Interface-driven topological Hall effect in oxide bilayers

Electron transport coupled with magnetism has attracted attention over the years. Among them, recently discovered is topological Hall effect (THE), originating from scalar spin chirality, that is, the solid angle subtended by the spins. THE is found to be a promising tool for probing the Dzyaloshinskii-Moriya (DM) interaction and consequent magnetic skyrmions. This interaction arises from broken inversion symmetry and hence can be artificially introduced at interface; this concept is lately verified in metal multilayers. However, there are few attempts to investigate such DM interaction at interface through electron transport. We clarified how the transport properties couple with interface DM interaction by fabricating the epitaxial oxide interface. We observed THE in epitaxial bilayers consisting of ferromagnetic SrRuO3 and paramagnetic SrIrO3 over a wide region of both temperature and magnetic field. The magnitude of THE rapidly decreases with the thickness of SrRuO3, suggesting that the interface DM interaction plays a significant role. Such interaction is expected to realize a 10-nm-sized Néel-type magnetic skyrmion. The present results established that the high-quality oxide interface enables us to tune the effective DM interaction; this can be a step toward future topological electronics.

inteface-driven skymion
Generation of skymion through interfaces

J. Matsuno et al., Science Advances 2, e1600304 (2016).

Engineering a spin-orbital magnetic insulator by tailoring superlattices

In 5d Ir oxides with an interplay of spin-orbit coupling and electron correlations, we have tailored a spin-orbital magnetic insulator out of a semimetal SrIrO3 by tuning the structure through superlattices [(SrIrO3)m, SrTiO3] (m = 1,2,3,4, and ∞). We observed the systematic decrease of the magnetic ordering temperature and the resistivity as a function of m. The transition from the semimetal to the insulator is found to be closely linked to the appearance of magnetism at m = 3. Long range magnetic ordering was realized even in the m = 1 single layer superlattice, implying that the design and realization of novel electronic phases is feasible at the level of a single atomic layer in complex Ir oxides.

Changing the number of atomic layers of strontium iridate (green) sandwiched between single layers of strontium titanate (blue) in a superlattice offers a means of controlling the phase of the material.

J. Matsuno et al., Physical Review Letters 114, 247209 (2015).

5d iridium oxide as a material for spin-current detection

Devices based on pure spin currents have been attracting increasing attention as key ingredients for low-dissipation electronics. To integrate such spintronics devices into charge-based technologies, electric detection of spin currents is essential. The inverse spin Hall effect converts a spin current into an electric voltage through spin-orbit coupling. Noble metals such as Pt and Pd, and also Cu-based alloys, have been regarded as potential materials for a spin-current injector, owing to the large direct spin Hall effect. Their spin Hall resistivity ρSH, representing the performance as a detector, is not large enough, however, due mainly because of their low charge resistivity. Here we report that a binary 5d transition metal oxide, iridium oxide, overcomes the limitations encountered in noble metals and Cu-based alloys and shows a very large ρSH ∼ 38 µΩcm at room temperature.

spin Hall effect
Soin-orbit coupling and spin Hall effect

K. Fujiwara et al., Nature Communications 4, 2893 (2013).