Spin electronics is a rapidly expanding field stimulated by a strong synergy between breakthrough basic research discoveries and industrial applications in the fields of magnetic recording, magnetic field sensors, nonvolatile memories [magnetic random access memories (MRAM) and especially spin-transfer-torque MRAM (STT-MRAM)]. In addition to the discovery of several physical phenomena (giant magnetoresistance, tunnel magnetoresistance, spin-transfer torque, spin-orbit torque, spin Hall effect, spin Seebeck effect, etc.), outstanding progress has been made on the growth and nanopatterning of magnetic multilayered films and nanostructures in which these phenomena are observed. Magnetic anisotropy is usually observed in materials that have large spin-orbit interactions. However, in 2002 perpendicular magnetic anisotropy (PMA) was discovered to exist at magnetic
metal/oxide interfaces [for instance Co(Fe)/alumina]. Surprisingly, this PMA is observed in systems where spin-orbit interactions are quite weak, but its amplitude is remarkably large—comparable to that measured at Co/Pt interfaces, a reference for large interfacial anisotropy (anisotropy ∼1.4 erg/cm2 = 1.4 mJ/m2). Actually, this PMA was found to be very common at magnetic metal/oxide interfaces since it has been observed with a large variety of amorphous or crystalline oxides, including AlOx, MgO, TaOx, HfOx, etc. This PMA is thought to be the result of electronic hybridization between the oxygen and the magnetic transition metal orbit across the interface, a hypothesis supported by ab initio calculations. Interest in this phenomenon was sparked in 2010 when it was demonstrated that the PMA at magnetic transition metal/oxide interfaces could be used to build out-of-plane magnetized magnetic tunnel junctions for STT-MRAM cells. In these systems, the PMA at the CoFeB/MgO interface can be used to simultaneously obtain good memory retention, thanks to the large PMA amplitude, and a low write current, thanks to a relatively weak Gilbert damping. These two requirements for memories tend to be difficult to reconcile since they rely on the same spin-orbit coupling. PMA-based approaches have now become ubiquitous in the designs for perpendicular STT-MRAM, and major microelectronics companies are actively working on their development with the first goal of addressing embedded FLASH and static random access memory-type of applications. Scalability of STT-MRAM devices based on this interfacial PMA is expected to soon exceed the 20-nm nodes. Several very active new fields of research also rely on interfacial PMA at magnetic metal/oxide interfaces, including spin-orbit torques associated with Rashba or spin Hall effects, record high speed domain wall propagation in buffer/magnetic metal/oxide-based magnetic wires, and voltage-based control of anisotropy. This review deals with PMA at magnetic metal/oxide interfaces from its discovery, by examining the diversity of systems in which it has been observed and the physicochemical methods through which the key roles played by the electronic hybridization at the metal/oxide interface were elucidated. The physical origins of the phenomenon are also covered and how these are supported by ab initio calculations is dealt with. Finally, some examples of applications of this interfacial PMA in STT-MRAM are listed along with the various emerging research topics taking advantage of this PMA.

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Perpendicular magnetic anisotropy at transition metal/oxide interfaces and applications

We investigate the relation between the magnetocrystalline anisotropy energy (MAE) and the electronic structure for transition metal thin films and surfaces which can display enhanced orbital magnetic moments. When the spin-orbit interaction is treated in second order, the MAE is proportional to the expectation value of the orbital magnetic moment as given by Bruno's model. However, there are additional terms which are related to the spin-subband orbital moment and to the magnetic dipole operator due to the anisotropy of the field of the spin. The latter term accounts for the spin-flip excitations between the exchange split majority and minority spin bands. A conjecture is proposed which relates the MAE to the expectation values of the orbital moments and the magnetic dipole term. It is shown how the different terms can be obtained experimentally with (transverse) magnetic circular x-ray dichroism. The model explains the experimentally observed perpendicular magnetic anisotropy in Co and Fe based multilayers and thin films.

https://iopscience.iop.org/article/10.1088/0953-8984/10/14/012/pdf

Microscopic origin of magnetocrystalline anisotropy in transition metal thin films

Electronic structure theory of surface, interface and thin-film magnetism

Magnetic surface anisotropies

The demagnetizing field and magnetostatic energy of a thin film with surface roughness has been calculated. It is shown that the surface roughness gives rise to an effective perpendicular anisotropy whose order of magnitude is evaluated as a function of the parameters characterizing the roughness. The results are discussed in connection with experimental situations.

Dipolar magnetic surface anisotropy in ferromagnetic thin films with interfacial roughness

A model calculation is presented for the computation of the magnetic surface anisotropy ${K}_{s}$ for transition metals Special reference is made to Ni It is demonstrated that the changes at the surface in the effective atomic potential of the different $d$ orbitals are very important in determining ${K}_{s}$ Furthermore it is shown that one must include all $d$ orbitals in the calculations The problem is studied by successively treating the weak-hopping limit, the one-layer problem, and the surface of a semi-infinite crystal The sign and the order of magnitude of the magnetic surface anisotropy which we find for Ni agree with the experimental results on Ni alloys A comparison is made to earlier work on the subject

Klaus-Peter Bohnen

Papers

Magnetic surface anisotropy of transition metals