Effect of the intergrain interactions on the coercivity and its angular dependence for Nd(FeCo)B sintered magnets

Effects of interactions between grains with different alignment degrees on the coercivity and its angular dependence for Nd₁₆(Fe_0.8Co_0.2)₇₈B₆ sintered magnets have been studied. The experiments show that the intrinsic coercivity _jH_c decreases with enhancing grain alignment (decreasing alignment coefficient σ), the coercivity _jH_c(θ) increases with increasing angle θ between the applied field and the texture axis of the magnets and the variation ratio is larger for the magnets with better grain alignment. The coercivity of the magnets should be determined by the critical field making the moment of individual grains reverse and the interactions between the grains. For the sintered magnets composed of the grains with μm size, the magnetostatic interaction between the grains is stronger than the exchange coupling interaction and it makes the coercivity of magnet increase with increasing alignment coefficient σ. Taking into account the intergrain interactions, the starting field theory of coercivity is in good agreement with the experimental results for Nd₁₆(Fe_0.8Co_0.2)₇₈B₆ sintered magnets.     

EFFECT OF α-Fe SURFACE LAYER ON THE MAGNETIC REVERSION PROCESS FOR Nd2Fe14B CRYSTAL

The demagnetization process for a Nd₂Fe₁₄B grain covered by an α-Fe layer was studied by use of the finite element technique of micromagnetics. μ_0iH_c decreases with the increase of α-Fe layer thickness t. With the increase of t from 0 to 6nm, μ_0iH_c decreases from ～7 T to ～3.5 T when the angle between the applied field and the c-axis of the Nd₂Fe₁₄B grain is 0.5° and from ～4 T to ～1.5 T when the angle is 30° or 60°. The effect is only slightly affected by the angle between the α-Fe layer and the field direction.     

INTER-GRAIN EXCHANGE INTERACTION FOR SINGLE-PHASE NANOCRYSTALLINE Nd-Fe-B MAGNETS

The demagnetization curve as a function of intensity of the inter-grain exchange interaction was calculated for single-phase nanocrystalline Nd-Fe-B magnets by use of the finite-element technique of micromagnetics. Also, the strength of the exchange interaction was estimated as a function of the Nd content x for the nanocrystalline Nd_xFe_94-xB₆ magnets by comparing the above result with the experimental relation between _iH_c and x for the magnets. We found that the inter-grain exchange interaction decreases with the increase of x, are ≈70% and ≈60% of the inter-grain exchange interaction for x=15.5(μ_0iH_c≈2.0T) and x=19(μ_0iH_c≈2.3T), respectively.     

Comparison of the texture functions describing the grain alignment in NdFeB magnets

Two types of Gaussian distribution function (`θ type’ and `tan θ type') describing the degree of grain alignment in sintered NdFeB magnets have been compared in the distribution coefficient σ (or σ_g), the distribution probability P(θ) and the grain alignment dependence of coercivity. The results show that when the grain alignment is good (the ratio of remanence-to-saturation polarization J_r/J_s⩾0.90), σ(σ_g) and P(θ) for the two types of Gaussian functions have similar variation tendencies, the calculated values of normalized coercivity based on the starting field theory are basically the same and are consistent with experiments. When the grain alignment is not good (J_r/J_s⩾0.80), the variation tendencies of σ and P(θ) are different. In addition, according to `tan θ type’ Gaussian function, the theoretical values of the normalized coercivity based on the starting field theory are still consistent with the experiments, but according to `θ type’ Gaussian function, the theoretical values seriously deviate from the experiments. This means that the `tan θ type’ Gaussian function is a better texture function for describing the grain alignment.     

交换弹簧磁性多层膜的磁矩取向及磁滞回线的解析研究

本文以界面交换耦合常数Jⁱ和软磁相厚度L^s为主要参变量,研究了易轴与膜面平行情况下的Nd₂Fe₁₄B/α-Fe磁性多层膜的磁矩随外场变化的取向及磁滞回线,并得到了成核场的解析公式.分析发现,Jⁱ对磁矩取向、钉扎场和矫顽力机理有着较大的影响.当L^s较小时,钉扎场等于成核场,随着Jⁱ的减小 关键词： 成核场 钉扎场 矫顽力 磁滞回线     

