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Register a new userLongitudinal optical and spin-Hall conductivities of Rashba conducting strips coupled to ferro- and antiferromagnetic layers
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Jose A. Riera
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A system composed of a conducting planar strip with Rashba spin-orbit coupling (RSOC), magnetically coupled to a layer of localized magnetic moments, at equilibrium, is studied within a microscopic Hamiltonian with numerical techniques at zero temperature in the clean limit. In particular, transport properties for the cases of ferromagnetic (FM) and antiferromagnetic (AFM) coupled layers are computed in linear response on strips of varying width. Some behaviors observed for these properties are consistent with the ones observed for the corresponding Rashba helical currents. The case of uncoupled Rashba strips is also studied for comparison. In the case of Rashba strips coupled to an AFM localized order, results for the longitudinal dc conductivity, for small strip widths, suggest the proximity to a metal-insulator transition. More interesting, in the proximity of this transition, and in general at intermediate values of the RSOC, it is observed a large spin-Hall conductivity that is two orders of magnitude larger than the one for the FM order for the same values of the RSOC and strip widths. There are clearly two different regimes for small and for large RSOC, which is also present in the behavior of Rashba helical currents. Different contributions to the optical and the spin-Hall conductivities, according to a new classification of inter- or intra-band origin proposed for planar strips in the clean limit, or coming from the hopping or spin-orbit terms of the Hamiltonian, are examined. Finally, the effects of different orientation of the coupled magnetic moments will be also studied.
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Magnetic and transport properties of a conducting layer with Rashba spin-orbit coupling (RSOC) magnetically coupled to a layer of localized magnetic moments are studied on strips of varying width. The localized moments are free to rotate and they acquire an order that results from the competition between the magnetic exchange energy and the kinetic energy of the conduction electrons. By minimizing the total Hamiltonian within the manifold of variational spiral orders of the magnetic moments, the phase diagram in the space of the interlayer exchange J_{sd}, and the ratio of the Rashba coupling to the hopping integral, lambda/t was determined. Two main phases with longitudinal spiral order were found, one at large interlayer coupling J_{sd} with uniform order in the transversal direction, and the other at small J_{sd} showing a transversal staggered order. This staggered spiral order is unstable against an antiferromagnetic (AFM) for large values of lambda/t. In both spiral phases, the longitudinal spiral momentum that departs from the expected linear dependence with the RSOC for large values of lambda/t. Then, various transport properties, including the longitudinal Drude weight and the spin Hall conductivity, inside these two phases are computed in linear response, and their behavior is compared with the ones for the more well-studied cases of a fixed ferromagnetic (FM) and AFM localized magnetic orders.
We report an investigation of temperature and IrMn layered thickness dependence of anomalous-Hall resistance (AHR), anisotropic magnetoresistance (AMR), and magnetization on Pt/Ir20Mn80/Y3Fe5O12 (Pt/IrMn/YIG) heterostructures. The magnitude of AHR is dramatically enhanced compared with Pt/YIG bilayers. The enhancement is much more profound at higher temperatures and peaks at the IrMn thickness of 3 nm. The observed spin-Hall magnetoresistance (SMR) in the temperature range of 10-300 K indicates that the spin current generated in the Pt layer can penetrate the entire thickness of the IrMn layer to interact with the YIG layer. The lack of conventional anisotropic magnetoresistance (CAMR) implies that the insertion of the IrMn layer between Pt and YIG efficiently suppresses the magnetic proximity effect (MPE) on induced Pt moments by YIG. Our results suggest that the dual roles of the InMn insertion in Pt/IrMn/YIG heterostructures are to block the MPE and to transport the spin current between Pt and YIG layers. We discuss possible mechanisms for the enhanced AHR.
Two-leg spin-1/2 ladder systems consisting of a ferromagnetic leg and an antiferromagnetic leg are considered where the spins on the legs interact through antiferromagnetic rung couplings $J_1$. These ladders can have two geometrical arrangements either zigzag or normal ladder and these systems are frustrated irrespective of their geometry. This frustration gives rise to incommensurate spin density wave, dimer and spin fluid phases in the ground state. The magnetization in the systems decreases linearly with $J^2_1$, and the systems show an incommensurate phase for $0.0<J_1<1.0$. The spin-spin correlation functions in the incommensurate phase follow power law decay which is very similar to Heisenberg antiferromagnetic chain in external magnetic field. In large $J_1$ limit, the normal ladder behaves like a collection of singlet dimers, whereas the zigzag ladder behaves as a one dimensional spin-1/2 antiferromagnetic chain.
We study the texture of helical currents in metallic planar strips in the presence of Rashba spin-orbit coupling (RSOC) on the lattice at zero temperature. In the noninteracting case, and in the absence of external electromagnetic sources, we determine by exact numerical diagonalization of the single-particle Hamiltonian, the distribution across the strip section of these Rashba helical currents (RHC) as well as their sign oscillation, as a function of the RSOC strength, strip width, electron filling, and strip boundary conditions. Then, we study the effects of charge currents introduced into the system by an Aharonov-Bohm flux for the case of rings or by a voltage bias in the case of open strips. The former setup is studied by variational Monte Carlo, and the later, by the time-dependent density-matrix-renormalization group technique. Particularly for strips formed by two, three and four coupled chains, we show how these RHC vary in the presence of such induced charge current, and how their differences between spin-up and spin-down electron currents on each chain, help to explain the distribution across the strip of charge currents, both of the spin conserving and the spin flipping types. We also predict the appearance of polarized charge currents on each chain. Finally, we show that these Rashba helical currents and their derived features remain in the presence of an on-site Hubbard repulsion as long as the system remains metallic, at quarter filling, and even at half-filling where a Mott-Hubbard metal-insulator transition occurs for large Hubbard repulsion.
We investigate thermodynamic properties like specific heat $c_{V}$ and susceptibility $chi$ in anisotropic $J_1$-$J_2$ triangular quantum spin systems ($S=1/2$). As a universal tool we apply the finite temperature Lanczos method (FTLM) based on exact diagonalization of finite clusters with periodic boundary conditions. We use clusters up to $N=28$ sites where the thermodynamic limit behavior is already stably reproduced. As a reference we also present the full diagonalization of a small eight-site cluster. After introducing model and method we discuss our main results on $c_V(T)$ and $chi(T)$. We show the variation of peak position and peak height of these quantities as function of control parameter $J_2/J_1$. We demonstrate that maximum peak positions and heights in Neel phase and spiral phases are strongly asymmetric, much more than in the square lattice $J_1$-$J_2$ model. Our results also suggest a tendency to a second side maximum or shoulder formation at lower temperature for certain ranges of the control parameter. We finally explicitly determine the exchange model of the prominent triangular magnets Cs$_2$CuCl$_4$ and Cs$_{2}$CuBr$_{4}$ from our FTLM results.
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