Anisotropy Field Distribution Research Articles

Micromagnetic studies of antiferromagnetism and resistance-field curves in giant magnetoresistive Co/Al/NiFe

Magnetic properties and magnetoresistance of a Co/Al/NiFe giant-magnetoresistance (GMR) spin valve are analyzed by a new micromagnetic simulation model for the strongly interacting soft magnetic materials. The coercivity is less than 10 Oe in an isolated Co or NiFe film. However, in the Co/Al/NiFe GMR structure with a 80 /spl Aring/ interlayer spacing, the coercivity of the Permalloy film increases to 208 Oe; and the remnant magnetization in the Co film even has an opposite sign with that in the NiFe film, which means the two magnetic films coupled anti-ferromagnetically in a zero external magnetic field. The looped resistance-magnetic field (R-H) curve is obtained. The noise during the switching process can be decreased and the linearity of the R-H curve can be improved by an orientation distribution of anisotropy fields for the GMR sample.

Deconvolution of anisotropy field distributions from transverse susceptibility measurements

The reversible transverse susceptibility is a measurement often used to determine the anisotropy field of magnetic materials. A maximum entropy approach issued to determine the dispersion of H k values for a system of randomly oriented CrO 2 particles.

Magnetic properties of the nanocrystalline bcc-Fe phase formed in FeZrBCu alloys investigated by using the radio-frequency collapse and sideband effects

An unconventional technique combining Mössbauer spectroscopy with the effects induced by magnetic radio-frequency (rf) fields (rf collapse and rf sidebands) is employed to study the microstructure and magnetic properties of nanocrystalline bcc-Fe phase formed by annealing amorphous Fe 93− x− y Zr 7B x Cu y alloys at 500–600 °C. The rf-Mössbauer experiments allow us to distinguish soft magnetic nanocrystals from magnetically harder microcrystalline phases. The complete rf collapse of the magnetic hyperfine structure occurs only in the amorphous and nanocrystalline phases and is suppressed by the formation of microcrystalline α-Fe. The rf sidebands disappear when the nanocrystalline phase is formed, what proves that magnetostriction vanishes. The rf-Mössbauer experiments performed as a function of the rf field intensity provide information regarding the distribution of anisotropy fields related to the size distribution of the bcc-Fe grains.

Distribution of anisotropy field in recording media deduced from the hysteresis curve

The distribution of anisotropy fields in magnetic recording media is carried out by performing the second derivative of the magnetization curve detected in perpendicular direction with respect to the easy axis of the system, following a procedure similar to that used for the soft magnetic materials. The method is applied to the CoCr films with the anisotropy direction perpendicular to the film plane. It was found that the field at the peak of the distribution function agrees with the anisotropy field measured by the torque method. For particulate media, the shape of the anisotropy distribution function depends both on the particle orientation ratio and on the interparticle interactions; as a consequence, the anisotropy distribution function determined on these materials does not represent the intrinsic anisotropy field distribution of the particles but the anisotropy distribution as it appears in that particular specimen. In any case, this function is useful for understanding the magnetic behavior of the system.

Surface magnetic microstructure of melt-spun magnetic ribbons

Scanning electron microscopy with polarization analysis (SEMPA) and photon-excited spin polarized secondary electron spectroscopy have been used to study respectively the lateral variations and depth profiles of the surface magnetic microstructure of as-cast amorphous melt-spun Fe80B20 ribbons. The 5 μm resolution SEMPA images showed very similar complex domain patterns for both remanent and field-on states, and spot mode hysteresis loops showed significant differences across the sample. These data are consistent with a very wide distribution of strain-induced anisotropy fields. Energy resolved hysteresis loops (ERHL) measured using photoemitted 1 and 20 eV secondary electrons showed clear differences in form, attributable to changes in probing depth. A unidirectional anisotropy found in the 20 eV ERHL is linked tentatively to the inhomogeneities revealed by SEMPA.

Magnetic properties of Ba- and Sr-hexaferrite prepared by mechanical alloying

Samples of Ba- and Sr-hexaferrite were prepared by mechanical alloying and subsequent heat treatment were found to consist of single domain particles of the single hexaferrite phase. The particles had a wide distribution of anisotropy fields. Study of irreversible magnetisation suggested, that the demagnetisation process is mainly controlled by the Wohlfarth rotation. It was deduced from the results of measurements of magnetic viscosity, that the activation volume was of same order of magnitude as the cube of the domain wall thickness.

