化合物Gd1-xLaxMn2Ge2(x=0.06, 0.07)中场诱导的反铁磁→铁磁一级磁相变

在脉冲强磁场中测量了Gd1-xLaxMn2 Ge2 (x =0 .0 6 ,0 .0 7)化合物在不同温度下的磁化曲线。结果表明 ,当这些化合物处于反铁磁状态的温度范围内时 ,Mn次晶格中发生了场诱导的从反铁磁状态到铁磁状态的一级磁相变。随着温度的降低 ,相变临界磁场逐渐增大 ,达到最大值后 ,随着温度的进一步降低 ,临界磁场很快减小 ;随La含量的增加 ,相变临界磁场也很快减小。在交换相互作用的分子场模型基础上 ,考虑层面间Mn Mn交换作用随晶格常数a以及温度的变化 ,从理论上计算了这种场诱导的反铁磁→铁磁一级磁相变所对应的临界磁场 ,理论计算结果较好地描述了临界磁场随温度的变化规律。理论模型也较好地解释了在这些化合物中发生的从亚铁磁结构到反铁磁结构再到铁磁结构的一级自发磁相变     

Tb0.3Dy0.7(Fe1-xAlx)1.95合金的结构、磁致伸缩性能和自旋重取向研究

本文系统研究了室温下Tb0.3Dy0.7(Fe1-xAlx)1.95(x=0,0.05,0.1,0.15,0.2,0.25,0.3,0.35)合金中金属Al替代Fe对晶体结构、磁致伸缩、内禀磁致伸缩、各向异性和自旋重取向的影响.结果发现,x＜0.4时,Tb0.3Dy0.7(Fe1-xAlx)1.95完全保持MgCl2立方Laves相结构,晶格常数a随Al含量x的增加而增大.磁致伸缩测量发现,随着替代量x的增多磁致伸缩减小,x＞0.15时超磁致伸缩效应消失;x＜0.15时磁致伸缩在低场下(H≤40kA/m)有小幅增加,高场下迅速减小,而且易趋于饱和,说明添加少量Al有助于减小磁晶各向异性.内禀磁致伸缩λ111随[111]替代量x的增加大幅度降低.穆斯堡尔效应表明,Tb0.3Dy0.7(Fe1-xAlx)1.95合金的易磁化方向随成份和温度在{110}面逐渐偏离了立方晶体的主对称轴,即自旋重取向.室温下,当x=0.15时,Tb0.3Dy0.7(Fe1-xAlx)1.95合金中出现了少量非磁性相;x＞0.15时,合金完全呈顺磁性.     

用攀枝花钛矿和云南钛矿冶炼钛渣工业试验研究

通过磁性测量和X-射线衍射研究了Gd2(Co，Al)17化合物的结构和磁晶各向异性。当x≤4时，样品具有菱方Th2znl7型结构，x=5时，具有六方CaCu5型结构。随着Al浓度增加，晶格常数和单胞体积单调增大，而居里温度和自发磁化强度近似线性地减小；在x≥l时，观察到Co亚点阵的磁晶各向异性变号，室温易磁化方向由易面转变为易轴，且易轴各向异性场和各向异性常数随Al原子浓度增加出现极大值；基于补偿温度(65K)下自由粉末颗粒的磁化曲线，导出了亚点阵间的分子场系数和Gd-co交换耦合常数，结果表明，Al原子对Gd-Co交换耦合作用的影响较小。根据Al原子在Co亚点阵4种晶位的择优占位，分析了磁晶各向异性的演变。     

Sm_2Co_(17-x)Ti_x化合物的结构和磁性

利用 Ｘ射线衍射和磁性测量研究了 Ｓｍ２Ｃｏ１７－ｘＴｉｘ化合物的结构和磁性实验表明这些化合物都具有 Ｔｈ２Ｚｎ１７型 结构, Ｔｉ替代 Ｃｏ不改变化合物的晶体结构,但引起晶格膨胀所有化合物在 Ｃｕｒｉｅ温度以下都表现为单轴磁晶各向异性随 着 Ｔｉ含量的增加,化合物的 Ｃｕｒｉｅ温度和饱和磁化强度都单调降低,而磁晶各向异性场在 Ｔｉ含量为 １．０时出现一极大值     

