Showing papers on "Magnetic anisotropy published in 2007"

Handbook of magnetism and advanced magnetic materials

Abstract: 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

Electric Field-Induced Modification of Magnetism in Thin-Film Ferromagnets

Abstract: A large electric field at the surface of a ferromagnetic metal is expected to appreciably change its electron density. In particular, the metal's intrinsic magnetic properties, which are commonly regarded as fixed material constants, will be affected. This requires, however, that the surface has a strong influence on the material's properties, as is the case with ultrathin films. We demonstrated that the magnetocrystalline anisotropy of ordered iron-platinum (FePt) and iron-palladium (FePd) intermetallic compounds can be reversibly modified by an applied electric field when immersed in an electrolyte. A voltage change of -0.6 volts on 2-nanometer-thick films altered the coercivity by -4.5 and +1% in FePt and FePd, respectively. The modification of the magnetic parameters was attributed to a change in the number of unpaired d electrons in response to the applied electric field. Our device structure is general and should be applicable for characterization of other thin-film magnetic systems.

Size-sorted anionic iron oxide nanomagnets as colloidal mediators for magnetic hyperthermia.

Abstract: Iron oxide colloidal nanomagnets generate heat when subjected to an alternating magnetic field. Their heating power, governed by the mechanisms of magnetic energy dissipation for single-domain particles (Brown and Neel relaxations), is highly sensitive to the crystal size, the material, and the solvent properties. This study was designed to distinguish between the contributions of Neel and Brownian mechanisms to heat generation. Anionic nanocrystals of maghemite and cobalt ferrite, differing by their magnetic anisotropy, were chemically synthesized and dispersed in an aqueous suspension by electrostatic stabilization. The particles were size-sorted by successive electrostatic phase separation steps. Parameters governing the efficiency of nanomagnets as heat mediators were varied independently; these comprised the particle size (from 5 to 16.5 nm), the solvent viscosity, magnetic anisotropy, and the magnetic field frequency and amplitude. The measured specific loss powers (SLPs) were in quantitative agreement with the results of a predictive model taking into account both Neel and Brown loss processes and the whole particle size distribution. By varying the carrier fluid viscosity, we found that Brownian friction within the carrier fluid was the main contributor to the heating power of cobalt ferrite particles. In contrast, Neel internal rotation of the magnetic moment accounted for most of the loss power of maghemite particles. Specific loss powers were varied by 3 orders of magnitude with increasing maghemite crystal size (from 4 to 1650 W/g at 700 kHz and 24.8 kA/m). This comprehensive parametric study provides the groundwork for the use of anionic colloidal nanocrystals to generate magnetically induced hyperthermia in various media, including complex systems and biological materials.

Large Magnetic Anisotropy of a Single Atomic Spin Embedded in a Surface Molecular Network

Abstract: Magnetic anisotropy allows magnets to maintain their direction of magnetization over time. Using a scanning tunneling microscope to observe spin excitations, we determined the orientation and strength of the anisotropies of individual iron and manganese atoms on a thin layer of copper nitride. The relative intensities of the inelastic tunneling processes are consistent with dipolar interactions, as seen for inelastic neutron scattering. First-principles calculations indicate that the magnetic atoms become incorporated into a polar covalent surface molecular network in the copper nitride. These structures, which provide atom-by-atom accessibility via local probes, have the potential for engineering anisotropies large enough to produce stable magnetization at low temperatures for a single atomic spin.

Electrically assisted magnetic recording in multiferroic nanostructures.

Abstract: We demonstrate the room-temperature control of magnetization reversal with an electric field in an epitaxial nanostructure consisting of ferrimagnetic nanopillars embedded in a ferroelectric matrix. This was achieved by combining a weak, uniform magnetic field with the switching electric field to selectively switch pillars with only one magnetic configuration. On the basis of these experimental results, we propose to use an electric field to assist magnetic recording in multiferroic systems with high perpendicular magnetic anisotropy.

