H. Kronmüller

Handbook of Magnetism and Advanced Magnetic Materials

VOLUME 1: Fundamentals and Theory Part 1: Electron Theory of Magnetism Density Functional Theory of Magnetism Hubbard Model Dynamical Mean Field Theory of Itinerant Electron Magnetism Quantum Monte Carlo Methods Part 2: Strongly Correlated Electronic Systems Heavy Fermions: electrons at the edge of magnetism The Kondo Effect Orbital physics in transition metal oxides: Magnetism and optics Part 3:Theory of Magnetic Spectroscopy and Scattering Magnetic Spectroscopy X-ray and Neutron Scattering by Magnetic Materials Part 4:Spin Dynamics and Relaxation Spin Waves History and A Summary of Recent Developments Dissipative Magnetization dynamics close to the adiabatic regime Part 5:Phase Transitions and Finite Temperature Magnetism Experiment and Analysis Electron Theory of Finite Temperature Magnetism Theory of Magnetic Phase Transitions Disordered and Frustrated Spin Systems Quantum Phase Transitions Part 6: Theory of Magneocrystalline Anisotropy and Magnetoelasticity Theory of Magnetocrystalline Anisotropy and Magnetoelasticity in transition metal systems Theory of Magnetocrystalline Anisotropy and Magnetoelasticity for 4f and 5f Metals Magnetostriction and Magnetoelasticity Theory: a Modern View Part 7: Theory of Transport and Exchange Phenomena in Layer Systems Exchange Coupling in Magnetic Multilayers Enhanced Magnetoresistance Berry phase in magnetism and the anomalous Hall effect Theory of Spin-Dependent Tunneling Part 8: Magnetism of Low Dimensions Magnetism of Low-dimensional Metallic Structures Magnetism of Low-Dimensional Systems: Theory Part 9: Molecular Magnets: Phenomenology and Theory Molecular Magnets: Phenomenology and Theory Part 10: Magnetism and Superconductivity Interplay of Superconductivity and Magnetism Magnetic Superconductors VOLUME 2: Micromagnetism Part 1: Fundamentals of Micromagnetism and Discrete Computational Models General Micromagnetic Theory Numerical Micromagnetics : Finite Difference Methods Numerical Methods in Micromagnetics (FEM) Magnetization dynamics including thermal fluctuations: basic phenomenology, fast remagnetization processes and transitions over high energy barriers Nonlinear Magnetization Dynamics in Nanomagnets Classical Spin Models Part 2: Micromagnetics Applications: Distribution of Equilibrium Configurations, Phase Diagrams and Hysteretic Properties- Small Objects Magnetization Configurations and reversal in small magnetic elements Magnetic Properties of Systems of Low Dimensions Part 3: Micromagnetics Applications: Distribution of Equilibrium Configurations, Phase Diagrams and Hysteretic Properties- Wall in Nanowires Domain Wall Propagation in Magnetic Wires Current Induced Domain-Wall Motion in Magnetic Nanowires The Motion of Domain Walls in Nano-Circuits and its Application to Digital Logic Part 4: Micromagnetics Applications: Distribution of Equilibrium Configurations, Phase Diagrams and Hysteretic Properties- Microstructure and Magnetization Processes Guided Spin Waves Micromagnetism-Microstructure Relations. Micromagnetism of the Hysteresis Loop Modelling of Non-linear Behaviour and Hysteresis in Magnetic Materials Part 5: Magnetization dynamics, solitons, Modes and Thermal Excitations Magnetization Dynamics: Thermal Driven Noise in Magnetoresistive Sensors Modes, Theory and Experiment Nonlinear Multi-dimensional Spin Wave Excitations in Magnetic Films Part 6: Micromagnetics of Spin angular transfer Theory of Spin-Transfer Torque Microwave Generation in Magnetic Multilayers and Nanostructures VOLUME 3: Novel Techniques for Characterizing and Preparing Samples Part 1: X-Ray and Neutron Diffraction Techniques Spin Structures and Spin Wave Excitations Domain States determined by Neutron Refraction and Scattering Polarized neutron reflectivity and scattering of magnetic nanostructures and spintronic materials Part 2: Synchrotron Radiation Techniques, Circular Dichroism of Hard & Soft X-Rays Synchrotron radiation techniques based on X-ray magnetic circular dichroism Part 3: Time and Space Resolved Magnetization Dynamics Ultrafast Magnetodynamics with Lateral Resolution: A View by Photoemission