Strange metamagnetism and magnetic transitions of TbCu_2Ge_2

四元化合物TbCu_2Ge_2表现出一种奇怪的变磁转变，在硬磁化过程中出现的变磁转变在TN以上约30K处持续存在。本研究已经进行，以了解这种变磁性转变的机制。1.单晶生长：一些单晶化合物已经生长，研究物理性质和中子衍射。然而，这些晶体太大，不能用中子衍射。2.电阻率：在TN= 12.3K和Tt= 9K时，电阻率随温度的变化有两个反常现象。转变方式为TN处的二级转变和Tt处的一级转变。在30 K附近没有观察到磁化率沿着c轴的宽峰。3.TbCu_2Si_2单晶：生长了类质同象的单晶化合物TbCu_2Si_2，研究其磁性。通过比较Ge和Si化合物的相变过程，可以更好地理解这种奇异的变磁相变。Si化合物的磁行为与Ge化合物的磁行为非常相似。Si和Ge之间的差异对磁性能没有影响。4.晶体电场效应的分析CEF效应对于理解磁行为是非常重要的。从磁化率、磁化强度和比心的测定结果，对这一现象进行了分析。

The tetragonal ternary compound TbCu_2Ge_2 shows a strange metamagnetic transition ; a metamagnetic transition, which appears in the hard magnetization [001] process, persists up to about 30 K above TN. This study has been carried out in order to understand the mechanism of this metamagnetic transition.1.Single crystal Growth :Some single crystal compounds have been grown to study physical properties and Neutron diffraction. The crystals are, however, too large mosaic to use the neutron diffraction. An attempt to growth a good crystal is now in progress.2.Electrical resistivity :Two anomalies are observed at TN=12.3 K and Tt=9 K in the temperature dependence of electrical resistivity. The way of transition shows the second order transition at TN and the first order one at Tt. No anomaly has been observed around 30 K where a broad peak appears in the magnetic susceptibility along the c-axis vs. temperature curve.3.TbCu_2Si_2 single crystal :The isomorphs single crystal compound TbCu_2Si_2 has been grown to study the magnetic properties. The comparison between those of the Ge and Si compound may lead better understanding for this strange metamagnetic transition. The magnetic behaviors of the Si compound is very similar to those of the Ge compound. The difference between Si and Ge does not affect the magnetic properties.4.Analysis for Crystalline electric field effectsThe CEF effects are very important for understanding the magnetic behaviors. From results of the magnetic susceptibility, magnetization and specific heart, the analysis is progress.