Synthesis and Characterization of Iron/Iron Oxide Core/Shell Nanocubes

Abstract: A simple method for the preparation of iron/iron oxide nanoparticles with core/shell cubic morphology is presented. The synthesis of the nanocubes was carried out through decomposition of a preformed iron oleate complex at high temperature. Although this procedure has been shown previously to produce monodisperse magnetite spheres,[1] the use of squalene as a solvent and the presence of sodium oleate was found to induce cube formation. A detailed high-resolution transmission electron microscopy (HRTEM) analysis of the nanocubes was performed for structural characterization. The core/shell structure, an iron core surrounded by magnetite (Fe3O4) shell, was confirmed by fast Fourier transform (FFT) filtering analysis. The results obtained by HRTEM analysis are in agreement with X-ray Photoelectron Spectroscopy (XPS) and magnetic analysis. The Fe nanocubes are superparamagnetic at room temperature with a saturation magnetization MS = 101 A m2 kg–1 and magnetic anisotropy density Keff = 1.6 × 105 J m–3 at low temperatures.

Structural and multiferroic properties of La-modified BiFeO3 ceramics

Abstract: The coexistence of the magnetic and the electrical properties in lanthanum (La)-modified bismuth ferrite (Bi1−xLaxFeO3, x=0.05, 0.1, 0.15, and 0.2) ceramics was studied and compared with those of bismuth ferrite (BiFeO3). The presence of a small secondary phase of BiFeO3 (arises due to excess Bi2O3) was removed on La substitution at the Bi site, as observed in x-ray diffraction (XRD). The effect of La substitution on dielectric constant, loss tangent, and remnant polarization of the samples was studied in a wide range of temperature (77–400K) and frequency (1kHz–1MHz). The variation of magnetization, coercive field, and exchange bias with temperature (2–300K) and La concentration were investigated. These changes in the magnetic parameters with La doping along with those of the electron magnetic resonance parameters measured at 300K and 9.28GHz are understood in terms of increase in the magnetic anisotropy and magnetization. These results also show that stabilization of crystal structure and nonuniformity ...

Experimental and theoretical studies of nanoparticles of antiferromagnetic materials

Abstract: The magnetic properties of nanoparticles of antiferromagnetic materials are reviewed. The magnetic structure is often similar to the bulk structure, but there are several examples of size-dependent magnetic structures. Owing to the small magnetic moments of antiferromagnetic nanoparticles, the commonly used analysis of magnetization curves above the superparamagnetic blocking temperature may give erroneous results, because the distribution in magnetic moments and the magnetic anisotropy are not taken into account. We discuss how the magnetic dynamics can be studied by use of magnetization measurements, Mossbauer spectroscopy and neutron scattering. Below the blocking temperature, the magnetic dynamics in nanoparticles is dominated by thermal excitations of the uniform mode. In antiferromagnetic nanoparticles, the frequency of this mode is much higher than in ferromagnetic and ferrimagnetic nanoparticles, but it depends crucially on the size of the uncompensated moment. Excitation of the uniform mode results in a so-called thermoinduced moment, because the two sublattices are not strictly antiparallel when this mode is excited. The magnetic dipole interaction between antiferromagnetic nanoparticles is usually negligible, and therefore such particles present a unique possibility to study exchange interactions between magnetic particles. The interactions can have a significant influence on both the magnetic dynamics and the magnetic structure. Nanoparticles can be attached with a common crystallographic orientation such that both the crystallographic and the magnetic order continue across the interfaces.

Analysis of anisotropy crossover due to oxygen in Pt/Co/MOx trilayer

Abstract: Extraordinary Hall effect and X-ray spectroscopy measurements have been performed on a series of Pt/Co/MOx trilayers (M=Al, Mg, Ta...) in order to investigate the role of oxidation in the onset of perpendicular magnetic anisotropy at the Co/MOx interface. It is observed that varying the oxidation time modifies the magnetic properties of the Co layer, inducing a magnetic anisotropy crossover from in-plane to out-of-plane. We focused on the influence of plasma oxidation on Pt/Co/AlOx perpendicular magnetic anisotropy. The interfacial electronic structure is analyzed via X-ray photoelectron spectroscopy measurements. It is shown that the maximum of out-of-plane magnetic anisotropy corresponds to the appearance of a significant density of Co-O bondings at the Co/AlOx interface.

