Evidence for a coupled magnetic-crystallographic transformation inNd5(Si0.6Ge0.4)4

Abstract

We have performed a detailed study of the magnetic and structural properties of the compound ${\mathrm{Nd}}_{5}{({\mathrm{Si}}_{0.6}{\mathrm{Ge}}_{0.4})}_{4}$ by means of neutron powder diffraction, magnetization, linear thermal expansion, and magnetostriction experiments. A coupled magnetic and crystallographic first-order transformation from a room-temperature monoclinic ${\mathrm{Gd}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$-type paramagnetic state to a low-temperature orthorhombic ${\mathrm{Gd}}_{5}{\mathrm{Si}}_{4}$-type ferromagnetic structure takes place on cooling at $\ensuremath{\sim}68\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This magnetostructural transition shifts towards higher temperatures by the application of an external magnetic field at a rate of $0.11(1)\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{kOe}$, which is lower than the $\ensuremath{\sim}0.6\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}\ensuremath{\sim}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{kOe}$ observed in the ${\mathrm{Gd}}_{5}{({\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x})}_{4}$ and ${\mathrm{Tb}}_{5}{({\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x})}_{4}$ compounds. The magnetostructural transformation cannot be completed in the isothermal regime by a $120\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ magnetic field. It is suggested that the single-ion anisotropy of the Nd ions hinders the completion of the field-induced transformation. The existence of a simultaneous magnetic and crystallographic transformation in this alloy, which is in sharp contrast with other previously studied ${\mathrm{Nd}}_{5}{({\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x})}_{4}$ alloys, is discussed in terms of the role of interstitial impurities in triggering and coupling/decoupling the crystallographic transition.