Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F47: Spin-Orbit Torque IFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Guoqiang Yu, UCLA Room: 394 |
Tuesday, March 14, 2017 11:15AM - 11:51AM |
F47.00001: Spin-orbit interactions in thin magnetic films: from doping and interfaces to transport and skyrmions Invited Speaker: Christopher Marrows The spin-orbit interaction is an inherent part of magnetism, which links up the independent world of spins to the atomic lattice, thus controlling many functional properties of magnetic materials. In the widely used 3$d$ transition metal ferromagnetic films, the spin-orbit interaction is relatively weak, due to low atomic number, and to obtain strong spin-orbit effects it is necessary to include heavy metals. Here we show that large spin-orbit effects can be obtained by two separate routes. First, it is possible to enhance and tune the spin-orbit interaction by adding 5$d$ platinum dopants into permalloy (Ni$_{\mathrm{81}}$Fe$_{\mathrm{19}})$ thin films by a cosputtering technique. This is achieved without significant changes of the magnetic properties, due to the vicinity of Pt to meeting the Stoner criterion for the ferromagnetic state. The spin-orbit interaction is investigated by means of transport measurements (the anisotropic magnetoresistance and anomalous Hall effect), ferromagnetic resonance measurements to determine the Gilbert damping, as well as by measuring the x-ray magnetic circular dichroism at the $L_{\mathrm{3}}$ and $L_{\mathrm{2}}$ x-ray absorption edges to reveal the ratio of orbital to spin magnetic moments. It is shown that the effective spin-orbit interaction increases with Pt concentration within the 0{\%}--10{\%} Pt concentration range in a way that is consistent with theoretical expectations for all four measurements. Second, we show how placing Pt in atomic contact with an ultrathin Co layer leads to strong spin-orbit-driven effects. As well as the well-known perpendicular magnetic anisotropy, a strong Dzyaloshinskii-Moriya interaction is induced, leading to homochiral N\'{e}el domain walls and chiral skyrmion bubbles. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:03PM |
F47.00002: Evidence for a common origin of spin-orbit torque and the Dzyaloshinskii-Moriya interaction at a Py/Pt interface Andrew Berger, Eric Edwards, Hans Nembach, Justin Shaw, Alexy Karenowska, Mathias Weiler, Tom Silva Spin-orbit coupling (SOC) can drive non-equilibrium spin-charge conversion through the reciprocal processes of current-driven spin torque and spin precession-driven current in ferromagnet/heavy metal (FM/HM) bilayers. Both damping-like and field-like spin-orbit torques (SOT) have been observed in the forward process of current-driven SOT, but details of the underlying physics are still debated. SOC also underlies the equilibrium antisymmetric exchange coupling, a.k.a. the interfacial Dzyaloshinskii-Moriya interaction (DMI). It was recently proposed that a Rashba Hamiltonian at FM/HM interfaces serves as a common origin for both SOT and DMI, with a simple quantitative relation between the two. Here, we verify this relation by comparing inverse SOT (iSOT) measurements with previous characterization of DMI via spin wave spectroscopy. To perform the iSOT measurements, we have developed a technique for quantitative vector network analyzer ferromagnetic resonance to inductively detect the AC charge currents produced by spin-charge conversion processes in FM/HM bilayers. Our findings reveal that Py/Pt bilayers exhibit both damping-like and field-like iSOT, consistent with the presence of inverse spin Hall effect and Rashba-Edelstein effect, respectively. [Preview Abstract] |
Tuesday, March 14, 2017 12:03PM - 12:15PM |
F47.00003: Switching of three-terminal MTJ devices by the giant spin Hall effect of Pt alloys Minh-Hai Nguyen, Shengjie Shi, Sriharsha Aradhya, Robert Buhrman Since the discovery of the large spin Hall ratios of Pt, beta-Ta and beta-W, the spin Hall effect (SHE) in normal metallic systems has attracted a great deal of attention due to the broad scientific interest in spin-orbit torques. In addition the high efficiency by which the SHE in such metals can convert an electrical current to a transverse spin current can enable a wide range of spintronics applications, including the fast, reliable, low energy switching of three-terminal nanoscale magnetic tunnel junction (MTJ) devices that has been recently demonstrated [1]. The performance of such three-terminal SHE devices is