Dynamic short-range order observed in the (Zn)[Fe2]O4 spinel by neutron diffraction, μSR, and Mössbauer experiments

Abstract

Using neutron diffraction (ND), muon-spin rotation/relaxation (μSR), and57Fe-Mossbauer spectroscopy (MS) we have investigated magnetic properties of the normal spinel (Zn)[Fe2]O4. In compounds which are slowly cooled from 1200°C to room temperature inversion is below detection limits. AtT 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 andμSR experiments show that already at temperatures of ∼ 10T N a short-range antiferromagnetic ordering (SRO) develops which extends through ∼70% of the sample volume just aboveT N . BelowT N SRO and LRO coexist. At 4.2 K still ∼25% of the sample is short-range ordered. The regions over which the SRO extends have a size of ∼ 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.