高压高温处理对YNi_2B_2C晶界效应的影响

在测量经压力处理的氩弧熔炼的YNi2 B2 C的金属间化合物超导体中场冷磁化率时 ,观察到升温磁化率曲线在临界温度附近有一个负的磁化率峰。通过研究高压高温处理对粉末样品和块状样品的场冷和零场冷却磁化率曲线的影响 ,认为场冷时反常负磁化率峰是由于样品中晶界的存在 ,从而导致晶粒间的弱连接所引起的 ,而不是由磁相变或超导和磁性共存现象所引起的。并认为有可能利用高压高温技术制备这类化合物和生长单晶。     

RUSr2EU1.4Ce0.6Cu2O10-δ超导体的磁特性研究

RuSr2Eu1.4Ce0.6Cu2O10-δ超导样品的交流磁化率曲线在TM2～125 K和Tp～75 K处出现峰值.我们通过测量不同频率下的交流磁化率和电子自旋共振谱,对这两个峰的起源进行了详细的研究.通过对铁磁共振线的强度、线宽和共振场与温度依赖关系的讨论,我们得出TM2附近的峰对应着体系的反铁磁转变.Tp处的峰随着测量频率的增加向高温移动,且峰的强度逐渐减小,这一结果证实了体系中存在自旋玻璃态.     

Ce_3Rh_3Si_2和CeRh_3Si_2的磁性模拟和晶场效应研究

根据晶场理论,通过对D.Kaczorowski等人测量的磁化率倒数与温度关系曲线的模拟,得到了稀土化合物CeRh3Si2和Ce3Rh3Si2的晶场分裂能和相应波函数.对于CeRh3Si2和Ce3Rh3Si2,它们的第一激发能和总的激发能分别为176K,855K和77.5K,428K.计算表明,Kramers离子Ce3+在晶场效应的作用下,基态简并部分消除得到了三个双重态,模拟得到的化合物的磁化率倒数与温度关系曲线与实验曲线吻合较好.     

取向Pr2Fe14B与Nd2Fe14B多晶磁化曲线的计算

基于单离子晶场模型 ,提出了计算稀土 Fe(Co)金属间化合物取向多晶样品磁化曲线的方法 .用此方法计算了取向Pr2 Fe14 B和Nd2 Fe14 B多晶的高场磁化曲线 ,计算中使用了拟合化合物单晶磁化曲线得到的交换场与晶场参数 .计算曲线与实验曲线相符合 .     

取向Pr2Fe14B与Nd2Fe14B多晶磁化曲线的 计算

基于单离子晶场模型,提出了计算稀土-Fe(Co)金属间化合物取向多晶样品磁化曲线的方法.用此方法计算了取向Pr₂Fe₁₄B和Nd₂Fe₁₄ B多晶的高场磁化曲线,计算中使用了拟合化合物单晶磁化曲线得到的交换场与晶场参数.计 算曲线与实验曲线相符合. 关键词： 磁化曲线 晶场 2Fe₁₄B')" href="#">R₂Fe₁₄B     

MgB_2超导体块材的高压合成

利用高压烧结的方法成功地合成出块状MgB2 超导体。所获得的样品具有良好的单相性。低温磁性测量实验和电阻测量实验均表明样品具有高于 39K的超导转变温度。实验结果表明 ,高压合成是制备和研究MgB2 新型金属间化合物超导体的一种非常有效的手段     

高压增氧T''相 R2CuO4+δ(R=Nd～Tm)低温磁性研究

T'相R2CuO4稀土铜氧化合物由于尺度效应而产生弱铁磁性行为已经被人们关注,报导了通过高温高氧压(6 GPa,1 000 ℃)合成稀土T'相R2CuO4(R＝Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er和Tm)化合物的结构和磁学性能.磁化率曲线显示,在低温下所有的高压增氧R2CuO4样品都出现新的低温弱铁磁性反常行为,转变温度在28 K附近.新的低温弱铁磁性行为是由于CuO2面上微量氧空穴的掺入,使处于反铁磁有序CuO2面形成局域化的铁磁性团簇造成.实验证明新发现的低温弱铁磁性行为与尺度效应产生弱铁磁性行为属于完全不同的物理机制.结果还预示T'相R2CuO4稀土铜氧化合物很难通过空穴掺杂而实现超导.     

MgB2超导体块材的高压合成

利用高压烧结的方法成功地合成出块MgB2超导体。所获得的样品具有良好的单相性,低温磁场测量实验和电阻测量实验均表明样品具有高于39K的超导转变温度。实验结果表明,高压合成是制备和研究MgB2新型金属间化合物超导体的一种非常有效的手段。     