Microscopy Part 4: Electron Microscopy and Electron Holography Lorentz Microscopy of Thin Film Systems Electron Holography Of Ferromagnetic Materials Spin-Polarized Low Energy Electron Diffraction Spin-polarized Low Energy Electron Microscopy (SPLEEM) Scanning Electron Microscopy with Polarisation Analysis Part 5: Magneto-optical Techniques Investigation of Domains and Dynamics of Domain Walls by the Magneto-optical Kerr-effect Magnetization-induced second harmonic generation technique Investigation of Spin Waves and Spin Dynamics by Optical Techniques Time-resolved Kerr-effect and spin dynamics in itinerant ferromagnets Part 6: Spin Polarized Electron Spectroscopies Investigation of Ultrathin Ferromagnetic Films by Magnetic Resonance Spin-Polarized Photoelectron Spectroscopy as a probe of Magnetic Systems High-energy surface spin-waves studied by Spin-polarized Electron Energy Loss Spectroscopy Part 7: Nano Magnetism- Application and Charaterisation Scanning Probe Techniques: MFM and SP-STM Alternative Patterning Techniques : Magnetic Interactions in Nanomagnet Arrays Chemical Synthesis of Monodisperse Magnetic Nanoparticles Nanoimprint Technology for Patterned Magnetic Nanostructures Part 8: Growth Techniques Growth of Magnetic Materials using Molecular Beam Epitaxy Epitaxial Heusler alloys on III-V semiconductors Crystal Growth of magnetic materials VOLUME 4: Novel Materials Part 1: Soft Magnetic Materials Amorphous Alloys Soft Magnetic Materials - Nanocrystalline Alloys Soft Magnetic Bulk Glassy and Bulk Nanocrystalline Alloys Advanced Soft Magnetic Materials for Power Applications Part 2 : Hard Magnetic Materials Rare earth intermetallics for permanent magnet applications Rare-earth (RE) Transition-Metal (T M) Magnets Rare earth nanocrystalline and nanostructured magnets Current Status of Magnetic Industry in China & its Future Part 3: Ferro- and ferrimagnetic oxides and alloys Ferrimagnetic Insulators Crystallography and Chemistry of Perovskites Chalcogenides and Pnictides Dilute Magnetic Oxides and Nitrides Heusler alloys Half Metals Part 4: Ferro- and ferrimagnetic particles Superparamagnetic Particles Novel Nanoparticulate Magnetic Materials and Structures Part 5: Micro- and Nanowires Advanced Magnetic Microwires Template-based Synthesis and Characterization of High-Density Ferromagnetic Nanowire Arrays Magnetic Carbon Part 6: Magnetic Thin Films Magnetic Ultra-hyphen thin Films Magnetic Thin Films Hard Magnetic Films Part 7: Magnetic Materials with outstanding properties Magneto-optical materials Magnetocaloric Materials Magnetostrictive Materials and Magnetic Shape Memory Materials Ferroelectricity in Incommensurate Magnets Magnetism and Quantum Critically in Heavy-Fermion Compounds: Interplay with Superconductivity Molecular nanomagnets Part 8: Biomagnetic Materials Spintronic Biochips For Biomolecular Recognition Application of Magnetic Particles in Medicine and Biology VOLUME 5: Spintronics and Magnetoelectronics Part 1: Metal Spintronics Magnetic Tunnel Junctions including Applications Spin angular momentum transfer in magnetoresistive nano-junctions Spin-transfer in high magnetic fields and single magnetic layer nanopillars Microwave Excitations in Spin Momentum Transfer Devices Theory of Spin-Polarized Current and Spin-Transfer Torque in Magnetic Multilayers Part 2: Exotic Materials High Temperature Superconductivity- Magnetic Mechanisms Ferromagnetic Manganite Films Magnetic Polarons Kondo Effect in Mesoscopic Quantum Dots Ferromagnetic Semiconductors Diluted ferromagnetic semiconductors - theoretical aspects Part 3: Semiconductor spintronics Spin Engineering in Quantum Well Structures Hot Electron Spintronics Spin-dependent transport of carriers in semiconductors Spintronic devices/spin relaxation Theory of Spin Hall Effects in Semiconductors Manipulation of Spins and Coherence in Semiconductors Quantum computing with spins in solids Part 4: Quantum computation The Magnetic Resonance Force Microscope Part 5: Magnetoresistance Tunneling Magnetoresistance in Semiconductors Spin-dependent Tunneling: Role of Evanescent and Resonant States Unusual magnetoresistance including extraordinary and Ballistic

