Temperature Dependence Of Magnetocrystalline Anisotropy Research Articles

First-Principles Study for Finite Temperature Magnetrocrystaline Anisotropy of L10-Type Ordered Alloys

We conducted the first-principles calculations for the temperature dependence of magnetic anisotropy energy K(T) for L10-type FePt, MnAl, and FeNi by using the coherent potential approximation for spin transverse fluctuation. The temperature dependence of magnetocrystalline anisotropy (MA) for FePt and MnAl almost follows the relation K(T) ∼ M(T)n (2 < n < 3) [M(T) is the magnetization], which has been suggested to be reproduced using the XXZ spin model, i.e., the Heisenberg model including the two-site anisotropy term \( - \sum\nolimits_{\langle ij\rangle }D_{ij} S_{i}^{z}S_{j}^{z}\). However, the temperature dependence of MA for FeNi cannot be fitted by the simple relation K(T) ∼ M(T)n. To analyze these results, we examined MA by using the XXZ spin model including the single-site anisotropy term \( - \sum\nolimits_{i}D_{i} (S_{i}^{z})^{2}\) with the mean field approximation. We confirm that the results from first-principles calculations are well explained by this spin model. We believe that the first-principles result for FeNi is the first case that can be reproduced using the spin model with Di < 0 and Dij > 0.

Temperature Dependence of Anisotropy in Ti and Gd Doped NiMnGa-Based Multifunctional Ferromagnetic Shape Memory Alloys.

The temperature dependence of magnetocrystalline anisotropy was investigated in detail for the polycrystalline Ni50Mn25Ga25, Ni50Mn25Ga20Ti5 and Ni50Mn25Ga20Gd5 ferromagnetic shape memory alloys in the temperature range of 50–400 K. The effective anisotropy constant was estimated from a series of high field magnetization curves based on the fitting procedure according to the law of approach to magnetic saturation. The low temperature martensitic phase was found to have a significantly higher anisotropy energy in comparison to a high temperature austenitic phase, which was observed through a sudden, distinct drop of anisotropy energy. The calculated values of the effective anisotropy constant were comparable to the results published by other authors. Moreover, the strong influence of chemical composition on the first-order phase transition and the second-order ferromagnetic to the paramagnetic transition was revealed. Finally, the strong coupling between the temperature dependence of the coercive field and the temperature dependence of magnetocrystalline anisotropy was also shown and discussed in the present study.

Torque magnetometry study of the spin reorientation transition and temperature-dependent magnetocrystalline anisotropy in NdCo5

We present the results of torque magnetometry and magnetic susceptibility measurements to study in detail the spin reorientation transition (SRT) and magnetic anisotropy in the permanent magnet NdCo5. We further show simulations of the measurements using first-principles calculations based on density-functional theory and the disordered local moment picture of magnetism at finite temperatures. The good agreement between theory and experimental data leads to a detailed description of the physics underpinning the SRT. In particular we are able to resolve the magnetization of, and to reveal a canting between, the Nd and Co sublattices. The torque measurements carried out in the ac and ab planes near the easy direction allow us to estimate the anisotropy constants, K1, K2 and K4 and their temperature dependences. Torque curves, τ(γ) recorded by varying the direction of a constant magnetic field in the crystallographic ac plane show a reversal in the polarity as the temperature is changed across the SRT (240 < T < 285 K). Within this domain, τ(γ) exhibits unusual features different to those observed above and below the transition. The single crystals of NdCo5 were grown using the optical floating zone technique.

Power law analysis for temperature dependence of magnetocrystalline anisotropy constants of Nd2Fe14B magnets

We perform phenomenological analysis of the temperature dependence of magnetocrystalline anisotropy (MA) in rare-earth magnets. We define the phenomenological power laws applicable to compound magnets using the Zener theory, and we apply these laws to study the magnetocrystalline anisotropy constants (MACs) of Nd2Fe14B magnets. The results indicate that the MACs closely obey the power law, and further, our analysis yields a better understanding of the temperature-dependent MA in rare-earth magnets. Furthermore, to examine the validity of the power law, we discuss the temperature dependence of the MACs in Dy2Fe14B and Y2Fe14B magnets as examples of cases wherein it is difficult to interpret the MA using the power law.

