The ability to reverse the magnetization of nanomagnets by current injection has attracted increased attention ever since the spin-transfer torque mechanism was predicted in 1996. In this paper, we review the basic theoretical and experimental arguments supporting a novel current-induced spin torque mechanism taking place in ferromagnetic (FM) materials. This effect, hereafter named spin–orbit (SO) torque, is produced by the flow of an electric current in a crystalline structure lacking inversion symmetry, which transfers orbital angular momentum from the lattice to the spin system owing to the combined action of SO and exchange coupling. SO torques are found to be prominent in both FM metal and semiconducting systems, allowing for great flexibility in adjusting their orientation and magnitude by proper material engineering. Further directions of research in this field are briefly outlined.

Individual Mn impurities deposited on Ge(100), Ge(111), and GaAs(110) substrates present magnetic moments significantly larger compared to the average Mn magnetization in bulklike ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$ and ${\mathrm{Mn}}_x{\mathrm{Ge}}_{1-x}$ dilute magnetic semiconductors. The Mn magnetic moment is shown to change considerably going from Ge(100), to GaAs(110), and Ge(111). Independently of the substrate, the Mn per atom moment decreases with increasing coverage owing to the formation of antiferromagnetic Mn clusters. We observe no evidence of magnetically ordered surface layers down to a temperature of $5\mathrm{K}$. The comparison of x-ray magnetic circular dichroism line shapes with that of a pure Mn $d^5$ configuration reveals the partial delocalization of the Mn $d$ states.

We show that double-resonance spectra recorded during the simultaneous absorption of x-ray and microwave (MW) photons are a fingerprint of the perturbed electronic configuration of atomic species driven to ferromagnetic resonance. X-ray absorption measurements performed as a function of x-ray energy and polarization over the $\text{Fe}{L}_{2,3}$ edges of single-crystal yttrium-iron garnet reveal MW-induced multiplet features related to angular momentum transfer from the MW field to localized $\text{Fe}3d$ magnetic sublevels. $\text{O}K$-edge absorption spectra demonstrate the formation of dynamic $2p$-orbital magnetization components at O sites coupled to the Fe magnetic moments at tetrahedral and octahedral sites. These results are compared with double-resonance x-ray absorption spectra of Permalloy, showing that the MW transition probability is distributed according to the hybridization character of the $3d$ states and proportional to the unperturbed unoccupied magnetic density of states of metals and insulators.

The magnetism of quench-condensed Ru and Rh impurities and metal films on Ag(100) and Pt(997) has been studied using x-ray magnetic circular dichroism. In the coverage range between 0.22 and 2.0 ML, no dichroic signal was detected at the $M_{3,2}$ absorption edges of Ru on Ag(100) at a temperature of $5\mathrm{K}$ in the presence of an applied magnetic field. The same was found for coverages between 0.12 and 0.5 ML of Rh on Ag(100) and Pt(997). It is concluded that the magnetic moments of single impurities, small clusters of various shape, and monolayers of the $4d$ metals are below the detection limit of $0.04{\mu }_B$ per atom. These results provide an unambiguous determination of the local magnetic moment of Ru and Rh deposited on nonmagnetic transition-metal surfaces, which are in contrast with theoretical predictions.

Correlations between magnetism and morphology of iron nanostructures of monatomic height on Pt(997) substrates are studied using x-ray magnetic circular dichroism as well as scanning tunneling microscopy and helium scattering. A drastic collapse of the average magnetization by more than a factor of 4 is observed when increasing the iron coverage from 0.1 to 0.2 ML. This effect goes along with a softening of the magnetic anisotropy energy and a gradual reorientation of the magnetic easy axis from in plane to out of plane. The experimental findings together with electronic density-functional calculations suggest the formation of a complex magnetic phase in corrugated rim regions of Fe islands, leading to both ferromagnetic and antiferromagnetic exchange couplings of Fe moments depending on their various local bonding configurations.