Distribution Of Easy Axes Research Articles

Magnetic properties of Ni-Mn-Ga ribbon prepared by rapid solidification

Magnetic shape memory Ni/sub 49.7/Mn/sub 28.7/Ga/sub 21.6/ alloy was prepared in the form of the thin ribbon by a planar-flow casting method. Optical microscopy and X-ray diffraction were used to check the microstructure of as-quenched and annealed ribbons (800C/24 h and 72 h). The as-quenched ribbon was microcrystalline with grain size about 1.5-3 /spl mu/m which after annealing increased to about 40 /spl mu/m. Annealing also increased the martensitic transformation and Curie temperatures. The annealed ribbon transformed to martensite at 299 K with a reverse transformation to austenite at about 308 K. Room-temperature martensite had a five-layered modulated structure similar to that observed in bulk material. Magnetization at room temperature, 63 Am/sup 2//kg and Curie temperature, 367 K, were close to that of the master alloy. The magnetization loop of the as-quenched ribbon was very flat due to the high level of quenched-in stresses. The loops of annealed ribbon were round indicating a random distribution of easy magnetization axes. The magnetocrystalline anisotropy constant of the annealed ribbon was K/sub u/=1.6/spl times/10/sup 5/ J/m/sup 3/. Thermoelastic strain due to martensitic transformation was about 0.3%.

Permanent-magnet properties of thermally processed FePt and FePt-Fe multilayer films

FePt single-layer and FePt-Fe multilayer thin films are prepared by magnetron sputtering. By varying the Pt content, FePt, Fe/sub 3/Pt, or a mixture of FePt and Fe/sub 3/Pt can be obtained in Fe-Pt single layers. In annealed FePt-Fe multilayers, the coercivities decrease with the introduction of Fe layers compared to FePt single layers, while the magnetization increases. The single-phase behavior of the hysteresis loops of FePt-Fe multilayers indicates the existence of exchange coupling in these materials. For one FePt-Fe sample with optimized exchange coupling, the intrinsic properties correspond to an energy product of 19 MGOe. The texture of the magnets is determined by the [111] orientation of the crystallites. This means that the easy magnetization directions of the FePt grains form an angle of 54.7/spl deg/ with the film normal, but are randomly oriented in the film plane. It is analyzed how this easy-axis distribution affects the magnetic hysteresis.

Influences of granular microstructure on the reversal mechanism of Co nanoelements

A two-dimensional micromagnetic simulation is developed using a spatial finite element method, with the dynamic Gilbert equations discretized by a Galerkin method. An algorithm is presented for the generation of granular microstructures via the Voronoi tessellation and rectangular nanoelements of 200×40×20 nm3 are constructed. Hysteresis simulations are performed on isolated nanoelements in order to determine the extent to which experimentally observed variations in coercivity are attributable to the microstructural properties rather than magnetostatic interactions with neighboring members of an array. Dependence of coercivity on grain size is examined, as well as the role of grain structure in the reversal process. Mean grain size is shown to dictate the mode of reversal, whereas grain irregularity is observed to influence specific intermediate magnetization configurations and therefore specific coercivity values. Low correlation between grain irregularity and coercivity indicates that the magnetization dynamics depend on a combination of physical microstructure and easy-axis distribution unique to each nanoelement.

Angular dependence of switching field of magnetic recording particles

In a magnetic recording media, anisotropic particles are placed with different directions in the head field. The switching field, H s of a particle at a certain direction is defined as the minimum field, which reverses its easy axis magnetization. To determine the position of magnetization transition in recording media, one needs the angular dependence of the switching field. To find this for a texture of anisotropic single domain particles, the acicular iron particles with 0.3 μm length have been grown in a magnetic field. Then, the textures of the particles have been oriented by another magnetic field. We have measured the remanent magnetization of samples m r( θ) and m r( H), respectively, as functions of angle and field by a VSM system. The easy axis distribution is obtained from m r( θ), then the distribution and m r( H) is used for determining the angular dependence of the switching field. Results show that H s for small angles is <20% of the anisotropy field of the particles H k. This difference from the Stoner–Wohlfarth model is explained by using particles interaction.