纳米晶复合Pr2Fe14B/α-Fe永磁材料磁性的研究

采用熔体快淬的方法制备Pr₂Fe₁₄B/α-Fe纳米晶复合永磁材料.使用振动样品磁强计(VSM)测量样品的室温磁性能.实验合金成分为(Pr_xFe_94.3-xB_5.7)_0.99Zr₁(其中x＝8.2，8.6，9.0，9.4，9.8，10.2，10.6，11.0，11.4(原子分数，%)).系统地研究了辊速及合金成分对快淬带磁性能的影响，当Pr原子分数由8. 关键词： 纳米复合永磁材料 熔体快淬 2Fe₁₄B/α-Fe')" href="#">Pr₂Fe₁₄B/α-Fe 磁性     

Nd-Fe-C(或BC)三元化合物的晶体结构与磁性

本文中研究了从Nd₁₄Fe₇₈B₈过渡到Nd₁₄Fe₇₈C₈时化合物的晶体结构与内禀磁性。指出轻稀土-Fe-C化合物形成P42/mnm型四方结构的结晶学条件在于适当的热处理温度。在Nd₁₄Fe₇₈B_8-xC_x系列中,随着C含量的增加,居里温度略有减少,饱和磁化强度σ_s出现一个平缓的峰值变化,室温下的剩磁σ_r、内禀矫顽力 _iH_e和磁晶各向异性场H_a都明显增大。含碳样品的各向异性行为与Nd-Fe-B相似,室温下易磁化方向沿e轴,低温下易轴各向异性转变成易锥型。自旋再取向温度随C的增加逐渐降低,但1.5K下的锥角有轻微的增大,这表明在Nd和Fe次晶格的各向异性之间存在着复杂的耦合。此外,适当富Nd的Nd-Fe-C化合物有强的各向异性。 关键词：     

Mössbauer analysis and magnetic properties of Invar Fe–Ni–C and Fe–Ni–Mn–C alloys

The saturation magnetization and the hyperfine magnetic field of different f.c.c. Fe–Ni based alloys containing nearby 29 at .% Ni were studied as a function of temperature and for different Carbon and Manganese contents. We have observed abnormal behaviors that are explained in terms of mixed exchange interactions between atomic spins: J _NiNi(r _i ) < 0, J _FeFe(r _i ) > 0, J _NiFe(r _i ) < 0.     

Contribution of superconducting grains and grain boundaries to the magnetoresistance of ceramic YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> high-temperature superconductors in weak magnetic fields

The current-voltage characteristics of granular YBa₂Cu₃O_6.95 high-temperature superconductor samples have been measured at a temperature of 77.3 K in external transverse magnetic fields H _ext with a strength of up to H _ext ≈ 500 Oe for low transport current densities (0.1 A/cm² ≤ j ≤ 0.6 A/cm²). The current-voltage characteristics obtained have been used to construct dependences of the magnetoresistance ρ on the quantities j (ρ(j) _Hext=const) and H _ext(ρ(H _ext) _{j = const}). It has been revealed that the current and field dependences of the magnetoresistance exhibit anomalies at H _ext ≥ H _c1g , where H _c1g is the lower critical field of superconducting grains. A comparative analysis of the dependences ρ(j)H _{ext = const} and ρ(H _ext) _{j = const} has made it possible to develop concepts regarding the influence of the processes of redistribution of the magnetic field between grain boundaries and superconducting grains on the transport and galvanomagnetic properties of granular high-temperature superconductors. It has been established that the field dependences of the magnetoresistance exhibit specific features associated with the beginning of penetration of Josephson vortices into grain boundaries in the magnetic field H _c1J and with the breaking of a continuous chain of Josephson junctions in the magnetic field H _c2J .     

Effects of nanoscale defects on critical current density of (Y1−xEux)Ba2Cu3O7−δ thin films

In pulsed laser deposition of YBa₂Cu₃O_7?δ films, defect introduction into the films tends to anisotropically improve the pinning along the H||c direction due to the columnar growth mode of the process. In Eu-substituted samples, however, even though an increase in critical current density (J_c) in the H||c direction was observed for low fields (H = 0.2 T), the improvement was more notable for the H||ab-plane at both low and higher fields. Herein we present detailed TEM microstructural studies to understand these new trends in J_c(H), which are markedly different than flux pinning increases achieved with other methods, for example, with nanoparticle additions. Threading dislocations, observed in the Eu-substituted samples along the c-axis, account for J_c enhancement with H||c at low field. The enhanced ab-planar pinning in the Eu-substituted samples is attributed to the extensive bending of the {0 0 1} lattice planes throughout the film, and the crystal lattice defects with excess Cu–O planes, that were effective in increasing the J_c for H||ab at both low and high fields.     

