Magnetvo1ume effects in the valence fluctuating and Kondo compounds

1.YbInCu4，YbAgCu4和YbAuCu4的磁致伸缩，测量了YblnCu4的磁致伸缩随温度的变化，发现YblnCu4的磁致伸缩与外场的平方成正比，YbAgCu4为非典型的致密近藤化合物，YbAuCu4为反铁磁性重费米子化合物。YbInCu4的高温态和YbAgCu4的低温态(价态涨落)的C值较大，而YbAgCu4的低温态和YbAgCu4的低温态的C值(-10^4/Mu_B^2)要小得多。EuNi2(Si&lt；1-x&gt；Ge_x)_2的磁致伸缩在东京大学ISSP上用高达40T的脉冲场测量了EuNi_2(Si_&lt；1-x&gt；Ge_x)_2的磁致伸缩。我们发现，除了x=0.82的化合物表现为变态转变外，体积磁致伸缩与磁化强度的平方成正比，其符号为正。对于局域反铁磁区(x&gt；0.85)，磁体耦合常数为C-10^&lt；-5&gt；/Mu_B^2；对于价涨落区(x&lt；0.8)，磁体耦合常数为C-10^&lt；-3&gt；/Mu_B^2。尽管Yb和Eu化合物的价不稳定性的微观来源不同，但磁体效应似乎是由相同的因素主导的。对磁体效应的理论研究似乎有助于理解f电子材料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.