Magnetic properties of GdFe 11− xSi xTi

Induction melted GdFe 11− x Si x Ti alloys ( x≤2) with ThMn 12-type structure have been investigated by X-ray diffraction, weak field thermomagnetometry from 300 to 850 K and isotherm magnetization measurements in magnetic fields up to 5.5 T from 4.5 to 300 K. The unit-cell volume decreases with increasing Si content. The reduction of the Curie temperature is correlated to the decrease of the Fe sublattice ordering temperature deduced from the Curie temperature of the homologous YFe 11− x Si x Ti series. The exchange coupling parameters were derived in the approximation of the molecular field. Their increase with x gives evidence for an enhancement of the indirect 3 d–4 f interaction via 5 d which results from the 3 d–5 d hybridization favored by the Si substitution. The magnetocrystalline anisotropy of aligned and fixed polycrystalline samples was deduced from the fits of the M ⊥( H) isotherms in the assumption of a Gaussian distribution of easy axis around the alignment direction. The anisotropy constants K 1, K 2 and the anisotropy field H A decrease with increasing temperature. The Si substitution is responsible for a reduction of K 1 but K 2, which increases slightly at low temperature, appears more sensitive to the exchange interaction Fe–Fe.

Approach to saturation in nanomagnetic systems: Easy axis distribution and interactions

Using the applied field angle dependence of the approach to saturation of the magnetization, a theory for the determination of the distribution of easy axes of anisotropy and the ratio between interaction and anisotropy in bidimensional nanomagnetic materials is proposed. The usual saturation process laws in two dimensions at high and intermediate field ranges are recovered and other dependencies between the former are found and justified in terms of scaling arguments. Finally, an approximate analytical model that provides a simpler method by which to determine the easy axis distribution and interaction strength is discussed.

Magnetic Coating Thickness Influence on the Texture in Metal Particle Systems

The magnetic characterization of four experimental advanced double-coated MP tapes with different magnetic coating thickness has been completed. Although standard vibrating sample magnetometer (VSM) measurements show very similar characteristics such as coercivity, squareness and orientation ratio (OR), alternative measurements show distinct differences in addition to their magnetic thickness. The in-plane distribution of easy axes (EAD) measured using a bi-axial VSM, show that the distribution width increased as the magnetic coating thickness decreased. A theoretical relation between OR and EAD was used to determine the out-of-plane distribution width. This was found to be narrower than the in-plane equivalent, as expected. The out-of-plane distribution width decreased as the magnetic thickness decreases, contrary to the in-plane variation.

DC modulation noise in clustered particulate media

Mallinson's theory of recording media noise is extended to ensembles where the particles have an easy axis distribution or are clustered. The effect of clusters is that there is a critical value of packing fraction at which noise in the demagnetised state switches to a minimum. Comparison is made with measurements of DC modulation noise in double layer MP tape.

Coercivity and fluctuation field in granular recording media

Fluctuation field as a function of applied field and coercivity data derived from magnetic measurements on two well-characterized CoPt/B (1 1 1) textured films is presented and compared with the predictions of a simple model based on Arrhenius–Néel type of thermal activation of Stoner–Wohlfarth particles that takes into account the specifics of the easy-axis distribution in hand. The model gives a good qualitative description of the results.

Anisotropy axis distribution and interactions in nanomagnet systems

We describe a mean-field theory applicable to nanostructured systems of single-domain magnetic particles. This theory provides expressions for the experimental determination of the easy-axis distribution and the interaction strength at remanence from measurements of the angular dependence of the maximum remanences parallel and perpendicular to the applied field. We have applied the model to barium ferrite tapes providing a suitable quantitative explanation for the difference experimentally observed between remanences, which could not be explained until now. The magnetocrystalline anisotropy strength of the nanoparticle is also determined. The values of the distribution function, the interaction parameter, and the magnetocrystalline anisotropy predicted by our model are in agreement with the results obtained by other methods.