determined by the spin Hall ratio and resistivity of the channel material, as well as by the demagnetization field and magnetic damping of the free ferromagnetic layer. Our earlier work [2] has shown that the spin Hall ratio of Pt can be enhanced when it is alloyed with Hf, due to the intrinsic nature of the SHE in Pt. In this talk, we will discuss the low switching current, nanosecond pulse switching speed and low write error rates of the three-terminal MTJ devices made with PtHf nano-channels. Our results suggest that further improvement of the three-terminal MTJ structure is possible by engineering the bulk and interfacial properties of the spin Hall channel. [1] Aradhya et al. Nano Letters 10, 5987 (2016) \newline [2] Nguyen et al. APL 108, 242407 (2016) [Preview Abstract] |
Tuesday, March 14, 2017 12:15PM - 12:27PM |
F47.00004: Spin scattering in Pt and at its interfaces Ryan Freeman, Andrei Zholud, Sergei Urazhdin We study spin transport in Pt utilizing current perpendicular-to-plane (CPP) giant magnetoresistance (GMR) in nanoscale Permalloy (Py)-based spin valves with Pt inserted in the nonmagnetic spacer. The spin diffusion length and interfacial spin flipping coefficients are extracted from the dependence of GMR on the Pt thickness. We interpret our results in terms of two relevant spin scattering mechanisms: the EY mechanism resulting in spin flipping due to orbital scattering, and the Dyakonov-Perel (DP) mechanism caused by spin precession around the effective spin-orbit field. Our results are consistent with the published value of the spin diffusion length, showing the dominance of the EY mechanism, but show an unexpected temperature dependence. We also measure GMR in the ballistic limit, where the Pt is made thinner than its mean free path. Anomalous temperature dependence of the GMR of these samples is consistent with the observation of DP relaxation. Finally, the interfacial scattering is found to be significantly smaller at Pt/Py interfaces than at Pt/Cu interfaces. We argue that DP relaxation is suppressed at Pt/Py interfaces, due to the dominance of the proximity-induced effective exchange field. [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 12:39PM |
F47.00005: Switching symmetry of in-plane current induced spin orbit effects in $\beta $-W/Ta/CoFeB/MgO/Ta multilayers AVYAYA jayanthi narasimham, Meng Zhu, Vincent LaBella Spin-orbit coupling in metastable $\beta $-W generates spin-orbit torques (SOT) strong enough to flip the magnetic moment of an adjacent magnetic layer. In a magnetic tunnel junction (MTJ) stack these torques can be used to switch between high and low resistive states. Inserting a 1 nm Ta insert-layer between the CoFeB and W induces PMA in these layers. $\beta $-W(5)/Ta(1) channel and the adjacent CoFeB/MgO/Ta layers are patterned into Hall bar and Hall cross structures. The effect of orthogonality between current and the external magnetic field will be presented. A comparison between switching symmetries of current sweeps under constant external magnetic field Vs magnetic field sweeps under constant current will be discussed. [Preview Abstract] |
Tuesday, March 14, 2017 12:39PM - 12:51PM |
F47.00006: First-principles calculations of Berry curvature for Tungsten metals and compound Sonny H. Rhim With rapid progress in spintronics, utilizing spin Hall effect or interface spin phenomena has become highly demanding for practical applications with high interests. In this sense, calculation of spin Hall angle ($\theta_{SH}$) or Berry curvature ($\Omega_z$) based on first-principles calculations is of great significant in exploration for materials search. Here, we adapt the method proposed by previous work \footnote{G. Y. Guo, S. Murakami, T.-W. Chen, and N. Nagaosa, Phys. Rev. Lett. {\bf 100}, 096401 (2008).}, which has been implemented in FLAPW method. As large spin Hall angle has been reported in tungsten \footnote{Kai-Uwe Demasius and {\em et al.} Nat. Comm. {\bf 7}, 10644 (2016).