Variation of local magnetic environments in olivine-type compounds: Na0.7FePO4 and FePO4

To investigate the microscopic magnetic nature of phospho-olivines, we have measured positive muon-spin rotation and relaxation (mu+SR) spectra for NaxFePO4 and FePO4, using powder samples in the temperature range between 2 and 200 K. For Na0.7FePO4, mu+SR measurements under zero external field (ZF) showed the appearance of static magnetic order below T-N similar to 61 K, at which the susceptibility chi(T) exhibits a sharp maximum due to an antiferromagnetic (AF) transition. The wide distribution found for the internal magnetic field (H-int) was reasonably explained by the formation of AF order with the spread in H-int due to a random distribution of Na vacancies. On the other hand, the ZF spectra for FePO4 suggested the formation of the canted AF order below T-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 mu+SR, whereas 0.22 was measured by ND, probably indicating anisotropic spin fluctuations in FePO4.