Si原子择优占位对R2Co17化合物中Co次晶格磁晶各向异性的影响

对R2Co17-xSix（R＝Ce,Gd和Y）化合物的X射线衍射和磁性测量研究结果表明,Si原子的加入使R2Co17化合物的室温磁晶各向异性由易面转变为易轴,各向异性常数随Si原子浓度的增加而出现极大值、重点讨论了Si原子在Co次晶格四个不同晶位择优占位与Co次晶格磁晶各向异性演化的关系,其中18h晶位对Co次晶格各向异性负贡献起重要作用     

Sm2Co17—xTix化合物的结构和磁性

利用X射线衍射和磁性测量研究了Sm2Co17-xTix化合物的结构和磁性，实验表明这些化合物都具有Th2Zn17型结构，Ti替代Co不改变化合物的晶体结构，但引起晶格膨胀，所有化合物在Curie温度以下都表现为单轴磁晶各向异性，随着Ti含量的增加，化合物的Curie温度和饱和磁化强度都单调降低，而磁晶各向异性场在Ti含量为1.0时出现一极大值。     

Si原子择优占位对R2Co17化合物中Co7次晶格磁晶各向异性的影响

对Ｒ２Ｃｏ１７－ｘＳｉｘ化俣物的Ｘ射线衍射和磁性测量研究结果表明,Ｓｉ原子的加入使Ｒ２Ｃｏ１７化合物的室温磁晶各向异性由易面转变为易轴,各向异性常数随Ｓｉ原子浓度的增加而出现在值,重点讨论了Ｓｉ原子在Ｃｏ次晶格四个不同晶位择优占位与Ｃｏ次晶格致晶各向异性演化的关系,其中１８ｈ晶位对Ｃｏ次晶格各向异性负贡献起重要作用。     

Tb_xDy_(1-x)(Fe_(0.6)Co_(0.4))_2合金的磁性和磁致伸缩性能

利用电弧熔炼法制备Tb_xDy_(1-x)(Fe_(0.6)Co_(0.4))_2合金(0.27≤x≤0.40),对合金的磁性和磁致伸缩性能进行研究。利用XRD、交流初始磁化率测试仪、超导量子干涉仪和标准应变测试仪,对样品的物相组成、居里温度、磁化曲线和磁致伸缩性能进行表征。结果表明:当x≤0.27时合金的易磁化方向为á100?方向,当x≥0.30时合金的易磁化方向变为〈 111〉方向;合金的居里温度随x的增加而增加;x=0.32附近时合金的磁晶各向异性常数K_1有极小值,室温时合金在x=0.32附近时达到各向异性补偿;当x=0.32时饱和磁致伸缩系数达到9.57×10~(-4);随Co含量的增加,合金的各向异性补偿点向Tb含量高的方向移动。Tb_(0.32)Dy_(0.68)(Fe_(0.6)Co(0.4))_2合金具有高磁致伸缩系数和低各向异性,是一种实用的磁致伸缩候选材料。     

退火温度对无取向电工钢磁晶各向异性能的影响

采用取向分布函数分析了无取向电工钢不同再结晶退火温度下的织构变化及织构对磁感应强度和铁损的影响,并计算了无取向电工钢的磁晶各向异性能。结果表明,随着实验钢的再结晶退火温度升高,Goss织构和立方织构组分显著增强,而{111}面织构强度却减弱。较高的退火温度有利于减小织构因子,提高磁感应强度。磁晶各向异性能计算结果显示,随着再结晶退火温度升高,无取向电工钢板的磁晶各向异性能降低。     

TbMn6—xCoxSn6赝三元化合物的结构和磁性

本文研究了ＴｂＭｎ６－ｘＣｏｘＳｎ和赝三元化合物的结构和磁性，结果表明：用Ｃｏ部分取代ＴｂＭｎ６Ｓｎ６中的Ｍｎ不改变其晶体结构，但随Ｃｏ含量的增加晶格常数和单胞体积单调减小；居里温度随Ｃｏ含昨增大而降低；自旋重取向温度也随Ｃｏ含量增大而降低，但Ｃｏ含≥０．６时自旋重取向消失；     

Magnetocrystalline anisotropy and spin reorientation transition of DyMn6Sn6 compound

The spin-reorientation transition of intermetallic compound DyMn6Sn6 was investigated by applying the molecular field theory. The temperature dependence of easy magnetization direction of compound and the magnetic moment directions of Dy and Mn ions were theoretically calculated and they have good agreement with the experimental data. In the framework of single ion model, the temperature dependence of magnetocrystalline anisotropic constants K1R and K2R of Dy ion were also calculated. The results show that the fourth-order crystal field parameter, B 40, and the corresponding second-order magnetocrystalline anisotropic constant, K2R, of Dy ion must be taken into account in order to explain the spin-reorientation transition satisfactorily. The competition between K2R and K1R plays a key role in the spin-reorientation transition of DyMn6Sn6.     

