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

众所周知，激发态一般通过相对论效应混合到f电子的基态中。由于Eu~(2+)离子具有球对称的基态S态，这种影响是显而易见的。在本研究中，我们试图通过观察Eu核的核磁共振谱中的四极分裂来发现这一效应。分裂与原子核的电场梯度成正比，电场梯度由晶格贡献和Eu原子自身电荷不对称引起的贡献组成。我们研究了晶格贡献很小的立方氯化钠晶体的EUX(X=O，Te)。用稳态法，用EuTe单晶，在磁场温度高达21kOe的液氦中观测到了~(151)Eu和~(153)Eu的核磁共振谱。在6 KOE以上，^&lt；151&gt；Eu核磁共振显示单线，而^&lt；153&gt；Eu核磁共振显示五线分裂，这归因于四极相互作用。测量了光谱随晶体方向的变化。[100]的光谱是对称的。随着温度的升高，裂解的幅度减小得很快。相对论效应是分不开的，但利用前人对CaF_2中Eu^&lt；2&gt；的Endor测量结果，我们分析了四极分裂的大小是相对论效应和磁弹性效应的贡献之和。分析给出了反铁磁性EuTe的磁致伸缩的重要信息。对于EuO，没有观察到分裂，这表明所用的样品不是很纯。文中还给出了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.