Strong stray fields in magnets with a nonuniform distribution of the magnetization

Stray fields produced by magnets in the form of a cylinder in which the vector of magnetization at each point is directed along the cylinder radius have been calculated. It was assumed that this distribution of magnetization was achieved due to the presence of a radial crystalline texture and the related magnetic anisotropy with a large field of the uniaxial anisotropy H _K . It has been shown that the greatest stray fields are reached on the surface of the magnet at points that are located near its axis, and the limiting value of the point field is equal to 4πM _S ln(a/r). It has been established that in magnets with a radial magnetization the region of localization of strong fields H > 4πM _S substantially exceeds that characteristic of the systems of magnets with a uniform magnetization.     

Effect of magnetic surface anisotropy on domain wall structure in magnetic triaxial films

The influence of the presence of surface anisotropy on the micromagnetic structure of the domain wall in magnetic-triaxial films has been investigated. The investigation method is the numerical minimization of the energy functional of the domain wall used in the two-dimensional model of the distribution of magnetization. Films with different orientations of crystallographic axes with respect to the film surface have been considered. The effect of the surface anisotropy type and surface anisotropy constant K _S value on the stability of different types of domain walls has been found.     

Magnetic field effect on the equilibrium hydrogen pressure for the PrCo5–H system

The equilibrium hydrogen pressures in the β_I+γ region of the ferromagnetic hydride PrCo₅H_x have been investigated as a function of the hydrogen composition x under the influence of magnetic fields up to 5 T at 273.2 K. The logarithmic pressure change is increased linearly with increasing magnetic fields and depends on the hydrogen composition. The value ΔM_S (the change in saturation magnetization per desorbed molH) is calculated from the relationship between the pressure change and the magnetic field. The results show a peculiar dependence of the ΔM_S on the hydrogen composition. The maximum value of ΔM_S of 25 J T⁻¹ molH⁻¹ at x=4.3 is much larger than that observed in other metal–hydrogen systems.     

Preparation and characterization of polypyrrole/nano-SrFe12O19 composites by in situ polymerization method

Polypyrrole (PPY)/nano-SrFe₁₂O₁₉ composites were synthesized by in situ polymerization method. The samples were characterized by TEM, SEM, XRD and IR technology. Spherical, conglobulation-like and arborization-like polypyrrole/SrFe₁₂O₁₉ composites were synthesized in an emulsion polymerization system for the first time. It was found that the morphology of PPY/nano-SrFe₁₂O₁₉ composites depended on the SrFe₁₂O₁₉ content of the reaction system. A possible mechanism for the formation of the different morphologic composites had been proposed. It was found that the saturation magnetization (M_S) and remanent magnetization (M_r) for the composites decreased with the decrease of the nano-SrFe₁₂O₁₉ content. The coercivity force (H_c) for the composites increased with the increase of PPY content when the PPY content was at a low value, and then decreased with the increase of PPY content. The possible mechanism for the phenomenon had been proposed. The conductivity was measured, and the variation mechanism of the composites conductivity was analyzed in the paper.     

Effect of frozen spin on the magnetocaloric property of La0.7Ca0.3CoO3 polycrystalline and single crystal samples

Polycrystalline (PC) and single crystalline (SC) samples of La_0.7Ca_0.3CoO₃ (LCCO) with the perovskite structure were synthesized by conventional solid-state reaction and the floating-zone growth method. We conducted isothermal magnetization measurements of the PC and SC samples at temperatures from 2.8 K to 140 K, and evaluated the magnetic entropy change (−ΔS_M) under zero field cooling (ZFC) and field cooling (FC) to 2.8 K. An interesting result has been obtained, where the −ΔS_M-T curves in the low temperature range show totally different features between the ZFC and FC cooling procedures. The −ΔS_M shows a large inverse irreversibility value for the ZFC process, while the −ΔS_M also shows a normal positive value, but one that is slightly larger at 2.8 K for the FC process for both samples. We also present the results of a comprehensive investigation of the magnetic properties of the LCCO system. Systematic measurements have been conducted on DC magnetization, AC susceptibility, and exchange-bias. These findings suggest that complex structural phases, including ferromagnetic and spin-glass/cluster-spin-glass (SG/CSG)) states and their transitions, exist in PC samples, while there is a much simpler magnetic phase regime in SC samples. It was also of interest to discover that the CSG induced a magnetic field memory effect and an exchange-bias-like effect.     