Angular dependences of critical current density in YBCO thin films with crossed columnar defects

We investigated the dependences of the critical current density J_c on the magnetic field angle θ in YBa₂Cu₃O_7−δ thin films with the crossed configurations of the columnar defects (CDs). To install the crossed CDs, the films were irradiated using the high energetic Xe ions at two angles relative to the c-axis. The additional peak around the c-axis appears in the J_c(θ) for all irradiated films. In lower magnetic fields, the height of the J_c(θ) peak caused by the crossed CDs with the crossing angles θ_i = ±10° was higher than that for the parallel CDs. It is considered that the correlation of the flux pinning by the crossed CDs along the c-axis occurs even in the case of θ_i = ±25°, which was also suggested by the kink behaviors of the scaling parameters of the current–voltage characteristics near 1/3 of the matching field. In higher magnetic fields, on the other hand, the height and width of the J_c(θ) peak for the crossed CD configurations rapidly reduce with increasing the magnetic field compared to the parallel ones. In the crossed CD configurations, the dispersion in the direction of CDs would prevent the correlation of flux pinning along the c-axis in high magnetic fields, which occurs in the parallel CD configurations due to the collective pinning of flux lines including the interstitial flux lines between the directly pinned flux lines by CDs.     

Magnetization and characteristic fields in Nd0.8Ba0.2FeAsO0.6F0.4 with binary doping

In the present work, the iron oxypnictide superconductor of the namely Nd_0.8Ba_0.2FeAsO_0.6F_0.4 is prepared with respect to the observation of distinct magnetization characteristics arising from both electron and hole doping. A magnetothermal phase diagram is given for the present iron oxypnictide system, based on the irreversibility fields (H_irr) and the upper critical fields (H_c2) obtained from magnetotransport measurements, as well as the lower critical fields (H_c1) evaluated by magnetization loops at various temperatures. High H_c2(0) are revealed at low temperature range, which is in consistent with the observations of comparatively high critical current (J_c) and the temperature dependent peak effect in the J_c vs H curves.     

A study of the plasticity in the vortex matter across the second magnetization peak in a YBCO crystal via measurements of minor hysteresis loops

Results of an investigation of the path dependence of the critical current density J _c due to the plastic deformation of the flux line lattice in a weakly pinned YBa₂Cu₃O_7-δ (YBCO) crystal for H‖c are reported. The procedure of minor hysteresis loops has been used to explore the path dependence of J _c and the metastability effects. Contrary to the behavior observed in low T _c systems. in YBCO it is found that at low temperatures, the multivaluedness in J _c(H) could persist beyond the notional peak field H _p, at which the anomalous variation in J _c(H) reaches its maximum value.     

Magneto-intersubband zener tunneling in a wide GaAs quantum well at high filling factors

The dependence of the differential resistance r _xx on the dc current density J _dc in a wide GaAs quantum well with two occupied size quantization subbands has been investigated at the temperature T = 4.2 K in the magnetic fields B < 1 T. A peak, whose position is given by the relation 2R _c eE _H = ħω_c/2, where R _c is the cyclotron radius, E _H is the Hall electric field, and ω_c is the cyclotron frequency, has been observed in the r _xx (J _dc) curves at high filling factors. The experimental results are attributed to Zener tunneling of electrons between the Landau levels of different subbands.     

Hard axis magnetic field dependence on current-induced magnetization switching in MgO-based magnetic tunnel junctions

We conducted a detailed study of hard axis magnetic field (H_hard) dependence on current-induced magnetization switching (CIMS) in MgO-based magnetic tunnel junctions (MTJs) with various junction sizes and various uniaxial anisotropy fields. The decreases in critical current density (J_c) and the intrinsic critical current density (J_c0) estimated from the pulse duration dependence on J_c in CIMS are observed when applying H_hard for all MTJs. The decrease in energy barrier of CIMS is also observed except for the largest sample. These results indicate that the reduction of J_c is attributable to both the increase of spin-transfer efficiency and the decrease in energy barrier in the case of applying H_hard. The J_c0 decreases with increase in the mutual angle between the direction of magnetization and the easy axis (θ_f), which is consistent with the theoretical prediction proposed by Slonczewski. The degree of the reduction of J_c0 for the same value of H_hard decreases with decreasing size of MTJs. This behavior is considered to be related to not only decrease in θ_f due to the increase in anisotropy field in MTJs, but also to the increase in the variance of the initial angle of magnetization due to the thermally activated magnon excitation. The stable switching endurance related to CIMS was observed in a wide range of MTJ sizes when applying H_hard. Moreover, we proposed a new architecture and a new switching method considering write disturbance. These results would be useful for application to spin memory and other spin-electronic devices.     