Large magnetocaloric effect in re-entrant ferromagnet PrMn1.4Fe0.6Ge2

Magnetocaloric effects (MCE) at multiple magnetic phase transition temperatures in PrMn_1.4Fe_0.6Ge₂ were investigated by heat capacity and magnetization measurements. PrMn_1.4Fe_0.6Ge₂ is of a re-entrant ferromagnet and performs multiple magnetic phase transitions in the temperature range from 5 to 340 K. A large magnetic entropy change (−ΔS_M) 8.2 J/kg K and adiabatic temperature change (ΔT_ad) 4.8 K are observed for a field change of 0-1.5 T around 25.5 K, associated with the field-induced first order magnetic phase transition (FOMT) from the antiferromagnetic to the ferromagnetic state with an additional Pr magnetic contribution. These results suggest that a re-entrant ferromagnet is probably promising candidate as working material in the hydrogen and nature gas liquefaction temperature range magnetic refrigeration technology.     

High-field magnetization process in Mn1.9Cr0.1Sb

The compound Mn_1.9Cr_0.1Sb shows a first-order transition from the ferrimagnetic (FI) to the antiferromagnetic (AF) state at T₁ = 265 K with decreasing temperature. High magnetic fields (up to 60 T) produce AF-FI transitions below T_t. The temperature dependence of the critical field is nonlinear and shows T² character at low temperatures (T < 150 K), in contrast to the exchange inversion model of Kittel. This behavior is connected with the itinerant magnetism of 3d-electrons of Mn atoms resulting in a large electronic contribution to the change in free energy at the field-induced first-order AF-FI transition. The field-induced phase transition from the AF to the FI state is accompanied by an increase in the electronic specific heat coefficient γ by 13 mJ K⁻² mol⁻¹ as estimated from the magnetization measurements using thermodynamic relation.     

FIR-spectroscopy of the FI-SDW excitation spectrum in a quasi-1D organic conductor

The anisotropic quasi one-dimensional conductor (TMTSF)₂X (X = ClO₄, PF₆) exhibits a cascade of field-induced spin density wave (FI-SDW) phase transitions. In this contribution, we present a magneto-optical study of the far infrared excitation spectrum of (TMTSF)₂ClO₄ as a function of magnetic field and temperature covering the low field part of the phase diagram of this system.

The observed transition temperatures of the various FI-SDW subphases follow closely the established phase diagram. Our results show that the magnitude of the SDW gap depends neither on magnetic field nor on temperature. In addition we find evidence for the existence of a collective mode excitation in the SDW state, indicating a relatively strong pinning to the lattice. Further indication of a large electron lattice coupling is given by the strong phonon renormalisation in the SDW state.     

Relation between stabilization energy, crystal field coefficient and themagnetic exchange interaction for Tb3+ ion

Based on a single ion model, Hamiltonian of the simplest form about magnetocrystalline anisotropy for Tb3+ ion was solved by using the numerical method. The relation between the stabilization energy, crystal field coefficient B20 and the magnetic exchange interaction was studied as temperature approaches to 0K. The results show that the stabilization energy contributed by Tb3+ is linear with crystal field coefficient B20 approximately, but it is insensitive to the change of magnetic exchange interaction for the strong magnetic substancessuch as TbCo5, Tb2Co17 and Tb2Fe14B compounds.     

