Micromagnetism
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Basic Magnetic Measurement Methods
Basic magnetic measurement methods Magnetic measurements in nanoelectronics 1. Vibrating sample magnetometry and related methods 2. Magnetooptical methods 3. Other methods Introduction Magnetization is a quantity of interest in many measurements involving spintronic materials ● Biot-Savart law (1820) (Jean-Baptiste Biot (1774-1862), Félix Savart (1791-1841)) Magnetic field (the proper name is magnetic flux density [1]*) of a current carrying piece of conductor is given by: μ 0 I dl̂ ×⃗r − − ⃗ 7 1 - vacuum permeability d B= μ 0=4 π10 Hm 4 π ∣⃗r∣3 ● The unit of the magnetic flux density, Tesla (1 T=1 Wb/m2), as a derive unit of Si must be based on some measurement (force, magnetic resonance) *the alternative name is magnetic induction Introduction Magnetization is a quantity of interest in many measurements involving spintronic materials ● Biot-Savart law (1820) (Jean-Baptiste Biot (1774-1862), Félix Savart (1791-1841)) Magnetic field (the proper name is magnetic flux density [1]*) of a current carrying piece of conductor is given by: μ 0 I dl̂ ×⃗r − − ⃗ 7 1 - vacuum permeability d B= μ 0=4 π10 Hm 4 π ∣⃗r∣3 ● The Physikalisch-Technische Bundesanstalt (German national metrology institute) maintains a unit Tesla in form of coils with coil constant k (ratio of the magnetic flux density to the coil current) determined based on NMR measurements graphics from: http://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_2/2.5_halbleiterphysik_und_magnetismus/2.51/realization.pdf *the alternative name is magnetic induction Introduction It -
Thermal Fluctuations of Magnetic Nanoparticles: Fifty Years After Brown1)
THERMAL FLUCTUATIONS OF MAGNETIC NANOPARTICLES: FIFTY YEARS AFTER BROWN1) William T. Coffeya and Yuri P. Kalmykovb a Department of Electronic and Electrical Engineering, Trinity College, Dublin 2, Ireland b Laboratoire de Mathématiques et Physique (LAMPS), Université de Perpignan Via Domitia, 52, Avenue Paul Alduy, F-66860 Perpignan, France The reversal time (superparamagnetic relaxation time) of the magnetization of fine single domain ferromagnetic nanoparticles owing to thermal fluctuations plays a fundamental role in information storage, paleomagnetism, biotechnology, etc. Here a comprehensive tutorial-style review of the achievements of fifty years of development and generalizations of the seminal work of Brown [W.F. Brown, Jr., Phys. Rev., 130, 1677 (1963)] on thermal fluctuations of magnetic nanoparticles is presented. Analytical as well as numerical approaches to the estimation of the damping and temperature dependence of the reversal time based on Brown’s Fokker-Planck equation for the evolution of the magnetic moment orientations on the surface of the unit sphere are critically discussed while the most promising directions for future research are emphasized. I. INTRODUCTION A. THERMAL INSTABILITY OF MAGNETIZATION IN FINE PARTICLES B. KRAMERS ESCAPE RATE THEORY C. SUPERPARAMAGNETIC RELAXATION TIME: BROWN’S APPROACH II. BROWN’S CONTINUOUS DIFFUSION MODEL OF CLASSICAL SPINS A. BASIC EQUATIONS B. EVALUATION OF THE REVERSAL TIME OF THE MAGNETIZATION AND OTHER OBSERVABLES III. REVERSAL TIME IN SUPERPARAMAGNETS WITH AXIALLY-SYMMETRIC MAGNETOCRYSTALLINE ANISOTROPY A. FORMULATION OF THE PROBLEM B. ESTIMATION OF THE REVERSAL TIME VIA KRAMERS’ THEORY C. UNIAXIAL SUPERPARAMAGNET SUBJECTED TO A D.C. BIAS FIELD PARALLEL TO THE EASY AXIS IV. REVERSAL TIME OF THE MAGNETIZATION IN SUPERPARAMAGNETS WITH NONAXIALLY SYMMETRIC ANISOTROPY 1) Published in Applied Physics Reviews Section of the Journal of Applied Physics, 112, 121301 (2012). -
Dynamic Symmetry Loss of High-Frequency Hysteresis Loops in Single-Domain Particles with Uniaxial Anisotropy
