Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B41: Electron Transport in Magnetic Materials |
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Sponsoring Units: GMAG Chair: Kyung-Jin Lee, Korea Univ Room: 707 |
Monday, March 2, 2020 11:15AM - 11:27AM |
B41.00001: Transport spin polarization of Pt/ferromagnetic insulator bilayers Michael Osofsky, Joseph Prestigiacomo, Peng Li, Yuri Suzuki It has been shown that the anomalous Hall effect can be observed in platinum films deposited on a ferromagnetic insulator which implies the existence of a magnetic proximity effect. One of the key features of ferromagnetic metals is a net spin polarization of the carriers at the Fermi energy which enable the functionality of spintronic devices. In this presentation we show that spin polarized carriers are indeed present in Pt films of various thicknesses that were deposited on magnesium aluminum ferrite (MAFO) using point contact Andreev scattering. The results were obtained by extracting conductance vs. voltage data from I-V curves taken through contacts formed by driving a sharpened superconducting Nb tip into the Pt/MAFO samples. The resulting spectra were then analysed using a modified BTK theory of supercurrent conversion at a normal/superconductor interface to obtain the values of the transport spin polarization. |
Monday, March 2, 2020 11:27AM - 11:39AM |
B41.00002: Strong transverse spin current generation in bulk ferromagnetic materials Rohit Nair, Kriti Gupta, Zhe Yuan, Gerrit Bauer, Paul Kelly The intrinsic spin Hall effect (SHE) is widely studied in non-magnetic metals and semiconductors. Recent studies have reported the observation of an inverse SHE (ISHE) in 3d ferromagnetic alloys where the underlying spin currents that generate the ISHE are oriented along the magnetization direction of permalloy (Py) [1]. Subsequent studies report a non-trivial SHE in Py [2,3] where the spin-polarization is not oriented along the magnetization; its origin in a 3d magnetic alloy is not known. To shed some light on the intrinsic SHE in magnetic materials we calculate transverse spin currents as a function of concentration x for fcc FexNi1-x alloys within a first-principles DFT-LDA scattering framework. For Py we find a spin Hall angle (SHA) of 4.8% compared to a value of 5% we calculate for platinum at room temperature [4]. We find a surprisingly large transverse spin current that is polarized along the same direction that could be useful in field-free switching applications. |
Monday, March 2, 2020 11:39AM - 11:51AM |
B41.00003: Electric current control of spin helicity in an itinerant helimagnet Nan Jiang, Yoichi Nii, Hiroki Arisawa, Eiji Saitoh, Yoshinori Onose The mechanism of homochirality in biological objects and other materials has been attracting great attention. The longitudinal helical magnetic structure is the chiral magnetic object, in which the ordered direction of the magnetic moment spatially rotates in the plane perpendicular to the propagation vector and the sense of rotation denoted as helicity determines the chirality. The robust magnetic degree of freedom may provide a new concept of magnetic memory if it can be electrically controlled and detected. Here we show that the helicity can be controlled by the magnetic field and electric current, and probed by the second-harmonic resistivity, which is sensitive to the chiral symmetry breaking, in an itinerant longitudinal helimagnet MnP. While the helicity can be controlled by the electric field in insulating multiferroic helimagnets, we have achieved the helicity manipulation in the conducting helimagnet compatible with electric circuits. The controllability may pave a new route to the spintronics. |
Monday, March 2, 2020 11:51AM - 12:03PM |
B41.00004: Experimental tests of spin transport models in organic semiconductors Henna Popli, Jingying Wang, Xiaojie Liu, Evan Lafalce, Taniya Hansika Tennahewa, Hans Malissa, Valy Vardeny, Christoph M Boehme It has been a long-standing question whether spin-transport and charge-transport in organic materials occur via the same electronic processes [1, 2], i.e. via charge excitations such as polaron states. If so, then the spin current that is produced by spin-pumping through Ferromagnetic resonance (FMR) excitation in an inverse spin-Hall-effect (ISHE) experiment on a NiFe/Alq3/Pt device [3] must be diminished under electron paramagnetic resonance (EPR) excitation of the charge carriers [4]. We have executed such experiments and do not observe such quenching, which shows that spin-transport is neither due to propagation of charge carriers nor due to spin-transfer between weakly coupled charge carriers. However, these observations are consistent with the idea of exchange mediated transport. We discuss the implications of our experimental results as well as various control experiments that have been done to scrutinize our results. [1] M. Groesbeck et al. (unpublished); [2] Z. Yue et al., Phys. Rev. B 92, 045405 (2015); [3] S.W. Jiang et al., Phys. Rev. Lett. 115, 086601 (2015); [4] D. R. McCamey et al., Nature Mat. 7, 723 (2008). |
