Understanding the effect of Mn substitution for Fe in Co ferrite presents a challenge because there are three different transition-metal ions distributed among two distinct crystallographic and magnetic sublattices with complicated superexchange and anisotropic interactions. In this study, a series of six powder samples with compositions Co 1.0 Mn x Fe 2−x O 4 were investigated using transmission Mössbauer spectroscopy. Mössbauer spectroscopy provides an excellent tool for probing the local environment of the Fe atoms present in such materials. Results show two sets of six-line hyperfine patterns for all samples, indicating the presence of Fe in both A and B sites. Identification of sites is accomplished by evidence from hyperfine distribution width, integrated intensity, and isomer-shift data. Increasing Mn concentration was found to decrease the hyperfine field strength at both sites, but at unequal rates, and to increase the distribution width. This effect is due to the relative strengths of Fe-O -X superexchange ͑X = Fe, Co, or Mn͒ and the different numbers of the next-nearest neighbors of A and B sites. Results are consistent with a model of Mn substituting into B sites and displacing Co ions onto A sites.

The magnetic and magnetocaloric properties of a series of minutely doped Mn5-xFexGe3 compounds that exhibit the D88-type hexagonal crystal structure at room temperature have been investigated. For all Fe concentrations, the alloys are ferromagnetic and undergo a second-order ferromagnetic-to-paramagnetic transition near room temperature. Although the small Fe doping had little effect on the ferromagnetic transition temperatures of the system, changes in the saturation magnetization and magnetic anisotropy were observed. For x≤0.15, all compounds exhibit nearly the same magnetic entropy change of ~7 J/kg K, for a field change of 50 kOe. However, the magnitude of the refrigerant capacities increased with Fe doping, with values up to 108.5 J/kg and 312 J/kg being observed for field changes of 20 kOe and 50 kOe, respectively. As second-order phase transition materials, the Mn5-xFexGe3 compounds are not subject to the various drawbacks associated with first-order phase transition materials yet exhibit favorable magnetocaloric effects.