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

Statistical mechanics and dynamics of solvable models with long-range interactions

The contribution that the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behaviour of ultrathin single films is reviewed. Experimental methods to measure FMR in situ in ultrahigh vacuum are presented. The temperature dependence of the magnetization, of the magnetic relaxation rate in the vicinity of the Curie temperature, and of the second- and fourth-order magnetic anisotropy energy (MAE) constants can be measured by FMR in situ for magnetic monolayers. Using the cases of Ni/Cu(001) and Gd/W(110) as examples, the role of the MAE for the quantitative description of temperature- and thickness-dependent reorientation transitions of the magnetization is discussed. Initial results for the anisotropy of the g-factor which is related to the anisotropy of the orbital moment (and the MAE) are presented.

https://iopscience.iop.org/article/10.1088/0034-4885/61/7/001/pdf

Ferromagnetic resonance of ultrathin metallic layers

The manganese oxides of general formula RE1−xMxMnO3 (RE = rare earth, M = Ca, Sr, Ba, Pb) have remarkable interrelated structural, magnetic and transport properties induced by the mixed valence (3+–4+) of the Mn ions. In particular, they exhibit very large negative magnetoresistance, called colossal magnetoresistance (CMR), in the vicinity of metal–insulator transition for certain compositions. In this review paper, we summarize the most important features of the physics of the CMR manganites. The growth techniques for manganese oxide thin films, which are the basic material for potential applications, are reviewed and their structure and morphology examined in relation to growth parameters. The effects of epitaxial strains on the physical properties are discussed. Early works on superlattices and devices are presented.

http://iopscience.iop.org/article/10.1088/0022-3727/36/8/201/pdf

CMR manganites: physics, thin films and devices

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

Ferromagnetic resonance measurements in Co films of 11.3, 20, and 80 A\r{} thickness sandwiched by Au have been made as a function of the dc magnetic field orientation in a plane perpendicular to the film. These polycrystalline films were measured to have the hcp structure with the c axis perpendicular to the film plane within a few degrees. The experimental results are well explained by a theoretical model where an axial magnetic anisotropy up to the second-order term is included.

Ferromagnetic resonance studies of very thin cobalt films on a gold substrate.

Single crystals of ${\mathrm{La}}_{0.85}{\mathrm{Sr}}_{0.15}{\mathrm{MnO}}_{3\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ were prepared with various oxygen vacancies from \ensuremath{\delta}=0 to \ensuremath{\delta}=0.055. The $b$ and $c$ lattice parameters were found to increase with \ensuremath{\delta} whereas the $a$ parameter remains practically constant. The oxygen vacancies are expected to reduce the hole concentration, and therefore the Curie temperature and the electrical conductivity. However, these effects are somewhat different from those observed in the hole-doped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ systems, owing to different lattice distortion. The lattice change of the sample is illustrated by a strong shift in the hyperfine field of ${\mathrm{Mn}}^{4+}$ ions, detected with the NMR method. The oxygen deficiency and all the above properties resulting from it were shown to be reversible.

Influence of controlled oxygen vacancies on the magnetotransport and magnetostructural phenomena in La 0.85 Sr 0.15 MnO 3-δ single crystals

We have performed magnetic-susceptibility measurements and Cu nuclear-magnetic-resonance studies on single crystals of the quasi-one-dimensional antiferromagnet CuGe1-xSixO3, with 0 ≤ x ≤ 0.1. The Si doping has a drastic effect on the spin-Peierls temperature TSP which decreases at TSP (x)/TSP (0) 1-50x at low Si concentrations. For x ≥ 0.005, three-dimensional antiferromagnetic order with an easy axis along the Cu chains takes place below a well-defined Neel temperature TN. A qualitative interpretation of the phase diagram (T, x) is given.

https://iopscience.iop.org/article/10.1209/0295-5075/30/8/006/pdf

Competition between Spin-Peierls Phase and Three-Dimensional Antiferromagnetic Order in CuGe1-xSixO3

We report investigations on the structure and the magnetic properties of (${\mathrm{CH}}_{3}$${)}_{4}$NNi(${\mathrm{NO}}_{2}$${)}_{3}$. From extended x-ray-absorption fine structure and x-ray-diffraction experiments, we show that (${\mathrm{CH}}_{3}$${)}_{4}$NNi(${\mathrm{NO}}_{2}$${)}_{3}$ has a chainlike structure of Ni(II) ions coupled via three ${\mathrm{NO}}_{2}$ anions. High-temperature susceptibility is consistent with one-dimensional antiferromagnetism with J/k=-12 K and g=2.25. Below 10 K, a strong decrease of susceptibility and proton relaxation rate is observed, in agreement with the Haldane prediction of a singlet ground state for the S=1, one-dimensional Heisenberg antiferromagnet. From the magnetization-versus-field data at low temperature, the Haldane gap is estimated to 4.5 K, close to the expected value 0.41 \ensuremath{\Vert}J\ensuremath{\Vert} for the S=1, one-dimensional Heisenberg antiferromagnet.

Structural and magnetic properties of (CH3)4NNi(NO2)3: A Haldane-gap system.

A semiconductor-metal transition occurring just below the Curie temperature TC was observed in single crystals of perovskite La1-xSrxMnO3 for x>or=0.175. For x or=0.13 whereas huge MR persists well below TC for the x=0.10 sample, which is probably due to a non-collinear spin configuration.

https://iopscience.iop.org/article/10.1088/0953-8984/7/35/008/pdf

K. Le Dang

Papers

Ferromagnetic resonance studies of very thin cobalt films on a gold substrate.

Influence of controlled oxygen vacancies on the magnetotransport and magnetostructural phenomena in La 0.85 Sr 0.15 MnO 3-δ single crystals

Competition between Spin-Peierls Phase and Three-Dimensional Antiferromagnetic Order in CuGe1-xSixO3

Structural and magnetic properties of (CH3)4NNi(NO2)3: A Haldane-gap system.

Transport properties and magnetic behaviour of La1-xSrxMnO3 single crystals