Hexagonally ordered 100 nm period nickel nanowire arrays

The magnetic behavior of 100 nm period arrays of Ni nanowires embedded in a highly ordered alumina pore matrix were characterized by magnetometry and magnetic force microscopy. Reducing the diameter of the nanowires from 55 to 30 nm while keeping the interwire distance constant leads to increasing coercive fields from 600 to 1200 Oe and to increasing remanence from 30% to 100%. The domain structure of the arrays exhibits in the demagnetized state a labyrith-like pattern. These results show that stray field interactions of single domain nanowires are entirely dependent on the nanowire diameter.

Analysis of the magnetic hardening mechanism in RE-FeB permanent magnets

Abstract For the interpretation of extremely high coercive fields of RE-NdB permanent magnets in general either models of pinning or nucleation mechanisms are considered. In both cases the magnetic properties of the phase boundaries between the grains are suggested to be responsible for the actual value of H c . In this paper a detailed analysis of the temperature dependence of H c is given on the basis of the predictions of micromagnetic theories for pinning and nucleation mechanisms. It is shown that the nucleation theory leads to a far more coherent interpretation of all relevant properties of H c than the pinning theory if the effects of misaligned grains, local stray fields and reduced anisotropies in grain boundaries are taken into account.

Nanocrystalline FeNdB permanent magnets with enhanced remanence

Isotropic nanocrystalline exchange coupled FeNdB magnets with enhanced remanence were produced using the melt spinning procedure. Starting from nearly single phase Fe14Nd2B magnets, the content of additional α‐Fe in composite magnets was stepwise increased up to 40 vol % by reducing the Nd and B content. The maximum remanence of JR=1.25 T was found in composite magnets containing 30 vol % α‐Fe, the maximum energy product reaches the value (BH)max=185 kJ/m3, whereas the coercive field is μ0HC=0.53 T for this composition. The microstructural investigations of the composite magnets reveal two characteristic maxima in the grain size distribution at about 15 and 25 nm corresponding to the α‐Fe phase and to the Fe14Nd2B phase, respectively. The influence of the exchange coupling on the coercive field can be described by a microstructural parameter αex. From the temperature dependence of the coercive field the microstructural parameters αKαex and Neff are determined.

Magneto-optical studies of current distributions in high-Tc superconductors

In the past few years magneto-optical flux imaging (MOI) has come to take an increasing role in the investigation and understanding of critical current densities in high-Tc superconductors (HTS). This has been related to the significant progress in quantitative high-resolution magneto-optical imaging of flux distributions together with the model-independent determination of the corresponding current distributions. We review in this article the magneto-optical imaging technique and experiments on thin films, single crystals, polycrystalline bulk ceramics, tapes and melt-textured HTS materials and analyse systematically the properties determining the spatial distribution and the magnitude of the supercurrents. First of all, the current distribution is determined by the sample geometry. Due to the boundary conditions at the sample borders, the current distribution in samples of arbitrary shape splits up into domains of nearly uniform parallel current flow which are separated by current domain boundaries, where the current streamlines are sharply bent. Qualitatively, the current pattern is described by the Bean model; however, changes due to a spatially dependent electric field distribution which is induced by flux creep or flux flow have to be taken into account. For small magnetic fields, the Meissner phase coexists with pinned vortex phases and the geometry-dependent Meissner screening currents contribute to the observed current patterns. The influence of additional factors on the current domain patterns are systematically analysed: local magnetic field dependence of jc(B), current anisotropy, inhomogeneities and local transport properties of grain boundaries. We then continue to an overview of the current distribution and current-limiting factors of materials, relevant to technical applications like melt-textured samples, coated conductors and tapes. Finally, a selection of magneto-optical experiments which give direct insight into vortex pinning and depinning mechanisms are reviewed.

