Chemical and magnetic properties of rapidly cooled metastable ferri-ilmenite solid solutions: implications for magnetic self-reversal and exchange bias-I. Fe-Ti order transition in quenched synthetic ilmenite 61

Quenched ferri-ilmenite solid solutions X FeTiO ₃+ (1 -X) Fe ₂O ₃ with X≈ 0.60 contain chemical and magnetic structures important for understanding the unusual magnetic properties in this series, including self-reversal in igneous rocks and exchange bias. Here we study a composition X= 0.61, annealed at 1055°C, above the Fe-Ti ordering temperature, then quenched. Presence of two interface-coupled phases is established by pot-bellied character of the room-temperature magnetic hysteresis loop, and large negative magnetic exchange bias below 30K. Transmission electron microscopy (TEM) dark-field imaging with the 003 reflection shows dominant Fe-Ti disordered antiferromagnetic and lesser ordered ferrimagnetic phases, the latter in lenses ≤8 nm thick. Parts of the ordered phase are in antiphase relationship, shown by high-resolution TEM imaging of Fe-rich and Ti-rich layers. TEM-EDX analyses indicate chemical phase separation during quench, with dominant compositions X= 0.56-0.63, extremes 0.50 and 0.70. Thermomagnetic experiments indicate compositions X= 0.56-0.61 are antiferromagnetic, X= 0.61-0.64 are ferrimagnetic. A sample held ~5 min at 1063K, increased in order, demonstrated by twofold increase in induced moment at 1T. This then acquired self-reversed thermoremanent magnetization between 490 and 440K. Progressive annealings of another sample at 773K, 973K, 1023K and 1063K, followed by cooling in a 1T field, produced positive room-temperature magnetic exchange bias, only for 1023K and 1063K runs. These properties suggest growth of ordered regions from disordered regions, and expansion of some ordered domains against others across antiphase boundaries, thus creating self-organized structures essential for magnetic self-reversal and magnetic exchange bias.