Journal of Magnetism and Magnetic Materials 324 (2012) 466–470 Contents lists available at SciVerse ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Dynamic symmetry loss of high-frequency hysteresis loops in single-domain particles with uniaxial anisotropy Gabriel T. Landi Instituto de Fı´sica da Universidade de Sao~ Paulo, 05314-970 Sao~ Paulo, Brazil article info abstract Article history: Understanding how magnetic materials respond to rapidly varying magnetic fields, as in dynamic Received 2 June 2011 hysteresis loops, constitutes a complex and physically interesting problem. But in order to accomplish a Available online 23 August 2011 thorough investigation, one must necessarily consider the effects of thermal fluctuations. Albeit being Keywords: present in all real systems, these are seldom included in numerical studies. The notable exceptions are Single-domain particles the Ising systems, which have been extensively studied in the past, but describe only one of the many Langevin dynamics mechanisms of magnetization reversal known to occur. In this paper we employ the Stochastic Landau– Magnetic hysteresis Lifshitz formalism to study high-frequency hysteresis loops of single-domain particles with uniaxial anisotropy at an arbitrary temperature. We show that in certain conditions the magnetic response may become predominantly out-of-phase and the loops may undergo a dynamic symmetry loss. This is found to be a direct consequence of the competing responses due to the thermal fluctuations and the gyroscopic motion of the magnetization. We have also found the magnetic behavior to be exceedingly sensitive to temperature variations, not only within the superparamagnetic–ferromagnetic transition range usually considered, but specially at even lower temperatures, where the bulk of interesting phenomena is seen to take place. -
Magnetic Switching of a Stoner-Wohlfarth Particle Subjected to a Perpendicular Bias Field
electronics Article Magnetic Switching of a Stoner-Wohlfarth Particle Subjected to a Perpendicular Bias Field Dong Xue 1,* and Weiguang Ma 2 1 Department of Physics and Astronomy, Texas Tech University, Lubbock, TX 79409-1051, USA 2 Department of Physics, Umeå University, 90187 Umeå, Sweden; [email protected] * Correspondence: [email protected]; Tel.: +1-806-834-4563 Received: 28 February 2019; Accepted: 21 March 2019; Published: 26 March 2019 Abstract: Characterized by uniaxial magnetic anisotropy, the Stoner-Wohlfarth particle experiences a change in magnetization leading to a switch in behavior when tuned by an externally applied field, which relates to the perpendicular bias component (hperp) that remains substantially small in comparison with the constant switching field (h0). The dynamics of the magnetic moment that governs the magnetic switching is studied numerically by solving the Landau-Lifshitz-Gilbert (LLG) equation using the Mathematica code without any physical approximations; the results are compared with the switching time obtained from the analytic method that intricately treats the non-trivial bias field as a perturbation. A good agreement regarding the magnetic switching time (ts) between the numerical calculation and the analytic results is found over a wide initial angle range (0.01 < q0 < 0.3), as h0 and hperp are 1.5 × K and 0.02 × K, where K represents the anisotropy constant. However, the quality of the analytic approximation starts to deteriorate slightly in contrast to the numerical approach when computing ts in terms of the field that satisfies hperp > 0.15 × K and h0 = 1.5 × K. Additionally, existence of a comparably small perpendicular bias field (hperp << h0) causes ts to decrease in a roughly exponential manner when hperp increases. -
Coey-Slides-1.Pdf
These lectures provide an account of the basic concepts of magneostatics, atomic magnetism and crystal field theory. A short description of the magnetism of the free- electron gas is provided. The special topic of dilute magnetic oxides is treated seperately. Some useful books: • J. M. D. Coey; Magnetism and Magnetic Magnetic Materials. Cambridge University Press (in press) 600 pp [You can order it from Amazon for £ 38]. • Magnétisme I and II, Tremolet de Lachesserie (editor) Presses Universitaires de Grenoble 2000. • Theory of Ferromagnetism, A Aharoni, Oxford University Press 1996 • J. Stohr and H.C. Siegmann, Magnetism, Springer, Berlin 2006, 620 pp. • For history, see utls.fr Basic Concepts in Magnetism J. M. D. Coey School of Physics and CRANN, Trinity College Dublin Ireland. 