Electrical currents at the surface or edge of a topological insulator are intrinsically spin-polarized. We show that such surface/edge currents can be used to switch the orientation of a molecular magnet weakly coupled to the surface or edge of a topological insulator. For the edge of a two-dimensional topological insulator as well as for the surface of a three-dimensional topological insulator the application of a well-chosen surface/edge current can lead to a complete polarization of the molecule if the molecule's magnetic anisotropy axis is appropriately aligned with the current direction. For a generic orientation of the molecule a nonzero but incomplete polarization is obtained. We calculate the probability distribution of the magnetic states and the switching rates as a function of the applied current.
Harnessing the electron's second fundamental property, its spin, is the basis of spintronic phenomena and devices [1] . These include recently discovered phenomena like the quantum anomalous Hall effect [2] in magnetic topological insulators [3] , spin transfer torque [4,5] effects in nonmagnetic metal / ferromagnetic metal / oxide heterostructures and spin transfer torque (SOT) switching of ferromagnets [6,7] , and ultimately of FMIs [8] . To realize novel circuit devices based on these effects a rich variety of specifically tailored magnetic materials has still to be developed. For instance, the study of exotic phenomena occurring at the boundary of topological insulators with ferromagnets requires the latter to be insulating, yet to retain magnetic properties including PMA.One of the most prominent FMIs classes is that of iron garnets, of which the most well studied is Y 3 Fe 5 O 12 (YIG). The ultra low magnon damping characteristics [9] and magneto-optical properties [10,11] of YIG are well known. The former makes YIG a suitable candidate for spin wave logic [12] and signal transmitters [13] due to the extremely large magnon decay length of several tens of millimeters. Epitaxial YIG thin films can, in principle, also possess PMA as a result of magnetization-lattice coupling [14] for thicknesses below nm [15,16] , but the fabrication of YIG films with complete out-of-plane remanence remains elusive because of its low magnetocrystalline anisotropy and magnetoelastic coefficients. In contrast, 50 nm thick thulium iron garnet (Tm 3 Fe 5 O 12 , TmIG), has been reported to show PMA [17,18] caused by magnetoelastic anisotropy when grown epitaxially on (111)-oriented gallium gadolinium garnet (Gd 3 Ga 5 O 12 or GGG) [19] .We recently demonstrated [8] reversible magnetization switching in 8 nm thick TmIG by utilizing SOT from an adjacent platinum layer, but a detailed structural and magnetic characterization of TmIG in the few-nm thickness regime is still lacking. In the present article, we provide a comprehensive description of the structural characteristics and magnetic properties of TmIG/GGG down to a thickness of nm. Furthermore we demonstrate that efficient spin transport can be achieved through the TmIG/Pt interface by measuring spin Hall magnetoresistance (SMR) in Pt. We exploit this method to measure the anisotropy field of the strained TmIG film electrically, which is inaccessible by conventional magnetometry measurements due to the dominant paramagnetic contribution of the GGG substrate. These results emphasize the potential of TmIG as a spintronic material.Structural characterization of the TmIG films are summarized in Fig.1. With xray reflectometry (XRR) scans (not shown), we measured film thicknesses of PMA epitaxial TmIG down to nm nm . To quantify the strain state of TmIG via XRD we measured a nm thick TmIG film, since thinner films could not be resolved with enough intensity using high resolution XRD. The symmetric XRD spectra shown in Fig.1a demonstrates a fully strained film with lattice spacing of ...
High quality epitaxial Sr 2 IrO 4 thin films with various thicknesses (9-300 nm) have been grown on SrTiO 3 (001) substrates, and their electric transport properties have been investigated. All samples showed the expected insulating behavior with a strong resistivity dependence on film thickness, that can be as large as three orders of magnitude at low temperature. A close examination of the transport data revealed interesting crossover behaviors for the conduction mechanism upon variation of thickness and temperature. While Mott variable range hopping (VRH) dominated the transport for films thinner than 85 nm, high temperature (>200 K) thermal activation behavior was observed for films with large thickness (≥85 nm), which was followed by a crossover from Mott to Efros-Shklovskii (ES) VRH in the low temperature range. This low temperature crossover from Mott to ES VRH indicates the presence of a Coulomb gap (~3 meV).Our results demonstrate the competing and tunable conduction in Sr 2 IrO 4 thin films, which in turn would be helpful for understanding the insulating nature related to strong spin-orbitalcoupling of the 5d iridates. In recent years, oxides with the 5d-element Ir have been established as a fertile ground for studying new physics arising from the large relativistic spin-orbit-coupling (SOC) [1][2][3]. So far, a variety of exotic properties related to the strong SOC such as intriguing insulating behavior [3][4][5][6][7][8][9][10] In addition to the newly raised controversy on the insulating nature of SIO, the conduction mechanism of the insulating state has been a long-standing issue. In an early work, variable range hopping (VRH) conduction was reported in single crystalline SIO [3]. A recent study also focusing on SIO single crystals revealed that the conduction well follows the thermal activation mechanism with an energy gap of Δ~107 meV perfectly matching the value of optical measurements [1,12]. Further investigations of the same group showed a VRH dominated conduction in doped SIO even though the doping content was small [19,20]. In Ba 2 IrO 4 (BIO), although it highly resembles SIO in terms of structural and electronic properties, good VRH conduction was observed in the whole measured temperature (T) range. Moreover this type of conduction is robust against carrier doping [6], which is very different from the SIO case. In fact, a close examination of the conduction mechanism in SIO and BIO can provide some more interesting details. While the doped SIO shows 3-dimentional (3D) VRH conduction [19,20] [19], which could probably be related to the SOC. These show intriguing conduction behavior in the single layered iridates, pointing to a rather poor understanding of the conduction mechanism in SIO. Moreover, this issue became even more puzzling in the case of SIO thin films, in which the thermal activation model was frequently used to determine the energy gap of SIO, although a linear fit on a plot of lnρ-1/T is questionable [21,22]. Therefore, it is of high interest and actuality to compreh...
We report fast and efficient current-induced switching of a perpendicular anisotropy magnetic insulator thulium iron garnet by using spin-orbit torques (SOT) from the Pt overlayer. We first show that, with quasi-DC (10 ms) current pulses, SOT-induced switching can be achieved with an external field as low as 2 Oe, making TmIG an outstanding candidate to realize efficient switching in heterostructures that produce moderate stray fields without requiring an external field. We then demonstrate deterministic switching with fast current pulses (≤20 ns) with an amplitude of ∼1012 A/m2, similar to all-metallic structures. We reveal that, in the presence of an initially nucleated domain, the critical switching current is reduced by up to a factor of five with respect to the fully saturated initial state, implying efficient current-driven domain wall motion in this system. Based on measurements with 2 ns-long pulses, we estimate the domain wall velocity of the order of ∼400 m/s per j = 1012 A/m2.
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