Magnetization and anisotropy of cobalt ferrite thin films

Abstract

The magnetization of thin films of cobalt ferrite frequently falls far below the bulk value of 455kAm−1, which corresponds to an inverse cation distribution in the spinel structure with a significant orbital moment of about 0.6μB that is associated with the octahedrally coordinated Co2+ ions. The orbital moment is responsible for the magnetostriction and magnetocrystalline anisotropy and its sensitivity to imposed strain. We have systematically investigated the structure and magnetism of films produced by pulsed-laser deposition on different substrates (TiO2, MgO, MgAl2O4, SrTiO3, LSAT, LaAlO3) and as a function of temperature (500−700∘C) and oxygen pressure (10−4−10Pa). Magnetization at room-temperature ranges from 60 to 440kAm−1, and uniaxial substrate-induced anisotropy ranges from +220kJm−3 for films on deposited on MgO (100) to −2100kJm−3 for films deposited on MgAl2O4 (100), where the room-temperature anisotropy field reaches 14 T. No rearrangement of high-spin Fe3+ and Co2+ cations on tetrahedral and octahedral sites can reduce the magnetization below the bulk value, but a switch from Fe3+ and Co2+ to Fe2+ and low-spin Co3+ on octahedral sites will reduce the low-temperature magnetization to 120kAm−1, and a consequent reduction of Curie temperature can bring the room-temperature value to near zero. Possible reasons for the appearance of low-spin cobalt in the thin films are discussed.