}, calculations on tungsten metals - for $bcc$ and $A15$ structure are presented. Also results on WTe$_2$ are shown further. [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F47.00007: Spin orbit torques in W(O) based three terminal magnetic memory devices. Jie Zhang, Timothy Phung, Chirag Garg, Charles Rettner, Brian. P Hughes, See-hun Yang, Stuart. S.P Parkin Recently, there has been a large interest in using spin orbit torques to controllably manipulate the magnetic order parameter in several promising magnetic memory devices such as racetrack memory and spin transfer torque MRAM. The efficient operation of such devices necessitates the finding of materials which exhibit efficient conversion of charge currents to spin orbit torques. This is typically quantified by the so-called spin Hall angle. The most efficient spin orbit torque generator to date based on the use of conventional metallic materials is W(O), wherein the effective spin hall angle is found to be -0.5. Here, we explore the use of W(O) to manipulate magnetization in three terminal magnetic memory devices. We find, consistent with the large spin orbit torques, observed in W(O), that the critical current required for switching a magnetic element is significantly smaller than compared to other metallic systems such as Pt, $\beta $-W, and Ta. Lastly, we shall discuss the technologically important high speed (\textasciitilde ns time scale) switching dynamics in these devices and the role of complex micromagnetic states upon the switching process. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F47.00008: Giant Spin Hall Effect of the Thick $\beta $-W: Temperature Study and Interface Property Wenzhe Chen, Gang Xiao The high quality (thick t) $\beta $-W/(1)CoFeB/(1.6)MgO/(1)Ta (in nanometer) with the robust perpendicular magnetic anisotropy is obtained using the sputtering system. The XRD pattern and HRTEM has clearly shown the existence of the $\beta $-phase. The Electrical and magnetic properties are studied in the temperature range from 10K to 300K. By employing the spin Hall measurement with the aid of ferromagnetic layer, we can calculate the spin Hall angle through the spin transfer torque. We report the largest ``internal'' spin Hall angle up to 50{\%} in our multilayer sample of 18nm-W, after the correction from the spin transmission probability. Our comprehensive study could promote the future development of the Spintronics device based on the $\beta $-W. [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F47.00009: Spin-orbit assisted transmission at 3d/5d metallic interfaces Henri Jaffres, Quentin Barbedienne, Augustin Jouy, Nicolas Reyren, Jean-Marie George We will describe the anatomy of spin-transport and spin-orbit torques (SOT) at spin-orbit active interfaces involving 5d transition metals (TM) as heavy metals spin-Hall effect (SHE) materials and 3d TM in [Co,Ni]/Pt, NiFe. NiFe/Au:W and Co/Pt/Au;W systems. In the case of Pt, recent studies have put forward the major role played by the spin-memory loss (SML), the electronic transparency at 3d/5d interfaces and the inhomogeneity of the conductivity in the CIP-geometry. Ingredients to consider for spin-transport and spin-Hall Magnetoresistance (SMR) are the conductivity, the spin-current profiles across the multilayers and the spin-transmission. We will present SMR measurements observed on these systems possibly involving interfacial Anisotropy of Magnetoresistance (AIMR) contributions. We analyze in large details our SMR signals in the series of samples owing: i) the exact conductivity profile across the multilayers via the Camley-Barnas approach and the spin current profile generated by SHE. We will discuss the role of the generalized spin-mixing conductance on the spin-transport properties and spin-orbit torques. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F47.00010: Ferromagnetism in platinum induced by field effect gating using paramagnetic ionic liquid Lei Liang, Qihong Chen, Jianming Lu, Wytse Talsma, Juan Shan, Graeme Blake, Thomas Palstra, Jianting Ye Field induced and controlled magnetism has attracted growing interest in spintronics, which requires the field effect manipulation of both charge and spin degrees of freedom. In this work, we report the reversible field effect switching of ferromagnetic states in a platinum (Pt) thin film by applying only a few volts. This highly effective control of ferromagnetic state is based on ionic gating using paramagnetic ionic liquids (MIL), a group of ionic media containing magnetic anions especially developed for gating in spintronics. Because of the magnetic anion, the field effect control of the ion transport in the MILs associates the ion movement with both electric charges and magnetic moments, which leads to emergent ferromagnetism with large coercivity in the Pt film with perpendicular anisotropy. While the induced itinerant ferromagnetic state extends to room temperature, a co-existing Kondo effect at low temperature sheds light on understanding the itinerant-localized duality of the gate-induced carriers. The present result reveals that MIL gating can serve as a versatile and highly efficient protocol to control electric and magnetic properties simultaneously, which is promising for the spintronics. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 1:51PM |