Dynamical magnetoelectric coupling in helical magnets.

Abstract: We develop a theory of collective mode dynamics in the helical magnets coupled to electric polarization via spin-orbit interaction. The low-lying modes associated with the ferroelectricity are not the transverse optical phonons, but are the spin waves hybridized with the electric polarization. This hybridization leads to the Drude-like dielectric function $\ensuremath{\epsilon}(\ensuremath{\omega})$ in the limit of zero magnetic anisotropy. There are two additional low-lying modes: phason of the spiral and rotation of helical plane along the polarization axis. Role of these low-lying modes in the neutron scattering and antiferromagnetic resonance is revealed, and a novel experiment to detect the dynamical magnetoelectric coupling is discussed.

Microwave photovoltage and photoresistance effects in ferromagnetic microstrips

Abstract: We investigate the dc electric response induced by ferromagnetic resonance in ferromagnetic Permalloy $({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20})$ microstrips. The resulting magnetization precession alters the angle of the magnetization with respect to both dc and rf current. Consequently the time averaged anisotropic magnetoresistance (AMR) changes (photoresistance). At the same time the time-dependent AMR oscillation rectifies a part of the rf current and induces a dc voltage (photovoltage). A phenomenological approach to magnetoresistance is used to describe the distinct characteristics of the photoresistance and photovoltage with a consistent formalism, which is found in excellent agreement with experiments performed on in-plane magnetized ferromagnetic microstrips. Application of the microwave photovoltage effect for rf magnetic field sensing is discussed.

Spin-torque influence on the high-frequency magnetization fluctuations in magnetic tunnel junctions.

Abstract: Voltage noise measurements were performed in the 3–7 GHz frequency range on magnetic tunnel junctions biased with a dc current. Magnetic noise associated with ferromagnetic resonance excitations is either amplified or reduced depending on the direction of the bias current. This effect is interpreted as the influence of spin transfer torque on the magnetization fluctuations and described using Gilbert dynamics equation including spin transfer torque and effective field terms.

Observation of exchange bias in the martensitic state of Ni50Mn36Sn14 Heusler alloy

Abstract: Exchange bias was observed in the Ni50Mn36Sn14 Heusler alloy after field cooling by means of hysteresis loop measurement. The hysteresis loops shift along the axis of an applied field and its magnitude significantly increased with decreasing temperature below 70K. This effect could be understood as a result of exchange anisotropy created at the interface between an antiferromagnet and a ferromagnet in the phase separated of martensitic state. Above 70K, however, the exchange bias field disappeared and the coercivity significantly reduced owing to the fact that the pinning between an antiferromagnet and a ferromagnet becomes weaker with increasing temperature.

Chiral Cyanide-Bridged MnIIMnIII Ferrimagnets, [MnII(HL)(H2O)][MnIII(CN)6]·2H2O (L = S- or R-1,2-diaminopropane): Syntheses, Structures, and Magnetic Behaviors

Abstract: Chirality plays a key role for exhibiting specific physical properties in functional materials. Molecule-based magnets can strategically introduce the chirality in their frameworks as a molecular property of co-ligand, and magnetic frameworks providing local magnetic anisotropy and noncentrosymmetric structure are expected to form a genuine chiral magnetic structure.

The effect of surface spin disorder on the magnetism of γ-Fe2O3 nanoparticle dispersions

Abstract: The nanomagnetism of monodisperse 7 nm γ-Fe2O3 nanoparticles exhibits unique features due to a significant amount of surface spin disorder. To correctly characterize the superparamagnetism of a dilute dispersion requires including the effects of the magnetic anisotropy and a shell of disordered spins surrounding the ordered core. The nanoparticle shell's disordered spin structure is exchange coupled to that of the ordered core. This enables an exchange bias loop shift, Hex, when the nanoparticle dispersion is field cooled. The surface spin disorder also leads to an unusual exponential-like decrease of the nanoparticle's total saturation magnetization with increasing temperature.