高压增氧T′相R_2CuO_(4+δ)(R=Nd～Tm)低温磁性研究

T′相R2CuO4稀土铜氧化合物由于尺度效应而产生弱铁磁性行为已经被人们关注,报导了通过高温高氧压(6GPa,1000℃)合成稀土T′相R2CuO4(R=Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er和Tm)化合物的结构和磁学性能。磁化率曲线显示,在低温下所有的高压增氧R2CuO4样品都出现新的低温弱铁磁性反常行为,转变温度在28K附近。新的低温弱铁磁性行为是由于CuO2面上微量氧空穴的掺入,使处于反铁磁有序CuO2面形成局域化的铁磁性团簇造成。实验证明新发现的低温弱铁磁性行为与尺度效应产生弱铁磁性行为属于完全不同的物理机制。结果还预示T′相R2CuO4稀土铜氧化合物很难通过空穴掺杂而实现超导。     

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     

Exchange bias with Fe substitution in LaMnO<Subscript>3</Subscript>

We observe the negative shift of the magnetic hysteresis loop at 5 K, while the sample is cooled in external magnetic field in case of 30% of Fe substitution in LaMnO₃. The negative shift and training effect of the hysteresis loops indicate the phenomenon of exchange bias. The cooling field dependence of the negative shift increases with the cooling field below 7.0 kOe and then, decreases with further increase of cooling field. The temperature dependence of the negative shift of the hysteresis loops exhibits that the negative shift decreases sharply with increasing temperature and vanishes above 20 K. Temperature dependence of dc magnetization and ac susceptibility measurements show a sharp peak (T_p) at 51 K and a shoulder (T_f) around 20 K. The relaxation of magnetization shows the ferromagnetic and glassy magnetic components in the relaxation process, which is in consistent with the cluster-glass compound.     

Magnetic anomalies in CeIn(2)

The magnetic behavior of binary compound CeIn(2) has been reported to be unusual in the sense that this compound appears to exhibit a first-order ferromagnetic transition at a rather high temperature of (T(C)=)22 K, which is not so common for Ce systems. In order to throw more light on the magnetic behavior of this compound, we have carried out detailed magnetization, and electrical resistivity studies as a function of temperature, magnetic field and external pressure, in addition to heat-capacity measurements. The plots of H/M versus M(2) at low fields are interestingly characterized by negative slopes, not only near T(C), but also at lower temperatures, a source of which could be attributed to magnetic-field-induced transitions at much lower temperatures. The sign of magnetoresistance tends to change from positive to negative with increasing temperature, as though there is a gradual change in the magnetic character. Finally, the magnetic ordering temperature increases with increasing pressure (until 20 kbar), as though this compound lies at the left-hand side of the peak in Doniach’s magnetic phase diagram.     

Comment on magnetism and superconductivity in rutheno cuprates: RuSr2GdCu2O8 and RuSr2Gd1.5Ce0.5Cu2O10

Both RuSr₂GdCu₂O_8-δ (Ru-1212) and RuSr₂Gd_1.5Ce_0.5Cu₂O_10-δ (Ru-1222) exhibits magnetism and superconductivity, as seen by magnetization vs. temperature behavior measured in 5 Oe field. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization data show branching at around 140 K and 100 K with a cusp at 135 K and 80 K and a diamagnetic transition around 20 K and 30 K in the ZFC part, for Ru-1212 and Ru-1222, respectively. The isothermal magnetization possesses a non-linear contribution due to a ferromagnetic component at low temperatures below 50 K for both samples. The resistance vs. temperature behavior of the samples in applied fields of 0, 3 and 7 T confirmed superconductivity, with a different type of broadening of the superconductivity transition under magnetic fields for Ru-1212 from that known for conventional high-T _c superconductors. The magnetoresistance (MR) is negative above the Ru magnetic ordering temperature at 135 K. Below the Ru magnetic ordering temperature, MR displays a positive peak at low fields and becomes negative at higher fields for Ru-1212. For Ru-1222, MR remains negative both above and below the ordering temperature. A maximum of 2% is observed for the negative MR value at the Ru magnetic ordering temperature. An electron diffraction pattern obtained for the Ru-1212 sample shows two types of superstructure: one has a weak spot at the centre of the a–b rectangle, and the other only along the b direction. Interestingly, Ru-1222 shows only clean a–b and a–c planes, without any superstructures.     

Temperature behavior of magnetization of DyCo2 compound

Zero field cooled (ZFC) and field cooled (FC) magnetization measurements were performed on the binary DyCo₂ cubic compound. Maxima for this compound were observed under ZFC magnetization. Below the broad maximum, irreversibility is observed. The magnetization curves for ZFC and FC regimes are split and magnetic moments for FC are higher than for ZFC. The dependence of the maxima upon the magnetic field and the time dependence of remanence is similar to the case of spin-glass-like systems.     

自旋为1/2的XY模型亚铁磁棱型链的物性和有序-无序竞争

利用不变本征算符法, 计算低温下自旋为1/2的XY模型一维亚铁磁棱型链系统的元激发谱, 讨论在此系统中不同的特殊情形下的元激发能量, 从而给出体系的三个临界磁场强度的解析解H_C1, H_C2, H_peak. 分析不同外磁场下 体系的磁化强度随温度的变化规律, 发现三个临界磁场强度的解析解H_C1, H_C2, H_peak是正确的, 并从三个元激发对磁化强度的贡献进行了说明. 低温下磁化强度随外磁场的变化呈现1/3磁化平台. 体系的磁化率随温度或者外磁场的变化都出现了双峰现象. 这说明双峰源于二聚体分子内电子自旋平行排列的铁磁交换作 用能和二聚体与单基体分子间电子自旋反平行排列的反铁磁交换作用能, 热无序能, 外磁场强度相关的自旋磁矩势能之间的竞争.     

