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Register a new userNew pathways towards efficient metallic spin Hall spintronics
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Matthias Benjamin Jungfleisch
Publication date
2015
fields
Physics
and research's language is
English
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Spin Hall effects interconvert spin- and charge currents due to spin-orbit interaction, which enables convenient electrical generation and detection of diffusive spin currents and even collective spin excitations in magnetic solids. Here, we review recent experimental efforts exploring efficient spin Hall detector materials as well as new approaches to drive collective magnetization dynamics and to manipulate spin textures by spin Hall effects. These studies are also expected to impact practical spintronics applications beyond their significance in fundamental research.
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We investigate the spin Hall effect (SHE) in a wide class of spin-orbit coupling systems by using spin force picture. We derive the general relation equation between spin force and spin current and show that the longitudinal force component can induce a spin Hall current, from which we reproduce the spin Hall conductivity obtained previously using Kubos formula. This simple spin force picture gives a clear and intuitive explanation for SHE.
It is proposed that the new generation of spintronics should be ideally massless and dissipationless for the realization of ultra-fast and ultra-low-power spintronic devices. We demonstrate that the spin-gapless materials with linear energy dispersion are unique materials that can realize these massless and dissipationless states. Furthermore, we propose four new types of spin Hall effects which consist of spin accumulation of equal numbers of electrons and holes having the same or opposite spin polarization at the sample edge in Hall effect measurements, but with vanishing Hall voltage. These new Hall effects can be classified as (quantum) anomalous spin Hall effects. The physics for massless and dissipationless spintronics and the new spin Hall effects are presented for spin-gapless semiconductors with either linear or parabolic dispersion. New possible candidates for Dirac-type or parabolic type spin-gapless semiconductors are demonstrated in ferromagnetic monolayers of simple oxides with either honeycomb or square lattices.
We predict spin Hall angles up to 80% for ultrathin noble metal films with substitutional Bi impurities. The colossal spin Hall effect is caused by enhancement of the spin Hall conductivity in reduced sample dimension and a strong reduction of the charge conductivity by resonant impurity scattering. These findings can be exploited to create materials with high efficiency of charge to spin current conversion by strain engineering.
Spin-charge conversion via inverse spin Hall effect (ISHE) is essential for enabling various applications of spintronics. The spin Hall response usually follows a universal scaling relation with longitudinal electric resistivity and has mild temperature dependence because elementary excitations play only a minor role in resistivity and hence ISHE. Here we report that the ISHE of metallic glasses shows nearly two orders of magnitude enhancements with temperature increase from a threshold of 80-100 K to glass transition points. As electric resistivity changes only marginally in the temperature range, the anomalous temperature dependence is in defiance of the prevailing scaling law. Such a giant temperature enhancement can be well described by a two-level thermal excitation model of glasses and disappears after crystallization, suggesting a new mechanism which involves unique thermal excitations of glasses. This finding may pave new ways to achieve high spin-charge conversion efficiency at room and higher temperatures for spintronic devices and to detect structure and dynamics of glasses using spin currents.
Electric currents carrying a net spin polarization are widely used in spintronics, whereas globally spin-neutral currents are expected to play no role in spin-dependent phenomena. Here we show that, in contrast to this common expectation, spin-independent conductance in compensated antiferromagnets and normal metals can be efficiently exploited in spintronics, provided their magnetic space group symmetry supports a non-spin-degenerate Fermi surface. Due to their momentum-dependent spin polarization, such antiferromagnets can be used as active elements in antiferromagnetic tunnel junctions (AFMTJs) and produce a giant tunneling magnetoresistance (TMR) effect. Using RuO$_{2}$ as a representative compensated antiferromagnet exhibiting spin-independent conductance along the [001] direction but a non-spin-degenerate Fermi surface, we design a RuO$_{2}$/TiO$_{2}$/RuO$_{2}$ (001) AFMTJ, where a globally spin-neutral charge current is controlled by the relative orientation of the Neel vectors of the two RuO$_{2}$ electrodes, resulting in the TMR effect as large as ~500%. These results are expanded to normal metals which can be used as a counter electrode in AFMTJs with a single antiferromagnetic layer or other elements in spintronic devices. Our work uncovers an unexplored potential of the materials with no global spin polarization for utilizing them in spintronics.
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