Neutron-Diffraction Study of NdFe2Si2

Abstract

A powder sample of the compound Nd${\mathrm{Fe}}_2$ ${\mathrm{Si}}_2$ was studied by neutron diffraction. The compound is found to be isostructural to the analogous actinide compounds $A{\mathrm{Fe}}_2{\mathrm{Si}}_2$, with $\frac{I4\mathrm{}}{\mathrm{mmm}}$ as a probable space group and ($2a;4d;4e$) as ionic positions. The bimolecular unit cell consistent with the room temperature (RT) pattern has the dimensions $a=3.983\mathrm{}$ Å, and $c=10.03\mathrm{}$ Å. A least-squares analysis of 20 lines in the RT pattern yielded a weighted $R$ factor of about 9% for two different distributions of the ions: (a) (Nd; Fe; Si); (b) [$\mathrm{Nd}; \mathrm{}(1-e)\mathrm{}\mathrm{Fe}e\mathrm{Si}; \mathrm{}(1-e)\mathrm{}\mathrm{Si}e\mathrm{Fe}$] with $e=9.8\mathrm{}$%. The refined $z$ parameter is 0.372 for the two distributions. Five superlattice lines were observed in the 4.2 °K pattern. These lines are consistent with a doubling of the unit cell in the $c$ direction. These lines are inconsistent with any collinear magnetic structure of tetragonal symmetry of the Fe ($4d$) sublattice but are consistent with an antiferromagnetic structure of the $\mathrm{Nd} \mathrm{}\left(2a\right)\mathrm{}$ sublattice. The structure consists of ferromagnetic sheets perpendicular to $c$ with the magnetic axis along $c$. These sheets are stacked according to the sequence ++--. This structure belongs to the magnetic space group $\frac{P_{2c}4}{n{m}^{\prime }{m}^{\prime }}$. The magnetic moment consistent with the diffracted intensities is $(3.1\mathrm{}\pm 0.3){\mu }_B$ per Nd ion. The Néel temperature, $T_N\sim 15.6$ °K, was determined from the intensity-temperature curve of the strongest superlattice line. This line exhibits a smooth temperature dependence but falls considerably above the $\left({\mathrm{Nd}}^{3+}\right) J=\frac{9}{2}$ Brillouin curve.