FeGaB (25 nm)/Al2O3/FeGaB(25 nm)多层薄膜结构：氧化铝(Al2O3)厚度变化对静态与动态磁性能的影响

铁镓(FeGa)薄膜与其它软磁材料相比具有较大的磁致伸缩常数,在设计集成磁性传感器芯片中具有独特的优势,本文通过采用非磁性掺杂和多层膜方法来控制这种合金薄膜的磁学与电学性能参数。我们实验发现在掺杂一定量硼(B)元素后,厚度小于30 nm的FeGa薄膜顽力可以得到显著降低,而对于较厚薄膜在插入超薄Al2 O3中间层后软磁性能可以得到同样程度显著改善,同时饱和磁化(Ms)变化可忽略。对于我们制备的FeGaB (25 nm)/Al2O3(0.5 nm)/FeGaB(25 nm)多层膜,其易轴矫顽力可以小到0.98 Oe,电阻率与50nm单层FeGaB膜相比增加了1.5倍,同时具有吉赫兹高磁导率谱。样品微结构分析表明,磁性颗粒结晶质量和物理尺寸的减小对软磁性改善起到重要作用,另外我们也讨论分析了静磁相互作用和表面形貌对磁畴运动及矫顽力的影响。本文发展的掺杂与多层膜混合方案来来增强电磁性能的方法,也可应用于其他类型的软磁材料系统。     

Nonlinear nonstationary dynamics of Néel domain walls in ultrathin films with an in-plane anisotropy

Nonlinear dynamic behavior of domain walls in ultrathin films (of thickness b < b _t , where b _t is a critical thickness below which only one-dimensional Néel walls exist in the stable state) has been investigated based on the numerical solution of the Landau-Lifshitz equation with an exact (modelless) allowance for all principal interactions, including dipole-dipole one (in the continuum approximation), in terms of a two-dimensional distribution of magnetization. It is shown that at film thicknesses close to b _t the nonlinear dynamic transformation of the Néel-wall structure occurs through the formation of vortex structures. In thinner films, the mechanism of the dynamic rearrangement of the wall proves to be similar to that in unbounded samples. Giant oscillations of the thicknesses of the domain walls in the process of their motion have been revealed. A monotonic decrease in the critical field with increasing b and its nonmonotonic dependence on the saturation induction have been found. The dependences of the period of dynamic transformations of the wall structure on the magnetic parameters and film thickness have been revealed.     

Synthesis and magnetocaloric characterization of rapidly solidified ErMn2 melt-spun ribbons

We have fabricated melt-spun ribbons of nominal composition ErMn₂ and studied their phase constitution and magnetocaloric (MC) properties by X-ray diffraction, scanning electron microscopy (SEM), magnetization and heat capacity measurements. The major phase formed shows the MgZn₂-type hexagonal structure (C14-type); however, both XRD and SEM analyses revealed the formation of impurity phases (i.e., Er₆Mn₂₃ and ErMn₁₂). Ribbons exhibit a saturation magnetization of 149 Am²kg⁻¹ at 2 K and a Curie temperature of T_C = 15 K. A field-induced metamagnetic transition at very low critical magnetic fields was observed below 8 K that leads to a change of sign in the magnetic entropy change ΔS_M below this temperature (ΔS_M^peak = 2.5 Jkg⁻¹K⁻¹ at 2 K and 5 T). For a magnetic field change of 5 T (2 T) applied along the ribbon length, the samples show a large peak value of the magnetic entropy change ΔS_M^peak of −20.5 (−10.8) Jkg⁻¹K⁻¹, a full-width at half-maximum δT_FWHM for the ΔS_M(T) curve of 20 (12) K, and a maximum adiabatic temperature change ΔT_ad^max of 7.4 (3.6) K. The obtained results are compared with the reported in literature by other authors for bulk alloys.     

Effect of Ni/Fe ratio of electrolyte salts on the magnetic property of electrodeposited Fe–Ni alloy

Electrodeposition of Fe–Ni thin films has been carried out on a copper substrate from simple as well as complex baths containing sulfate salts with Ni/Fe ratio of 1 : 1 and 12 : 1. Complex baths consistedeither all of ascorbic acid, citric acid and saccharine in addition to the salts viz. NiSO₄ · 7H₂O; FeSO₄ · 7H₂O; H₃BO₃ and Na₂SO₄ in simple bath. The chemical composition of the deposit was determined by an energy dispersive X-ray analyzer. Magnetic properties of the Fe–Ni films were measured by avibrating sample magnetometer (VSM). The X-ray diffraction was done on the electrodeposited thin films to determine Fe–Ni alloy phases. Magnetic properties of films were studied before and after heat treatment of the samples. It is found that the saturation magnetization decreases with increasing Ni content in the films obtained from simple baths with low Ni/Fe ratio (1 : 1) while the saturation magnetization increases with increasing Ni content obtained from complex bath with high Ni/Fe ratio (12 : 1). Among different baths with high Ni/Fe ratio of 12 : 1, the saturation magnetization of deposited film is higher deposited from a bath containing three complexing agents, namely, ascorbic acid, citric acid and saccharine than from a bath containing a single complexing agent–ascorbic acid. The ideal nature of the M _s–H (saturation magnetization vs. applied field) curve was obtained from complex baths with a high Ni/Fe ratio (12 : 1).     

