Emergence of ferromagnetism due to charge transfer in compressed ilmenite powder using super-high-energy ball milling

Ilmenite, FeTiO(3), is a common mineral in nature, existing as an accessory phase in the most basic igneous and metamorphic rocks, for example, it is derived from the upper mantle. Therefore, an understanding of the high-pressure physics of FeTiO(3) is of fundamental importance in the study of rock magnetization. Here, we provide experimental evidence of lattice compression of FeTiO(3) powder using super-high-energy ball milling, enabling the very high collision energy of 420 times gravitational acceleration. A sample obtained as an ilmenite- hematite 0.5FeTiO(3)·0.5Fe(2)O(3) solid solution showed a decrease in molar volume of approximately 1.8%. Consequently, the oxidation state in FeTiO(3) powder was changed into almost Fe(3+)Ti(3+), corresponding to 87% Fe(3+) of the total Fe for FeTiO(3), resulting in the emergence of ferromagnetism. This new ferromagnetic behaviour is of crucial importance in the study of rock magnetization which is used to interpret historical fluctuations in geomagnetism. In addition, the super-high-energy ball mill can be used to control a range of charge and spin states in transition metal oxides with high pressure.