Magnetization process related to the singular point detection signal observed at high fields on Pr2Fe14B and Nd2Fe14B

The singular point detection signal, d²M/dt², observed on Pr₂Fe₁₄B and Nd₂Fe₁₄B at high fields and low temperatures is explained on the basis of one sublattice crystal field model. The signal is attributed to a steeper reversible magnetization process and a first order magnetization process for Pr₂Fe₁₄B and Nd₂Fe₁₄B respectively.     

Sm2(TM)17合金的磁性(自旋再取向相变)研究

本文研究了Sm₂(FeNiCoM)₁₇合金(M为非磁性组元)的磁性。样品由六角结构无序型的2∶17主相及少量FeNi合金杂相组成。在六角结构的e轴方向(易磁化方向)观察到下述异常现象:低温(273K以下)时的磁化及反磁化曲线发生明显的跃变,跃变时相应的磁场H_r随温度下降而增大;磁滞迴线是蜂腰型的,温度愈低蜂腰愈明显;升温时磁化强度随温度变化(1.5K至居里点T_C)的曲线上出现极大值,其相应的温度T_t随磁场增大而降低;降温时观察到了热磁滞后现象。但在基面(难磁化方向)上及Co含量增多(>18at％)时,样品却表现了正常的铁磁行为。本文提出用磁矩非共线结构排列的自旋再取向相变来解释上述异常现象,并给出自旋倒向所需越过的能垒高度U=9.2×10^-15erg,用设想磁结构的模型得到的磁化强度的计算值与实验值也符合得较好。 关键词：     

Magnetization studies in niobium and YBa2Cu3O7 samples

The results of experimental studies on hysteresis in magnetization, thermomagnetic history effects, anomalous variations in magnetic hysteresis curves and the decay rates of magnetization obtained under different thermomagnetic histories in specimens of conventional and high temperature superconductors are presented. The Bean’s critical state model is considered adequate to explain magnetic behaviour in conventional hard superconductors. The similarity in the general features of the results of different experiments on specimens of the two families of superconductors underscores the efficacy of the said model to understand some aspects of the macroscopic magnetic response of high temperature superconductors as well. For instance, the isothermal magnetization hysteresis loop which comprises of magnetization curves along forward (−H _max to +H _max) and reverse (+H _max to −H _max) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There exist inequality relationship between various field values identified asH _peak,H _I,H _II etc. in isothermal magnetization hysteresis as well as magnetic relaxation data. The entire field span of an isothermal magnetization hysteresis data set can be considered to comprise of three parts corresponding to (M _rem(H)−M _FC(H)+M _ZFC(H)) being equal to, less than or greater than zero, whereM _rem(H) are the remanent magnetization values obtained on reducing field to zero after having the specimen in different applied field (H) values. There are, however some situations amongst thermomagnetic history effects in specimens which show incomplete flux trapping on field cooling, where the critical state model has been found inadequate.     

Prediction of a domain-drag effect in uniaxial,non-compensated,ferromagnetic metals

An electric current is predicted to exert a dragging force on ferromagnetic domain walls. The effect arises from the non-uniform current distribution, recently predicted and observed at 4·2 K in the neighborhood of a wall in metals (Co, Ni) with large Hall angle. When the average current density j? exceeds a certain “coercive” value j?_c, the domains are set into motion in the same direction as the charge carriers. When j?j?_c, the wall speed approaches the drift speed of the carriers. Ohm's law fails when the walls move. The limits of very small and of very large Hall angle are both considered, using the single-wall model of Williams, Shockley and Kittel, and also the stratified-medium model of Herring. We assume B = M_s inside each domain; this is appropriate if the sample thickness t in the direction of the saturation magnetization M_s, and the wall spacing a, obey t > a. Materials with large anisotropy field H_A (h.c.p. Co, Gd) are necessary if the field H of the current is not to remove all walls normal to the current. For similar reasons, the sample should be in the shape of a flat ribbon (or rod) normal to M_s, of thickness (or diameter) t obeying t < dH_A/H_c, where H_c is the coercive field and d the wall length in the direction normal to the current and to M_s.     

