Magnetic-field-induced structural phase transition and giant magnetoresistance in La0.85Ce0.15Fe12B6

Abstract

Magnetoelastic coupling, structural, magnetic, electronic transport, and magnetotransport properties of ${\mathrm{La}}_{0.85}{\mathrm{Ce}}_{0.15}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ have been studied by a combination of macroscopic [magnetization, electrical resistivity, and magnetoresistance (MR)] and microscopic temperature- and magnetic-field-dependent x-ray powder diffraction measurements. The itinerant-electron system ${\mathrm{La}}_{0.85}{\mathrm{Ce}}_{0.15}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ exhibits an antiferromagnetic (AFM) ground state and multiple magnetic transitions, AFM-ferromagnetic (FM) and FM-paramagnetic (PM), triggered by changes in both temperature and magnetic fields. At low temperatures, the field-induced first-order AFM-FM metamagnetic phase transition is discontinuous, manifesting itself by extremely sharp steps in magnetization as well as in MR and is accompanied by large magnetic hysteresis. A remarkably large negative MR of \ensuremath{-}73% was discovered. In addition, the time evolution of the electrical resistivity displays a colossal spontaneous jump when both the applied magnetic field and temperature are constant. Diffraction data reveal a magnetic-field-induced structural phase transition associated with the AFM-FM and PM-FM transformations. The lattice distortion is driven by magnetoelastic coupling and converts the crystal structure from rhombohedral ($R\overline{3}m$) to monoclinic $(C2/m)$. The AFM and PM states are related to the rhombohedral structure, whereas the FM order develops in the monoclinic symmetry. A huge volume magnetostriction of \ensuremath{\sim}0.9% accompanies this symmetry-lowering lattice distortion. Meanwhile, a highly anisotropic thermal expansion involving giant negative thermal expansion with an average volumetric thermal expansion coefficient ${\ensuremath{\alpha}}_{V}=\ensuremath{-}195\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ was observed. The consistency seen in these different experimental data constitutes direct evidence of the strong correlations between charge, magnetic, and crystallographic degrees of freedom in this material.