Ultrafast magnetization dynamics in high perpendicular anisotropy †Co/Pt‡ n multilayers

Abstract: We present time-resolved magneto-optical Kerr effect measurements of picosecond laser induced precessional dynamics in out-of-plane magnetized [Co(4A)∕Pt(8A)]n multilayer films. A fast precession of magnetization (up to 81GHz) emerges 6ps after pumping and decays within a 100ps time scale. The uniform precessional frequency was studied for varying external magnetic bias field and number of Co∕Pt repeats n. The variation of the precession frequency with external bias field is quantitatively understood using the macrospin model of the Landau-Lifshitz-Gilbert equation of motion [Phys. Z. Sowjetunion 8, 153 (1935); Phys. Rev. 100, 1243 (1955)], yielding a large perpendicular anisotropy of up to 1.0×107ergs∕cm3 for our samples. The precession frequency increases sharply with reduced n and almost saturates below n=5, suggesting a commensurate variation of the perpendicular anisotropy. A heavy damping of the precessional motion is observed with increasing n and can be explained by enhanced spin-orbit coupling in...

Reorientation of magnetic anisotropy in epitaxial cobalt ferrite thin films

Abstract: Spin reorientation has been observed in CoFe2O4 thin single crystalline films epitaxially grown on (100) MgO substrate upon varying the film thickness. The critical thickness for such a spin-reorientation transition was estimated to be 300 nm. The reorientation is driven by a structural transition in the film from a tetragonal to cubic symmetry. At low thickness, the in-plane tensile stress induces a tetragonal distortion of the lattice that generates a perpendicular anisotropy, large enough to overcome the shape anisotropy and to stabilize the magnetization easy axis out of plane. However, in thicker films, the lattice relaxation toward the cubic structure of the bulk allows the shape anisotropy to force the magnetization to be in plane aligned.

Magnetic and electric excitations in split ring resonators

Abstract: We studied the electric and magnetic resonances of U-shaped SRRs. We showed that higher order excitation modes exist in both of the electric and magnetic resonances. The nodes in the current distribution were found for all the resonance modes. It turns out that the magnetic resonances are the modes with odd-number of half-wavelength of the current wave, i.e. λ/2, 3λ/2 and 5λ/2 modes, and the electric resonances are modes with integer number of whole-wavelength of current wave, i.e. λ, 2λ and 3λ modes. We discussed the electric moment and magnetic moment of the electric and magnetic resonances, and their dependence to the length of two parallel side arms. We show that the magnetic moment of magnetic resonance vanishes as the length of side arms of the SRR reduces to zero, i.e. a rod does not give any magnetic moment or magnetic resonance.

Color-flavor locked superconductor in a magnetic field

Abstract: We study the effects of moderately strong magnetic fields on the properties of color-flavor locked color superconducting quark matter in the framework of the Nambu-Jona-Lasinio model. We find that the energy gaps, which describe the color superconducting pairing as well as the magnetization, are oscillating functions of the magnetic field. Also, we observe that the oscillations of the magnetization can be so strong that homogeneous quark matter becomes metastable for a range of parameters. We suggest that this points to the possibility of magnetic domains or other types of magnetic inhomogeneities in the quark cores of magnetars.

Magnetic recording element and magnetic memory

Abstract: A magnetic recording element according to an example of the present invention includes a magnetic free layer whose magnetization is variable in accordance with a current direction passing through a film and whose direction of easy axis of magnetization is a direction perpendicular to a film plane, a magnetic pinned layer whose magnetization is fixed to a direction perpendicular to the film plane, and a non-magnetic barrier layer between the magnetic free layer and the magnetic pinned layer. In the magnetic free layer, a relation between a saturated magnetization Ms (emu/cc) and an anisotropy field Han (Oe) satisfies Han>12.57 Ms, and Han<1.2 E7 Ms−1+12.57 Ms.