Influence of the structural instabilities on the anisotropy fields in Co-based amorphous alloys

The influence of structural instabilities, produced by thermal treatments on the magnetic anisotropy and initial permeability, is examined in a set of Co-based amorphous alloys. The analysis is made on square strips cut out from as-received ribbons in order to study the magnetic properties along two in-plane perpendicular directions having the same demagnetizing factor. Generally, in the as-received state the samples show an easy magnetization direction parallel to the original longitudinal direction of the ribbon. Annealing, performed at various temperatures, in the absence of applied magnetic fields and mechanical stresses, enhances the original anisotropy. Some exceptions are observed: in one of these alloys the influence of the annealing is very weak, whereas in another alloy the annealing treatment gives rise to a change of the easy axis from the longitudinal direction to the in-plane transverse direction. The evolution of the anisotropy field distribution with the annealing features is also evaluated. The results are discussed on the basis of irreversible changes of local atomic order.

Mössbauer study of amorphous and nanocrystalline FeZrBCu alloys

The microstructure and magnetic properties of the nanocrystalline bcc phase formed by annealing at 500–600°C of the amorphous Fe 81Zr 7B 12 and Fe 79Zr 7B 12Cu 2 alloys are studied using Mössbauer spectroscopy. The unconventional rf-Mössbauer experiments in which the radio frequency induced effects (rf collapse and rf sidebands) are used allow the nanocrystalline α-Fe phase to be distinguished from the magnetically harder microcrystalline α-Fe formed at higher annealing temperatures. The complete rf collapse of the magnetic hyperfine structure occurs only in the amorphous and nanocrystalline phases and is suppressed by the formation of microcrystalline α-Fe. The rf sidebands disappear when the nanocrystalline phase is formed revealing that magnetostriction vanishes. The rf-Mössbauer experiments performed as a function of the rf field intensity allowed the determination of the distribution of anisotropy fields related to the size distribution of the α-Fe grains.

Magnetic Aftereffect and Magnetization Reversal of Sr-Ferrite Fine Particles

the magnetic aftereffect constant (S v ), the anisotropy field (H A ) distribution and the rotational hysteresis integral (R h ) were measured for M-type hexagonal strontium ferrite (SrM) fine particles with various average particle diameters (D) and coercive forces (H cJ ). The relations between S v , the activation volume (v), H cJ , H A , and R h were studied, as well as the effect of milling. S v is constant and independent of the magnetic field. In S v varies in proportion to In H cJ , and the relation H cJ ?v?2 was found to obtain in SrM particles not extensively subjected to milling. S v increases and v decreases for SrM fine particles (D=0.3 ?m) whose H A is widely distributed by milling, even though H cJ decreases markedly. S v and v are related to the distribution of H A . As H cJ increases, R h and v decrease, which indicates that the magnetization reversal mechanism of SrM fine particles approaches that of a coherent rotation mode.

Improvement of anisotropy of perpendicular magnetic recording tape by Ta addition and Kr sputtering gas (abstract)

A perpendicular magnetic recording tape requires a high magnetic anisotropy field Hk and a small distribution of the perpendicular magnetic anisotropy field σhk/Hk. It can be expected that by the extension of the interplane distance of (002) plane d(002) in Co-Cr, a perpendicular magnetic anisotropy energy will be increased and Hk becomes higher. It can be also expected that the σhk/Hk becomes smaller by the improvement of the crystallinity of Co-Cr film. Therefore, in order to achieve the above, the following experiments were attempted. At first, a small amount of Ta was added to Co-Cr. This, as a result, extended the d(002) of the Co-Cr from 2.033 to 2.046 Å. Consequently, Hk was increased from 4.4 to 5.1 kOe and σhk/Hk was reduced from 0.14 to 0.11. Second, as a sputtering gas, Kr gas was used instead of Ar gas. The use of Kr resulted in greater crystallinity in the film, increasing the peak intensity I(002) of x-ray diffraction of the Co-Cr-Ta from 2090 to 2500 cps. Consequently, σhk/Hk was reduced up to 0.07. Last, a recording and reproducing experiment with the Co-Cr-Ta tape was attempted with the use of a hilical-scan VTR machine. As a result, a reproduced output of 56 nVO-P (μm T m/s) was obtained at the recording wavelength of 0.33 μm. This figure is equivalent to the output at the recording wavelength of 0.66 μm of metal-powder tape.

