单晶NdMnO3的比热研究

研究了低温下NdMnO3单晶的比热随温度和磁场的变化(2K≤T≤200K,0T≤H≤8T).对应于Mn磁矩亚晶格的A型反铁磁(A-AF)相变,零场下的比热曲线在85K附近出现尖锐的λ形峰,随着磁场的增加,此λ峰降低展宽而且平滑变化,这与此温度附近磁化强度的变化规律一致.与磁有序相变相关的熵变约为理论值的26%,这可能是由于磁有序涨落延续在较大温区造成的.在20K以下,比热曲线出现了明显的肩膀形状的Schottky反常,其峰值随着磁场的增加而逐渐向高温移动.考虑了低温下比热的各种贡献,根据Nd3+位有效分子场(Hmf)引起的Nd3+基态双重态(GSD)劈裂对上述现象进行了解释.通过对2K≤T≤20K,0T≤H≤8T范围内比热数据的拟合,得到了样品的GSD劈裂,德拜温度和A-AF自旋波劲度系数以及它们对磁场的依赖关系.发现GdFeO3型八面体旋转引起的A-AF结构中Mn磁矩亚晶格的铁磁成分可能是Hmf的来源.     

PrNi$lt;sub$gt;2$lt;/sub$gt;的磁和磁热性能研究

在一些磁性材料内, 磁性离子间交换作用和磁性离子的自旋涨落对材料磁性有影响. 本文根据磁比热实验值确定了晶场参数后, 利用包含自旋涨落的交换作用有效场H_m= n₀ (1 + γ T + β e^{ω T})M, 计算了PrNi₂晶体晶场能级的Zeeman劈裂. 在温度为3.8 K ≤T≤ 30 K范围内, 计算了该晶体多晶磁矩随外磁场的变化, 以及外磁场H=5000 Oe时磁化率倒数随温度的变化, 计算结果和实验值符合较好. 当外磁场在0–50000 Oe时, 计算的该晶体的磁熵变与已有文献的理论结果相似. 计算结果说明, 提出的包含自旋涨落的交换作用有效场不仅适合亚铁磁性晶体, 而且也适合顺磁性晶体. 关键词： 2')" href="#">PrNi₂ 磁比热 交换作用有效场 磁矩 磁熵变     

Heusler合金Cu2VAl磁性及输运性质的研究

对Heusler 合金Cu₂VAl多晶甩带样品进行了磁性与输运性质方面的研究. 实验 发现Cu₂VAl在温度T为210K附近发生铁磁—顺磁相变，为弱铁磁体. 输运性质的 测量表 明在72K时电阻因局域杂质超导相变而发生突变，电子和声子之间的散射是主要的散射机 理. 居里温度T_C以下存在侧跃导致的反常霍尔效应，并且7K附近的相变导致霍 尔电阻率发生异常. 关键词： 磁性 输运 2VAl')" href="#">Cu₂VAl     

一种具有铁磁与反铁磁相互作用Ising模型的热力学性质

对最近邻自旋间具有反铁磁相互作用Ｊ_s，相隔偶数个自旋间又具有指数型长程铁磁相互作用Ｊ₁的一维Ｉｓｉｎｇ模型，利用积分方程的方法得出内能和比热的解析表达式。在Ｊ₁＝１，Ｊ_s分别为０．０５，０．１和０．３的条件下，作出零场时的内能和比热随温度参量变化的图形，并指出其相变临界点。在Ｊ₁＝１，Ｊ_s＝０．１的条件下，作出外磁场变化对内能和比热的影响图。鉴于系统在低温、零场或弱场以及较弱 关键词：     

关于La_2CuO_(4-y)反铁磁性的研究

本文测量了三个不同制备条件的 La_2CuO_(4-y)样品以及原材料 CuO 样品在液氮温度以上的比热,实验发现四个样品在200K 附近都有明显的比热反常峰,分析表明 La_2CuO_(4-y)的比热反常并非 CuO 杂质相引起,而是样品发生反铁磁有序相变所致.在 Nèel 温度 T_N 附近,磁比热按 C_M∝1n|T-T_N|规律变化,与二维伊辛模型基本吻合,很小的熵变则表明大的零点量子自旋涨落的存在.其磁化率变化服从居里-外斯定律,并计算出 Cu^(2+)的有效磁矩为～0.6μ_B.低温下电阻按 InR∝(T_c/T)^(1/3)规律变化,表现为二维空穴导电机制.     