Bi_2Sr_2CaCu_2O_8单晶中的反常尖锋效应

使用牛津震动样品磁强计 (VSM)研究了Bi2 Sr2 CaCu2 O8单晶的磁滞回线 .在 2 0到 40K温度之间发现了反常的尖锋效应 ,随样品O含量的增加 ,发生尖锋效应的外场也相应提高 .可以认为在尖峰效应处发生了由涡漩物质的有序固态到无序固态的相变 ,在有少量点缺陷存在的BSCCO单晶相图上 ,Bsp线终止于 2 0K温度处 ,在 2 0K以下温区没有发生准格子到涡漩玻璃的相变 ,涡漩固相始终以准格子形式存在 ;可以认为尖峰效应是外场、温度、无序的复杂函数     

Electric-field-induced in-plane effective 90° magnetization rotation in Co_2FeAl/PMN-PT structure

The in-plane effective 90° magnetization rotation of Co_2 Fe Al thin film grown on PMN-PT substrate induced by the electric field is investigated at room temperature. The magnetic hysteresis loops under different positive and negative electric fields are obtained, which reveals remanent magnetization can be mediated by the electric field. Moreover, under positive electric fields, the obvious 90° magnetization rotation can be observed, while remanent magnetization is nearly unchanged under negative electric fields. The result is consistent with the electric field dependence of effective magnetic field,which can be attributed to the piezostrain effect in Co_2 Fe Al/PMN-PT structure. In addition, the piezostrain-mediated 90° magnetization rotation can be demonstrated by the result of resonance field changing with electric field in the measurement of ferromagnetic resonance, which is promising for the design of future multiferroic devices.     

Sol-gel derived nanoparticles of Zn substituted lithium ferrite (Li0.32Zn0.36Fe2.32O4): magnetic and Mössbauer effect measurements and their theoretical analysis

Nanoparticles of Zn substituted lithium ferrite (Li_0.32Zn_0.36Fe_2.32O₄) have been prepared by a sol-gel method where the ultra-sonication technique has been adopted to reduce the agglomeration effect among the nanoparticles. The samples were heat-treated at three different temperatures and the formation of the nanocrystalline phase was confirmed by X-ray diffractograms (XRD). The average particle size of each sample has been estimated from the (311) peak of the XRD pattern using the Debye-Scherrer formula and the average sizes are in the range of 10-21 nm. The average particle size, crystallographic phase, etc. of some selected samples obtained from the high-resolution transmission electron microscopy are in agreement with those estimated from the XRD patterns. Static magnetic measurements viz., hysteresis loops, field cooled and zero field cooled magnetization versus temperature curves of some samples carried out by SQUID in the temperature range of 300 to 5 K clearly indicate the presence of superparamagnetic (SPM) relaxation of the nanoparticles in the samples. The maximum magnetization of the SPM sample annealed at 500 °C is quite high (68 Am²/Kg) and the hysteresis loops are almost square shaped with very low value of coercive field at room temperature (827.8 A/m). The particle size, magneto-crystalline anisotropy, etc. have been estimated from the detailed theoretical analysis of the static magnetic data. The dynamic magnetic behavior of the samples was also investigated by observing the ac hysteresis loops and magnetization versus field curves with different time windows at room temperatures. The different soft magnetic quantities viz., coercive field, magnetization, remanance, hysteresis losses, etc. were extracted from dynamic measurements. Dynamic measurements confirmed that the samples are in their mixed state of SPM and ordered ferrimagnetic particles, which is in good agreement with the results of static magnetic measurements. Mössbauer spectra of the samples recorded at room temperature (300 K) and at different temperatures down to 20 K confirmed the presence of the SPM relaxation of the nanoparticles of the samples.     

混合场中 (Pr1－yNdy)2/3Sr1/3MnO3体系磁转变行为研究

对(Pr1-yNdy)2/3Sr1/3MnO3(y=2/8,4/8,6/8)体系在42K—300K温度范围内混合场（即交流驱动场Hac与附加直流磁场H dc的叠加）下的磁转变行为进行了系统的实验研究. 结果表明，随着外加直流磁场强 度的增大，样品的直流磁化强度增大，交流磁化率逐渐减小；在混合场作用下，磁转变温度 （居里温度）附近样品交流磁化率的实部出现一个特征的尖峰. 对于(Pr1-yNdy)2/3Sr1/3MnO3体系，随着Nd掺杂量的增加，交流磁化率实部 的相对变化 (Eχ′=Δχ′/χ′dc)的峰值对应的温度降低，这一点与该系 统的磁电阻的变化规律一致. 对锰氧化物体系在混合场中的磁转变现象进行了讨论. 关键词： 锰氧化物 混合场 磁特性