H c enhancement of Co-adsorbed γ-Fe2O3 particles via surface treatment with sodium polyphosphate

The coercive force HC of partially reduced, acicular γ-Fe2O3 particles having an adsorbed Co layer (4 wt % Co; Fe+2/Fe+3≊0.09; 2000 Å×240 Å particle dimensions; 36 m2/g specific surface area) increases substantially following surface treatment with (NaPO3)n≊14⋅Na2O. The maximum observed effect occurs at a polyphosphate-to-iron oxide weight ratio P/Fe near 0.5, denoted (P/Fe)max, where HC of the Co-adsorbed oxide increases from 687 to 1096 Oe immediately following treatment. The latter value increased to 1215 Oe after a four month storage in an argon environment containing less than 1 ppm H2O and O2, suggesting that the polyphosphate-oxide interaction is sensitive to moisture or oxygen. Co-adsorbed particles containing no Fe+2 in the oxide core also exhibit HC enhancement following surface treatment, but the change in HC is substantially less than that of particles containing Fe+2. The general behavior of polyphosphate-treated Co-adsorbed particles is observed to be similar to that of treated berthollide particles having no adsorbed Co layer: (i) the magnitude and sign of change in HC varies with P/Fe, reaching a peak value at (P/Fe)max, (ii) the magnitude of increase in HC at (P/Fe)max depends on the Fe+2 content of the particle core, (iii) saturation magnetization of the particles treated at (P/Fe)max is essentially unchanged from that of the untreated precursor, (iv) HC of specimens treated at (P/Fe)max returns to the value of the untreated precursor when the polyphosphate coating is removed via washing with water, and (v) particle dissolution occurs when P/Fe≫(P/Fe)max. Studies of anisotropy field distributions show that there is no obvious bimodal character to the very broad distribution of anisotropy fields found in randomly oriented specimens of surface-treated particles.

Static magnetic properties of nanocrystalline Co-Ti doped barium ferrite BaFe/sub 12-2x/Co/sub x/Ti/sub x/O/sub 19/ (x=0.8)

We have studied the static magnetic properties of nanocrystalline Co-Ti doped barium ferrite BaFe/sub 10.4/Co/sub 0.8/Ti/sub 0.8/O/sub 19/. The zero-field-cooled (ZFC) and field-cooled (FC) processes show the typical features of a small particle system. The dependence on temperature of the coercive field is discussed within the scope of an aligned and random assembly of particles. The distribution of blocking temperatures is obtained from the remanent-to-saturation magnetization ratio, leading to the mean volume of the magnetic unit (/spl ap/(6/spl plusmn/3)/spl middot/10/sup 4/ /spl Aring//sup 3/), in agreement with the mean particle volume determined by TEM (/spl ap/1.1/spl middot/10/sup 5/ /spl Aring//sup 3/). Remanence curves are used to derive the anisotropy field distribution and the interparticle magnetic interaction.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Properties of barium hexaferrite powders for magnetic recording

Substituted barium hexaferrite powders prepared by the glass crystallization method are investigated geometrically and magnetically. Evaluating a special kind of remanence curves allows the anisotropy field distribution of the powders to be determined. In order to obtain the intrinsic anisotropy field distribution the influence of thermal fluctuations must be separated from the data. The magnetic properties of powders of different composition are discussed.

Spin canting, surface magnetization, and finite-size effects in gamma -Fe2O3 particles.

Small \ensuremath{\gamma}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ acicular particles (250-\AA{} diam by 2000 \AA{}) with and without surface coating of $^{57}\mathrm{Fe}$ were examined by M\ossbauer spectroscopy in large (to 60 kOe) longitudinal applied fields. As has been previously seen, the Fe moments are not aligned in large fields. However, unlike previous results, the canting is found to be a finite-size effect rather than a surface property. Magnetization versus field curves for the uncoated small \ensuremath{\gamma}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ particles and, as a comparison, large \ensuremath{\gamma}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ particles were analyzed with log-normal anisotropy field distributions within the Stoner-Wohlfarth model. The small particles show a large paramagnetic susceptibility that increases with decreasing temperature. Using a Langevin-function analysis, one finds that the susceptibility is not due to superparamagnetism, and the susceptibility correlates well with the M\ossbauer canting. Hyperfine-field tailing increases with temperature for the small particles, especially for the coated samples, indicating weaker surface exchange interactions.

