Magnetvo1ume effects in the valence fluctuating and Kondo compounds

1. YbInCu_4, YbAgCu_4，和YbAuCu_4的磁致伸缩量。我们测量了YblnCu_4的磁致伸缩量的温度依赖关系，其中YblnCu_4表现为一阶价跃迁，非典型致密近道化合物YbAgCu_4和反铁磁性重费米子化合物YbAuCu_4，对于所有物质，我们发现体积和各向异性磁致伸缩量都与外场的平方成正比。所有材料的体积磁致伸缩量均为正数，但随近藤温度T_K的变化，其大小有显著不同。应用关系d V/V = C(XH)^2，我们估计YbInCu_4的高温状态和YbAgCu_4的低温部分（价涨落区）的C值较大（-10^<-3>/mu_B^2），而YbInCu_4的低温状态和YbAgCu_4的高温部分（近藤区）的C值较小（-10^4/mu_B^2） .2。EuNi_2(Si_<1-x> Ge_x)_2的磁致伸缩率在东京大学ISSP用高达40 t的脉冲场测量了EuNi_2(Si_<1-x> Ge_x)_2的磁致伸缩率。我们发现，除了x=0.82的化合物表现为超磁致转变外，体积磁致伸缩率与磁化强度的平方成正比，且其符号为正。定矩反铁磁区（x>0.85）的磁体积耦合常数估计为C-10^<-5>/mu_B^2，价态涨落区（x<0.8）的磁体积耦合常数估计为C-10^<-3>/mu_B^2。尽管这些Yb和Eu化合物的价态不稳定性的微观起源不同，但磁体积效应似乎是由相同的因素主导的。对磁体积效应的理论研究似乎有助于理解f电子材料中价态不稳定性的起源。

1. Magnetostriction of YbInCu_4, YbAgCu_4, and YbAuCu_4, We measured the temperature dependence of magnetostriction of YblnCu_4, which show the first-order valence transition, atypical dense Kondo compound YbAgCu_4 and antiferromagnetic heavy fermion compound YbAuCu_4, For all substances, we found the both volume and anisotropic magnetostriction is in proportion to the square of the external field. The volume magnetostriction has positive sign for all the materials but has markedly different magnitude depending on the Kondo temperature T_K. Applying a relation d V/V = C(XH)^2, we estimated large C values (-10^<-3>/mu_B^2) for the high-temperature state of YbInCu_4 and a low-temperature part of YbAgCu_4 (valence fluctuation regime), and much smaller C values (-10^4/mu_B^2) for the low-temperature state of YbInCu_4 and a high-temperature part of YbAgCu_4 (Kondo regime).2. Magnetostriction of EuNi_2 (Si_<1-x> Ge_x)_2 The magnetostriction of EuNi_2(Si_<1-x> Ge_x)_2 was measured at ISSP, the University of Tokyo, using pulse field up to 40 T.We found that, except the compound with x=0.82, which shows a metamagenetic transition, the volume magnetostriction is in proportion to the square of the magnetization and that its sign is positive. The magnetovolume coupling constant was estimated to be C-10^<-5>/mu_B^2 for localized-moment antiferromagnetic region (x>0.85) and C-10^<-3>/mu_B^2 for the valence fluctuation regime (x<0.8).In spite of the different microscopic origin of the valence instability in these Yb and Eu compounds, the magnetovolume effect seems to be dominated by the same factor. A theoretical approach to the magnetovolume effect seems to be helpful to understand the origin of the valence instability in the f-electron materials.