Monday, March 2, 2020 12:03PM - 12:15PM |
B41.00005: Magneto-transport in half metallic CrO2 nanowires with spin texture Lijuan Qian, Wenzhe Chen, Gang Xiao Magneto-transport in ferromagnetic metals (FM) is influenced by spin textures. In this work, we study the magneto-transport of spin textures in epitaxial half-metallic CrO2 nanowires. We quantitatively define the spatial non-uniformity of spin textures as spin curvature. We design geometric notches along the edges of the 300nm-width wires to introduce the spin curvature. The magnitude of the spin curvature is controlled via both the depth of notches and the magnetic fields. Through micromagnetic simulations and magneto-transport measurements, we uncover an empirical relation between the spin curvature and the magnetoresistance. This provides a quantitative method to predict the magneto-transport behavior of any spin texture. The thermal effect is studied from 10K and 250K. Results show that the thermal magnons worsen the spin asymmetry and suppresses the spin curvature induced resistivity at temperatures much lower than the ferromagnetic ordering temperature Tc. |
Monday, March 2, 2020 12:15PM - 12:27PM |
B41.00006: Non-local voltage measurements with wide F/I/N tunnel contacts are strongly affected by generation of circular electric currents Yaroslaw Bazaliy, Revaz Ramazashvili Electric spin injection is routinely tested by non-local voltage measurements in lateral spin-valves. The advantage of this method is the absence of electric current entering the part of the device where the detecting F/N contact is located. Ideally this leads to the independence of the measured Johnson-Silsbee voltage from the contact size and shape. However, it was predicted [1] that if the detecting contact is wide enough, then circular electric currents are excited in it, altering the non-local voltage and making it dependent on the device geometry. Calculations [1] were performed in the regime of highly transparent F/N boundary. At the same time, modern measurements often use low transparency F/I/N tunnel contacts, in which circular currents should be strongly suppressed. Thus it seems that tunnel contact measurements should conform to the classic Johnson-Silsbee formula. Here we show that while circular electric currents are indeed suppressed by the tunnel barrier, the voltage modifications persist, may be significant, and have to be accounted for in the data analysis. |
Monday, March 2, 2020 12:27PM - 12:39PM |
B41.00007: Magnetotransport in amorphous cobalt silicon and amorphous cobalt germanium thin films Christopher Fuchs, Dinah Simone Bouma, Zhanghui Chen, Frank Bruni, Arnoud Everhardt, Paul Corbae, Neal Reynolds, Lin-Wang Wang, Frances Hellman The transition-metal group IV compounds iron germanium and iron silicon show ferromagnetic properties in amorphous Fex(Ge,Si)1-x films [1], and the recently shown non-trivial topology in crystalline CoSi [2] suggests a study of amorphous cobalt silicon (a-CoxSi1-x) and cobalt germanium (a-CoxGe1-x) might lead to advances in the field of topological materials. A comparative study of a-CoxSi1-x and a-CoxGe1-x thin films has been performed for 0.4<x<0.7. At low temperatures, a-Cox(Ge,Si)1-x shows no magnetism for x<0.6 and weak ferromagnetism above x≈0.6, well over an order of magnitude smaller than in a-Fex(Ge,Si)1-x. While the Fe-based alloys show hole carriers, the Co-based alloys show electron carriers. Resistivity measurements show that ρxx ≥ 150µΩcm and dependents mainly on the carrier concentration, which is characteristic for amorphous metals. Hall measurements probe whether the anomalous Hall effect stems from the intrinsic (Berry curvature) mechanism, equal to the integral over occupied energy states of the density of Berry curvature, which is the sum of the spin-orbit correlations of local orbital states [1]. |
Monday, March 2, 2020 12:39PM - 12:51PM |