MOKE spectra and ultrahigh density data storage perspective of FePt nanomagnet arrays

Superlattices of self-assembled, monodisperse FePt nanomagnet arrays have been studied using magneto-optical (MOKE) spectroscopy in the photon energy range 0.8-5.3 eV. The nanomagnets are chemically synthesized and subsequently deposited on SiO/sub 2/ substrates for structural, magnetic and optical characterization. Large room temperature coercivities up to 9000 Oe are obtained after annealing to temperatures up to 580/spl deg/C. They are attributed to the transformation from the chemically disordered fcc phase to the chemically ordered L1/sub 0/ fct phase of FePt. The chemical ordering process is accompanied by changes in the electronic structure of the materials, which leads to characteristic MOKE spectral changes. In particular, the occurrence of a strong MOKE peak at 2 eV photon energy is observed. Polar and transverse (out-of-plane and in-plane) Kerr hysteresis studies indicate 3D random distribution of the magnetic easy axes in these superlattices. These nanomagnet assemblies with the control on magnet spacing and spatial order are prospective candidates for future ultrahigh density magnetic recording media with potential areal densities beyond Tbit/in/sup 2/.

Demagnetization in an inclusive model of ferromagnetic hysteresis

The magnetic tensor response in computational demagnetization simulations of hard magnetic materials is modeled by a new inclusive macroscopic model of ferromagnetic hysteresis, a two-dimensional vector generalization of the classic Stoner-Wohlfarth model. The model includes a mean field interaction and pinning extensions. Both rotating and oscillating demagnetization cycles and isotropic and anisotropic polycrystalline easy axis distributions are treated.

Effect of grain alignment on magnetic structure in nanoscale material

Intergranular exchange interactions act to bind nanograins in nanoscale magnetic materials into larger interaction domains with a magnetic structure considerably larger than that of a single grain. To investigate the effect of differing relative grain orientation on the scale of the interaction domains in melt-quenched Nd/sub 2/Fe/sub 14/B, a two-dimensional micromagnetic model has been used. The model consists of 18 grains characterized by the intrinsic properties of Nd/sub 2/Fe/sub 14/B. Two cases are considered; one with a narrow distribution of easy axis orientations centered along the applied field direction and one with a nearly isotropic easy axis distribution. Computational results confirm that the scale of the magnetic structure for well-aligned material is very large, while for isotropic material only a few grains can be coupled Into an interaction domain before the intergranular misalignment is large enough to pin domain walls and impede reversal.

Study of the origin of orientation ratio in high-density longitudinal magnetic recording media

A study on the origin of orientation ratio in high-density recording media on mechanically textured substrates is reported. C-axis distribution of Co grains in textured case observed using high resolution TEM images shows its preferred orientation along the circumferential (along the groove) direction. The asymmetrical easy axis distribution is seen by maximum torque observed in different directions along the plane of the film. Diffraction patterns from several individual grains of the Cr underlayer are analyzed and indicated the preference of Cr along the circumferential direction. This could be the reason for C-axis of Co grains to show the preferred anisotropy along circumferential direction.

Rotational hysteresis of torque curves in polycrystalline ferro/antiferromagnetic systems

Rotational hysteresis of ferromagnetic (F)/antiferromagnetic (AF) exchange coupled systems was studied by using NiFe/IrMn, NiFe/FeMn, and NiFe/NiMn samples sputter deposited under almost the same conditions, although the sample of NiFe/NiMn was annealed later to obtain the antiferromagnetic phase for the NiMn layer. The rotational hysteresis of each sample exhibited quite a different feature from each other, especially in the rotational hysteresis–magnetization angle curves. The NiFe/IrMn bilayer showed a dip at around the antiparallel direction to the applied field direction during sample preparation (pinning direction), while the NiFe/NiMn bilayer exhibited a large peak in that direction. The NiFe/FeMn bilayer did not show any noticeable structure in the rotational hysteresis–magnetization curve. Based on the analysis of those data done by using the model recently proposed by the authors, the following was inferred: (1) The distribution of the anisotropy easy axes of IrMn of the NiFe/IrMn bilayer grains is limited in some range of angles around the pinning direction. Some direct exchange coupling exists between the IrMn grains. (2) The distribution of the anisotropy easy axes of FeMn grains of the NiFe/FeMn bilayer is almost random and the direct exchange coupling between the FeMn grains is negligible. (3) In the NiFe/NiMn bilayer, substantial strength of direct exchange coupling exists between some of the grains which switch and the ones which do not switch with the rotation of the magnetization of the NiFe layer, although the volume fraction of the former is much smaller than the latter.