1. Magnetostatics 2. Magnetism of multi-electron atoms 3. Crystal field 4. Magnetism of the free electron gas 5. Dilute magnetic oxides Comments and corrections please: [email protected] www.tcd.ie/Physics/Magnetism 1 Introduction 2 Magnetostatics 3 Magnetism of the electron 4 The many-electron atom 5 Ferromagnetism 6 Antiferromagnetism and other magnetic order 7 Micromagnetism 8 Nanoscale magnetism 9 Magnetic resonance Available November 2009 10 Experimental methods 11 Magnetic materials 12 Soft magnets 13 Hard magnets 14 Spin electronics and magnetic recording 15 Other topics 1. Magnetostatics 1.1 The beginnings The relation between electric current and magnetic field Discovered by Hans-Christian Øersted, 1820. ∫Bdl = µ0I Ampère’s law 1.2 The magnetic moment Ampère: A magnetic moment m is equivalent to a current loop. Provided the current flows in a plane m = IA units Am2 In general: m = (1/2)∫ r × j(r)d3r where j is the current density; I = j.A so m = 1/2∫ r × Idl = I∫ dA = m Units: Am2 1.3 Magnetization Magnetization M is the local moment density M = δm/δV - it fluctuates wildly on a sub-nanometer and a sub-nanosecond scale. -
Magnetization and Demagnetization Studies of a HTS Bulk in an Iron Core Kévin Berger, Bashar Gony, Bruno Douine, Jean Lévêque
Magnetization and Demagnetization Studies of a HTS Bulk in an Iron Core Kévin Berger, Bashar Gony, Bruno Douine, Jean Lévêque To cite this version: Kévin Berger, Bashar Gony, Bruno Douine, Jean Lévêque. Magnetization and Demagnetization Stud- ies of a HTS Bulk in an Iron Core. IEEE Transactions on Applied Superconductivity, Institute of Electrical and Electronics Engineers, 2016, 26 (4), pp.4700207. 10.1109/TASC.2016.2517628. hal- 01245678 HAL Id: hal-01245678 https://hal.archives-ouvertes.fr/hal-01245678 Submitted on 19 Dec 2015 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 1PoBE_12 1 Magnetization and Demagnetization Studies of a HTS Bulk in an Iron Core Kévin Berger, Bashar Gony, Bruno Douine, and Jean Lévêque Abstract—High Temperature Superconductors (HTS) are large quantity, with good and homogeneous properties, they promising materials in variety of practical applications due to are still the most promising materials for the applications of their ability to act as powerful permanent magnets. Thus, in this superconductors. paper, we have studied the influence of some pulsed and There are several ways to magnetize HTS bulks; but we pulsating magnetic fields applied to a magnetized HTS bulk assume that the most convenient one is to realize a method in sample. -
Module 2C: Micromagnetics
Module 2C: Micromagnetics Debanjan Bhowmik Department of Electrical Engineering Indian Institute of Technology Delhi Abstract In this part of the second module (2C) we will show how the Stoner Wolfarth/ single domain mode of ferromagnetism often fails to match with experimental data. We will introduce domain walls in that context and explain the basic framework of micromagnetics, which can be used to model domain walls. Then we will discuss how an anisotropic exchange interaction, known as Dzyaloshinskii Moriya interaction, can lead to chirality of the domain walls. Then we will introduce another non-uniform magnetic structure known as skyrmion and discuss its stability, based on topological arguments. 1 1 Brown's paradox in ferromagnetic thin films exhibiting perpendicular magnetic anisotropy We study ferromagnetic thin films exhibiting Perpendicular Magnetic Anisotropy (PMA) to demonstrate the failure of the previously discussed Stoner Wolfarth/ single domain model to explain experimentally observed magnetic switching curves. PMA is a heavily sought after property in magnetic materials for memory and logic applications (we will talk about that in details in the next module). This makes our analysis in this section even more relevant. The ferromagnetic layer in the Ta/CoFeB/MgO stack, grown by room temperature sput- tering, exhibits perpendicular magnetic anisotropy with an anisotropy field Hk of around 2kG needed to align the magnetic moment in-plane (Fig. 1a). Thus if the ferromagnetic layer is considered as a giant macro-spin in the Stoner Wolfarth model an energy barrier equivalent to ∼2kG exists between the up (+z) and down state (-z) (Fig. 1b). Yet mea- surement shows that the magnet can be switched by a field, called the coercive field, as small as ∼50 G, which is 2 orders of magnitude smaller than the anisotropy field Hk, as observed in the Vibrating Sample Magnetometry measurement on the stack (Fig. -
Magnetically Multiplexed Heating of Single Domain Nanoparticles
