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.     

Crystal structure and magnetic properties of hard magnetic Sm2Fe17Nδ thin films with Co substitution

The crystal structure and magnetic properties of the hard magnetic Sm₂(Fe_1−xCo_x)₁₇N_δ thin films prepared by dc magnetron sputtering and subsequent nitrogenation process were investigated. It is found that the N content and crystal structure determine the non-monotonic dependence of the coercivity H_C on nitriding temperature for the films with x=0. With nitriding temperature exceeding 300 °C, N atoms can enter the Sm₂Fe₁₇ phase and the N content increases with increasing nitriding temperature, which leads to an increased coercivity. However, the maximum value of the H_C is observed at 400 °C. The α-Fe soft phase appears with nitriding temperature further increasing to 500 °C, which is responsible for the decreased H_C. When x is between 0 and 0.36, the films exhibit single Th₂Zn₁₇-type structure. Co atoms are found to go into the lattice of the 2:17 phase, generating an enhanced exchange coupling interaction between the nano-grains, which is responsible for the improved hard magnetic properties of the films with Co substitution at a certain range. Especially, the optimal value of the coercivity H_C and remanence ratio M_R/M_S reaches 4.0 kOe and 0.70 for the films with x=0.17 and 0.36, respectively.     

Effect of sputtering process on magnetic properties and crystal structure of Sm2Fe17Nx thin films

Hard magnetic Sm₂Fe₁₇N_x thin films were prepared by dc magnetron sputtering and subsequent nitrogenation process. The results show that the sputtering parameters determine the film composition, which determines the crystal structure and magnetic properties. When the gas pressure varies from 1.2 to 2.1 Pa and power varies from 40 to 60 W, higher Sm content (>11.3 at%) is obtained, giving rise to improved coercivity H_C and remanence ratio M_R/M_S. The optimal H_C of 2127.8 Oe and M_R/M_S of 0.53 are obtained when the gas pressure and power reach 1.2 Pa and 50 W, respectively. In addition, it is found that the pure single Sm₂Fe₁₇ phase can be observed when the ratio of Fe/Sm exceeds 7.1 by controlling the sputtering parameters to adjust the composition.     

Angular dependence of the coercive force of textured rare earth permanent magnets

On the basis of model calculations experimental results on the angular dependence of the coercivity H_c and the remanence coercivity H_R of hard magnetic materials of the type SmCo₅, Sm₂(Co, Fe, Cu, Zr)₁₅ and Nd₂Fe₁₄B are discussed. In the model coherent rotation as well as incoherent magnetization jumps (e.g. 180°-Bloch walls) are included. The texture is described by an axial symmetric distribution of the easy axes with only oneparameter. For Sm₂(Co, Fe, Cu, Zr)₁₅ the model explains irreversible (H_R (θ)-curves) as well as reversible (H_R(θ)−H_c(θ)) magnetization processesin good agreement with the experiments, whereas stronger deviations exist for SmCo₅ and Nd₂Fe₁₄B, especially in the H_c(θ)-curves. These deviations should be caused by other reversible magnetization processes     

Study of the magnetic and structural properties of Mn-, Fe-, and Co-doped ZnO powder

A study of the magnetic and structural properties of Zn_1−xM_xO powder (where x=0 or 0.01, and M=Mn, Fe or Co) produced by the proteic sol–gel process was undertaken. The sample crystal structure was analyzed by XRD and magnetic measurements were carried out in a SQUID magnetometer. Of the XRD analysis, all samples had hexagonal wurtzite crystal structure with P63mc space group, and no secondary phase was observed. It is observed of the M(H) measures at 2 K, that the Co- and Mn-doped ZnO displayed saturation magnetizations (M_s) of approximately 2 and 3.2 emu/g, respectively, and no remanence (M_r) was observed, indicating a superparamagnetic behavior in these samples. However, the Fe-doped sample showed a ferromagnetic behavior with M_s∼0.34 emu/g, M_r∼0.05 emu/g, and coercivity (H_c)∼1090 Oe. Already at room temperature, the M(H) measurements reveal a purely paramagnetic behavior for Mn- and Fe-doped ZnO, indicating that the Curie temperature (T_c) is below 300 K. However, a weak superparamagnetic behavior was observed in the Co-doped sample, indicating that T_c>300 K.     

