Reorientation of magnetic anisotropy in epitaxial cobalt ferrite thin films

Abstract

Spin reorientation has been observed in CoFe 2 O 4 thin single crystalline films epitaxially grown on ͑100͒ MgO substrate upon varying the film thickness. The critical thickness for such a spin-reorientation transition was estimated to be 300 nm. The reorientation is driven by a structural transition in the film from a tetragonal to cubic symmetry. At low thickness, the in-plane tensile stress induces a tetragonal distortion of the lattice that generates a perpendicular anisotropy, large enough to overcome the shape anisotropy and to stabilize the magnetization easy axis out of plane. However, in thicker films, the lattice relaxation toward the cubic structure of the bulk allows the shape anisotropy to force the magnetization to be in plane aligned. The importance of magnetic anisotropy is well recognized in many technical applications such as magnetic and magneto-optic recording. The large interest for high anisotropies is motivated by technological demands such as increasing the magnetic recording density. With large anisotropy, the superparamagnetic limit can be pushed down, and a stable magnetization can be promoted in ultrasmall nanosized magnetic structures, which are needed in advanced media for ultrahigh density recording. Besides the intrinsic anisotropy of the bulk, other sources of anisotropy may be enhanced in artificial structures and contribute to their magnetic properties. Depending on their relative orientations and magnitudes, the involved anisotropies may compete between each other, leading to spin-reorientation phenomena in the system. For example, the broken symmetry at the interfaces in ultrathin films generates a perpendicular anisotropy, which overcomes the shape anisotropy.1 However, increasing the layer thickness reduces the ratio between the surface and the volume atoms, leading to an in-plane alignment of the easy axis.2 In obliquely sputtered metallic thin films, we established the existence of an in-plane reorientation of magnetic anisotropy.3 Depending on the film thickness and due to the shadow effect during the growth, the layer can develop columns or nuclei able to confine the anisotropy parallel or perpendicular to the longitudinal direction ͑projection of the incident beam in the film plane͒.Ferrites cover a large family of oxides, including soft as well as hard magnetic materials. Hard ferrites such as the hexagonal ͑BaFe 12 O 19 ͒ and the spinel ͑CoFe 2 O 4 ͒ are particularly attractive for magnetic and magneto-optic recording applications due to their large magnetocrystalline anisotropy and high chemical stability. Recent studies demonstrated that integrating cobalt ferrite as a pinning layer in the spin valve architecture can strongly enhance the magnetoresistance effect of the sandwiched structure. 4 In epitaxial hexaferrite thin films, the uniaxial magnetocrystalline anisotropy is strong enough to dominate all the other sources of anisotropy and to keep the spin alignment constant regardless the film thickness and the preparation conditions. 5 However, cobalt ferrite rep...

The magnetization of epitaxial nanometric CoFe2O4(001) layers

“…[10][11][12][13] These results are both compatible with the reported negative magnetostriction constant ͓100͔ Ͻ 0. On the other hand, it has been claimed that the antiphase boundaries ͑APBs͒ are the origin of the reduced magnetization and differential susceptibility quite often observed in ferrimagnetic spinel thin films.…”

“…For instance, the recent observation of a spin reorientation from out-of-plane to in-plane type of anisotropy that takes place when increasing the thickness of CFO films grown on MgO has been interpreted as due to the dominating role of lattice strain. 13 Other than this, quite often the hysteresis loops of CFO thin films display an unexpected shrinking at low fields, [8][9][10]14 which although being of relevance for any practical application of CFO-based devices, has remained unexplained.…”

Spin Hall Magnetoresistance as a Probe for Surface Magnetization inPt/CoFe2O4Bilayers

“…Figure 5a shows the hysteresis loops M(H) of the Pt(2 nm)/CFO(40 nm) sample obtained by VSM when applying the magnetic field H along t and n directions at 50 K. As can be seen from the hysteresis loop when the field is applied in plane, M(H||t) curve, the large coercive fields  0 H c (t) ≈ ±1.2 T and the fact that hysteresis only disappears at ≈ 5 T are signatures of the strong magnetic anisotropy typical of CFO thin films [54,55]. The shape of the hysteresis loop when the field is applied out of plane, M(H||n), indicates a harder magnetization axis and, correspondingly, the coercive field  0 H c (n) ≈ ±0.44 T and the magnetic remanence are smaller.…”

Observation of longitudinal spin-Seebeck effect in cobalt-ferrite epitaxial thin films

“…22 Therefore, the strength and direction of a magnetic anisotropy in CFO can be tailored simply by tuning the composition of Co and/or the strain (e.g., a compressive strain in the [100] direction results in the formation of a magnetic easy axis in the CFO film due to the inverse effect of the magnetostriction). [23][24][25][26][27][28] In this study, we focus on epitaxially-strained CFO thin films as anisotropy-tailored SSE materials, which meet a recent demand for external-field-free TE generation.…”