Spin-Fluctuation Mechanism of Anomalous Temperature Dependence of Magnetocrystalline Anisotropy in Itinerant Magnets.

The origins of the anomalous temperature dependence of magnetocrystalline anisotropy in (Fe_{1-x}Co_{x})_{2}B alloys are elucidated using first-principles calculations within the disordered local moment model. Excellent agreement with experimental data is obtained. The anomalies are associated with the changes in band occupations due to Stoner-like band shifts and with the selective suppression of spin-orbit "hot spots" by thermal spin fluctuations. Under certain conditions, the anisotropy can increase, rather than decrease, with decreasing magnetization due to these peculiar electronic mechanisms, which contrast starkly with those assumed in existing models.

Low field magnetoresistance of gadolinium nanowire

We report low field (μ0H &amp;lt; 0.2 T) magnetoresistance (MR) studies on a single Gd nanowire patterned from a nano-structured film (average grain size ∼ 35 nm) by focused ion beam. For comparison, we did similar MR measurements on a polycrystalline sample with large crystallographic grains (∼4 μm). It is observed that in the low field region where the MR is due to motion of magnetic domains, the MR in the large grained sample shows a close relation to the characteristic temperature dependent magnetocrystalline anisotropy including a sharp rise in MR at the spin reorientation transition at 235 K. In stark contrast, in the nanowire, the MR shows complete suppression of the above behaviours and it shows predominance of the grain boundary and spin disorder controlling the domain response.

Spin reorientation transition: phase diagrams and entropy change

ABSTRACTIn this paper we review the phase diagram and derive the entropy change for spin reorientation transitions by considering first order magnetization process theory with temperature dependent magneto-crystalline anisotropy constants. We derive the magnetic field-induced entropy change Δs for a transition between easy axis and easy plane, showing that for alternating magnetic field, Δs has a change of sign at the reorientation temperature, while for rotating magnetic field its sign is definite. We apply the model to CoZn W-type barium ferrite.

Magnetization reversal of two-dimensional superlattices of Mn3O4 nanocubes and their collective dipolar interaction effects

Magnetic properties of two-dimensional (2D) paddy-field like superlattices of Mn3O4 cubic nanoparticles have been investigated by magnetization measurements. The 2D ordered structure extends over several microns in size. Each nanocube is of single-crystal about 6 nm in size. The magnetic properties are investigated with the powders dispersed in nonmagnetic n-eicosane to “dilute” the dipolar interaction. By accounting for the temperature variation effect of the magnetocrystalline anisotropy, Kmag(T), the temperature dependent coercivity, HC(T), can be well described by the equation, HC(T)=H0kmag(T)/mS(T){1−[kBT ln(t/t0)/E0kmag(T)]3/4}, in which kmag(T)=Kmag(T)/Kmag(0) is the reduced temperature dependent magnetocrystalline anisotropy and mS(T)=MS(T)/MS(0) is the reduced saturation magnetization. The effects of collective dipolar interaction on the magnetic properties are also studied with the as-prepared powder sample. The apparent magnetic anisotropy is seriously reduced with the presence of dipolar interaction. The switching volume is determined by the analysis on the magnetic measurements both with and without the dipolar interaction effect. There is a discrepancy in the value of switching volume determined by the two different analysis methods. Possible reasons are discussed.

