. This increase is comparable sensor’s transfer function. The noise signals can, generally, be to the growth in semiconductor industry described by Moore’s described by an equivalent circuit that contains two additional law. The areal density—the number of discrete bits of infor- generators. One is a voltage noise generator e and the other n mation that can be squeezed onto a square inch of disk real is a current noise generator i . One contribution to the sensor n estate—has been increased at a pattern of magnetic fields that noise is thermal resistance noise (also called Johnson noise), magnetized bits on a circular track directly below on the spin- which is always present in resistive devices as a voltage noise. ning disk. To read back the data, the head was placed above This noise source contributes a background to the voltage spec- the track. As the bits spun beneath the head, the sweep of their tral density, which is representative of noise power, considered magnetic fields generated opposing voltages in the head. Since equal to S = 4k TRf (in units of volts squared per Hertz), V B the detectable magnetic flux (stimulus) from a bit decreases as where k is the Boltzmann constant, T is the temperature, R is B the bit gets smaller, a scaling approach to increased density the total resistance of the sensor, and f is the bandwidth, in requires reducing the read–write head’s dimensions while in- Hertz, over which the measurement is made. creasing its sensitivity. To sustain such progress, the technology From the physical point of view, a sensor is a converter of of recording-head fabrication will need to continue to advance generally nonelectrical effects into electrical signals. Often sev- at a rapid pace. eral transformation steps are required before the electric out- In 1969, the IBM T. J. Watson Research Center, invented a put signal can be generated. These steps involve changes of method for making the wire coils by the same photolithographic types of energy where the final step must produce an electri- thin-film techniques used to make semiconductor chips [3]. cal signal of desirable format. Since there are several physical That method led to very small and sensitive read heads that IBM effects which cause generation of electrical signals in response first used in products [3] in 1979. However, the increased ten- to nonelectrical influences, sensor classification schemes range dency for miniaturization in hard-disk drives required more sen- from very simple to the complex. Thus, depending of what it sitive sensors than the inductive coils in the reading heads, with measures (stimulus), what its specifications are, what physical an ability to measure smaller magnetic fields. Thus, the read- phenomenon it is sensitive to, what conversion mechanism is ing coils had to be replaced by an alternative material, able to employed, what material it is fabricated from, and what is its operate as an electromagnetic sensor. The electrical resistivity field of application, a broad and representative classification of a magnetoresistive (MR) thin ferromagnetic film [4] changes scheme [1] can be constructed. Magnetic memories and sen- according to the strength and the orientation of the magnetic sors are one of the oldest, yet one of the most widely used, field it experiences. In ferromagnetic materials, the “ordinary” solid-state devices. Already at an annual sale of about 40 bil- anisotropic transport effects, observed in nonmagnetic films, lion dollars, the market is still growing rapidly thanks to ever- are present but are accompanied by stronger phenomena having increasing demands in data storage and to new applications of similar geometrical dependences and symmetries. The galvano- magnetic devices in the field of sensors. magnetic effects unique to ferromagnets are called “extraordi- nary,” “spontaneous,” or “anomalous” because of their greater strength relative to the ordinary effects. The extraordinary gal- 1.2. All-Metal Thin-Film Magnetoresistive Sensors vanomagnetic effects derive their strength from the fact that the Today’s computers and their precursors are based on an idea role of the external field is replaced by an internal field propor- known as stored program electronic computer, which relies on tional to the magnetization M, which is generally much stronger electronic logic gates and memory. The speed of computers has than an applied field. The mechanism by which the microscopic doubled every three years, a trend known as Moore’s law. Thus, internal field associated with M couples to the current density MAGNETOTRANSPORT PROPERTIES