Magnetic properties of metastable Gd-Cr alloys

We report on the magnetization, magnetocaloric effect, magnetic ordering temperatures, saturation magnetic moments and anisotropy of sputter-deposited Gd_xCr_1−x alloys with Gd atomic concentrations, x, ranging from 0.13 to 0.52. The complex magnetic nature of the Gd-Cr films was revealed from the M×H isotherms, which do not show saturation even at an applied field of 70 kOe and a temperature of 2 K and do not exhibit a linear behavior at higher temperatures. For some of the samples, the isotherms were used to determine the isothermal entropy variation as a function of temperature, for a change of 50 kOe in the applied magnetic field. The saturation magnetic moment varies with x and follows the dilution law, implying that the Cr atoms do not contribute to the total moment of the Gd-Cr alloys. Both static magnetization and dynamic susceptibility measurements reveal the existence of a magnetic glassy behavior in the alloys, which occurs below a freezing temperature. The existence of anisotropy at low temperatures for all samples was revealed by their M×H hysteresis loops from which the in-plane coercive fields, H_c, were determined. A monotonical increase in H_c with increasing Gd concentration was observed.     

Effect of thermal treatment on the magnetization processes of nanocrystalline Fe80Ge3Nb10B7 alloys

The dynamic magnetization processes of nanocrystalline Fe₈₀Ge₃Nb₁₀B₇ alloys after annealing at different temperatures are studied through the permeability spectroscopy. Three steps of crystallization are found when amorphous Fe₈₀Ge₃Nb₁₀B₇ alloys are heated from 300to 1200 K. The dominant magnetization process varies with different annealing temperatures. Domain wall bulging is the main magnetization mechanism under weak applied field. When the applied field exceeds pinning field H_p, the depinning-involved domain wall displacement occurs. Different annealing temperature results in different H_p. The lower value of μ′ and high relaxation frequency after heating at 923 and 973 K are due to the strengthened domain wall pinning and the increase of magnetocrystalline anisotropy.     

Magnetization reversal dependence on magnetic properties of a spin torque oscillator with in-plane anisotropy free layer and orthogonal polarizer

The effect of magnetic properties on magnetization dynamics is studied for a spin torque oscillator (STO) composed of a free layer with an in-plane magnetic anisotropy and a reference layer with a fixed out-of plane magnetization. A transition from damped to uniform oscillations is observed for a critical value of saturation magnetization M_S). In the uniform oscillations regime, the frequency is inversely proportional to M_S. Similarly, the critical current for achieving uniform oscillations is investigated as a function of free layer intrinsic properties. In a second part of the study, the magnetostatic field (H_m) from the reference layer is considered and it is revealed that the out-of plane component of magnetization has a strong dependence on H_m. For a particular configuration, H_m could reduce the out-of plane component maximizing thus the out-put signal of the STO.     

Breakdown of an intermediate plateau in the magnetization process of anisotropic spin-1 Heisenberg dimer: Theory vs. experiment

The magnetization process of the spin-1 Heisenberg dimer model with the uniaxial or biaxial single-ion anisotropy is particularly investigated in connection with recent experimental high-field measurements performed on the single-crystal sample of the homodinuclear nickel(II) compound [Ni₂(Medpt)₂(μ-ox)(H₂O)₂](ClO₄)₂·2H₂O (Medpt=methyl-bis(3-aminopropyl)amine). The results obtained from the exact numerical diagonalization indicate a striking magnetization process with a marked spatial dependence on the applied magnetic field for arbitrary but finite single-ion anisotropy. It is demonstrated that the field range, which corresponds to an intermediate magnetization plateau emerging at a half of the saturation magnetization, basically depends on a single-ion anisotropy strength as well as a spatial orientation of the applied field. The breakdown of the intermediate magnetization plateau is discussed at length in relation to the single-ion anisotropy strength.     