Influence of Co substitution on magnetoelastic properties of Er 2Fe 14− xCo xB ( x = 1, 3 and 5) intermetallic compounds

The magnetostriction and thermal expansion of Er 2Fe 14− x Co x B ( x = 1, 3 and 5) intermetallic compounds were measured, using the strain gauge method in the temperature range 75–450 K under applied magnetic fields up to 1.5 T. For all samples the longitudinal magnetostriction ( λ l) undergoes an anomaly around the spin reorientation temperature ( T SR). It is also observed that λ l decreases with increasing the Co content. All compounds show saturation type behaviour in their anisotropic magnetostriction curves at different temperatures and applied fields. The saturation behaviour of the compound with x = 3 occurs at higher temperatures than with x = 1 and 5. The volume magnetostriction strongly increases below μ 0 H = 0.3 T, then monotonically rises with applied field up to the spin reorientation temperature. An invar type behaviour is observed above 200 K in the compound with x = 1. The results are discussed based on the temperature dependence of magnetocrystalline anisotropy of compounds below and above their T SR.

Observation of unusual magnetic behavior: Spin reorientation transition in thick Co–Fe–Ta–B glassy films

Atomically smooth Co–Fe–Ta–B glassy films were deposited on variety of substrates (Si, SiO2, and keptone). An extensive magnetic characterization in the temperature range from 5to330K is reported for the films of thickness up to ∼5.5μm. A reversible spin reorientation transition (SRT) from in-plane single domainlike state to out-of-plane multidomain state with increase in measuring temperature from 5to330K was observed in the films of thickness up to ∼2.5μm, in contrast to previously reported ultrathin ferromagnetic films of transition metals consisting of about half a dozen of monolayers. The SRT temperature (TSRT) is dependent on the film thickness and the applied magnetic field and is not governed by the temperature dependent magnetocrystalline anisotropy or the anisotropy at the film-substrate interface, which are the most common cause for the SRT in magnetic materials. Atomic relaxation has significant influence on SRT. The relaxed state results in a shift in TSRT to higher temperature or disappearance of SRT. We have compared our results with the reported data on ultrathin ferromagnetic films and shown that the atomic randomness and the strains/stress are responsible for the SRT in present case.

A comparative study on magnetostrictive strain in GdCo 5 and Gd 0.9 Pr 0.1 Co 5

Results of the magnetostriction measurements on exchange coupled GdCo5-type alloys, (with insignificant magnetic anisotropy of L-quenched Gd+3 ions) show lower values compared to Gd0.9Pr0.1Co5 compound. In fact, partial substitution of Pr for Gd creates an orbital moment for the rare earth sublattice and therefore the magnetocrystalline anisotropy (MA) increases. The temperature dependence of MA introduces the spin reorientation transition in Gd0.9Pr0.1Co5 compound. Simple model calculations indicate that a negative contribution in the first order anisotropy constant (K1) appears after Pr substitution. The sign of K1 changes from negative to positive at ∼160 K with increasing temperature. The latter introduces a large increase in the anisotropic magnetostriction. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of coercivity in Sm/sub 2/TM/sub 17/ magnets and domain wall motion in magnetic materials

It was observed in Sm/sub 2/TM/sub 17/ magnets that decreasing Fe content did not strongly affect room temperature coercivity but led to a significantly higher coercivity at high temperatures; increasing Sm content resulted in much lower room temperature coercivity but substantially higher coercivity at high temperatures; increasing Cu content led to higher coercivity at all temperatures. In this paper, the effects of Fe, Sm, and Cu on coercivity are explained by using the temperature dependence of magnetocrystalline anisotropy of the 2:17 cell phase and 1:5 cell boundary phase. In order to explain complex temperature dependence of coercivity, thermal activation of domain walls must be taken into account. It is believed that the energy necessary for domain walls to overcome the energy barrier between the 1:5 cell boundary phase and 2:17 cell phase can be acquired not only from the magnetic field, but also from thermal energy k/sub B/T. This concept may apply to all magnetic materials in which domain wall motion is involved during magnetization and demagnetization processes.