Relevance of transverse remanence for textured recording media

The derivative of the transverse remanent (TR) magnetization curve for diluted, randomly oriented particle systems is known to give the anisotropy field distribution. The goal of this Letter is to investigate the TR relevance for textured (oriented) recording media. A method for measuring the easy axis distribution, that uses the TR saturation values for different directions of the applied field, is presented. Particle interactions are shown to have a strong influence on the TR. Experimental results on tape recording media show that the TR curves measured by rotating the applied field towards the perpendicular to the tape axis (the texture symmetry axis) saturates faster than those measured by rotating the field towards the tape axis. Monte Carlo simulations indicate as possible explanation for this effect the negative (demagnetizing) character of particle interaction.

Orientation dependence of the dynamic coercivity of Stoner-Wohlfarth particles

The time dependence of the remanent coercivity for noninteracting Stoner-Wohlfarth particles with uniaxial anisotropy has been calculated numerically as a function of easy axis angle and distribution width. The results have been interpreted in terms of a simplified effective energy barrier function of the form /spl Delta/E=CKV(1-H/H/sub 0/)/sup n/ and a modified Sharrock equation. For a system with identical easy axis angles, Sharrock law behavior occurs at all angles over a wide range of times with C=1 and n a function of angle. For a distribution of easy axes, Sharrock law behavior also occurs with C=1 and n/spl ap/1.5 for squareness values ranging from random to at least 0.98. The dependence of the energy barrier on field and easy axis angle is compared with the Victora expansion and with the Pfeiffer model.

Calculations of the susceptibility of interacting superparamagnetic particles

A model of the magnetic properties of a dispersion of interacting superparamagnetic particles in a solid matrix is presented. The model uses Monte Carlo techniques and is capable of predicting the time and temperature dependence of the magnetic properties. The model is applied to the study of the magnetic behavior of a cobalt granular system, particularly the low-field susceptibility. It is shown that strongly interacting systems at high density exhibit non-Langevin behavior and give a strongly nonlinear variation of susceptibility with packing density. The temperature dependence of the initial susceptibility shows the characteristic peak observed experimentally, with the peak temperature increasing with packing density. The field cooled (FC) and zero field cooled (ZFC) magnetization are also studied. The field dependence of the FC magnetization is shown to depend on the interparticle interactions and also on the orientational easy axis distribution. The FC magnetization is found to exhibit a peak resulting from the interactions. This behavior is finally related to the energy barrier distribution of the system (and its dependence on the interactions) using the temperature decay of remanence. It is also shown that the remanence calculated from the complete hysteresis loop at each temperature differs from the values obtained by increasing the temperature of a system initially at saturation remanence. The evolution of magnetic properties as a function of the magnetic state and history points to the importance of collective phenomena. Calculations of a spin-spin correlation function show the existence of a state with short-ranged order at low temperatures.

A hysteresis model for planar Hall effect in thin films

Based on the single-domain theory, a hysteresis model of the magnetoresistance ratio of magnetic thin films has been studied with different current-voltage configurations. The influence of the distribution of easy axes on the magnetoresistance was also studied, and results indicated that the peak value of magnetoresistance does not occur at coercivity. Results also suggested that both the multidomains and nonuniform current reduce the MR ratio.

Easy axis distribution in interacting fine-particle systems: Two and three dimensions

We describe a two- and three-dimensional theory applicable to fine-particle systems, which directly includes interactions between particles through a mean field model. Expressions that allow for the experimental determination of the easy-axis distribution are provided. When the driving magnetic field is decreased from a saturation value, interaction and disorder give rise to a rotation of the integration region that must be considered for the evaluation of the average values. Differences between parallel and transverse remanences are predicted, resulting in good agreement with experiments found in the literature. Finally, this model reproduces the results of the preceding noninteracting theories when interaction between particles is not taken into account.