GRAIN-SIZE DEPENDENCE OF REMANENCE AND COERCIVE FIELD OF ISOTROPIC NANOCRYSTALLINE COMPOSITE PERMANENT MAGNETS

Abstract Micromagnetic calculations using a finite element technique are useful to investigate magnetization processes in nanocrystalline ferromagnetic materials. In particular numerical calculations reveal the microstructural conditions required for high remanence and high coercivity isotropic permanent magnets. composite materials of Nd 2 Fe 14 B and α-Fe are excellent candidates for such high performance permanent magnets. The soft magnetic α-Fe grains cause a large spontaneous magnetization and the hard magnetic grains induce a large coercive field, provided that both phases are strongly exchange coupled. The numerical investigations on realistic three-dimensional grain arrangements suggest an optimal microstructure consisting of small soft magnetic grains ( D ≈ 10 nm, V soft ≈ 40%) embedded between hard magnetic grains with a mean grain diameter of about D ≈ 20 nm. Additionally, a microstructure with regular shaped grains improves the magnetic properties.

Determination of intrinsic magnetic material parameters of Nd2Fe14B from magnetic measurements of sintered Nd15Fe77B8 magnets

Abstract The magnetic anisotropy constants K 1 and K 2 of Nd 2 Fe 14 B have been determined in the temperature range between 4.2 and 575 K using aligned sintered Nd 15 Fe 77 B 8 magnets. Magnetization curves measured perpendicular to the aligning direction were analysed taking into account the volume fraction of the Nd 2 Fe 14 B phase and the angular distribution of grain axes. The grain alignment was determined by means of magnetic domain observation and X-ray analysis. It is shown that for sintered magnets an analysis under the assumption of the validity of the so-called Sucksmith-Thompson plot leads to significant errors especially of K 2 . As a further method for the determination of the anisotropy constant K 1 measurements of the magnetic initial susceptibility parallel to the alignment direction are proposed. The experimental results of both methods are compared with each other.

Magnetic properties of amorphous ferromagnetic alloys

Abstract For the application of amorphous, ferromagnetic alloys as soft magnetic materials a detailed knowledge of the domain structure, magnetization processes , the magnetic microstructure and thermodynamic properties is required. A review of the present state of the research work will be given. It is shown that the displacement of domain walls is described fairly well by the conventional potential theory developed previously for crystalline materials. Within the framework of the theory of micromagnetism the effect of quenching stresses on the domain structure as well as of short range stresses on the magnetic microstructure are discussed in detail.

The coercive field of sintered and melt-spun NdFeB magnets

Abstract The magnetic hysteresis loops have been investigated in the temperature range between 4.2 and 575 K for aligned sintered permanent magnets of nominal composition Nd 15 Fe 77 B 8 and for isotropic melt-spun ribbons of composition Nd 15 Fe 76 B 9 . The measured temperature and field dependence of the coercive field is analysed within the framework of theoretical results for nucleation fields of the ideal Nd 2 Fe 14 B matrix and, of disturbed surface regions of Nd 2 Fe 14 B grains. Furthermore the pinning of domain walls at thin soft magnetic grain boundary phases is considered for the high temperature range. It is concluded that for both types of NdFeB magnets the relevant magnetic hardening mechanisms at lower temperatures are nucleation processes in magnetically inhomogeneous regions whereas at higher temperatures the pinning of domain walls at grain boundaries predominates. The critical temperature where the change of nucleation hardening to pinning hardening occurs depends sensitivity on the crystal anisotropy, the grain boundary microstructure and the macroscopic grain- and multi phase arrangements.

Two‐ and three‐dimensional calculation of remanence enhancement of rare‐earth based composite magnets (invited)

Micromagnetic calculations using a finite element technique rigorously describe the magnetic properties of novel, isotropic rare‐earth‐based composite magnets. Numerical results obtained for a composite material of Nd2Fe14B, SmCo5 or Sm2(Fe0.8Co0.2)17N2.8 and α‐Fe particles show that remanence, coercivity, and coercive squareness sensitively depend on microstructural features. Interparticle exchange interactions enhance the remanence by about 60% with respect to noninteracting particles for a mean‐grain size approaching the exchange length of the soft magnetic phase and a significant percentage of α‐Fe. On the other hand, exchange interactions between the phases suppress the nucleation of reversed domains and thus preserve a high coercive field. Therefore, optimally structured, isotropic composite magnets show remarkably high energy products exceeding 400 kJ/m3.