Magnetic properties of hematite with large coercivity

P. G. BERCOFF; H. R. BERTORELLO

APPLIED PHYSICS. A - MATERIALS SCIENCE AND PROCESSING

SPRINGER

Año: 2010 vol. 100 p. 1019 - 1027

0947-8396

The magnetic properties of hematite powders produced by asolid state nucleation-and-growth process are studied as afunction of temperature $T$ and applied field $H$. Independentlyof the temperature, there exists a soft magnetic contribution thatis assigned to the canting of spins at the superficial shell ofeach particle and is not affected by the Morin transition. At $220< T < T_M$ a magnetic contribution with high coercivity isobserved, due to spin--flop in the anti ferromagnetic state andabove $T_M=248$ K the weakly ferromagnetic state has a coercivitythat ranges from 6 kOe to 4 kOe when raising $T$ up to roomtemperature. Different sub-grain structures were obtained by meansof isochronal and isothermal annealing. Changes in thesusceptibility are directly related to the sub-particle size. Itis concluded that sub-boundaries are the defects responsible forthe high coercivities observed in the weakly ferromagnetic state.