B41.00008: Two transient time in the charge and spin current Sung Po Chao, Narasimha Raju Chebrolu Using magnetic tunnel junction formed by two leads made of ferromagnetic metals and a thin insulating barrier, we study its time depedent charge and spin current following a square voltage pulse. Magnetic fluctuations in the tunnel junction is modeled as magnons, and its influence on the tunneling is treated as a local bosonic mode described by the spinful Anderson-Holstein model. We compute the time dependent spin and charge current perturbatively with Lang-Frisov transformation to handle the strong local electron-magnon or electron phonon interaction at finite temperature. Two transient behaviors are seen in the charge and spin currents. Those two time scales due to different spin and charge responses within one junction could be useful for parallel computing within a single unit. |
Monday, March 2, 2020 12:51PM - 1:03PM |
B41.00009: Sublattice Magnetization Driven Planar and Anomalous Hall Effect in β-W/Gdx(FeyCo1-y)1-x Heterostructures Anthony Johnson, Ezana Negusse, Dan Anyumba, Deandre McAmont, Ramesh Budhani The antiferromagnetic coupling between 4f and 3d spin sublattices in amorphous films of GdFeCo alloys is known to affect the temperature dependence of magnetization and magnetic anisotropy dramatically. Multilayers of these alloys also reveal non-trivial spin textures driven by dipolar and chiral exchange interactions. Here we present measurements of anomalous Hall (AH) and planar Hall (PH) voltages in β -W/Gdx(FeyCo1-y)1-x bilayer films across the sublattice magnetization compensation point and the temperature at which the magnetic anisotropy changes from in-plane to out-of-plane configuration. We observe a robust planar Hall Effect in these samples that tracks the behavior of magnetization measured with vibrating sample magnetometry. The magnetization switching fields extracted from the PH data allow calculations of anisotropy energy as a function of temperature and 4f/3d moment ratios in the films. The anomalous Hall data in the reversible and irreversible regimes of response are analyzed to extract the Berry phase and spin orbit torque contributions to Hall resistivity. |
Monday, March 2, 2020 1:03PM - 1:15PM |
B41.00010: Chirality-induced Spin Selectivity in a Two-terminal Semiconductor Device Tianhan Liu, Xiaolei Wang, Hailong Wang, Gang Shi, Fan Gao, Honglei Feng, Haoyun Deng, Longqian Hu, Eric Lochner, Pedro Schlottmann, Stephan Von Molnar, Yongqing Li, Jianhua Zhao, Peng Xiong Electrical generation of spin polarization in semiconductors is of great interest for the device potential in spintronics. One such mechanism is chirality-induced spin selectivity (CISS), with which structural chirality leads to different electric conductivities for electrons of opposite spins. CISS has been widely reported for chiral molecule assemblies on metal surfaces. However, theoretical understanding of the microscopic mechanism and manifestation of CISS in specific device structures remain controversial.1,2 Here, we report direct evidence of CISS in two-terminal devices of heterojunctions of (Ga,Mn)As/AHPA-L molecules/Au. The (Ga,Mn)As layer acts as a spin analyzer for electrons injected from the Au electrode through the AHPA-L monolayer. The CISS of AHPA-L and spin injection into the (Ga,Mn)As are manifested as sharp changes in the junction conductance at the coercive fields of the (Ga,Mn)As. The observations provide a definitive signature of CISS-induced spin valve effect in a two-terminal device. Theoretical implications of the effect and its bias dependence will be discussed. |
Monday, March 2, 2020 1:15PM - 1:27PM |
B41.00011: Resistivity Minimum in Dilute Itinerant Magnets Zhentao Wang, Cristian Batista Resistivity minima are commonly seen in itinerant magnets and they are often attributed to the Kondo effect. However, recent experiments are revealing an increasing number of materials showing resistivity minima in absence of the Kondo singlet formation. In a previous work, we demonstrated that the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction can produce a classical spin liquid state at finite temperature, whose resistivity increases with decreasing temperature. In this work, we investigate the robustness of the RKKY-induced resistivity upturn against site dilution. By series expansions and molecular dynamics simulation, we show that site dilution competes with thermal fluctuations and further stabilizes the upturn, which applies to both 2D and 3D dilute itinerant magnets. Under a magnetic field, the resistivity upturn is reduced due to suppression of the electron-spin scattering. |