Nonmonotonic magnetic-field dependence of transport coefficients for n-Bi-Sb semiconducting alloys

The transport coefficients of tellurium-doped n-Bi_{1 ? x} Sb _x semiconducting alloys (0.07 ≤ x ≤ 0.15) are studied for single-crystal samples in the temperature range 1.5 ≤ T ≤ 40 K and in magnetic fields 0 ≤ H < 20 kOe. The theory developed in this study attributes the specific features in the behavior of the transport coefficients observed in a magnetic field to a strong anisotropy of the electron spectrum and anisotropy in the electron relaxation time. It is found that the dependences of the transport coefficients on the magnetic field for H ∥ C ₃ can be theoretically expressed through one anisotropy parameter δ, and those for H ∥ C ₂, by means of several anisotropy parameters, namely, γ, η, ζ, and m ₃/m ₁. It is established that the anisotropy parameter δ in the n-Bi-Sb semiconducting alloys can be estimated from measurements of the electrical resistivity ρ₂₂(∞)/ρ₂₂(0) ? δ and the Hall coefficient R _12.3(∞)/R _12.3(H → 0) ? δ in a magnetic field H ∥ C ₃. It is shown that the observed increase in the thermoelectric efficiency by a factor of 1.5–2.0 in the transverse magnetic fields H ∥ C ₃ and H ∥ C ₂ originates from the nonmonotonic dependence of the diffusion component of the thermopower Δα₂₂(H)(?T ∥ C ₁) on the magnetic field. The nonmonotonic dependence of the diffusion thermopower in n-Bi-Sb semiconducting alloys is associated with the strong anisotropy of the electron spectrum, the anisotropy in the electron relaxation time, and the many-valley pattern of the spectrum.     

Magnetic Resonance in Gadolinium Nanoparticles near the Curie Point

The Influence of temperature in the range from 275 to 320 K on ESR spectra and magnetization m of ensembles of spherical gadolinium nanoparticles with the diameter from 89 to 18 nm was studied. The particles with d = 18 nm had a cubic face centered structure and no magnetic transition. At T > T_C all particles were paramagnetic, and their g factors were g = 1.98 ± 0.02 irrespective of their size and structure. At T = T_C the particles having 28 to 89 nm in size experienced a magnetic and orientation transition; at T < T_C their m(H) dependences were described by the Langevin function, and the FMR lines broadened and shifted towards H = 0. FMR lines of the Gd particle ensembles showed a hysteresis behavior during magnetization reversal, which did not correlate with the coercivity of the particles. Dependences of the Gd nanoparticles FMR linewidth ΔH(T) changed proportionally to |T–T_C|.     

Coercivity of anisotropic γ-Fe2O3 particles at high temperatures

The coercivity of five different samples of anisotropic γ-Fe₂O₃ particles is studied in the temperature range 0°–600°C. It is found that their relative coercive force h _c = H _c (T)/H _c (0) is almost a linear function of the relative magnetization of the particles m _s = M _s (T)/M _s (0), where H _c (0) and M _s (0) are the values of H _c and M _s of the particles at 0°C. It is experimentally found that h _c = βm _s + α, where β = 1.103 ± 0.015 and α = ?0.114 ± 0.009. This character of the dependence of h _c on m _s suggests that, at high temperatures, H _c of anisotropic γ-Fe₂O₃ particles can depend on both their shape anisotropy and other factors. It is assumed that, as the temperature increases, anisotropic γ-Fe₂O₃ particles in a zero magnetic field are divided into small structurally and magnetically unstable nanoclusters with magnetization spontaneously changing its direction. As a result, H _c disappears near the Curie temperature, although the saturation magnetization of the particles in a field of 1 T is still retained at this temperature.     

Study of critical exponents in doped La2/3Ca1/3Mn1-y Fe y O3 (y = 0, 0.03) manganite films

In this work, we investigated close to the Curie temperature T _C the critical exponents of the magnetization of doped manganite La_2/3Ca_1/3Mn_0.97Fe_0.03O₃ (LCMFO) thin films, as well as undoped La_2/3Ca_1/3MnO₃ (LCMO). Using a T _C distribution given by the intrinsic magnetic inhomogeneities in these ferromagnets enables the determination of β and δ critical exponents [corresponding to M(T) and M(H) respectively], average Curie temperature < T _C > , and the T _C distribution width, ΔT _C. Additionally, we extracted the critical exponent η = βδ from the fits of ΔT _C as a function of the external applied magnetic field. We found a value of 1.74 ± 0.09 for this exponent, close to that reported in undoped La_2/3Ca_1/3MnO₃ thin films. Even though the substitution effects of the Mn ions by Fe affect the magnetotransport and structural properties of LCMO system, these results suggest that around T _C, the magnetic phase transition, governed by the critical exponents, is similar in both magnetic systems, and belongs to the same universality class.     