Magnetically Multiplexed Heating of Single Domain Nanoparticles M. G. Christiansen,1) R. Chen,1) and P. Anikeeva1,a) 1Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA Abstract: Selective hysteretic heating of multiple collocated sets of single domain magnetic nanoparticles (SDMNPs) by alternating magnetic fields (AMFs) may offer a useful tool for biomedical applications. The possibility of “magnetothermal multiplexing” has not yet been realized, in part due to prevalent use of linear response theory to model SDMNP heating in AMFs. Predictive successes of dynamic hysteresis (DH), a more generalized model for heat dissipation by SDMNPs, are observed experimentally with detailed calorimetry measurements performed at varied AMF amplitudes and frequencies. The DH model suggests that specific driving conditions play an underappreciated role in determining optimal material selection strategies for high heat dissipation. Motivated by this observation, magnetothermal multiplexing is theoretically predicted and empirically demonstrated for the first time by selecting SDMNPs with properties that suggest optimal hysteretic heat dissipation at dissimilar AMF driving conditions. This form of multiplexing could effectively create multiple channels for minimally invasive biological signaling applications. Text: Magnetic fields provide a convenient form of noninvasive electronically driven stimulus that can reach deep into the body because of the weak magnetic properties and low -
Ncomms5548.Pdf
ARTICLE Received 30 Oct 2013 | Accepted 27 Jun 2014 | Published 22 Aug 2014 DOI: 10.1038/ncomms5548 Magnetic force microscopy reveals meta-stable magnetic domain states that prevent reliable absolute palaeointensity experiments Lennart V. de Groot1, Karl Fabian2, Iman A. Bakelaar3 & Mark J. Dekkers1 Obtaining reliable estimates of the absolute palaeointensity of the Earth’s magnetic field is notoriously difficult. The heating of samples in most methods induces magnetic alteration—a process that is still poorly understood, but prevents obtaining correct field values. Here we show induced changes in magnetic domain state directly by imaging the domain configurations of titanomagnetite particles in samples that systematically fail to produce truthful estimates. Magnetic force microscope images were taken before and after a heating step typically used in absolute palaeointensity experiments. For a critical temperature (250 °C), we observe major changes: distinct, blocky domains before heating change into curvier, wavy domains thereafter. These structures appeared unstable over time: after 1-year of storage in a magnetic-field-free environment, the domain states evolved into a viscous remanent magnetization state. Our observations qualitatively explain reported underestimates from otherwise (technically) successful experiments and therefore have major implications for all palaeointensity methods involving heating. 1 Paleomagnetic laboratory Fort Hoofddijk, Department of Earth Sciences, Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlands. 2 NGU, Geological Survey of Norway, Leiv Eirikssons vei, 7491 Trondheim, Norway. 3 Van’t Hoff Laboratory for Physical and Colloid Chemistry, Department of Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands. Correspondence and requests for materials should be addressed to L.V.dG. -
Apparatus for Magnetization and Efficient Demagnetization of Soft
3274 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 9, SEPTEMBER 2009 Apparatus for Magnetization and Efficient Demagnetization of Soft Magnetic Materials Paul Oxley Physics Department, The College of the Holy Cross, Worcester, MA 01610 USA This paper describes an electrical circuit that can be used to automatically magnetize and ac-demagnetize moderately soft magnetic materials and with minor modifications could be used to demagnetize harder magnetic materials and magnetic geological samples. The circuit is straightforward to replicate, easy to use, and low in cost. Independent control of the demagnetizing current frequency, am- plitude, and duration is available. The paper describes the circuit operation in detail and shows that it can demagnetize a link-shaped specimen of 430FR stainless steel with 100% efficiency. Measurements of the demagnetization efficiency of the specimen with different ac-demagnetization frequencies are interpreted using eddy-current theory. The experimental results agree closely with the theoretical predictions. Index Terms—Demagnetization, demagnetizer, eddy currents, magnetic measurements, magnetization, residual