Magnetic properties of Cu_1+xM_xFe_2-2xO ₄ mixed ferrites (M = Ge,Ti, 0≤x≤0.4)

Spinnel copper ferrites Cu_1+xM_xFe _2-2xO₄ (M = Ge, Ti) with 0≤x≤0.4 have been investigated using Mössbauer spectroscopy and magnetic measurements. Mössbauer measurements performed at 77 and 300K show broadened magnetic sextets superimposed on a paramagnetic doublet. The magnetic components have been assigned to Fe³⁺ in two non-equivalent crystallographic tetrahedral and octahedral sites. The Mössbauer data show that Ge ⁴⁺ has strong preference to occupy the tetrahedral site in contrast to Ti⁴⁺ which occupy the octahedral site. Saturation magnetization (M_S) measurements were performed using a vibrating sample magnetometer in the temperature range of 77-750K, while magnetic susceptibility (χ_m) measurements were performed using a Faraday microbalance susceptometer in the range of 500-1000K. Both M_S and χ _m variations with temperature indicate that the Curie temperature T_C decreases almost linearly with increasing Ti and Ge concentrations. The magnetic moments calculated from the M_S data and the effective paramagnetic moments calculated from χ_m data are discussed on the basis of Neel's two sublattice molecular field model and the cations distribution obtained from the Mössbauer analysis.