Session K18: Spin-Hall III

Anomalous Hall effect in antiferromagnetic GdPtBi

The Berry phase of the electronic wave function is responsible for a transverse velocity of conducting carriers which results in anomalous Hall conductivity. This effect has been extensively investigated in ferromagnetic systems, but less is known in antiferromagnets. We have synthesized single crystals of GdPtBi, a metallic system which exhibits antiferromagnetic ordering below the transition temperature $T_{\rm N}$ = 9 K. We have investigated the electrical transport and magnetic properties of these crystals and found a distinct anomalous Hall effect response. We will discuss these observations in the context of the known mechanisms for anomalous velocity in ferromagnets and recent models unique to antiferromagnetic systems.   

Spin transport in antiferromagnetic insulator detected by spin pumping

Spin transport in antiferromagnetic insulators has drawn attention recently. Prior work has been done on the spin diffusion length of different antiferromagnetic materials via inverse spin hall effect. In this work, we measure the spin pumping of Py/Cu/CoO to characterize the absorption of spin current in the CoO layer. The series of Py/Cu/CoO (t) with changing the thickness of CoO layer indicates that there is a Gilbert damping enhancement of 0.001 in saturation at about 2 nm at room temperature. The spin mixing conductance obtained from this experimental series and from Py (t)/Cu/CoO series is 2.4 $nm^{-2}$ and 3.2 $nm^{-2}$, respectively. We also measured the spin pumping of the Py/Cu/CoO sample at low temperatures. The Gilbert damping exhibits a positive peak at about 280 K, which is close to the Néel temperature of CoO. Our work shows a finite spin mixing conductance in Py/Cu/CoO and the spin diffusion length of CoO is quite small at room temperature. We also find that its Gilbert damping reaches its maximum value at Néel temperature.   

Spin current control of damping in YIG/Pt nanowires

Understanding of spin transport at ferromagnet/normal metal interfaces is of great importance for spintronics applications. We report the effect of pure spin currents in YIG(30 nm)/Pt(6 nm) nanowires. The samples show magneto-resistance from two distinct mechanisms: (i) spin Hall magnetoresistance (SMR) and (ii) inverse spin Hall effect (iSHE) along with spin Seebeck current (SSC) induced by Ohmic heating of the Pt layer. Using the SMR and iSHE effects, we measure the spin wave eigenmodes by spin-torque ferromagnetic resonance (ST-FMR). Direct current applied to the Pt layer results in injection of spin Hall current into YIG that acts as damping or anti-damping spin torque depending on the polarity. In addition, Ohmic heating gives rise to a SSC acting as anti-damping regardless of current polarity. ST-FMR reveals current-induced variation of the spin wave mode linewidth that is asymmetric in the bias current and decreases to zero for anti-damping spin Hall current. Near this current, we observe complex interaction among the spin wave eigenmodes that we asses using micromagnetic simulations. Our results advance understanding of magnetization dynamics driven by pure spin currents.   

Spin Circuit Representation of Spin Pumping in Topological Insulators

Earlier we developed spin circuit representation of spin pumping and combined it with the spin circuit representation for the inverse spin Hall effect to show that it reproduces the established results in literature [1]. Here we construct the spin circuit representation of spin pumping in topological insulators. The discovery of spin-polarized surface states in three-dimensional (3D) topological insulators (TIs) with strong spin-orbit coupling is promising for the development of spintronics. There is considerable bulk conduction too in 3D TIs (e.g., Bi$_{\mathrm{2}}$Se$_{\mathrm{3}})$ apart from possessing the surface states. We utilize the spin circuit model for spin orbit torques in topological insulator surface states [2] to develop the equivalent circuit model of spin pumping in topological insulators. Such equivalent circuit model developed here can be utilized to analyze available experimental results and evaluate more complex structures. [1] K. Roy et al., Spin Circuit Representation for Spin Pumping Phenomena, in APS March Meeting, Session Y28.12 (2015). [2] S. Hong, Spin Circuit Model for Spin-Orbit Torque in 2D Channels, in APS March Meeting, Session G52.1 (2015).   

