The dynamics of magnetic ordering in a natural hemo‐ilmenite solid solution

SUMMARY We investigated the micromagnetic properties of hemo‐ilmenite particles in an alluvial soil. All magnetic accessory minerals except the weathering resistant hemo‐ilmenite grains were removed from the soil matrix by chemical treatment with concentrated acid followed by magnetic separation. X‐ray diffraction revealed hemo‐ilmenite grains with single crystal properties and an ilmenite mole fraction of y = 0.86 . Magnetization versus applied magnetic field plots in a temperature range between 6 and 300 K were recorded in order to study the hysteresis and the exchange properties. In addition, field and frequency‐dependent AC susceptibility measurements were performed with and without a DC bias field in order to analyse the dynamic magnetization of the sample down to 3 K. The hemo‐ilmenite particles are considered as a mixed system with nano‐sized cation‐ordered areas (COA) and cation‐disordered areas (CDA), which differ in their local Fe (III) concentration. Ferrimagnetic single‐domain order in the Fe (III) ‐enriched CDA started at about 220 K. Upon cooling gradual transdomain transformation generates multidomain order. A maximum in the blocking distribution was reached at 44 K, followed by the onset of spin‐glass dynamics. At lower temperature, blocking of superparamagnetic clusters in the COA created antiferromagnetic (AFM) ordering, which became more prominent with decreasing temperature. The interaction between the spin‐glass like CDA and the AFM areas was documented by the onset of exchange bias at T < 20 K . The occurrence of exchange bias as well as spin‐glass dynamics in the hemo‐ilmenite grains is probably an effect of the inhomogeneity of the local Fe (III) concentration. This effect leaves a magnetically competitive regime with areas showing ilmenite‐like magnetic properties, and Fe (III) rich disordered areas with magnetic long‐range ordering up to 220 K and frustration near the ordering temperature of ilmenite.