Barkhausen effect in magnetic thin films: General behaviour and stationarity along the hysteresis loop

Several aspects of the Barkhausen effect can be investigated by using a measurement equipment that records the Barkhausen signal as a function of time in digital form on computer tape. Measurements on thin magnetic films containing a small number of domain walls are presented. The behaviour of the wall jumps during a reversal is investigated. During the steep part of the loop the stationarity of the signal is measured with the magnetic field as a parameter. The influence of the coercive fieldH _c and the thicknessd _m is considered and compared with results obtained by others on bulk samples.     

Magnetic and microstructural properties of the assembly of Ni(C) nanoparticles

Superparamagnetic properties of the assembly of carbon encapsulated Ni nanoparticles (Ni(C) nanoparticles) with an average particle size of 10.5 nm are studied, the blocking temperature (T_B) is determined to around 115 K at l000 Gs applied field. Above T_B, the magnetization M(H,T) can be described by the standard Langevin function L using the relation M/M_S(T=0)=coth(μH/kT)−kT/μH. Magnetization measurements suggest, this assembly of carbon encapsulated Ni nanoparticles have been exhibited typical single-domain, field-dependent superparamagnetic relaxation properties.     

Effect of deformation on Barkhausen jumps of fine wires of iron,nickel and iron-nickel alloy

The induction technique was used to measure the potential difference of Barkhausen jumps (V_B) for fine wires of technical iron, nickel and iron-nickel alloy. The effect of deformation and mechanical stresses on the dispersal band and on the behaviour of the measured values was studied. The theory of the measurement process is described in terms of magnetic domain theory. The behaviour of V_B reflects the magnetude of the applied stress.     

Distortions of the statistical distribution of Barkhausen noise measured by magneto-optical Kerr effect

In magneto-optical Kerr measurements of the Barkhausen noise, a magnetization jump ΔM due to a domain reversal produces a variation ΔI of the intensity of a laser beam reflected by the sample, which is the physical quantity actually measured. Due to the non-uniform beam intensity profile, the magnitude of ΔI depends both on ΔM and on its position on the laser spot. This could distort the statistical distribution p(ΔI) of the measured ΔI with respect to the true distribution p(ΔM) of the magnetization jumps ΔM. In this work the exact relationship between the two distributions is derived in a general form, which will be applied to some possible beam profiles. It will be shown that in most cases the usual Gaussian beam produces a negligible statistical distortion. Moreover, for small ΔI the noise of the experimental setup can also distort the statistical distribution p(ΔI), by erroneously rejecting small ΔI as noise. This effect has been calculated for white noise, and it will be shown that it is relatively small but not totally negligible as the measured ΔI approaches the detection limit.     

A new type of peak effect in the magnetization of anisotropic superconductors

Experimental and theoretical studies of the magnetization curves M(_H) obtained for plates of an anisotropic hard superconductor are performed. The samples are YBCO single-domain textured crystals with the c axis lying in the sample plane. It is shown that, for some orientations of the magnetic field in the sample plane, the magnetization curves contain additional extrema; at the same time, the virgin magnetization curve M(H) exhibits features characteristic of the peak effect. This proves that the anisotropy of the current-carrying ability can give rise to a new type of peak effect.     

A note on magnetization of high temperature superconductors (YBCO, BSCCO) in mixed state

We present a critical study of the temperature and field dependence magnetization of high temperature superconductors (HTSCs). The controversial field dependence of dM/d ln B for YBa₂Cu₃O₇ (YBCO) and Bi₂Sr₂CaCu₂O₈ (BSCCO) is discussed using different models. Moreover, for both the systems the magnetization (M(H)) dependence is compared with field (H) dependence.     

Application of the Stoner-Wohlfarth model with interaction for the determination of the saturation magnetisation, anisotropy field, and mean field interaction in bulk amorphous ferromagnets