Spin pumping from a ferromagnet into a hopping insulator: Role of resonant absorption of magnons

Motivated by recent experiments [1,2,3] on spin pumping from a ferromagnet into organic materials in which the charge transport is due to hopping, we study theoretically the generation and propagation of spin current in a hopping insulator. Unlike metals, the spin polarization at the boundary with ferromagnet is created as a result of magnon absorption within pairs of localized states and it spreads following the current-currying resistor network (although the charge current is absent). We consider a classic resonant mechanism of the ac absorption in insulators and adapt it to the absorption of magnons. A strong enhancement of pumping efficiency is predicted when the Zeeman splitting of the localized states in external magnetic field is equal to the frequency of ferromagnetic resonance. Under this condition the absorption of a magnon takes place within {\em individual} sites. \\ \\ \noindent [1] K. Ando {\em et al.,} Nat. Mater. {\bf 12}, 622 (2013). \\ \noindent [2] S. Watanabe {\em et al.,} Nat. Phys. {\bf 10}, 308 (2014).\\ \noindent [3] Z. Qiu {\em et al.,} AIP Advances {\bf 5}, 057167 (2015).   

Detection of topological surface states by spin pumping at room temperature

Spin pumping on heterostructures made of ferrimagnetic YIG film and topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ films has been performed at room temperature. In the presence of topological interface states, spin pumping induced non-equilibrium spin density caused significant resonance field shifts (H$_{\mathrm{res}}$ shifts) of YIG/Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ with respect to bare YIG. The uncommon H$_{\mathrm{res}}$ shifts correspond to clearly resolved changes of gyromagnetic ratio of YIG. As the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ thickness varied from 4 nm to 20 nm, increasing H$_{\mathrm{res}}$ shifts were observed, while the enhancement of damping constant saturated at the spin diffusion length of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, suggesting the two parameters were of different origins. Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ thickness dependence of spin pumping revealed that Rashba-split 2DEG has comparable effects on the magnetization dynamics. From the change of gyromagnetic ratio, we calculated the imaginary part of spin mixing conductance to be one order of magnitude larger than the real part. Our results showed that with clean and well-defined interface, spin pumping may serve as an effective way to detect spin-polarized surface states.   

Current-induced spin and orbital magnetizations in tellurium

Tellurium has a characteristic helical lattice structure, and lacks inversion and mirror symmetries. Such chiral crystals lead to various novel phenomena. For example, we have shown that spin and orbital magnetizations are induced by an electric current in chiral crystals[1]. In our presentation, we calculate the current-induced spin and orbital magnetization in tellurium by using first-principles calculations. The calculations show that both spin and orbital magnetizations are induced parallel to the electric current. In tellurium we found that the orbital magnetization is larger than the spin magnetization by two orders of magnitude. The spin magnetization is induced by the current via the spin-orbit coupling. Therefore, the induced spin magnetization is limited by the size of the spin-orbit coupling. On the other hand, the orbital magnetization is determined by crystal structure without spin-orbit coupling. By using a chiral crystal, a magnetization can be induced by an electric current without ferromagnets and the spin-orbit coupling. [1]Yoda, T. et al. Sci. Rep. 5, 12024   

Enhanced spin Hall ratios by Al and Hf impurities in Pt thin films

The spin Hall effect (SHE) in Pt has been reported to be strong and hence promising for spintronic applications. In the intrinsic SHE mechanism, which has been shown to be dominant in Pt, the spin Hall conductivity $\sigma_{SH}$ is constant, dependent only on the band structure of the spin Hall material. The spin Hall ratio $\theta_{SH} = \sigma_{SH} \cdot \rho$, on the other hand, should be proportional to the electrical resistivity $\rho$ of the spin Hall layer. This suggests the possibility of enhancing the spin Hall ratio by introducing additional diffusive scattering to increase the electrical resistivity of the spin Hall layer. Our previous work has shown that this could be done by increasing the surface scattering by growing thinner Pt films in contact with higher resistivity materials such as Ta. In this talk, we discuss another approach: to introduce impurities of metals with negligible spin orbit torque into the Pt film. Our PtAl and PtHf alloy samples exhibit strong enhancement of the spin Hall torque efficiency with impurity concentration due to increased electrical resistivity.   

