Sign change of the spin Hall effect due to electron correlation in nonmagnetic CuIr alloys.

Recently, a positive spin Hall angle (SHA) of 0.021 was observed experimentally in nonmagnetic CuIr alloys [Niimi et al, Phys. Rev. Lett. 106, 126601 (2011)] and attributed predominantly to an extrinsic skew scattering mechanism, while a negative SHA was obtained from ab initio calculations [Fedorov et al, Phys. Rev. B 88, 085116 (2013)], using consistent definitions of the SHA. We reconsider the SHA in CuIr alloys, with the effects of the local electron correlation U in 5d orbitals of Ir impurities, included by the quantum Monte Carlo method. We found that the SHA is negative if we ignore such local electron correlation, but becomes positive once U approaches a realistic value. This may open up a way to control the sign of the SHA by manipulating the occupation number of impurities.

Temperature can enhance coherent oscillations at a Landau-Zener transition.

We consider sweeping a system through a Landau-Zener avoided crossing, when that system is also coupled to an environment or noise. Unsurprisingly, we find that decoherence suppresses the coherent oscillations of quantum superpositions of system states, as superpositions decohere into mixed states. However, we also find an effect we call "Lamb-assisted coherent oscillations," in which a Lamb shift exponentially enhances the coherent-oscillation amplitude. This dominates for high-frequency environments such as super-Ohmic environments, where the coherent oscillations can grow exponentially as either the environment coupling or temperature are increased. The effect could be used as an experimental probe for high-frequency environments in such systems as molecular magnets, solid-state qubits, spin-polarized gases (neutrons or He3), or Bose condensates.

Quantum renormalization of the spin Hall effect.

By quantum Monte Carlo simulation of a realistic multiorbital Anderson impurity model, we study the spin-orbit interaction (SOI) of an Fe impurity in Au host metal. We show, for the first time, that the SOI is strongly renormalized by the quantum spin fluctuation. Based on this mechanism, we can explain why the gigantic spin Hall effect in Au with Fe impurities was observed in recent experiments, while it is not visible in the anomalous Hall effect. In addition, we show that the SOI is strongly renormalized by the Coulomb correlation U. Based on this picture, we can explain past discrepancies in the calculated orbital angular momenta for an Fe impurity in an Au host.

Author Correction: Record thermopower found in an IrMn-based spintronic stack.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Record thermopower found in an IrMn-based spintronic stack.

The Seebeck effect converts thermal gradients into electricity. As an approach to power technologies in the current Internet-of-Things era, on-chip energy harvesting is highly attractive, and to be effective, demands thin film materials with large Seebeck coefficients. In spintronics, the antiferromagnetic metal IrMn has been used as the pinning layer in magnetic tunnel junctions that form building blocks for magnetic random access memories and magnetic sensors. Spin pumping experiments revealed that IrMn Néel temperature is thickness-dependent and approaches room temperature when the layer is thin. Here, we report that the Seebeck coefficient is maximum at the Néel temperature of IrMn of 0.6 to 4.0 nm in thickness in IrMn-based half magnetic tunnel junctions. We obtain a record Seebeck coefficient 390 (±10) µV K-1 at room temperature. Our results demonstrate that IrMn-based magnetic devices could harvest the heat dissipation for magnetic sensors, thus contributing to the Power-of-Things paradigm.

Model for vacancy-induced d0 ferromagnetism in oxide compounds.

We propose a model with few parameters for vacancy-induced ferromagnetism based on a correlated model for oxygen orbitals with random potentials representing cation vacancies. For certain potentials, moments appear on oxygen sites near defects. Treating the randomness exactly, we calculate the magnetic couplings between moments, the Curie temperature and spin and charge densities as a function of the potential, the density of vacancies, and correlation strength. For physically reasonable parameters this predicts Curie temperatures well above room temperature for small concentrations of vacancies. We discuss our results in relation to questions of stability and reproducibility raised in experiments. To circumvent the difficulties of controlling intrinsic defects, we propose specific nonmagnetic host doping that could be, for example, substituted for cations in HfO2 or ZrO2.