Coercivity of Fe2+in octahedral sites of Fe-Ti spinels

Abstract

High coercivities (∼1.8 to 8.1 kOe) have been observed in polycrystalline titanium substituted magnetites at 77°K. This property and unusually large ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sim10^{7}</tex> ergs. cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ) anisotropy constants observed by others in single crystals of the same compound are interpreted on the basis of a new model for cation distribution. Electrical resistivity and low temperature saturation magnetization measurements confirm this model, according to which new Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions occupy tetrahedral sites for most part of the solid solution series Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-2x</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+x</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> +Ti <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-</sup> . This causes a negative trigonal field to be superimposed on octahedral Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions, hence a twofold degeneracy of the orbital ground state resulting in an unquenched orbital angular momentum and a large anisotropy. Mössbauer absorption spectra provide direct confirmation of the trigonal distortion in the shape of a large quadrupole splitting. For definite proof of the twofold degeneracy, however, such polycrystalline specimens with disordered octahedral cations and ultrabroad magnetic spectra are not useful. It is hoped that single crystal work would prove more definitive.