Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L47: Spin-Orbit Torque IIFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Joseph Sklenar, University of Illinois at Urbana-Champaign Room: 394 |
Wednesday, March 15, 2017 11:15AM - 11:27AM |
L47.00001: Spin Torques Arising from the Spin Hall Effect in Ferromagnets Jonathan Gibbons, David MacNeill, Robert Buhrman, Daniel Ralph The anomalous Hall effect arising within a ferromagnet possessing strong spin-orbit interactions is expected to generate a spin current whose spin orientation can be manipulated by altering the magnetic state of the generating layer. This capability could provide a new level of functionality for controlling magnetic memory devices using spin-transfer torque. We report the generation of spin currents by the anomalous Hall effect from a Gadolinium-doped Iron layer, and verify that the in-plane spin polarization can indeed be controlled by varying the magnetization direction. We use a pinned ferromagnetic layer/spacer/free magnetic layer stack and an in-plane second harmonic detection method to obtain quantitative measurements of the spin transfer torque exerted by these spin currents. We will also discuss the conditions required to use the anomalous Hall effect to generate an out-of-plane anti-damping spin-orbit torque. [Preview Abstract] |
Wednesday, March 15, 2017 11:27AM - 11:39AM |
L47.00002: Spin-Orbit Torque Control of Dipole Field-Localized Spin Wave Modes. Chi Zhang, Yong Pu, Sergei A. Manuilov, Shane P. White, Michael R. Page, Erick C. Blomberg, Denis V. Pelekhov, Chris Hammel Auto-oscillation of a ferromagnet due to spin-orbit torques in response to a dc current is of wide interest as a flexible mechanism for generating controllable high frequency magnetic dynamics. We use localized spin wave modes that are confined by strongly inhomogeneous dipole magnetic field of a nearby micromagnet to manipulate their response to spin-orbit generated spin currents. This provides variable spatial confinement and systematic tuning of magnon spectrum, which offers a new approach to study the impact of multi-mode interactions on auto-oscillations by continuously tuning the spectral separations of dipole field-localized modes. Here we first demonstrate electrical spin-torque ferromagnetic resonance (ST-FMR) detection of well-resolved dipole field-localized modes in a Py/Pt strip. We observe clear damping control, and find that localized modes can be controlled as efficiently as the uniform mode. We further drive localized modes into auto-oscillations with dc current only. We study their characteristic behaviors, and discuss the role of reduced scattering channels. [Preview Abstract] |
Wednesday, March 15, 2017 11:39AM - 11:51AM |
L47.00003: Magnetization-Induced Electromagnetic Spin-Orbit Coupling in Magneto-Optic Media Miguel Levy, Dolendra Karki We present a formulation of nonreciprocal electromagnetic transverse-spin to orbital angular momentum coupling in magneto-optic media. Transverse-spin angular-momentum-density shifts for evanescent waves in magneto-optic waveguides are shown to result in nonreciprocal unidirectional coupling into orbital momenta in silicon-on-insulator waveguides with iron garnet claddings. Experimental results of geometrical confinement of light in ultra-thin magnetic garnet films evince significant enhancement of Faraday rotation and magnetic circular dichroism and impact the electromagnetic spin-orbit coupling. We describe the diamagnetic electronic transition processes responsible for these results. Transverse-spin to orbital angular momentum coupling into magneto-optic waveguide media are shown to engender magneto-optic-gyrotropy-dependent unidirectional propagation. We demonstrate that magnetization-induced electromagnetic spin-orbit coupling as a result of Faraday rotations in waveguide media leads to nonreciprocal spin to orbital angular momentum conversions. The spin-helicity- and magnetization-gyrotropy dependence of free-space helicoidal beams based on this conversion are described. [Preview Abstract] |
Wednesday, March 15, 2017 11:51AM - 12:27PM |
