Physical properties of the ferrites NiFe 2− x Mn x O 4 (0 ≤ x ≤ 2) prepared by sol–gel method

NiFe 2− x Mn x O 4 (0 ≤ x ≤ 2) is prepared by sol–gel route and the physical properties are studied for the first time. The thermal analysis shows that the phases are formed at 600 °C. The oxides crystallize in an inverse spinel with a cubic symmetry whose lattice constant ( a : 0.83381–0.83985 nm) augments slightly up to x = 1.6, according to the Vegard’s law; such result is corroborated by the FTIR spectroscopy. The diffuse reflectance shows direct (1.00–1.56 eV) and indirect (0.39–1.65 eV) optical transitions due to the lift of degeneracy of 3d orbital. Field-dependent magnetization of NiFe 2− x Mn x O 4 was measured at 300 K in the range (± 20 kOe) and NiFe 2 O 4 shows a strong magnetism with a saturation magnetization of 20 emu/g. The saturation magnetization varies from 0.5 to 20 emu/g, whereas the coercivity fluctuates between 110 and 239 Oe. The variation of the electrical conductivity with temperature shows a conduction mechanism by small polaron hopping which obeys an exponential law with activation energies ( E a : 0.12–033 eV). The thermo-power is positive and almost constant ( S 300K : 144–130 µV K −1 ), indicating p type comportment with a mobility more a less constant (1.5–8 × 10 –6 V 2 cm −1 s −1 ). The hole concentrations ( N A × 10 16 : 0.26–2.17) are computed from the capacitances measurements in Na 2 SO 4 solution (0.5 M). The flat band potentials ( E fb : − 0.04 to − 0.29 V), determined electrochemically, does not vary significantly indicating that the valence band derives mainly from 3d character.