Impurity Effect and Size Effect on Itinerant-Electron Ferromagnets

对ZrZn_2、YNi_3、Y_2Ni_7等弱流动电子铁磁体进行了实验研究，研究了在弱流动电子铁磁体中掺杂钆原子或将部分组成金属原子替换为不同过渡金属原子的效果。研究了co_2铁磁体中阴离子的取代对其磁性能的影响。在很强的磁场中测量了磁化曲线。在平均场近似的基础上，考虑到局域自旋引起的磁矩，可以解释钆掺杂弱巡回电子铁磁体磁化强度的温度变化。然而，磁场对磁化强度的依赖表明平均场近似的不足，并表明场对自旋涨落的影响抑制和各向异性的影响。作为一阶近似，过渡金属原子的取代似乎使费米能级发生了位移，但某些复杂的能带结构效应似乎主导了磁化的变化。以硒取代硫后，流动电子铁磁体CoS_2的磁态发生了剧烈变化。它表现为铁磁体向流动电子超磁体的转变，并通过取代进一步向顺磁体转变。特别地，在10%取代的样品中，通过增加温度，观察到从铁磁状态到亚磁性状态的明显的一级相变和从亚磁性状态到顺磁性状态的二阶相变。这是由于我们的样品的相当均匀的状态。这种转变可以用朗道相变理论进行定性分析。在非常弱的流动电子铁磁体中可能存在尺寸效应，但没有观察到明显的变化。

Effect of doping gadlinium atoms into weak itinerant-electron ferromagnets or substituting a part of constituent metal atoms for different transition-metal atoms in weak itinerant-electron ferromagnets have been studied experimentally for ZrZn_2, YNi_3, Y_2Ni_7 ferromagnets. Effect of substituting anions in CoS_2 ferromagnet on its magnetic property has also been examined. Measuremnts have been made of magnetization curves in a very strong magnetic field. The temperature variation of magnetization in the Gd-doped weak itinerant-electron ferromagnets can be explained by taking into consideration of the induced magnetic moment in itinerant-electrons due to localized spins, on the basis of the mean field approximation. However, the magnetic field dependence of magnetization shows an inadequacy of the mean field approximation and suggests the effect of field on spin fluctuations to suppress and also an effect of anisotropy. The substitution by transition-metal atoms appears to shift a Fermi level as a first order approximation, but some complicated band structure effet seems to dominate the variaton of magnetization. The substitution of selenium for sulphur results in a drastic change of magnetic states in the itinerant-electron ferromagnet CoS_2. It appears a change from a ferromagnet to an itinerant-electron metamagnet, and further to a paramagnet by the substitution. Especially, in the 10 % substituted sample, it has been observed a clear first order phase transiton from ferromagnetic state to a metamagnetic state and a second order phase transiton from a metamagnetic state to a paramagnetic state by increasing a temperature. This is due to a quite homogeneous state of our samples. This transiton can be qulitatively analyzed by the Landau's theory of phase transiton. A possible size effect in the very weak itinerant-electron ferromagnets has been examined experimentally, but no appreciable change has been observed.