  The low temperature lattice structure and magnetic properties of
Co$_{2.75}$Fe$_{0.25}$O$_4$ ferrite have been investigated using experimental
results from synchrotron x-ray diffraction (SXRD), dc magnetization, ac
susceptibility, neutron diffraction and neutron depolarization techniques. The
samples have been prepared by chemical co-precipitation of the Fe and Co
nitrates solution in high alkaline medium and subsequent thermal annealing of
the precipitates in the temperature range of 200- 900 $^\circ$C. Rietveld
refinement of the SXRD patterns at room temperature indicated two-phased cubic
spinel structure for the samples annealed at temperatures 200-600 $^\circ$C.
The samples annealed at temperatures 700 $^\circ$C and 900 $^\circ$C (CF90)
have been best fitted with single phased lattice structure. Refinement of the
neutron diffraction patterns in the temperature range of 5-300 K confirmed
antiferromagnetic (AFM) Co$_3$O$_4$ and ferrimagnetic (FIM)
Co$_{2.75}$Fe$_{0.25}$O$_4$ phases for the sample annealed at 600 $^\circ$C and
single FIM phase of Co$_{2.75}$Fe$_{0.25}$O$_4$ for the CF90 sample. Magnetic
measurements have shown a non-equilibrium magnetic structure, consisting of the
high temperature FIM phase and low temperature AFM phase. The magnetic phases
are sensitive to magnetic fields, where high temperature phase is suppressed at
higher magnetic fields by enhancing the low temperature AFM phase, irrespective
of annealing temperature of the samples.