Ferromagnetic resonance study of sputtered Co|Ni multilayers

Abstract: We report on room temperature ferromagnetic resonance (FMR) studies of [ t Co|2t Ni] × N sputtered films, where 0.1 ≤ t ≤ 0.6 nm. Two series of films were investigated: films with the same number of Co|Ni bilayer repeats (N = 12), and samples in which the overall magnetic layer thickness is kept constant at 3.6 nm (N = 1.2/t). The FMR measurements were conducted with a high frequency broadband coplanar waveguide up to 50 GHz using a flip-chip method. The resonance field and the full width at half maximum were measured as a function of frequency for the field in-plane and field normal to the plane, and as a function of angle to the plane for several frequencies. For both sets of films, we find evidence for the presence of first and second order anisotropy constants, K1 and K2. The anisotropy constants are strongly dependent on the thickness t, and to a lesser extent on the total thickness of the magnetic multilayer. The Lande g-factor increases with decreasing t and is practically independent of the multilayer thickness. The magnetic damping parameter α, estimated from the linear dependence of the linewidth ΔH, on frequency, in the field in-plane geometry, increases with decreasing t. This behaviour is attributed to an enhancement of spin-orbit interactions with decreasing Co layer thickness and in thinner films, to a spin-pumping contribution to the damping.

Magnetic irreversibility and the Verwey transition in nanocrystalline bacterial magnetite

Abstract: The magnetic properties of biologically produced magnetite nanocrystals biomineralized by four different magnetotactic bacteria were compared to those of synthetic magnetite nanocrystals and large, high-quality single crystals. The magnetic feature at the Verwey temperature ${T}_{V}$ was clearly seen in all nanocrystals, although its sharpness depended on the shape of individual nanoparticles and whether or not the particles were arranged in magnetosome chains. The transition was broader in the individual superparamagnetic nanoparticles for which ${T}_{B}l{T}_{V}$, where ${T}_{B}$ is the superparamagnetic blocking temperature. For nanocrystals organized in chains, the effective blocking temperature ${T}_{B}g{T}_{V}$ and the Verwey transition is sharply defined. No correlation between particle size and ${T}_{V}$ was found. Furthermore, measurements of $M(H,T,\text{time})$ suggest that magnetosome chains behave as long magnetic dipoles where the local magnetic field is directed along the chain. This result confirms that time-logarithmic magnetic relaxation is due to the collective (dipolar) nature of the barrier for magnetic moment reorientation.

Threshold currents to move domain walls in films with perpendicular anisotropy

Abstract: The authors quantify the impact of the pinning potential on the current driven domain wall depinning process in wires with spin valves having perpendicular magnetic anisotropy. By artificially controlling the strength of the pinning field Hp, the threshold current Jth is found to decrease when reducing Hp. In addition, the efficiency of the depinning process is observed to correlate with the polarization and the damping parameter of the magnetic layer. This suggests that the engineering of pinning sites together with well suited materials provides a pathway for efficient current-induced domain wall manipulation in films with perpendicular anisotropy.

Magnetic anisotropy in the ferromagnetic Cu-doped ZnO nanoneedles

Abstract: Copper-doped ZnO (ZnO:Cu) nanoneedles exhibiting room-temperature ferromagnetism were fabricated by an ion beam technique using Cu plate and ZnO film. A saturated magnetization moment of 0.698emu∕cm3 was found in the nanoneedles when a field of 10kOe was applied perpendicular to the substrate, which was 15% larger than the field applied parallel to the substrate. The magnetic ordering of the nanoneedles was enhanced significantly to 0.968emu∕cm3 after annealing of 400°C for 20min. However, the magnetic anisotropy at high field is vanished but an “easy plane” ferromagnetism becomes apparent at low field region. The possible mechanisms of the magnetic ordering and anisotropy in the ZnO:Cu nanoneedles are discussed.