R3(Fe,Mo)29Nx的晶体学特性和内禀磁性

研究了R₃(Fe,Mo)₂₉(R=Ce,Nd,Sm,Gd,Tb,Dy,Y)氮化物的晶体学特性和内禀磁性.主要研究内容为：氮化对R₃(Fe,Mo)₂₉金属间化合物的点阵参数、居里温度T_C、饱和磁化强度σ_s和各向异性场B_a的影响. 关键词：     

Superparamagnetism and spin-glass like state for the MnFe2O4 nano-particles synthesized by the thermal decomposition method

MnFe₂O₄ nano-particles with an average size of about 7 nm were synthesized by the thermal decomposition method. Based on the magnetic hysteresis loops measured at different temperatures the temperature-dependent saturation magnetization (M_S) and coercivity (H_C) are determined. It is shown that above 20 K the temperature-dependence of the M_S and H_C indicates the magnetic behaviors in the single-domain nano-particles, while below 20 K, the change of the M_S and H_C indicates the freezing of the spin-glass like state on the surfaces. By measuring the magnetization–temperature (M–T) curves under the zero-field-cooling (ZFC) and field-cooling procedures at different applied fields, superparamagnetism behavior is also studied. Even though in the ZFC M–T curves peaks can be observed below 160 K, superparamagnetism does not appear until the temperature goes above 300 K, which is related with the strong inter-particle interaction.     

Magnetic properties and magnetocaloric effect of Sr-doped Pr0.7Ca0.3MnO3 compounds

In this work, we pointed out that Sr substitution for Ca leads to modify the magnetic and magnetocaloric properties of Pr_0.7Ca_0.3-xSr_xMnO₃ compounds. Analyzing temperature dependence of magnetization, M(T), proves that the Curie temperature (T_C) increased with increasing Sr content (x); T_C value is found to be 130–260 K for x = 0.0–0.3, respectively. Using the phenomenological model and M(T,H) data measured at several applied magnetic field, the magnetocaloric effect of Pr_0.7Ca_0.3-xSr_xMnO₃ compounds has been investigated through their temperature and magnetic field dependences of magnetic entropy change ΔS_m(T,H) and the change of the specific heat change ΔC_P(T,H). Under an applied magnetic field change of 10 kOe, the maximum value of -ΔS_m is found to be about 3 J/kg·K, and the maximum and minimum values of ΔC_P(T) calculated to be about ±60 J/kg·K for x = 0.3 sample. Additionally, the critical behaviors of Pr_0.7Ca_0.3-xSr_xMnO₃ compounds around their T_C have been also analyzed. Results suggested a coexistence of the ferromagnetic short- and long-range interactions in samples. Moreover, Sr-doping favors establishing the short-range interactions.     

Magnetic properties of Fe<Subscript>3</Subscript>C ferromagnetic nanoparticles encapsulated in carbon nanotubes

The low-temperature dependences of magnetic characteristics (namely, the coercive force H _c , the remanent magnetization M _r , local magnetic anisotropy fields H _a, and the saturation magnetization M _s ) determined from the irreversible and reversible parts of the magnetization curves for Fe₃C ferromagnetic nanoparticles encapsulated in carbon nanotubes are investigated experimentally. The behavior of the temperature dependences of the coercive force H _c (T) and the remanent magnetization M _r (T) indicates a single-domain structure of the particles under study and makes it possible to estimate their blocking temperature T _B = 420–450 K. It is found that the saturation magnetization M _s and the local magnetic anisotropy field H _a vary with temperature as ～T ^5/2.     

时效工艺对PtCo合金磁性能的影响

研究了时效温度对PtCo合金磁性能的影响.磁性能测量结果显示,随着时效温度的增加合金的剩磁逐渐减小,时效温度为690 ℃时,合金的矫顽力达到最大值.第一步时效处理后,合金的有序度S随时效温度的提高而增加,长程有序度的增加说明合金有序相的晶粒尺寸也在增加,这是合金矫顽力变化的根本原因.第二步时效处理后,PtCo合金的微观组织结构不发生改变,有序相的各向异性场增大,使合金的矫顽力增大. 关键词： 时效温度 PtCo合金 磁性能 有序度     

Anisotropy dispersion in (CoCrPt)1−x(SiO2)x perpendicular recording media

The distribution of easy axis orientation in perpendicular media is of technological importance because it affects the value of S^* (see Fig. 1), which quantifies the switching field distribution (SFD) and hence partially determines the data density achievable on a given medium. The distribution is controlled by the crystallographic orientation of grains and factors such as intergranular exchange and dipolar coupling. Due to strong demagnetising fields in the perpendicular orientation, traditional measurements of remanence as a function of angle are difficult to interpret and have required the use of large-scale computational models. In this work we have utilised the variation of coercivity H_C with angle, which has the advantage that at H_C the global demagnetising field is zero. Additionally, since such materials follow essentially the Stoner–Wohlfarth mode of reversal, the variation of H_C with angle, H_C(θ), is much greater than that for the remanence. We find that for (CoCrPt)_1−x(SiO₂)_x, where the level of exchange coupling is controlled, the distribution of magnetic easy axes is narrower when the exchange coupling is reduced, but dipolar coupling between the grains is strong and affects the magnetisation reversal significantly.     

