Magnetic anisotropy in the ferromagnetic insulating state ofLa1−xCaxMnO3

Abstract

Ferromagnetic resonance (FMR) has been applied to study the variation of magnetic anisotropy and phase separation tendencies in the ferromagnetic insulating state of stoichiometric hole-doped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}\phantom{\rule{0.3em}{0ex}}(0.125\ensuremath{\leqslant}x\ensuremath{\leqslant}0.19)$. A strongly anisotropic ferromagnetic phase is identified in the ferromagnetic insulating regime. The magnetic anisotropy evolves from positive uniaxial at $x=0.125$ and 0.15 to negative cubiclike at $x=0.175$ with increasing magnitude, whereas it decreases appreciably at $x=0.19$, which is close to the ferromagnetic metallic phase boundary. Minor contributions from ferromagnetic inhomogeneities characterized by weaker magnetic anisotropy are traced at low temperatures for $x=0.125$ and 0.15, while their temperature dependence implies coupling with the major anisotropic phase. A growing tendency towards the formation of spatially separate ferromagnetic regions in the paramagnetic regime is observed at $x\ensuremath{\geqslant}0.175$, as the ferromagnetic metallic phase is approached. Persistent anomalies in the temperature dependence of the FMR parameters are traced concurrently with the structural transformation at $60--70\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.