Variation of local magnetic environments in olivine-type compounds:Na0.7FePO4andFePO4

Abstract

To investigate the microscopic magnetic nature of phospho-olivines, we have measured positive muon-spin rotation and relaxation (${\ensuremath{\mu}}^{+}$SR) spectra for ${\mathrm{Na}}_{x}\mathrm{Fe}\mathrm{P}{\mathrm{O}}_{4}$ and $\mathrm{Fe}\mathrm{P}{\mathrm{O}}_{4}$, using powder samples in the temperature range between 2 and 200 K. For ${\mathrm{Na}}_{0.7}\mathrm{Fe}\mathrm{P}{\mathrm{O}}_{4}$, ${\ensuremath{\mu}}^{+}$SR measurements under zero external field (ZF) showed the appearance of static magnetic order below ${T}_{\mathrm{N}}\ensuremath{\sim}61$ K, at which the susceptibility $\ensuremath{\chi}(T)$ exhibits a sharp maximum due to an antiferromagnetic (AF) transition. The wide distribution found for the internal magnetic field (${H}_{\mathrm{int}}$) was reasonably explained by the formation of AF order with the spread in ${H}_{\mathrm{int}}$ due to a random distribution of Na vacancies. On the other hand, the ZF spectra for FePO${}_{4}$ suggested the formation of the canted AF order below ${T}_{\mathrm{N}}=113$ K, as proposed by neutron diffraction (ND), but the ratio between the $y$ and $x$ components of the ordered moment (${M}_{y}/{M}_{x}$) was determined as 0.6 by ${\ensuremath{\mu}}^{+}$SR, whereas 0.22 was measured by ND, probably indicating anisotropic spin fluctuations in $\mathrm{Fe}\mathrm{P}{\mathrm{O}}_{4}$.