F47.00011: The spin-Hall effect and spin-orbit torques in epitaxial Co$_2$FeAl/platinum bilayers T. A. Peterson, C. Liu, T. McFadden, C. J. Palmstr{\O}m, P. A. Crowell We have performed magnetoresistance measurements on epitaxially grown Co$_2$FeAl/platinum (CFA/Pt) ultrathin ferromagnet/heavy metal bilayers to study the spin-Hall effect in Pt and the accompanying spin-orbit torque (SOT) exerted on the magnetic CFA layer. Specifically, we measure the spin-Hall magnetoresistance in the Pt layer by changing the orientation of the CFA magnetization with respect to the spin current orientation created in the Pt, and we determine the SOT efficiency using a second-harmonic detection technique. Because the latter of the two measurements is proportional to the spin-Hall ratio $\theta_{SHE}$ while the former is proportional to $\theta_{SHE}^2$, we are able to extract the bare Pt spin-Hall ratio with no assumptions about the CFA/Pt interface spin mixing conductance. Furthermore, by varying the Pt thickness we show that the results are consistent with resistivity-independent spin-Hall conductivity. Finally, the two measurements in combination allow us to infer a spin-mixing conductance at the CFA/Pt interface of $2\pm1\times10^{15}~\Omega^{-1}~m^{-2}$. The combination of spin-Hall magnetoresistance and SOT measurements allows for a determination of the spin-mixing conductance using only low-frequency transport techniques. [Preview Abstract] |
Tuesday, March 14, 2017 1:51PM - 2:03PM |
F47.00012: Temperature dependence of the spin Hall effect in mixed-valent YbAl$_{\mathrm{3}}$ Neal Reynolds, Shouvik Chatterjee, Ariel Seidner, Darrell Schlom, Kyle Shen, Daniel Ralph The spin Hall effect results in a spin current that flows transverse to an applied electric field in non-magnetic materials, and that can be used to apply an efficient spin-transfer torque in magnetic memory devices. Theoretical predictions suggest that the strength of the spin Hall effect might be enhanced by rare-earth $f$-electron elements, if the $f$ levels are hybridized with itinerant states and are sufficiently close in energy to the Fermi level. YbAl$_{\mathrm{3}}$ is a rare-earth mixed-valence system, where Yb 4$f$ states become increasingly itinerant at low temperatures. This is accompanied by a shift in binding energy of the 4$f$ derived heavy bands towards the Fermi level and an enhanced 4$f$ contribution to the YbAl3 Fermi surface, as temperature is lowered. We report on temperature dependent measurements of the spin Hall effect using spin-torque ferromagnetic resonance in YbAl3/Fe bilayers, and discuss implications for the spin Hall effect of increasing 4$f$ density of states at the Fermi level. [Preview Abstract] |
Tuesday, March 14, 2017 2:03PM - 2:15PM |
F47.00013: Theory of unidirectional spin Hall magnetoresistance in heavy-metal/ferromagnetic-metal bilayers. Shulei Zhang, Giovanni Vignale Recent experiments have revealed a unidirectional spin Hall magnetoresistance (USMR) effect in metallic bilayers consisting of a heavy-metal (HM) and a ferromagnetic-metal (FM). Modulation in the longitudinal resistance of the bilayer has been observed when reversing the direction of either the applied in-plane current or the magnetization of the FM layer. In this work, we attribute the USMR effect to the modification of conductivity by spin accumulation induced by spin Hall (SH) effect, which may be best understood by thinking of the spin accumulation at the FM side as an artificial FM layer. In analogy to the current-in-plane giant-magnetoresistance effect, change in longitudinal resistance is expected when the ``magnetization'' of the artificial FM layer switches from parallel to antiparallel (or vice versa) to that of the ``natural'' FM layer. The nonlinear character of the magnetoresistance arises from the fact that ``magnetization'' of the artificial FM layer is generated by the electric current itself via the SH effect. An explicit expression for the USMR is derived, and the numerical estimation is in order of magnitude agreement with the experimental observations. We also discuss possible ways to control the sign and magnitude of the effect. [Preview Abstract] |
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