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

Details 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 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

Sep 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011geoji.186..997f&link_type=abstract

Geophysical Journal International, Volume 186, Issue 3, pp. 997-1014.

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1

Magnetic Mineralogy And Petrology, Rock And Mineral Magnetism, Microstructures, Phase Transitions

Scientific paper

Quenched ferri-ilmenite solid solutions X FeTiO3+ (1 -X) Fe2O3 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 30 K. 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 1063 K, increased in order, demonstrated by twofold increase in induced moment at 1 T. This then acquired self-reversed thermoremanent magnetization between 490 and 440 K. Progressive annealings of another sample at 773 K, 973 K, 1023 K and 1063 K, followed by cooling in a 1 T field, produced positive room-temperature magnetic exchange bias, only for 1023 K and 1063 K 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.

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