金属间化合物DyMn2Ge2的自发磁相变和场诱导的磁相变

在10—800K的温度范围内用X射线衍射方法测量了DyMn₂Ge₂化合物的晶格常数与温度的变化关系,观察到高温时DyMn₂Ge₂由顺磁状态到反铁磁状态的自发磁相变伴随着晶格常数a的负的磁弹性异常现象.在4.2K—200K的温度范围内测量了DyMn₂Ge₂的交流磁化率.在交换相互作用的分子场模型近似下,从理论上分析讨论了DyMn₂Ge₂的低温自发磁相变和场诱导的磁相变.计算了DyMn₂Ge₂单晶的磁化强度与温度的变化关系以及不同温度下外磁场沿晶轴c方向时的磁化曲线.理论分析和计算结果表明,温度低于33K时在DyMn₂Ge₂中观察到的场诱导的一级磁相变为由亚铁磁状态(Fi)到中间态(IS)相变. 关键词： 稀土-过渡族金属间化合物 磁结构 磁相变     

哈斯勒合金Ni50Mn35In15的马氏体相变及其磁热效应

利用电弧炉熔炼了Ni₅₀Mn₃₅In₁₅多晶样品,根据磁性测量对其马氏体相变和磁热效应进行了系统研究.结果表明,随着温度的降低,样品在室温附近先后发生了二级磁相变与一级结构相变特征的马氏体相变,导致它的磁化强度产生突变. 同时通过低温下的磁滞回线的测量发现样品存在交换偏置行为,表明低温下马氏体相中铁磁和反铁磁共存. 此外,根据Maxwell方程,计算了样品在马氏体相变温度附近的磁熵变,当温度为309K,磁场改变5 T时,样品的磁熵变可达22.3J/kgK. 关键词： 哈斯勒合金 50Mn₃₅In₁₅')" href="#">Ni₅₀Mn₃₅In₁₅ 马氏体相变 磁热效应     

三维Ising模型的蒙特卡罗模拟

采用蒙特卡罗(Monte Carlo)重点抽样法对三维Ising模型进行计算机模拟,测量无外磁场时三维Ising模型中自旋键链的能量、磁化强度、比热及磁化率的统计平均值与标准误差(不确定度).结果表明,三维Ising模型在无外磁场时存在自发磁化现象,铁磁→非铁磁相变临界点在J/(k_BT_C)=0.222 0,或居里温度T_C=4.500 0处.并研究存在外磁场时上述物理量随温度与外磁场的变化规律,给出物理解释.     

Mn位W掺杂对La0.3Ca0.7MnO3体系磁结构的影响

通过对La_0.3Ca_0.7Mn_1-xW_xO₃(x=0.00，0.04，0.08，0.12，0.15)多晶样品M-T曲线、M-H曲线及ESR谱的测量，研究了Mn位W掺杂对电荷有序体系La_0.3Ca_0.7MnO₃磁结构的影响.结果表明，当掺杂量为0.00≤x≤0.08时，体系存在电荷有序(CO)相，AFM/CO态共存于相变温度以下，电荷有序温度T_CO随着W掺杂量的增加而增加；x＝0.04时，样品在低温下为FM相与AFM/CO相共存，在CO相建立前、后均有FM从PM中分离出来；当x≥0.12时，CO态融化，在极低温度下存在顺磁-铁磁(PM-FM)相变. 关键词： 磁结构 电荷有序 融化 Mn位掺杂     

掺杂对Ni51.5Mn25Ga23.5相变行为和磁性的影响

通过往母合金Ni_51.5Mn₂₅Ga_23.5掺入7种IVA, VA和VIA 过渡族元素得到系列掺杂合金Ni_51.5Mn₂₃M₂Ga_{23 .5}.M为掺杂元素.实验结果表明，掺杂效应一般引起马氏体相变温度的下降，其中，W 的掺杂是7种元素中唯一使相变温度升高的特例，且出现了中间马氏体相变.同时，在价电子 浓度不变的情况下，相变更敏感于原子的尺度效应.实验发现，Ti，Zr，Hf，V四种非磁性元 素的掺杂使Mn原子磁矩减小，而Nb，Ta，W三种非磁性元素的掺杂却可以明显地增大Mn原子 的磁矩.在考察掺杂效应时，不能忽略马氏体相变引起的晶格变化对材料磁性的影响. 关键词： NiMnGa 掺杂 马氏体相变 磁性     