Low-temperature study of the magnetization reversal and magnetic anisotropy of Fe, Ni, and Co nanowires

We present an extensive study of the magnetic reversal mechanism of Fe and Ni nanowires with diameters down to 6 nm, i.e. smaller than the domain wall width. The coercive field at 5 K is a factor of 3 lower than the prediction for rotation in unison. We also observe that the activation energy associated with the reversal process is proportional to the cross-section of the wires and nearly independent of the wire length. From the temperature dependence of the coercive field and the magnetic viscosity we can conclude that magnetization reversal takes place via a nucleation of a small magnetic domain, probably at the end of the wire, followed by the movement of the domain wall. For Co wires, we observe a different behavior that is dominated by the competition between the shape anisotropy and the temperature-dependent magnetocrystalline anisotropy.

Magnetization reversal and magnetic anisotropy of Fe, Ni and Co nanowires in nanoporous alumina membranes

ABSTRACTThe magnetization reversal and magnetic anisotropy of Fe, Ni and Co nanowires is studied at low temperatures. All nanowires show a strong shape anisotropy with the easy axis being parallel to the long axis of the wires. Co nanowires additionally show a temperature dependent magnetocrystalline anisotropy along the hexagonal c-axis, which is directed nearly perpendicular to the long axis of the wires, as is confirmed by X-Ray diffraction measurements [1] and reported by Strijkers et al. who performed NMR measurements on samples prepared in a similar way [2]. Therefore, at low temperatures and for large wire diameters a competition between magnetocrystalline and shape anisotropies can be observed. Co wires with a small diameter, however, do not show a significant magnetocrystalline anisotropy. Fcc-Co, which is only known as a high-temperature Co modification and which does not have a large magnetocrystalline anisotropy constant, becomes the predominant Co modification here [1,3]. Investigations on the size dependence of the switching field for Fe and Ni nanowires provide information about the magnetization reversal process, which takes place via a nucleation of small magnetic domains probably at the end of the wires, and subsequent propagation of the domain wall along the wire.

Magnetic Hysteresis Properties of Fine Particles of Monoclinic Pyrrhotite Fe7S8.

The magnetic hysteresis properties have been studied of synthetic Fe7S8 separated into a suite of 8 particle size fractions in the range 1 to 30μm. Between-196°C to about 260°C, both saturation isothermal remanence (Mrs) and coercive force (Hc) can be represented as analytical functions of particle size (L) of the form (Mrs, Hc) ∝ L-n or (Mrs, Hc) ∝ exp(-kL1+2) although the particle size dependence of Mrs is much weaker than that of Hc. The parameters n and k show a small but systematic fall in values as temperature rises, but may be acceptably constant to furnish analytic functions for physical models of more complex magnetic properties such as thermoremanent magnetization or domain dynamics. The temperature dependence of Hc below room temperature is weaker than that above, possibly due to an increasing single-ion contribution to magnetocrystalline anisotropy. The inferred temperature dependence of magnetocrystalline anisotropy above room temperature up to about 250°C suggests that temperature-induced expulsion or nucleation of domain walls is not expected in this temperature range. The temperature dependence of Mrs and Hc can be expressed as (1 - T/Tc)q where q shows a small but systematic fall as particle size increases in the range 1-30μm. The several properties of the finest, monodomain, fraction (

Temperature dependence of magnetocrystalline anisotropy for Sn substituted Ba ferrite particles

Temperature dependence of magnetocrystalline anisotropy was measured for Co-Ti-Sn substituted Ba ferrite particles, to clarify the origin of a small temperature dependence of coercivity (dHc/dT) for this material. For comparison, the same measurement was carried out for Co-Ti substituted particles which have positive dHc/dT near room temperature. First and second anisotropy constants K/sub 1/ and K/sub 2/ were found to be both positive for these materials. In the Co-Ti-Sn particles, K/sub 1/ was dominant over K/sub 2/ over the whole temperature range, and K/sub 1/ decreased rapidly with increasing temperature. The rapid decrease in K/sub 1/ explains the experimental small dHc/dT. In the case of Co-Ti particles, K/sub 2/ is not negligible below 0 degrees C. The decrease in Hc with increasing temperature for the Co-Ti particles at a low temperature range is due to the decrease in K/sub 2/. The predicted temperature dependencies of Hc derived by using the K/sub 1/ and K/sub 2/ values for the two kinds of materials explain the material results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Exchange and crystal field interactions in RCo4B (R  Gd, Dy, Ho, Er)