Comparative study of one-dimensional NiCo alloy nanostructures assembled by in situ and ex situ applied magnetic fields

For the study of magnetic field-assisted assembly behavior, one-dimensional (1D) NiCo alloy nanostructures were solvothermally obtained at 180 °C under an in situ magnetic field (the magnetic field as applied during the chemical reduction) and ex situ field (after the chemical reduction was finished). Microscopic morphology and magnetic properties differences were investigated using scanning electronic microscope (SEM) and vibrating sample magnetometer (VSM) for these products. Magnetic measurement results show that 1D ordered microstructures under in situ magnetic field possess higher saturation magnetization M_s, remnant magnetization M_r, coercivity H_c and reduced magnetization M_r/M_s than 1D ordered microstructures under ex situ field, and the four magnetic parameters of the two ordered microstructures are much higher than those randomly distributed alloy particles prepared in the absence of external magnetic field.     

Magnetocaloric effect in HoMn2Si2 compound with multiple magnetic phase transitions

The structural, magnetic and magnetocaloric properties of the polycrystalline compound HoMn₂Si₂ have been studied. The temperature variation of magnetization and heat capacity show that below room temperature the compound under goes multiple magnetic transitions, at about 80, 15 and 3.5 K respectively. Moreover, the presence of the interesting thermomagnetic irreversibility in HoMn₂Si₂ is detected in the magnetization versus temperature plot, which can be ascribed to the narrow domain wall pinning effect. A broad and asymmetric peak is observed for the MCE response which might suggest the underlying first-order nature of the transition and/or the spin fluctuations of the Mn subsystem. The density of states N(E_F) at the Fermi level and the Debye temperature have been determined and analyzed. Large magnetocaloric effect (ΔS_M = −9 J/kg-K for a field change of 5 T around 9 K) suggest that this material is suitable for the low temperature magnetic refrigeration.     

Magnetostatic field in a thin plate with a stripe-domain structure

Magnetostatic fields inside and outside of a plate with a stripe-domain structure have been calculated based on the formulas derived depending on the domain and domain-wall parameters and the plate thickness. In a plate with equal-size domains, the induction B _z perpendicular to the plate surface nearly repeats the magnetization (without extremes) in thick samples, while in a thin plate, the maximum B _z values correspond to domain-wall edges. It has been shown that the maximum value of the field component H _x perpendicular to the domain-wall plane is, on the order of magnitude, close to the logarithm of the ratio of the wall thickness to the period of the domain structure; for thin domain walls, it can exceed the saturation magnetization by several times.     

Nonlinear dynamics of domain walls in three-layer magnetic films with nanosized layers

Nonstationary dynamics of domain walls in thin multilayer magnetic films with nanosized layers has been investigated using numerical solution of the nonlinear Landau-Lifshitz equation with exact consideration of all major interactions: exchange, magnetoanisotropic, and dipole-dipole (in continuum approximation). Scenarios have been established of nonlinear dynamic transformation of internal wall structure in three-layer films with layers with different saturation magnetization. It has been demonstrated that any velocities of nonstationary motion of the walls (average over the period, maximum over the period, preset at fixed fields) in layered films with the magnetizations of middle layers lower than the magnetization of outer layers are always higher than the velocities of one-layer films of the same thickness with the same average saturation magnetization as in multilayer films.     

Ferromagnetism of Mn-doped TiO2 nanorods synthesized by hydrothermal method

Mn-doped TiO₂ nanorods which showed room temperature ferromagnetic (RTFM) behavior have been synthesized by a hydrothermal method. Analysis of X-ray diffraction and Raman spectral data reveal the formation of anatase phase without any impurity phase. From the hysteresis loop measurements, it is possible to estimate the two parameters, magnetization (M_S) and coercivity (H_C) which are 33 memu/g and 89 Oe respectively, for 5 at% Mn-doped TiO₂. The two parameters (M_S, H_C) are strongly dependent on the Mn doping concentration. The origin of the RTFM is understood in terms of the concentration of oxygen vacancies and/or defects which is created by Mn doping.     

