Magnetvo1ume effects in the valence fluctuating and Kondo compounds

1.测量了YbInCu_4、YbAgCu_4和YbAuCu_4的磁致伸缩随温度的变化，发现YbInCu_4的磁致伸缩随温度的变化具有一级相变，非典型致密近藤化合物YbAgCu_4和反铁磁重费米子化合物YbAuCu_4的磁致伸缩随温度的变化，发现它们的体积磁致伸缩和各向异性磁致伸缩都与外场的平方成正比。所有材料的体积磁致伸缩系数都为正值，但随近藤温度T_K的不同而有明显的变化。应用d V/V = C（XH）^2关系，我们估计了<-3>YbInCu_4高温态和YbAgCu_4低温部分（价态涨落区）的大C值（-10^4/mu_B^2），以及YbInCu_4低温态和YbAgCu_4高温部分（近藤区）的小得多的C值（-10^4/mu_B^2）。EuNi_2（Si_<1-x>Ge_x）_2的磁致伸缩在<1-x>东京大学的ISSP实验室用高达40 T的脉冲磁场测量了EuNi_2（Si_ Ge_x）_2的磁致伸缩。我们发现，除了x=0.82的化合物表现出变晶转变外，其体积磁致伸缩与磁化强度的平方成正比，并且其符号为正。在<-5>局域磁矩反铁磁区（x&gt;0.85），磁体积耦合常数为C-10^ /μ_B^2;<-3>在价态涨落区（x&lt;0.8），磁体积耦合常数为C-10^ /μ_B^2.尽管Yb和Eu化合物的价态不稳定性的微观起源不同，但磁体积效应似乎受同一因素支配.磁体积效应的理论研究有助于理解f电子材料中价不稳定性的起源。

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.