Schottky-like anomaly in the low-temperature specific heat of single-crystal NdMnO3

The specific heat of single-crystal NdMnO₃ was investigated from 2 to 20 K under different magnetic fields up to 8 T. All the specific heat data show a Schottky-like anomaly, which becomes more indistinctive as increasing magnetic field. The experiment data were successfully fitted by taking into account factors such as crystal-field splitting, the two-level Schottky anomaly, the lattice vibration, and type-A antiferromagnetic (A-AF) spin waves. It was found that the splitting of the ground state doublet of Nd³⁺ ion increases linearly with magnetic field. The above phenomena can be interpreted in terms of the model of unchanged effective molecular field at Nd³⁺ site caused by the ferromagnetic component of A-AF structure of Mn spins. This ferromagnetic component is likely caused by the GdFeO₃-type octahedron rotation. In addition, it was also found that the magnetic field increases the spin-wave stiffness coefficient, but reduces the Debye temperature.     

The Nd–Mn exchange interaction,low temperature specific heat and magnetism of Nd2/3Ca1/3MnO3

The low temperature specific heat and magnetic characteristics of Nd_2/3Ca_1/3MnO₃ perovskite are studied in a wide range of magnetic fields (up to 9 T). Temperature dependent specific heat data show a broadened Schottky-like anomaly below 20 K caused by splitting of the Nd³⁺ ions ground-state doublet in the effective molecular field H_ex, determined by exchange interaction between Nd and Mn spin systems supplemented by an applied external magnetic field. Existence of the splitting at zero magnetic field and expressed field dependence is the evidence of a strong exchange coupling between Nd and Mn magnetic subsystems. The Nd-ions magnetic ordering leads to an additional contribution to the magnetic moment of the system below 30 K, producing anomalies of the magnetic loss and field-cooled and zero-field-cooled magnetizations. The observed broadened Schottky-like anomalies are fitted for each applied magnetic field by the sum of three Schottky functions. Applied magnetic field extends the anomaly region and shifts it to higher temperatures. Splitting of the higher crystal field Kramers doublets gives an additional contribution to the heat capacity in magnetic fields. The ground state doublet g-factors g_|| and g_⊥ were estimated to be 3.4 and 2.2, respectively, and H_ex was estimated to be 9 T. The Nd³⁺ ions magnetic moment estimated from the magnetization data agrees with the value obtained from the specific heat data.     

Behavior of the magnetic subsystems in Nd2BaNiO5

The temperature dependences of the heat capacity, the magnetic susceptibility, and the splitting of the ground Kramers doublet of the Nd³⁺ ion in the chain magnet Nd₂BaNiO₅ are studied. An antiferromagnetic phase transition manifests itself as anomalies in all these dependences. The parameters of the Nd-Ni and Nd-Nd interactions are estimated. The field dependence of the magnetization has two anomalies. A strong magnetic anisotropy prevents the magnetic moments of the Nd³⁺ ion from deviating from axis c in the crystal even in an external magnetic field. The processes of magnetization and the internal specific features of a chain of spins S = 1 are discussed.     

Possible observation of the coexistence of superconductivity and long-range magnetic order in NdRh4B4

The ternary rare earth compound NdRh₄B₄ has been studied by means of critical field, low temperature heat capacity, and static magnetic susceptibility measurements. Features in the upper critical field and heat capacity data at 1.31 K and 0.89 K suggest the occurrence of long-range magnetic order in the superconducting state. The temperature dependence of the static magnetic susceptibility follows a Curie-Weiss law with an effective magnetic moment μ_eff = 3.58 ± 0.05 μ_B and a Curie-Weiss temperature θ_p = ?6.2 ± 1.0 K between 20 K and room temperature. However,, magnetization vs. applied magnetic field isotherms suggest the development of a ferromagnetic component in the Nd³⁺ magnetization at low temperatures.     