Relationship Between Impact Velocity of Al2O3 Particles and Deposition Efficiency in Aerosol Deposition Method

Aerosol deposition method (ADM) is a technique to form dense films by impacting solid particles to a substrate at room temperature. To improve the deposition efficiency in ADM, the relationship between the impact velocity of Al₂O₃ particles and the deposition efficiency was investigated in this study. Relative difference in impact particle velocity was evaluated by the increment percentage of the substrate surface area after deposition (ΔS). It is thought that the increase of ΔS means the increase of the impact particle velocity. When ΔS was lower than 10 %, the deposition efficiency increased from 0.082 to 0.104 % as ΔS increased from 3.46 to 9.25 %. Increasing impact particle velocity could promote the bonding between the particles themselves. On the other hand, when ΔS was higher than 10 %, the erosion of the film was observed and the deposition efficiency decreased to about 0.02 % as ΔS increased to about 40 %. SEM observation revealed that cracks parallel to the film surface were propagated. There is a possibility that this tendency of the deposition efficiency toward the impact particle velocity is common among the methods for forming ceramic films by impacting solid ceramic particles.     

Effect of surface anisotropy on the dynamic properties of Néel domain walls in the films with planar anisotropy

This work is devoted to the investigation into the effect of surface magnetic anisotropy on the motion of domain walls in magnetic uniaxial films with an in-plane anisotropy in the range of thicknesses corresponding to the stability of one-dimensional Néel walls. It has been established that the surface anisotropy with a constant K _s > 0 (K _s < 0) increases (decreases) the velocity of both stationary and nonstationary motion of the domain wall, as well as the period of oscillations of the velocity of nonstationary motion. The magnitude of the critical field as a function of K _s has been determined. Effect of surface anisotropy with K _s < 0 on the scenario of the dynamic transformation of the domain-wall structure has been revealed.     

Intrawall configurations of magnetization near Bloch-Néel transitions in magnetically triaxial films

The micromagnetic approach is used to study the structure of domain walls and find their energies depending on values of magnetically anisotropic interactions and thicknesses b of magnetically triaxial films with various orientation of the surface relative to (100) plane. This is performed based on the two-dimensional magnetization distribution with the exact consideration of the principal interactions, including dipoledipole interaction. The calculations are carried out for film thicknesses b _N that correspond to the transitions of domain wall structures from Néel to Bloch walls. It is shown that the magnetic anisotropy and orientation of film surfaces become significant at great distances from the point b _N . Initially, with increasing b, the transition of the one-dimensional Néel domain wall to the asymmetric vortex-free Néel domain wall occurs; the energy of the latter is lower than that of the asymmetric Bloch wall. As b continues to increase, the vortex-free asymmetric Néel wall transforms to a double-vortex wall and its energy begins to exceed that of the asymmetric Bloch wall.     

Effect of Single and Double Sintering on Microstructure and Magnetic Properties of Gd-Substituted Ni-Cd Ferrites

Two different groups, I and II, which are fired one and two times, respectively, from Ni-Cd ferrites of chemical formula Ni_0.7Cd_0.3Gd _x Fe_2?x O₄ (x = 0 to x = 0.1 with step = 0.025) have been prepared by conventional ceramic method. The effect of Gd-substitution on the microstructure and magnetic properties of the two groups has been studied. X-ray patterns indicated the presence of a minor secondary phase with the spinel phase at Gd-concentrations with x = 0.075 and 0.1. SEM and VSM are used to investigate the microstructure and to measure the magnetization of the samples at room temperature, respectively. The initial permeability is measured as a function of temperature and Curie temperature is determined. It was found that there are considerable differences in the microstructure and the magnetic properties of the two groups. Although the Gd-substitution decreased the values of both saturation magnetization M _s and initial permeability μ_i, the value of Curie temperature T _C has been improved. Moreover, the double sintering process improved the magnetization and densification of samples.     

Structural state and magnetic properties of cementite alloyed with manganese

Using X-ray diffraction analysis, M?ssbauer spectroscopy, and magnetic measurements, the structure, parameters of hyperfine interactions, localization of Mn atoms in the lattice, coercive force, and specific saturation magnetization have been investigated in the mechanically alloyed and annealed cementite (alloyed with manganese) of compositions (Fe_{1 ? x} Mn _x )₃C (x = 0?C0.12). It has been shown that strongly deformed cementite resides in the low-coercivity state and, after annealing in the vicinity of 500°C, in the high-coercivity state. Alloying with manganese reduces the coercive force, the specific saturation magnetization, and the Curie temperature of cementite. Inhomogeneities of the distribution of manganese atoms indicate the temperature dependence of the coercive force of mechanically alloyed and annealed cementite samples.