ATOMIC GEOMETRY AND MAGNETISM IN FERRI‐ILMENITE SOLID SOLUTIONS: MAGNETIC SELF-REVERSAL AND EXCHANGE BIAS

Ferri-ilmenite solid solutions xFeTiO₃-(1-x)Fe₂O₃ with x=0.6-0.7 (Ilm60 – Ilm70) display two exotic magnetic properties: self-reversal during cooling in an external field, and exchange bias which generates asymmetry of the magnetic hysteresis loop. Both magnetic behaviors reflect minute details of the geometric arrangement of iron and titanium on the atomic level. The intricate relationship between ordered, anti-ordered and disordered domains inside ferri-ilmenite solid solutions was a major theme of Peter Robinsons recent work. Chemical phase separation during quenching is driven by the disorder/order transition at temperatures above the tricritical point and by spinodal decomposition at temperatures below the tricritical point. The former explains microtextures and chemical features in samples quenched from high temperature; the latter produces textural and chemical evolution during subsequent annealing. Geometrically, the annealing of quenched Ilm60-Ilm70 leads to coarsening of ordered and anti-ordered domains until they impinge along antiphase domain boundaries. Growth of ordered or anti-ordered phase regions reduces the surface area of antiphase domain boundaries, such that Fe becomes enriched in shrinking domains and depleted in growing domains. Within each synthetic sample, nano-scale variations in local composition x and ordering state Q depend on minute details of the cooling and annealing history, and strongly influence the magnetic properties. Using MPMS measurements up to 7 T, and high-field measurements up to 60 T, together with TEM images we characterize nanostructure formation in terms of average composition and order parameters <x>, <Q>. These characteristics are then linked to the magnetic properties, based on average mean-field models for the saturation magnetization M_s(T). The high-field measurements prove and quantify the extremely strong exchange coupling in nanotextured ferri-ilmenite that is required to explain the observed exchange bias and self-reversal, and are necessary to obtain the first reliable data of <Q> for quenched ferri-ilmenite solid solutions.