Ferromagnetic materials exhibit intrinsic `easy' and `hard' directions of the magnetization. This magnetic anisotropy is, from both a technological and fundamental viewpoint one of the most important properties of magnetic materials. The magnetic anisotropy in metallic magnetic multilayers forms the subject of this review article. As individual layers in a multilayer stack become thinner, the role of interfaces and surfaces may dominate that of the bulk: this is the case in many magnetic multilayers, where a perpendicular interface contribution to the magnetic anisotropy is capable of rotating the easy magnetization direction from in the film plane to perpendicular to the film plane. In this review, we show that the (in-plane) volume and (perpendicular) interface contribution to the magnetic anisotropy have been separated into terms related to mechanical stresses, crystallographic structure and the planar shape of the films. In addition, the effect of roughness, often inherent to the deposition techniques used, has been addressed theoretically. Several techniques to prepare multilayers and to characterize their growth as well as methods to determine the magnetic anisotropy are discussed. A comprehensive survey of experimental studies on the perpendicular magnetic anisotropy in metallic multilayers containing Fe, Co or Ni is presented and commented on. Two major subjects of this review are the extrinsic effects of strain, roughness and interdiffusion and the intrinsic effect of the crystallographic orientation on the magnetic anisotropy. Both effects are investigated with the help of some dedicated experimental studies. The results of the orientational dependence studies are compared with ab initio calculations. Finally, the perpendicular surface anisotropy and the in-plane step anisotropy are discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/59/11/002/pdf

Magnetic anisotropy in metallic multilayers

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

https://link.aps.org/accepted/10.1103/RevModPhys.89.025006

Interface-induced phenomena in magnetism

We prepared by vapor deposition at room temperature thin (500 A) Co/Pt multilayers or layered structures directly onto glass or Si substrates. They show a preferential magnetization perpendicular to the film plane for Co thicknesses below 12 A and a 100% perpendicular remanence for Co thicknesses below 4.5 A. The magnetic anisotropy can be explained by an interface contribution to the anisotropy. We also investigated the magneto‐optical (MO) polar Kerr effect of these multilayers. Because of their excellent magnetic properties and their potentially high oxidation and corrosion resistance, these Co/Pt‐layered structures are very promising candidates for MO recording. The Kerr rotation θk at λ=820 nm for a 35×(4.0 A Co+12.7 A Pt)‐layered structure, which has 100% magnetic remanence, is modest (−0.12°), but the reflectivity R is high (70%), which results in a respectable figure of merit Rθ2k. Furthermore, the Kerr effect increases towards shorter wavelengths and thus favors future higher‐density recording.

Perpendicular magnetic anisotropy and magneto-optical Kerr effect of vapor-deposited Co/Pt-layered structures

Magnetic anisotropy of multilayers

(1993). Ultrathin metallic magnetic films: magnetic anisotropies and exchange interactions. Advances in Physics: Vol. 42, No. 5, pp. 523-639.

Ultrathin metallic magnetic films: magnetic anisotropies and exchange interactions

Co-Au multilayers with Co layer thicknesses between 22 and 5 \AA{}A, prepared by ion-beam sputtering, have an easy-plane magnetic anisotropy Their saturation magnetization indicates the presence of diffuse interfaces, caused by ion-beam mixing Annealing of the multilayers at 250-300\ifmmode^\circ\else\textdegree\fi{}C leads to a perpendicular anisotropy for Co thicknesses below about 14 \AA{}A This effect is attributed to a stronlgy enhanced interface anisotropy, which is due to sharpening of the interfaces as revealed by x-ray diffraction

Perpendicular magnetic anisotropy of Co-Au multilayers induced by interface sharpening.

Superlattices of [001]fcc Co/Pd with varying Co thicknesses from one to eight atomic layers per modulation period were epitaxially grown on NaCl by vapour deposition in UHV. Transmission electron diffraction indicates lattice coherence between the Co and the Pd layers for Co thicknesses up to six atomic layers. If deposited at a substrate temperatureTs=50°C, only the superlattices containing Ci-monolayers show perpendicular magnetization. By raisingTs to 200°C, the perpendicular anisotropy for Co monolayers is increased, and is also observed for Co bilayers. We suggest that this is due tolayer smoothening, which increases Neel's interface anisotropy. For more than 6 atomic layers of Co a loss of coherence is observed atTs=50°C, accompanied by a structure transformation to hcp Co with a (0001)Co∥(111)Pd orientation.

A comparison of the magnetic anisotropy of [001] and [111] oriented Co/Pd Multilayers

Polycrystalline Pd/Co multilayers containing Co layers in the monolayer regime and having perpendicular magnetic anisotropy were subjected to diffusion anneals up to 400°C and to 600 keV Kr+ bombardment. As a result of Pd-Co mixing these treatments led to a decrease of the perpendicular anisotropy, coercivity and remanence. Diffusion was studied quantitatively by means of X-ray diffraction. The results indicate that diffusion occurs primarily along grain boundaries, which initially are migrating and finally become stationary. In this latter stage the activation energy of diffusion is 1.1 eV.

Effects of annealing and ion implantation on the magnetic properties of Pd/Co multilayers containing ultrathin Co

Preparation des superreseaux par depot en phase vapeur sous ultra-vide. Determination de la structure par rayons X. Etude de la variation de l'anisotropie perpendiculaire avec la temperature du support, dans le cas des superreseaux contenant des couches monomoleculaires de Co

D. Kuiper

Papers

Perpendicular magnetic anisotropy of Co-Au multilayers induced by interface sharpening.

A comparison of the magnetic anisotropy of [001] and [111] oriented Co/Pd Multilayers

Effects of annealing and ion implantation on the magnetic properties of Pd/Co multilayers containing ultrathin Co

STRUCTURE AND ANISOTROPY OF [001] Co/Pd ARTIFICIAL SUPERLATTICES