Evidence for a coupled magnetic-crystallographic transition in La0.9Ce0.1Fe12B6

Competition between antiferromagnetic (AFM), paramagnetic (PM), and ferromagnetic (FM) states in ${\mathrm{La}}_{0.9}{\mathrm{Ce}}_{0.1}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ compound is investigated by means of temperature- and magnetic field-dependent x-ray diffraction, magnetization, linear thermal expansion, and magnetostriction experiments. It is shown that both AFM and PM phases get converted into the FM phase via a first-order metamagnetic transition, which is accompanied by a huge forced-volume magnetostriction $\mathrm{\ensuremath{\Delta}}V/V(25\phantom{\rule{0.16em}{0ex}}\mathrm{K},\phantom{\rule{0.16em}{0ex}}6\phantom{\rule{0.16em}{0ex}}\mathrm{T})=1.15%$. X-ray powder diffraction reveals a magnetic field-induced crystallographic phase transition from a $R\overline{3}m$ rhombohedral (AFM, PM) to a $C2/m$ monoclinic (FM) structure. A peculiarly anisotropic lattice expansion as well as giant negative thermal expansion with a volumetric thermal expansion coefficient ${\ensuremath{\alpha}}_{V}=\ensuremath{-}193\ifmmode\times\else\texttimes\fi{}{10}^{--6}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{--1}$ are observed. These findings point to the significance of magnetoelastic effects in this metamagnet and illustrate the strength of the coupling between lattice and spin degrees of freedom in the ${\mathrm{La}}_{0.9}{\mathrm{Ce}}_{0.1}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ intermetallic compound.