Polyol synthesis and magnetic study of Mn3O4 nanocrystals of tunable size

Nanosized manganese oxide particles were prepared by the so-called polyol process. The average diameter of the particles was controlled by the growth time. X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photon spectroscopy (XPS) show that the particles are well crystallized, pure, stoichiometric Mn₃O₄ single crystals of uniform size ranging from about 5 to 12 nm. The variation of their dc-magnetization, M, as a function of the magnetic field, H, and temperature, T, clearly corresponds to ferromagnetic ordering at low temperature, with a Curie temperature slightly higher than 40 K. The evidence for superparamagnetism in these particles, due to their very small size, has been discussed in the light of their M(H) and M(T) for zero-field-cooled (ZFC) and field-cooled (FC) plots.     

Study of the field dependence of the magnetocaloric effect in Nd1.25Fe11Ti: A multiphase magnetic system

The field dependence of magnetic entropy change ΔS_M(T,H) has been studied in the crystalline sample Nd_1.25Fe₁₁Ti, a multiphase system constituted by three phases: Fe₁₇Nd₂, Fe₇Nd and Fe₁₁TiNd. The magnetic entropy change has been calculated from the numerical derivative of magnetization curves M(T,H) with respect to temperature and subsequent integration in field. To determine the field dependence of the experimental ΔS_M, a local exponent n(T,H) can be calculated from the logarithmic derivative of the magnetic entropy change vs. field. In contrast with the results for single phase materials, where n at the Curie temperature T_C is field independent, it is shown that for a multiphase system n evolves with field both at the Curie temperature of the system and the Curie temperatures of the constituent phases. This is in agreement with numerical simulations using the Arrott-Noakes equation of state.     

Influence of annealing on magnetic properties of Co-based metallic glasses

The influence of low-temperature annealing on the magnetic hysteresis loop parameters of magnetostrictive Co-Si-B and nonmagnetostrictive Co-Fe-Si-B glasses was studied. The dependence of Curie temperature (T_C), crystallization temperature (T_x), full-stress-relaxation temperature, saturation magnetostriction constant, saturation magnetization and coercive field H_c on metalloids contents for as quenched Co_100-x(Si_0.5B_0.5)_x glasses was determined.It was found that annealing enhances remanence magnetization for all investigated Co-Si-B glasses. The coercive field of these glasses is influenced by annealing owing to: stress relaxation (resulting in an H_c decrease) and domain structure stabilization (resulting in an H_c increase). Significant H_c reduction for both magnetostrictive and non-magnetostrictive glasses was observed, after annealing above T_C. For glasses with T_C<>T_x, it was necessary to apply an external magnetic field in ord er to decrease H_c.It was found that non-magnetostrictive metallic glasses with low Curie temperatures (T_C ? 450 K) exhibit the most stable magnetic hysteresis loop parameters.     

Temperature dependence of coercivity and metamagnetism in Tb0.5Y0.5Ni

The temperature dependences of magnetic properties of the FeB structure compound Tb_0.5Y_0.5Ni are examined with the aim to elucidate the origin of intrinsic magnetic hardness in this compound. The easy a axis coercivity of an aligned powder sample is found to be strongly temperature dependent. A domain wall activation model is employed to obtain the parameters H₀(0)=30kOe, V_V(0)=0.24K^?1, and β=0.016K^?1, where H₀(0) is the coercive force at absolute zero, V_T(0) the domain wall activation parameter, and β the coefficient for the linear temperature dependence of the reduced anisotropy. From polycrystalline data, we find a strong temperature dependence of the c axis metamagnetic transition only for the ferromagnetic (F) to antiferromagnetic (AF) direction. From the relative constancy of the AF to F c axis metamagnetic transition compound to the a axis coercivity we conclude that the coercive force is not directly related to the local molecular fields as had been previously suggested.     

Effect of Mn on the magnetic properties of Fe3B/Nd2Fe14B nanocomposites

The magnetic properties of Nd_4.5Fe_77−xMn_xB_18.5 (x=0, 1 and 2) nanocomposites prepared by the crystallization of amorphous precursors were investigated. Addition of Mn is found to decrease the crystallization temperature of the amorphous ribbons. The intrinsic coercivity _iH_c and maximum energy product (BH)_max increase from 2.6 kOe and 9.1 MGOe for x=0 to 3.1 kOe and 10.3 MGOe for x=1, respectively, and the remanence ratio M_r/M_s increases from 0.70 to 0.72. The effect of Mn on Curie temperature T_C and the thermal stability of M_r and _iH_c were also studied. ⁵⁷Fe Mössbauer spectra have been recorded for x=0, 1 and 2 ribbons at room temperature and site preference of the Mn atoms in Fe₃B and Nd₂Fe₁₄B phases is discussed using the Mössbauer spectroscopy.