Dynamic short-range order observed in the (Zn)[Fe-2]O-4 spinel by neutron diffraction, mu SR, and Mossbauer experiments

Using neutron diffraction (ND), muon-spin rotation/relaxation (mu SR), and Fe-57-Mossbauer spectroscopy (MS) we have investigated magnetic properties of the normal spinel (Zn)[Fe-2]O-4. In compounds which are slowly cooled from 1200 degrees C to room temperature inversion is below detection limits. At T-N = 10.5 K the spinel exhibits long-range antiferromagnetic order (LRO). The transition as seen in thermal-scan spectra by MS is very sharp. However, ND and mu SR experiments show that already at temperatures of similar to 10 T-N a short-range antiferromagnetic ordering (SRO) develops which extends through similar to 70% of the sample volume just above T-N. Below T-N SRO and LRO coexist. At 4.2 K still similar to 25% of the sample is short-range ordered. The regions over which the SRO extends have a size of similar to 3 nm. Their fluctuation rates are in the GHz range. Modern ab initio cluster calculations successfully describe the magnetic hyperfine fields as well as the electric field gradient (EFG) tensor at the Fe sites. Covalency of the Fe-O and Zn-O bonds is important. The physical origin of the regions exhibiting SRO, however, remains unresolved at this point.