L47.00004: Spin-Orbit Torque-Assisted Switching in Magnetic Insulator Thin Films with Perpendicular Magnetic Anisotropy Invited Speaker: Mingzhong Wu As an in-plane charge current flows in a heavy metal film with spin-orbit coupling, it produces a torque that can induce magnetization switching in a neighboring ferromagnetic metal film. Such spin-orbit torque (SOT)-induced switching has been studied extensively in recent years and has shown higher efficiency than switching using conventional spin-transfer torque. This presentation reports the SOT-assisted switching in heavy metal/magnetic insulator systems.$^{\mathrm{1}}$ The experiments made use of Pt/BaFe$_{\mathrm{12}}$O$_{\mathrm{19}}$ bi-layered structures. Thanks to its strong spin-orbit coupling, Pt has been widely used to produce pure spin currents in previous studies. BaFe$_{\mathrm{12}}$O$_{\mathrm{19}}$ is an M-type barium hexagonal ferrite and is often referred as BaM. It is one of the few magnetic insulators with strong magneto-crystalline anisotropy and shows an effective uniaxial anisotropy field of about 17 kOe. It's found that the switching response in the BaM film strongly depends on the charge current applied to the Pt film. When a constant magnetic field is applied in the film plane, the charge current in the Pt film can switch the normal component of the magnetization ($M_{\mathrm{\bot }})$ in the BaM film between the up and down states. The current also dictates the up and down states of the remnant magnetization when the in-plane field is reduced to zero. When $M_{\mathrm{\bot }}$ is measured by sweeping an in-plane field, the response manifests itself as a hysteresis loop, which evolves in a completely opposite manner if the sign of the charge current is flipped. When the coercivity is measured by sweeping an out-of-plane field, its value can be reduced or increased by as much as about 500 Oe if an appropriate charge current is applied. 1. P. Li, T. Liu, H. Chang, A. Kalitsov, W. Zhang, G. Csaba, W. Li, D. Richardson, A. Demann, G. Rimal, H. Dey, J. S. Jiang, W. Porod, S. Field, J. Tang, M. C. Marconi, A. Hoffmann, O. Mryasov, and M. Wu, Nature Commun. 7:12688 doi: 10.1038/ncomms12688 (2016). [Preview Abstract] |
Wednesday, March 15, 2017 12:27PM - 12:39PM |
L47.00005: Spin-orbit torque induced switching in a magnetic insulator thin film with perpendicular magnetic anisotropy J. X. Li, G. Q. Yu, C. Tang, K. L. Wang, J. Shi Spin-orbit torque (SOT) has been demonstrated to be efficient to manipulate the magnetization in heavy-metal/ferromagnetic metal (HM/FMM) heterostructures. In HM/magnetic insulator (MI) heterostructures, charge currents do not flow in MI, but pure spin currents generated by the spin Hall effect in HM can enter the MI layer to cause magnetization dynamics. Here we report SOT-induced magnetization switching in Tm$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$/Pt heterostructures, where Tm$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$ (TmIG) is a MI grown by pulsed laser deposition with perpendicular magnetic anisotropy. The anomalous Hall signal in Pt is used as a probe to detect the magnetization switching. Effective magnetic fields due to the damping-like and field-like torques are extracted using a harmonic Hall detection method. The experiments are carried out in heterostructures with different TmIG film thicknesses. Both the switching and harmonic measurements indicate a more efficient SOT generation in HM/MI than in HM/FMM heterostructures. Our comprehensive experimental study and detailed analysis will be presented. [Preview Abstract] |
Wednesday, March 15, 2017 12:39PM - 12:51PM |
L47.00006: Sign reversal of Hall signals in ${\mathrm{Tm}}_{\mathrm{3}}{\mathrm{Fe}}_{\mathrm{5}}\mathrm{O}_{\mathrm{12}}$ /Pt with perpendicular magnetic anisotropy Yawen Liu, Chi Tang, Yadong Xu, Zhong Shi, Jing Shi Robust interface strain-induced perpendicular magnetic anisotropy is produced in atomically flat ferromagnetic insulator ${\mathrm{Tm}}_{\mathrm{3}}{\mathrm{Fe}}_{\mathrm{5}}\mathrm{O}_{\mathrm{12}}$ (TIG) films grown with pulsed laser deposition on both substituted-${\mathrm{Gd}}_{\mathrm{3}}{\mathrm{Ga}}_{\mathrm{5}}\mathrm{O}_{\mathrm{12}}$ and ${\mathrm{Nd}}_{\mathrm{3}}{\mathrm{Ga}}_{\mathrm{5}}\mathrm{O}_{\mathrm{12}}$ (NGG). In TIG/Pt bilayers, we observe large hysteresis loops over a wide range of Pt thicknesses and temperatures. Both the ordinary Hall effect and anomalous Hall effect undergo a sign reversal as the temperature is lowered. The temperature dependence of the Hall signals in bilayers with different thickness of Pt indicates the existence