The exchange and crystalline-electric field effects in RCo4B compounds with heavy rare earth are analysed using a series of experiments. These experiments include magnetic moment behaviour, Curie temperature, temperature dependence of magnetocrystalline anisotropy and reorientation of spin magnetization in Ho and Dy compounds. The molecular field coefficient nRCo is evaluated from temperature dependence of magnetization in two-sublattice model and from the analysis of the Curie temperature. The latter depends on the nature of the rare earth. The value of the RCo exchange field is discussed. The magnetocrystalline anisotropy behaviour is treated in terms of the exchange and crystal field interactions which may differ at a and b sites. The increase of the crystal effects in comparison with RCo5 compounds results in a reduction of the 4f magnetocrystalline anisotropy especially for the b site.

Growth induced magnetic anisotropy in Co-doped garnet and its capability of application to magneto-optical memory materials

Growth-induced magnetic anisotropy (Kug) normal to the film surface in Co-doped iron garnet films grown on GGG substrates by the LPE method has been quantitatively discussed. Kug is increased with increasing Co content and growth rate (Vg). From the measurement of temperature dependence of magnetocrystalline anisotropy K <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> , the parameter αλ is evaluated to be 80 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> through a curve fitting procedure using Slonczewski's theory of Co single-ion model for the anisotropy. After the annealing at temperatures above 900°C, Kug is disappeared and K <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> is increased: These changes are successfully interpreted by the theory using the above value αλ. Also shown is the magnetic field effect on the perpendicular anisotropy of poly-crystalline Co-doped BiDyGeIG films prepared by the pyrolysis method on glass substrates for the purpose of application to magneto-optical memory materials. In these films, (Ge(CH <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> COOH) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> is found to be a suitable solute for adding tetravalent Ge ions. In this method, an increase of Co amount raises the crystallization temperature, while doping of Ge lowers it. A magnetic field perpendicularly applied to the film during the heat treatment for crystallization enhances the anisotropy, whereby Ku is increased to about 170% of those of films prepared with no field.

Magnetocrystalline anisotropy of light rare-earth cobalt compounds

The magnetocrystalline anisotropy constants of Y-, Ce-, Pr-, Nd-, Sm- and CeMMCo 5 were determined from magnetization and torque curve measurements on single crystals in the temperature range between 77 K and the ferromagnetic Curie point and in fields up to 16 kOe. The anisotropy of Y-, Ce-, Sm- and CeMMCo 5 can be described by the constant K 1 alone, whereas K 2 and K 4 are necessary for NdCo 5 and PrCo 5, for temperatures below 300 K. A comparison of the temperature dependence of K 1 with the classical temperature dependence of magnetocrystalline anisotropy shows that the intrinsic anisotropy itself is a function of temperature. The change in type of anisotropy from NdCo 5 and PrCo 5 is consistent with crystal field calculations.

Temperature Dependence of Magnetocrystalline Anisotropy

A well-known theoretical treatment of the temperature dependence of magnetocrystalline anisotropy predicts the equality of the reduced first anisotropy constant and the nth power of the reduced spontaneous magnetization where n=10 at low temperatures. To allow for the effect of thermal expansion, the experimental values of this anisotropy constant should be corrected, according to Brenner (1957), by an amount dependent on b3, the third magnetoelastic coupling constant of Becker and Döring (1939). Measurements of the temperature dependence of magnetostriction constants and of elastic constants are used to calculate b3 as a function of temperature for nickel and it is then found that the experimental value of n is effectively reduced, from about 50 at low temperatures (Carr 1958), to much nearer the theoretical value of 10. A critical examination of possible errors is presented, particularly with reference to experiments in which the pressure dependence of anisotropy is measured.