magnetization. I. INTRODUCTION to magnetize a magnetic sample by delivering a current propor- HERE is a widespread need for a convenient and eco- tional to an input voltage provided by the user and can be used T nomical apparatus that can ac-demagnetize magnetic ma- to measure magnetic properties such as - hysteresis loops terials. It is well known that to accurately measure the mag- and magnetic permeability. netic properties of a material, it must first be in a demagnetized Our apparatus is simple, easy to use, and economical. It state. For this reason, measurements of magnetization curves uses up-to-date electronic components, unlike many previous and hysteresis loops use unmagnetized materials [1], [2] and it designs [9]–[13], which therefore tend to be rather compli- is thought that imprecise demagnetization is a leading cause for cated. -
Spin Wave Dispersion in a Magnonic Waveguide G
1 Proposal for a standard micromagnetic problem: Spin wave dispersion in a magnonic waveguide G. Venkat, D. Kumar, M. Franchin, O. Dmytriiev, M. Mruczkiewicz, H. Fangohr, A. Barman, M. Krawczyk, and A. Prabhakar Abstract—We propose a standard micromagnetic problem, of Micromagnetic Framework (OOMMF) [12], LLG [13], Micro- a nanostripe of permalloy. We study the magnetization dynamics magus [14] and Nmag [15]. We rely on the finite difference and describe methods of extracting features from simulations. method (FDM) adopted by OOMMF and the finite element Spin wave dispersion curves, relating frequency and wave vector, are obtained for wave propagation in different directions relative method (FEM) used in Nmag. The latter is more suitable for to the axis of the waveguide and the external applied field. geometries with irregular edges [16]. However, the compu- Simulation results using both finite element (Nmag) and finite tation overhead and management of resources become major difference (OOMMF) methods are compared against analytic issues in FEM simulations. To compare different numerical results, for different ranges of the wave vector. solvers, the Micromagnetic Modeling Activity Group (µMag) Index Terms—Computational micromagnetics, spin wave dis- publishes standard problems for micromagnetism [17]–[19]. persion, exchange dominated spin waves A more recent addition included the effects of spin transfer torque [20]. However, there has thus far been no standard INTRODUCTION problem that includes the calculation of the spin wave dis- There have been steady improvements in computational persion of a magnonic waveguide. We believe that specifying micromagnetics in recent years, both in techniques as well as a standard problem will promote the use of micromagnetic in the use of graphical processing units (GPUs) [1]–[4]. -
Advanced Micromagnetics and Atomistic Simulations of Magnets
Advanced micromagnetics and atomistic simulations of magnets Richard F L Evans ESM 2018 Overview • Micromagnetics • Formulation and approximations • Energetic terms and magnetostatics • Magnetisation dynamics • Atomistic spin models • Foundations and approximations • Monte Carlo methods • Spin Dynamics • Landau-Lifshitz-Bloch micromagnetics (tomorrow) Micromagnetics source: mumax Why do we need magnetic simulations? Demagnetization factors for different shapes N = 0 N = 1/3 N = 1/2 N = 1 Infinite thin film Infinitely long Sphere cylinder Infinitely long cylinder Short cylinder Why do we need magnetic simulations? Jay Shah et al, Nature Communications 9 1173 (2018) Why do we need magnetic simulations? • Most magnetic problems are not solvable analytically • Complex shapes (cube or finite geometric shapes) • Complex structures (polygranular materials, multilayers, devices) • Magnetization dynamics • Thermal effects • Metastable phases (Skyrmions) Analytical micromagnetics • An analytical branch of micromagnetics, treating magnetism on a small (micrometre) length scale • Mathematically messy but elegant • When we talk about micromagnetics, we usually mean numerical micromagnetics Numerical micromagnetics • Treat magnetisation as a continuum approximation <M> • Average over the local atomic moments to give an average moment density (magnetization) that is assumed to be continuous • Then consider a small volume of space (1 nm)3 - (10 nm)3 where the magnetization (and all atomic moments) are assumed to point along the same direction The micromagnetic