of exchange interaction at the interface. Our results provide a clue to further understand the origin of the anomalous Hall effect in ferromagnetic insulator/normal metal bilayer systems. [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L47.00007: Spin-orbit torques and charge pumping in YIG/Pt bilayers Cecilia Holmqvist, Arne Brataas Efficient control of magnetic elements using charge currents is essential for spintronics applications. The magnetization of a ferromagnetic insulator (FI) thin-film can be controlled by a current through an adjacent normal metal (N) thin film. We theoretically investigate the spin-orbit torques in Pt/YIG bilayers taking the crystal symmetries of the materials into account. Besides the damping torque in bilayers with C$_{\infty v}$ symmetry, the crystal symmetry of the clean Pt/YIG bi-layers also allows additional spin-orbit torque contributions. We show that the spin-orbit torque terms can be differentiated via charge pumping, in which the precession of a ferromagnet generates a charge current. The additional spin-orbit torque terms refine the understanding of the control of the magnetic moments [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L47.00008: Tunable sign change of spin Hall magnetoresistance in Pt/NiO/YIG structures Dazhi Hou, Zhiyong Qiu, Joseph Barker, Koji Sato, Kei Yamamoto, Saul Velez, Juan M. Gomez-Perez, Luis E. Hueso, Felix Casanova, Eiji Saitoh Spin Hall magnetoresistance (SMR) has been investigated in Pt/NiO/YIG structures in a wide temperature range. The SMR changes sign at a temperature which increases with the NiO thickness. This is contrary to a conventional SMR theory picture applied to Pt/YIG bilayer which always predicts a positive SMR. We explain the negative SMR at low temperatures by the NiO 'spin-flop' coupled with YIG, which is overridden at higher temperature by positive SMR contribution from YIG. This highlights the role of magnetic structure in antiferromagnets for transport of pure spin current in multilayers. [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L47.00009: Spin-torque oscillation in magnetic insulator probed by single-electron spins Mark Ku, Huiliang Zhang, Francesco Casola, Toeno van der Sar, Chunhui Du, Keigo Arai, Caroline Ross, Amir Yacoby, Ronald Walsworth Spin-torque oscillation (STO) in magnetic insulators is under intensive investigation because of excellent material properties (low damping, no electromigration, etc.), with potential applications in storage devices, spintronics, and magnonics. Coherent auto-oscillation inside magnetic insulators (e.g., Yttrium Iron Garnet, YIG) has been observed recently; however many questions remain due to lack of a proper diagnostic tool. Nitrogen-Vacancy (NV) centers in diamond provide a versatile magnetic sensing modality with nanoscale spatial resolution. We used NV centers to probe~a~Platinum/YIG hybrid~microstructure and~quantify the magnetic damping variation under pure spin current spin injection, detect magnetization auto-oscillation, and observe STO mutual interaction and mode hopping.~ [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L47.00010: Spin-Orbit Torque Efficiency in Compensated Ferrimagnetic Cobalt-Terbium Alloys Joseph Finley, Luqiao Liu We report spin-orbit torque induced magnetization switching of ferrimagnetic Co$_{\mathrm{1-x}}$Tb$_{\mathrm{x}}$ films with perpendicular magnetic anisotropy. Current induced switching is demonstrated in all studied film compositions, including those near the magnetization compensation point. A divergent behavior that scales with the inverse of magnetic moment is confirmed for the spin-orbit torque efficiency, which is consistent with angular momentum conservation. Moreover, we also quantified the Dzyaloshinskii-Moriya interaction energy in the Ta/Co$_{\mathrm{1-x}}$Tb$_{\mathrm{x}}$ system and found that the energy density increases with Tb concentration. The large effective spin-orbit torque, the previously demonstrated fast dynamic, and the minimal net magnetization in these ferrimagnetic systems promise spintronic devices that are faster and with higher density than traditional ferromagnetic systems. [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L47.00011: Decreasing the Spin Orbit Torque-critical current density to reverse perpendicular magnetization in Co$_{\mathrm{x}}$Tb$_{\mathrm{1-x}}$ ferrimagnetic based alloys. J-Carlos Rojas-Sanchez, T.