Investigation of spin ordering in antiferromagnetic Fe1−xMnxPO4 with Mössbauer spectroscopy

We have investigated the spin ordering in Fe1−xMnxPO4, which is a possible cathode material for rechargeable lithium ion battery, with antiferromagnetic structure below Néel temperature (TN). The prepared Fe1−xMnxPO4 (x = 0.0, 0.1, and 0.3) samples have orthorhombic structures with space group of Pnma. These samples show the magnetic phase transition, caused by the strong crystalline field at the MO6 octahedral sites. According to the temperature dependence of magnetic susceptibility of Fe1−xMnxPO4, all samples show antiferromagnetic behaviors. The Néel temperature (TN) decreases from 114 K at x = 0.0 to 97 K at x = 0.3 with Mn concentrations. The magnetization of Fe1−xMnxPO4 decreases until the temperature reaches the spin-reorientation (TS) temperature, and then starts increasing as the temperature increases up to TN. The TS of the Fe1−xMnxPO4 were found to be 30, 27, and 24 K for x = 0.0, 0.1, and 0.3. In order to investigate the hyperfine interaction of Fe3+ ions in FeO6 octahedral sites, Mössbauer spectra of Fe1−xMnxPO4 have been taken at various temperatures from 4.2 to 295 K. The isomer shift (δ) values of the Fe1−xMnxPO4 were between 0.31 and 0.43 mm/s, indicating the high spin state of Fe3+ at all temperatures. The magnetic hyperfine field (Hhf) and electric quadrupole splitting (ΔEQ) values of Fe0.9Mn0.1PO4 at 4.2 K were determined to be Hhf = 498 kOe and ΔEQ = 2.1 mm/s. We have also observed the abrupt changes in Hhf and ΔEQ at 27 K for Fe0.9Mn0.1PO4, and decrease the value of TS of Fe1−xMnxPO4 with Mn concentrations. Our study suggests that these changes in Fe1−xMnxPO4 are originated from the strong electric crystalline field and spin-orbit coupling of FeO6 octahedral site.