Deviation from the S-state of Eu^<2+> ion in magnetic materials due to relativistic effect of f electrons

众所周知，受相对论效应的影响，激发态通常混合到f电子的基态。由于Eu^<2+>离子具有球对称的基态S，因此可以很容易地观察到这种效应。在本研究中，我们试图通过观察铕核的核磁共振谱中的四极分裂来发现这种效应。分裂与原子核处的电场梯度成正比，其中包括晶格贡献和Eu原子本身的不对称电荷所产生的贡献。我们研究了立方NaCl型的EuX (X= 0, Te)，其晶格贡献很小。利用EuTe单晶，在21 kOe的磁场下，用稳态方法观测了液氦温度下^<151>Eu和^<153>Eu的核磁共振谱。在6 kOe以上，^<151>的Eu NMR表现为单线，而^<153>的Eu NMR表现为五线分裂，这是由于四极相互作用。测量了光谱随晶体方向的变化。[100]的谱是对称的。劈裂的幅度随着温度的升高而迅速减小。相对论效应是不可分割的，但我们利用先前在CaF_2中Eu^<2+>的ENDOR测量值，分析了四极分裂的大小，作为相对论效应和磁弹性效应贡献的总和。该分析提供了反铁磁EuTe的磁致伸缩的重要信息。对于EuO没有观察到分裂，这表明所使用的样品不是那么纯净。研究了EuSe中分裂的温度依赖性。

It is known that the excited states generally mix into the ground state of f electrons by the relativistic effect. Since Eu^<2+> ion has the ground S state which is spherically symmetric, the effect can be easily seen. In this study we tried to find this effect through the observation of the quadrupole splitting in the NMR spectrum of the Eu nuclei. The splitting is proportional to the electric field gradient at the nuclei, which consists of the lattice contribution and the contribution arising from the asymmetry of the electronic charge of the Eu atom itself. We studied EuX (X=O, Te) of the cubic NaCl type, in which the lattice contribution is very small. Using a single crystal of EuTe, we observed ^<151>Eu and ^<153>Eu NMR spectrum in liquid helium temperatures in magnetic fields up to 21 kOe by means of the steady state method. Above 6 kOe, ^<151>Eu NMR exhibited a single line, but ^<153>Eu NMR exhibited a five-line splitting which attributable to the quadrupole interaction. The variation of the spectrum with the crystal direction was measured. The spectrum for [100] was symmetric. The magnitude of the splitting decreased rather rapidly with increasing temperature. The relativistic effect could not be separated, but with the use of the value from the previous ENDOR measurements of Eu^<2+> in CaF_2 we analyzed the magnitude of the quadrupole splitting as a sum of the relativistic effect and the contribution from the magnetoelastic effect. The analysis gave important information of the magnetostriction in antiferromagnetic EuTe. For EuO the splitting was not observed, which indicates that the sample used was not so pure. The temperature dependence of the splitting in EuSe was also presented.