Monday, March 2, 2020 1:27PM - 1:39PM |
B41.00012: Nonequilibrium noise measurements using hBN tunnel barriers Xuanhan Zhao, Liyang Chen, Panpan Zhou, Douglas Natelson Spin-orbit coupling, a relativistic effect coupling the spin angular momentum of an electron with its orbital angular momentum, has shown its significance for potential “spintronic” device applications. Nonequilibrium noise (or “shot”) noise, which comprises the inherent fluctuations in the electrical current of system driven out of equilibrium, is predicted to be an outstanding experimental probe for underlying physics in the spin-orbit coupling, since predictions shows the tunneling shot noise will be modified in the presence of spin accumulation. Here we report initial nonequilibrium charge current noise measurements in a variety of tunnel junctions prepared using hBN as monolayer tunnel barriers and strong spin hall angle materials as electros. We will also report flicker noise found during low-frequency noise measurement in Au/hBN/Au tunneling structures due to the presence of thermally excited dynamic defects. |
Monday, March 2, 2020 1:39PM - 1:51PM |
B41.00013: Theory of nonlinear Hall effects: modified semiclassics from quantum kinetics Cong Xiao, Zongzheng Du, Qian Niu We propose a modified Boltzmann nonlinear electric-transport framework which differs from the nonlinear generalization of the linear Boltzmann formalism by a contribution that has no counterpart in linear response. This contribution follows from the interband-coherence effect of dc electric-fields during scattering and is related to the interband Berry connection. As an |
Monday, March 2, 2020 1:51PM - 2:03PM |
B41.00014: Spin valves with exfoliated 2D materials: MoS2 Marta Galbiati, Florian Godel, Aymeric Vecchiola, Victor Zatko, Sergio Tatay, Regina Galceran, Samuel Mañas-Valero, Maëlis Piquemal-Banci, Marie-Blandine Martin, Alicia Forment-Aliaga, Eugenio Coronado, Bruno Dlubak, Pierre Seneor In the last years 2D materials have attracted a huge attention for spintronics thanks to the amazing properties that arise when thickness approaches the single layer level and thanks to the large number of functionalities that they offer. The recent introduction of 2D materials in magnetic tunnel junctions (2D-MTJs) offers very promising properties such as atomically defined interfaces, spin filtering, perpendicular anisotropy and spin-orbit torques modulation. Nevertheless, the difficult integration of exfoliated 2D materials in spintronic devices has limited so far their exploration and performances, maintaining experimental results still far from theoretical expectations [1]. Here, we will show successful fabrication of NiFe/MoS2/Co MTJs thanks to an in-situ fabrication process leading to the highest results reported so far for MTJs based on TMDCs 2D family [2]. Moreover, we will further discuss a path to alleviate fundamental technological and physics issues encountered for the integration of 2D-MTJs [2,3]. |
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B41.00015: Coherent Resonant Tunneling through Double Metallic Quantum-Well States Yuan LU, Bingshan Tao, Caihua Wan, Ping Tang, Jiafeng Feng, Hongxiang Wei, Xiao Wang, Stéphane Andrieu, Hongxin Yang, Mairbek Chshiev, Xavier Devaux, Thomas Hauet, Francois Montaigne, Stephane Mangin, Michel Hehn, Daniel Lacour, Xiufeng Han Study of resonant tunneling through multi-metallic quantum well (QW) structure is not only important for the fundamental understanding of quantum transport, but also for the great potential to generate advanced functionalities of spintronic devices. However, it remains challenging to engineer such structure due to the short electron phase coherence length in metallic QW system. Here, we demonstrate the successful fabrication of double-QW structure in a single fully epitaxial MTJ heterostructure, where two Fe QW layers are sandwiched between three MgAlOx tunnel barriers. We show clear evidence of the coherent resonant tunneling through the discrete QW states in the two QWs. Compared to the single QW structure, the resonant tunneling in double-QW MTJ produces strong conductivity oscillations with much narrower peak width (about half) owing to the enhanced energy filtering effect. This study presents a comprehensive understanding of the resonant tunneling mechanism in MTJ with multiple QWs, which is essential for future development of new spintronic devices operating in the quantum tunneling regime. |
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