Mossbauer study of copper-nickel and copper-manganese ferrites

The Mossbauer spectra have been recorded from room temperature to above the Neel temperature for the spinel ferrites Cu 1-x Ni x Fe 2 O 4 and Cu 1-x Mn x Fe 2 O 4 for x=0.2, 0.4, 0.6 and 0.8. The spectra have been fitted with two sextets in the ferrimagnetic state. Iron is found to be in the trivalent high-spin state. The values of the hyperfine magnetic fields at 57 Fe nuclei at A and B sites increase steadily with the increase of Ni concentration. In the case of the Cu 1-x Mn x Fe 2 O 4 series of ferrites they first increase with increase of Mn concentration and then decrease with further increase of Mn concentration. Thus a canted spin structure is expected to be present in the latter ferrites. The quadrupole splitting is zero within experimental error for both the ferrites. The temperature dependence of the hyperfine magnetic field has been determined for all the compositions. The lattice constant decreases with increase of x in the Cu-Ni ferrite whereas it increases with the increase of x in the Cu-Mn ferrite. The Neel temperature increases in Ni concentration whereas it decreases with the increase in Mn concentration.