Magnetic hysteresis curves of bulk amorphous ferromagnet alloys of composition Nd₆₀Fe₃₀Al₁₀, Nd₆₀Fe₂₀Co₁₀Al₁₀ and Pr₅₈Fe₂₄Al₁₈ have been measured in applied magnetic fields up to 9 T at temperatures in the range 10-350 K. The behaviour of the demagnetisation curve in the first quadrant is interpreted using a mean field interaction model as proposed by Callen et al. [Phys. Rev. B 16 (1977) 263], which extends the Stoner-Wohlfarth model [Philos. Trans. Roy. Soc. A 240 (1948) 599] for a random distribution of non-interacting uniaxial grains. Application of the mean field interaction model enables the determination of the saturation magnetisation M_s, anisotropy field H_a, and interaction parameter d, and from these other magnetic parameters, such as the anisotropy constant, K, are deduced. For the three alloys, the temperature dependent behaviour of M_s, H_a, d and K over the range 20-350 K are found to be qualitatively similar, though there are quantitative differences. In all cases M_s increases with decreasing temperature, both H_a and K increase with decreasing temperature, reaching a peak in the range 75-120 K, and then decreasing, and d decreases approximately linearly as the temperature decreases. The physical mechanisms responsible for coercivity in these materials are discussed in the context of random anisotropy and a strong pinning model of domain walls.     

Effect of frustration on the magnetization in nickel ferrite-chromites

A study is reported on the behavior of the isotherms of the magnetization σ(H) and of the longitudinal λ_‖(H), transverse λ_⊥(H), volume ω(H), and anisotropic λ_t(H) magnetostrictions measured at T=80 K in the Cu_0.4Fe_0.6[Ni_0.6Cr_1.4]O₄ and Zn_0.4Fe_0.6[Ni_0.6Cr_1.4]O₄ ferrite-chromites having a frustrated magnetic structure. It has been established that these ferrite-chromites do not undergo technical magnetization and that the growth of the magnetization with the field is accounted for by two paraprocesses of different natures.     

The possibility of the “giant” isotope effect for ultrasound absorption in crystals

The Zeeman effect, magnetization M(H), and differential magnetic susceptibility dM/dH of ErVO₄ crystals in a pulsed magnetic field have been experimentally and theoretically studied. In magnetic fields H ∥ [001] and H ∥ [100], the energy levels of Er³⁺ ions exhibit mutual approach and crossing (the crossover effect), which results in the peaks in dM/dH and the jumps in M(H) curves at low temperatures. The anomalies in the magnetic properties related to the crossover in ErVO₄ for H ∥ [001] are highly sensitive to the electronic structure of Er³⁺ ion, which allows this effect to be used for refining the crystal field parameters. The influence of the temperature, field misorientation from the symmetry axis, parameters of pair interactions, and other factors on the magnitude and character of magnetic anomalies in ErVO₄ crystals is considered.     

Magnetization reversal in bulk and thin films of the ferrimagnetic ErCo0.50Mn0.50O3 perovskite

We present herein a comparison of the magnetic properties of bulk ceramics and thin films of the ferrimagnetic ErCo_0.50Mn_0.50O₃ compound. Epitaxial thin films were deposited onto (1 0 0) SrTiO₃ substrates by pulsed-laser ablation while bulk ceramics were prepared by solid state reaction. When cooling under low applied fields, a spin reversal is observed in both thin film and bulk due to the competition between two magnetic sublattices (Co/Mn and Er) coupled by a negative exchange interaction. Original features are observed in the M(H) loops for bulk materials: abrupt jumps at 4 T due to a reorientation of domains, while in the low field region, the increasing and decreasing branches of the magnetization intersect each other. In the thin film, the ordering temperature increased from 69 to 75 K, and the ZFC anomaly (AF transition) became sharper, compared to the bulk specimen. The oxygen content and the microstructure are crucial to observe the intersection of the magnetization branches.     

Improvement of the remanence properties and the weakening of interparticle interactions in BaFe12O19 particles by B2O3 addition

In the present study, the effects of B₂O₃ addition on the remanence properties of barium ferrite magnets are examined. The relationship between isothermal magnetization remanence M_R(H) and demagnetization remanence M_D(H) for non-interacting single domain particles, M_D(H)=M_R(H_max)−2M_R(H), was used in order to investigate the interactions between particles. We have found that remanence magnetization M_R increased by 40% in magnitude with B₂O₃ addition in addition to the weakened couplings between particles. The B₂O₃ addition seems to supply the required conditions for usage of these materials in the magnetic recording media.