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

已知激发态通常通过相对论效应混入f电子的基态。由于Eu^<2+>离子具有球对称的S基态，因此可以很容易地看到这种效应。在这项研究中，我们试图通过观察Eu核的NMR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原子本身电子电荷不对称性产生的贡献。我们研究了立方NaCl型EuX（X=O，Te），其中晶格贡献很小。本文<151><153>用稳态方法，在液氦温度和21 kOe的磁场下，用EuTe单晶观测了~ 3Eu和~ 3Eu的NMR谱。在6 kOe以上，~ 1 <151>Eu NMR谱线为单线，而~ 1 <153>Eu NMR谱线为五线分裂，这归因于四极相互作用。测量了光谱随晶向的变化。[100]的光谱是对称的。随着温度的升高，分裂的幅度下降得相当快。相对论效应是不可分离的，但我们利用以前对CaF_2中Eu^&lt;2+&gt;的ENDOR测量值，分析了四极分裂的大小，它是相对论效应和磁弹性效应的贡献之和。分析结果给出了反铁磁EuTe磁致伸缩的重要信息。对于EuO，没有观察到分裂，这表明所用样品不是那么纯。还提出了EuSe的分裂随温度的变化关系。

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.