Calorimetric and spectroscopic study of quasi-one-dimensional Haldane magnets (Y1 ? xNdx)2BaNiO5 (x = 1, 0.75, 0.50, 0.25)

The temperature dependences of the heat capacity and optical transmission spectra of quasi-one-dimensional Haldane magnets (Y_{1 − x} Nd _x )₂BaNiO₅ (x = 1, 0.75, 0.50, 0.25) have been studied. All the compounds studied undergoes antiferromagnetic ordering. In the ordered state, the internal magnetic field produces splitting of the ground-state Kramers doublet of Nd³⁺ ion. The temperature dependences of the magnitude of splitting, as determined from spectroscopic data, were used to calculate the Schottky anomaly on the heat capacity curve. Nonequivalent centers related to neodymium ions have been found in compounds with x = 0.75, 0.50, and 0.25.     

Low temperature studies on magnetic properties of Tm2O3

Magnetic properties of Tm₂O₃ were studied by means of magnetometry as well as by heat capacity measurements. The specific heat data were collected down to 0.35 K, and yielded some new information about crystal field splitting of the ³H₆ thulium multiplet. Some anomalous behaviour of the specific heat below 5 K was observed. It turned out, that this anomaly can be nicely described by Schottky contribution originating from crystal field levels distribution of the Tm³⁺ multiplet. No magnetic ordering was evidenced down to the lowest temperatures studied.     

Optical absorption spectra and magnetic phase transitions in TbFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Optical absorption spectra of the trigonal crystal of TbFe₃(BO₃)₄ in the vicinity of the ⁷F₆ → ⁵D₄ transition in a Tb³⁺ ion were studied as a function of temperature (2–70 K) and magnetic field strength (0–60 kOe) at 2 K. The splitting of the excited states of Tb³⁺ due to both the magnetic ordering of iron and an external magnetic field was determined. Abrupt splitting of the absorption lines of Tb³⁺ at temperature T_N of the magnetic ordering of the subsystem of iron was revealed, suggesting that the nature of such splitting is not entirely magnetic.     

On the behaviour of Er3+ ion in tetragonal crystalline field

Crystal field parameters for ErGaG and Er³⁺ YAlG and used to compute the temperature dependence of Schottky specific heat, paramagnetic susceptibility, magnetic anisotropy,μ _eff and quadrupole splitting in the range 10–400 K. The hyperfine interaction parametersA andB for¹⁶⁶Er and¹⁶⁷Er in both the systems are also obtained and in turn used to estimate the nuclear specific heat contribution. The studied parameters compare well with the available experimental results.     

Low temperature specific heats of erbium dihydrine. Magnetic transition and crystal field splitting effects

The specific heat of erbium dihydride, ErH_1.99, was measured in the temperature range from 1.8–230 K. A λ-type specific heat anomaly, most probably due to an antiferrómagnetic transition has been found at T_N = 2.13 ± 0.03 K. The influence of the crystal field of cubic symmetry on the splitting of the ground state ⁴I_{$\text{15}\text{2}$} of the Er³⁺ ion was analysed and the conclusion was drawn, that the doublet Γ₇ is the lowest lying crystal-field level.     

Optical measurements in single crystal ErCrO3

The optical⁴ I _15/2→⁴ S _3/2 transition of Er³⁺ in single crystal ErCrO₃ has been studied with polarized light as a function of temperature between 1.7 and 140 °K and of applied magnetic fields in the temperature region between 1.7 and 10 °K. The temperature dependence of the ground state splitting has been deduced from these measurements. ErCrO₃ undergoes a spin reorientation with a reorientation temperature depending on applied external field. The reorientation temperature in zero field was determined to beT _r =9.4±0.5 °K. The experiments in external magnetic field along thez-axis have given the magnetic moment of the Er³⁺ ions to be μ _z =5.55 (±0.45) μ _b and the reorientation fieldH _z ^r as a function of temperature. Specific heat and magnetization data have been calculated from the optical data of the Er³⁺ ground state and compared with recent specific heat and magnetization studies in ErCrO₃ by other authors.