Magnetization and anisotropy of cobalt ferrite thin films

The magnetization of thin films of cobalt ferrite frequently falls far below the bulk value of 4550.16em0exkAm^-1, which corresponds to an inverse cation distribution in the spinel structure with a significant orbital moment of about 0.60.16em0exμB 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 (500-7000.16em0ex^∘C) and oxygen pressure (10^-4-100.16em0exPa). Magnetization at room-temperature ranges from 60 to 4400.16em0exkAm^-1, and uniaxial substrate-induced anisotropy ranges from +2200.16em0exkJm^-3 for films on deposited on MgO (100) to -21000.16em0exkJm^-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 1200.16em0exkAm^-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.