Ultra-fine grained Nd–Fe–B by high pressure reactive milling and desorption

This paper reports on the grain refinement in dynamic hydrogenation disproportionation desorption and recombination (d-HDDR) processed Nd-rich Nd₂Fe₁₄B and stoichiometric Nd₂Fe₁₄B powders using high pressure reactive milling (HPRM) followed by a subsequent desorption and recombination. In contrast to the dynamic-HDDR processed anisotropic powder with a grain size of the Nd₂Fe₁₄B phase of 300 nm, the new approach yields a further reduction of the Nd₂Fe₁₄B₁ grain size to less than 70 nm. Nd-rich Nd₂Fe₁₄B powder produced by HPRM and subsequent desorption exhibits a coercivity μ_0iH_c=1.35 T and a remanence of 0.80 T. In the stoichiometric material, the reduction of the Nd-content leads to an increase in remanence to 0.85 T. Additionally, it is demonstrated that highly anisotropic powders can also be obtained by dynamic-HDDR processing of stoichiometric Nd₂Fe₁₄B powders.     

Effects of surface damage on critical current density in MgB2 thin films

We investigated the influence of surface damage on the critical current density (J_c) of MgB₂ thin films via 140-keV Co-ion irradiation. The J_c(H) of the surface-damaged MgB₂ films was remarkably improved in comparison with that of pristine films. The strong enhancement of J_c(H) caused by a surface damage in MgB₂ films can be ascribed to additional point defects along with an atomic lattice displacement introduced through low-energy Co-ion irradiation, which is consistent with the change in the pinning mechanism, from weak collective pinning to strong plastic pinning. The irreversible magnetic field (H_irr) at 5 K for surface-damaged MgB₂ films with a thickness of 850 and 1300 nm was increased by a factor of approximately 2 compared with that of a pristine film. These results show that the surface damage produced by low energy ion irradiation can serve as an effective pinning source to improve J_c(H) in a MgB₂ superconductor.     

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.     

Effect of sorbic acid doping on flux pinning in bulk MgB2 with the percolation model

In this paper, we study the doping effect of sorbic acid (C₆H₈O₂), from 0 to 20 wt.% of the total MgB₂, on critical temperature (T_c), critical current density (J_c), irreversibility field (H_irr) and crystalline structure. The XRD patterns of samples show a slightly decrease in a-axis lattice parameter for doped samples, due to the partial substitution of carbon at boron site. On the other hand, we investigate the influence of doping on the behavior of flux pinning and J_c(B) in the framework of percolation theory and it is found that the J_c(B) behavior could be well fitted in high field region. The two key parameters, anisotropy and percolation threshold, play very important roles. It is believed that the enhancement of J_c is due to the reduction of anisotropy in high field region.     

Effect of post-sintering annealing on microstructure and coercivity of Al85Cu15-added Nd-Fe-B sintered magnets

Effects of post-sintering annealing on the microstructure and coercivity have been investigated for the Al₈₅Cu₁₅-added (Pr, Nd)_14.8Fe_78.7B_6.5 sintered magnets. It is found that the optimum annealing temperature at which the coercivity _iH_c reaches a maximum decreases from 550 °C for the magnets added with 0.3% and 0.6% Al₈₅Cu₁₅ to 480 °C for the magnets added with 0.9% and 1.2% Al₈₅Cu₁₅. The decrease in optimum annealing temperature is related to the precipitation of Al-Cu or (Pr, Nd)−Cu liquid phase among (Pr, Nd)-rich phases during annealing. Existence of Al-Cu or (Pr, Nd)−Cu liquid phase is beneficial to dissolve the irregularities of (Pr, Nd)₂Fe₁₄B grain interface and increase the quantities of (Pr, Nd)-rich phases at the grain boundary, thus optimizing the grain boundary microstructure. The modifications of the microstructure are helpful to decouple the exchange interaction between (Pr, Nd)₂Fe₁₄B hard magnetic grains, thereby increasing the coercivity.