SPIN FLUCTUATIONS IN URANIUM COMPOUNDS

(1)稀铀杂质体系：用两种晶格参数明显不同的非磁性基体，分别研究了稀UAl_2和U_2Zn_2中铀离子的单格位性质<17>，并与相应的致密化合物的磁行为作了对比。在所有的情况下，观察到的行为可以半定量地解释在晶场下的近藤效应。但与基于Ce的4f系统不同，致密5f化合物的行为不能被描述为U杂质在相应基质中的简单收集。此外，该系统的特征能量尺度的变化是不是在所有单调的晶格参数。(2)铀化合物自旋动力学的系统学：反铁磁的U_3Pd_Si_6<20>（T_N= 19K）是典型的定域5f矩系统。在NMR测量中，我们发现在30K以上，核纵向弛豫时间T_1随T几乎呈线性增加。这表明，从NMR的观点来看，低能自旋动力学也可以用局域化的5f矩来理解，但5f矩的强烈T依赖性、大弛豫速率表明，由于5f和传导电子之间的杂化而导致的单位点动力学在顺磁状态下主导低能自旋动力学。这些行为与具有较高有序温度的U化合物如UP、UAs和U_3P_4有许多相似之处，被认为是接近局域磁矩极限的磁有序U系统的共同特征。(3)U化合物的多Q磁性结构：我们在<75>4.2K下的磁性有序的双Q型IA状态下的1H As NMR谱中发现了精细结构。这些结构很可能与多Q结构有关，并可作为双Q态四极序的证据。

(1) Dilute U Impurity Systems : Single-site properties of U ions have been investigated in dilute UAI_2 and U_2Zn_<17>, with counterpart dense compounds showing quite contrasting magnetic behavior, by using respectively two different non-magnetc matrices with significantly different lattice parameters. In all the cases, the observed behavior can be explained semi-quantitatively in terms of Kondo effect under crystal field. But differently from Ce-based 4f systems, the behavior of the dense 5f compounds cannot be described as simple collection of the U impurities in the corresponding matrices. Moreover, the variation of the characteristic energy-scale of the system is not at all monotonous against that in the lattice parameters.(2) Systematics of the Spin Dynamics of U Compounds : The antiferromagnetic U_3Pd_<20>Si_6 (T_N=19 K) was previously characterized as a typical localized 5f moment system. In NMR measurements, we have found that the nuclear longitudinal relaxation time T_1 increases almost linearly with T above about 30 K. This indicates that the low-energy spin dynamics can be understood in terms of localized 5f moment also from the NMR viewpoint, but a strongly T-dependent, large relaxation rate of the 5f moment shows that single-site dynamics due to hybridization between 5f and conduction electrons dominate the low-energy spin dynamics in the paramagnetic state. The observed behavior has many similarities to those on U compounds with relatively high ordering temperatures such as UP, UAs and U_3P_4, and are considered as common characteristics in magnetically ordering U systems that are close to the localized moment limit.(3) Multi-Q Magnetic Structure of U Compounds : We have found fine structures in the ^<75>As NMR spectrum at 4.2 K in the magnetically ordered double-Q type IA state. These structures are very likely to be related to the multi-Q structure, and can be an evidence for the quadrupole order in the double-Q state.