Spin-wave propagation on imperfect ultrathin ferromagnetic films.

The effects of localized imperfections on spin-wave propagation in very thin ferromagnetic films are examined. The imperfections are assumed to be localized to a few lattice sites and cause local changes in anisotropy and exchange fields. These imperfections may be due to thickness variations or other geometrical imperfections. We find that in very thin films the lifetime of long-wavelength spin waves is relatively insensitive to scattering from even large numbers of imperfections, and therefore cannot explain large observed linewidths observed in Brillouin light-scattering experiments. On the other hand, we find that a band of long-wavelength spin-wave modes can exist in an inhomogeneous film with a distribution of effective anisotropy fields. It is possible to have large bands with bandwidths on the order of 10 GHz in rough ultrathin films due to the sensitive dependence of effective anisotropy fields on thickness.

Studies of anisotropy mechanisms in polyphosphate-treated magnetic iron oxide particles

The coercive force of small, acicular (⋍ 2000 Å length, ⋍8:1 length-to-diameter ratio) spinel-type iron oxide particles increases substantially following surface treatment with sodium polyphosphate. Regardless of Fe 2+/Fe 3+ cation ratio, H c always attains a peak value when the polyphosphate/iron oxide weight ratio P/Fe is in the range 0.5–0.6, denoted (P/Fe) max. The maximum change in H c is observed when Fe 2+/Fe 3+ ≈ 0.10–0.15. When P/Fe⪢0.5–0.6, however, both magnitude and sign of the change in coercive force show strong dependence on the Fe 2+ content of the oxide, suggesting that the H c changes are caused by a magnetostrictive mechanism at these high treatment ratios. Calculated anisotropy field distributions of treated specimens show that both the mean anisotropy field δ H kδ G and predicted H c reach a peak when particles are treated at (P/Fe) max, where the distribution becomes very broad. At high treatment ratios both δ H kδ G and predicted H c decrease to values below that of the untreated oxide. Mössbauer studies of treated and untreated particles show no significant change in the environment of surface iron ions following treatement at P/FE = 0.5, but indicate a small increase in the Fe 3+ concentration of the particle core. When P/Fe = 2.5, however, the Fe 2+ concentration of the core increases markedly.

Fluctuation fields of recording media particles, the role of anisotropy dispersion

The authors report on the field variation of the fluctuation field (H/sub f/) in recording media. From theoretical calculations the field variation of H/sub f/ is found to be governed by the distribution of particle volumes and the distribution of anisotropy fields. The decrease in H/sub f/ arises from the volume distribution, while the presence of an anisotropy distribution results in an increase in H/sub f/. Thus, when these two effects are present in the sample, the field variation of H/sub f/ exhibits a minimum.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Calculation of the anisotropy field distribution in magnetic particle assemblies: the influence of thermal fluctuations

A theory is set up to calculate the anisotropy field distribution in assemblies of single domain particles from a special type of remanence curves. The formalism takes into account thermal fluctuations which are essential for small particle volumes.

Preparation of Co-Cr-Ta Hard Disk High Density Recording Media

In order to improve the anisotropy field distribution of Co-Cr recording media, the effect of Ta addition was investigated. It was found that by adding Ta the initial Co-Cr layer with poor crystal orientation is eliminated, and crystal orientation and crystallinity are drastically improved. It was also confirmed that the anisotropy field distribution of the Co-Cr-Ta layer is narrowed to one-third that of the Co-Cr layer. To clarify the effect of Ta addition on recording characteristics, Co-Cr-Ta/Nb-permalloy double-layer media were prepared and their recording characteristics were measured. The Co-Cr-Ta media showed a sharp magnetization distribution and higher reproduced voltage at high densities than did the Co-Cr media.

Anisotropy field distribution in amorphous ferromagnetic alloys from second harmonic response

We present a new procedure to obtain the anisotropy distribution in a ferromagnetic amorphous alloy. The method is based on the existence of second-order harmonics in the differential magnetization around the anisotropy field. We derive a mathematical expression relating the strength of the second-harmonic component and the anisotropy distribution function. An experimental setup is described and the optimum operating conditions discussed. The apparatus is based on the detection of the voltage induced in a coil using a lock-in amplifier tuned to a frequency double to the exciting one. Results obtained on a stress annealed sample show the existence of well defined values of the anisotropy in addition to a continuous background.