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 455 kAm -1 , which corresponds to an inverse cation distribution in the spinel structure with a significant orbital moment of about 0.6 µB that is associated with the octahedrally-coordinated Co 2+ 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 -10 Pa). Magnetization at room-temperature ranges from 60 to 440 kAm -1 , and uniaxial substrate-induced anisotropy ranges from +220 kJm -3 for films on deposited on MgO (100) to -2100 kJm -3 for films deposited on MgAl2O4 (100), where the roomtemperature anisotropy field reaches 14 T. No rearrangement of high-spin Fe 3+ and Co 2+ cations on tetrahedral and octahedral sites can reduce the magnetization below the bulk value, but a switch from Fe 3+ and Co 2+ to Fe 2+ and low-spin Co 3+ on octahedral sites will reduce the lowtemperature magnetization to 120 kAm -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.

Ultrafast Spin Crossover in a Room-Temperature Ferrimagnet: Element-Specific Spin Dynamics in Photoexcited Cobalt Ferrite

“…This is observed in the XUV spectrum as a reduction of Fe 3+ to Fe 2+ and a corresponding oxidation of Co 2+ to Co 3+ . These oxidation state changes are evident 39 1.5 × 10 3 (16 K) 43 5.0 × 10 1 (5 K) 30 Saturation Magnetization / cm 3 mol −1 G 1.1 -2.2 × 10 5 (300 K) 39 6.4 × 10 3 (16 K) 43 6.9 × 10 4 (5 K) 30 Remanence / % 32 -35 (300 K) 39 33 ( To confirm this assignment, Figure 2 compares the transient spectrum with the results of similar measurements made on monometallic oxides, Fe 2 O 3 (red line) and Co 3 O 4 (blue line), which both experience a ligand-to-metal charge transfer and subsequent reduction of their respective M 3+ metal cations (M = Fe or Co) upon photoexcitation. 46,55 In the Fe 2 O 3 transient spectrum, this reduction is observed as an excited state absorption by Fe 2+ at 52.5 eV and a ground state bleach from Fe 3+ at 55 eV.…”

Enhanced piezomagnetic coefficient of cobalt ferrite ceramics by Ga and Mn doping for magnetoelectric applications

“…Magnetic metallic alloys or spinel oxides are used for the former component, while ferroelectric polymers or perovskite phases are preferred for the latter one. 2,[4][5][6][7][8] Interest in cofired ceramic composites has grown due to their low cost, easy manufacturing, possibility of co-sintering the magnetostrictive and piezoelectric oxide phases, and high electrical resistivity when compared to metallic alloys as Metglas or Terfenol-D. [9][10][11][12][13][14] Among magnetostrictive phases, CoF e 2 O 4 (CFO) stands out because of its very high magnetostriction (λ). 15 However, studies in ME composites formed by CFO-based compositions mostly resulted in ME coefficients lower than those using N iF e 2 O 4 -based magnetostrictive phases.…”

Large negative uniaxial magnetic anisotropy in highly distorted Co-ferrite thin films

“…CFO thin films deposited on various substrates have previously been investigated considering the various effects of lattice distortion inducing MA via the ME effect. [9][10][11] Unfortunately, the saturation magnetization value is usually far below the bulk value of 425 kA/m, which corresponds to an inverse cation distribution in the spinel structure with a significant orbital moment of approximately 0.6 l B that is associated with the octahedrally coordinated Co 2þ ions. 2 However, Niizeki et al recently developed a technique to grow high-quality CFO thin films on MgO(001) by reactive RF magnetron sputtering.…”