-H. Pham, P. Vallobra, D. Lacour, G. Malinowski, M. Hehn, S. Mangin, M.-C. Cyrille, S.-G. Je, G. Gaudin, O. Boulle We show the SOT-switching of perpendicularly magnetized //W(3nm)/Co$_{\mathrm{x}}$Tb$_{\mathrm{1-x}}$(3.5nm)/AlOx(3nm) structure. Ferrimagnetic alloys allow us to tune the net magnetic moment by varying the Co concentration [1]. Samples were grown by sputtering on Si-SiO2 substrates and characterized by MOKE and SQUID measurements which allow us to estimate the critical compensation where the net moment reaches zero at room temperature. Then we studied the magnetization reversal in Hall crosses by analyzing the anomalous Hall Effect (AHE) when applying in-plane current pulses and an in-plane field H$_{\mathrm{x}}$ parallel to the current direction [2]. A minimum H$_{\mathrm{x}}$ of the order of 1 mT is needed for fully reverse the magnetization. The full reversal of magnetization has been observed in all our samples. More importantly, critical current densities for switching were found to decrease systematic when decreasing the Co concentration even in the Tb rich compositions samples. Our results bring out the role of different concentration of Co in Co$_{\mathrm{x}}$Tb$_{\mathrm{1-x}}$ alloys on SOT-switching providing a method to reduce the current density using ferrimagnetic alloys. [1] M. Gottwald et al., JAP 111, 083904 (2012) [2] J.C. Rojas-S\'{a}nchez et al., APL 108, 082406 (2016). [Preview Abstract] |
Wednesday, March 15, 2017 1:51PM - 2:03PM |
L47.00012: Temperature dependence of spin-orbit torques in a bulk perpendicularly magnetized Tb$_{\mathrm{x}}$Co$_{\mathrm{1-x}}$ alloy film Kohei Ueda, Maxwell Mann, Geoffrey Beach Current-driven spin-orbit torques (SOTs) in ultra-thin ferromagnet/heavy metals with strong spin orbit coupling has been shown to be efficient way for manipulating magnetization [1,2]. Damping-like (DL) SOT is most relevant to highly efficient magnetization switching due to spin Hall effect [2]. Recently, some groups have reported SOT efficiency in ferrimagnet alloy films with bulk perpendicular magnetic anisotropy, which becomes potential candidate for the futures spin-orbit device with large thermal stability [3-5]. However, mechanism of spin transport still remains questionable issues in magnetic compensated ferrimagnet alloy film strongly depending on the temperature and composition. Here, we report DL-SOT efficiency ($\chi_{\mathrm{DL}})$ for various temperatures in a Ta/Tb$_{\mathrm{x}}$Co$_{\mathrm{1-x}}$ bi-layers film, characterized by conventional harmonic voltage measurement. As decreasing temperature, coercively field ($H_{\mathrm{c}})$ showed an increase and decrease below 200 K, whereas the saturation magnetization ($M_{\mathrm{s}})$ does opposite behavior to the $H_{\mathrm{c}}$, since two sub-lattices of Co and Tb are equal in magnitude due to the magnetic compensation point ($T_{\mathrm{M}})$. Furthermore, it is found that $\chi _{\mathrm{DL}}$ showed the maximum, approaching to $T_{\mathrm{M}}$. This result is consistent with a relation that $\chi_{\mathrm{DL}}$ is inversely proportional to $M_{\mathrm{s}}$ [5]. [Preview Abstract] |
Wednesday, March 15, 2017 2:03PM - 2:15PM |
L47.00013: Spin-Orbit Torques in ferrimagnetic GdFeCo Niklas Roschewsky, Charles-Henri Lambert, Sayeef Salahuddin Recently spin-orbit torques in antiferromagnets received a lot of attention due to intrinsic high frequency dynamics as well as robustness against perturbations from external magnetic fields. Here, we report on spin-orbit torque (SOT) switching in ferrimagnetic \linebreak \mbox{Gd$_{x}($Fe$_{90}$Co$_{10})_{100-x}$} films on both sides of the magnetic compensation point~[1]. In addition to current driven switching experiments we performed harmonic Hall measurements of the effective SOT fields. We find that both the Slonczewski torque as well as the field-like torque diverge at the magnetization compensation point. However, the effective spin Hall angle $\xi=(2|e|/\hbar)M_S t_{FM} (H_{eff}/|j_{HM}|)$ is found to be roughly constant across the investigated composition range. This provides important insight into the the angular momentum transfer process in ferrimagnets.\\ \\ $[1]$ Roschewsky, N. et al. \textit{Applied Physics Letters}, 109(11), 2016. [Preview Abstract] |
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