Large Spin Hall Angle in Vanadium Film

We report the large spin Hall angle observed in Vanadium film with small grain size and distorted lattice parameter. The spin Hall angle is quantified by measuring current-induced spin-orbit torque in V/CoFeB bilayer using optical spin torque magnetometer based on polar magneto-optical Kerr effect (MOKE). The spin Hall angle as large as $\theta _{\mathrm{SH}}=$-0.071 has been observed in V/CoFeB bilayer Structural analysis, using X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED), confirms films grown at room temperature have very small grain size and enlarged lattice parameter. The Vanadium films with distorted crystal structure also have high resistivity (\textgreater 200 $\mu \Omega $\textbullet cm) and long spin diffusion length (\textasciitilde 16.3 nm) measured via spin pumping experiment. This finding of spin Hall effect enhancement in more disordered structure will provide insights for understanding and exploiting materials with strong spin orbit interaction, especially in light 3d transition metals which promise long spin diffusion length.   

Spin-orbit torques and charge pumping in crystalline magnets

In magnetic crystals with an inversion asymmetric unit cell a non-zero global spin-polarization is generated by an electrical current, which acts with a torque on the magnetisation exciting magnetic dynamics [1]. This relativistic non-equilibrium spin phenomenon also has a reciprocal effect in which the excitation of magnons results in the pumping of a charge current [2]. The possibility to manipulate/read magnetism with electrical currents is highly relevant for magnetic memories and other spintronic devices. I will start by reviewing our recent research on spin-orbit torques (SOTs) in crystalline magnets, in particular our very recent measurements of the crystalline SOT at room temperature in half-Heusler NiMnSb thin films. With this experiment we are able to fully characterise magnitude and symmetry of the SOTs [3, 4]. I will then talk about the first demonstration of magnonic charge pumping in crystal magnet GaMnAs [2]. In this effect, which is the reciprocal effect of SOTs, the precessing ferromagnet pumps a charge current. Differently from spin pumping, which is commonly used to electrically detect magnetization dynamics, in charge pumping magnons are converted within the ferromagnet into high-frequency currents via the relativistic spin-orbit interaction, without the need of a secondary spin-charge conversion element, such as heavy metals with large spin Hall angle. References 1. Chernyshov et al., Nature Physics 5, 656 (2009). 2. Ciccarelli et al., Nature nanotechnology 10, 50 (2014). 3. Fang et al., Nature Nanotechnology 6, 413 (2011). 4. Kurebayashi et al., Nature Nanotechnology 9, 211 (2014).   

Moderate Positive Spin Hall Angle in Uranium

We will present results on FMR and voltage measurements of magnetic damping and the inverse spin Hall effect, respectively, in Ni$_{80}$Fe$_{20}$/Uranium bilayers. A pure spin current is injected into an Uranium film from the ferromagnetic resonance dynamics of the magnetization of an adjacent Ni$_{80}$Fe$_{20}$ (permalloy) film. The spin current generated is then converted into an electric field by the inverse spin Hall effect. Our results suggest a spin mixing conductance of order 2x10$^{19}$ m$^{-2}$ and a positive spin Hall angle of 0.004, which are both unexpected based on trends in d-electron systems. These results support the idea that materials with unfilled f-electron orbitals may require additional exploration for spin physics.   

Thickness dependence of inverse spin Hall effect in Au and W studied using YIG-based spin pumping

Yttrium iron garnet (YIG) is an excellent material for generating pure spin currents due to its narrow ferromagnetic resonance (FMR) linewidth and low damping. High quality YIG thin films are deposited by off-axis magnetron sputtering, followed by in-situ deposition of Au and W layers of varying thicknesses. Using the inverse spin-Hall effect (ISHE) in the Au and W layers, we study FMR-driven spin pumping from YIG thin films (16nm) into each metal at thicknesses of 2-50nm. Gilbert damping of these bilayers are obtained with variable frequency FMR measurements. Spin transport parameters, including the spin diffusion length in metal, spin mixing conductance at the interfaces and spin hall angles, are also determined.