Magnetic anisotropy of the Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B compound and its hydride Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>BH<Subscript>4</Subscript>

Magnetization curves of single crystals of Nd₂Fe₁₄B and its hydride Nd₂Fe₁₄BH₄ have been measured along their principal crystallographic directions in a temperature range of 4.2–280 K. The magnetic anisotropy constants, which allow one to describe experimental magnetization curves as well as the low-temperature “easy-cone”-type magnetic structure and field-induced first-order magnetic phase transitions, have been determined in terms of a collinear-ferrimagnetic-ordering model. The anisotropy constants were shown to decrease in magnitude upon hydrogenation. In this case, the ratios of the effective fourth-and sixth-order constants to the second-order constant increase, whereas the constants responsible for the basal-plane anisotropy exhibit a more than threefold decrease. At the same time, the spin-reorientation temperature and opening of the magnetization cone at 4.2 K remain virtually unchanged. The results obtained are discussed in terms of the single-ion-anisotropy theory.     

Hyperfine interactions studied by Sn Mössbauer spectroscopy in TbAuSn and TmAuSn compounds

X-ray diffraction, magnetic susceptibility measurements and ¹¹⁹Sn Mössbauer spectroscopy were used to study structure and hyperfine interactions in intermetallic TbAuSn and TmAuSn compounds. It was shown that the first compound orders antiferromagnetically at low temperatures with Tb magnetic moments inclined at an angle of about 25° to the crystallographic c-axis. The second compound stays non-magnetic in the whole measured region. The occurrence of a magnetic hyperfine field distribution in TbAuSn suggests that this compound crystallizes in the disordered CaIn₂-type of structure.     

Step-like and first-order characters of Tb0.2Pr0.8(Fe0.4Co0.6)1.93 with low interstitial carbon content

In this study we report detailed magnetic property of the 4f-3d pseudo-quaternary Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.88C_0.05 compound by detailed magnetization measurements. Very sharp magnetization jumps across the antiferromagnetic–ferromagnetic transition are observed below 3.0 K, and the number of jump-like transitions increases with decreasing temperature. The time-dependent magnetic relaxation, field sweep rate and cooling field dependence of magnetization jumps resemble the martensitic scenario. The number and occurrence of magnetization jumps are mainly determined by the competitions between the thermal fluctuation energy, elastic energy and Zeeman energy, and the field-induced antiferromagnetic to ferromagnetic phase transition at low temperatures is of first-order in nature.     

Magnetic ordering in Th3P4-type Tb4Sb3 compound

The nature of the magnetic ordering of Tb₄Sb₃ compound (Th₃P₄-type, cubic; cI28, space group , No. 220, a = 0.91518(7) nm) has been investigated by using the techniques of magnetization and neutron diffraction. AC and DC magnetisation measurements indicate antiferromagnetic ordering at 108 K in zero magnetic field that is accompanied by a field-induced metamagnetic transition to a ferromagnetic state, in fields above 0.3 T. Neutron diffraction experiment in zero applied magnetic field shows that below T_N = 112(4) K Tb₄Sb₃ exhibits an antiferromagnetic flat spiral-type ordering with propagation vector K₁ = [±1/8, ±1/8, ±1/8]. The magnetic moment of Tb atoms is found to be M_Tb = 6.7(3) μ_B at 80 K. The magnetic moment of Tb atoms lie in the (1 1 1) plane of Tb₄Sb₃ unit cell (the cone axis arranges along [1 1 1] direction with cone angle β = 90°). Below T_N2  50 K, Tb₄Sb₃ shows second antiferromagnetic transition with K₂ = [1/2, 1/2, 1/2] with possible re-orientation of Tb magnetic moments.     

Effects of high temperature annealing on the chemical homogeneity and Magnetostriction of Tb0.3Dy0.7Fe1.90 Intermetallic Compound

A rare earth-iron intermetallic compound Tb_0.3Dy_0.7Fe_1.90, also called Terfenol-D, has a gigantic room temperature magnetostriction at a relatively small magnetic field. In this study microsegregation of Tb and Dy in directionally solidified Terfenol-D was investigated. It was predicted from a calculated phase diagram and experimentally verified that Tb concentration at the center of the cell which solidified earlier was lower than that at the cell boundary. In order to homogenize this microsegregation, specimens were annealed at 1000°C for 40 hours, and various magnetic properties were measured as a function of applied field under uniaxial compressive stress in the range from 0.5 to 1.2 MPa. After the annealing, the maximum magnetostriction increased at a compressive stress below 9 MPa, magnetostrictive coefficient d₃₃ and magneto-elastic coupling coefficient k₃₃ increased, but permeability decreased. These results may be due to the reduction of magnetocrystalline energy which was locally very high before annealing due to microsegregation.