Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T47: Invited Session: The Effect of Electric Fields on Magnetism |
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Sponsoring Units: GMAG Chair: Chia-Ling Chien, Johns Hopkins University Room: Hilton Baltimore Holiday Ballroom 6 |
Thursday, March 21, 2013 8:00AM - 8:36AM |
T47.00001: Voltage controlled magnetic anisotropy in magnetic tunnel junctions Invited Speaker: Weigang Wang Recently, voltage controlled magnetic anisotropy (VCMA) in 3d transitional ferromagnets (FM) has attracted a great deal of attentions. VCMA has traditionally been explored in multiferroic materials and diluted magnetic semiconductors, but not in metals because of the anticipated negligible effects since the electric field would be screened within 1-2 {\AA} at the metal surface. However, a voltage may exert marked effects if the magnetic properties of ultrathin films are dominated by interfacial magnetic anisotropy. Here we demonstrate a large VCMA effect in perpendicular MgO magnetic tunnel junctions (p-MTJs) with very thin CoFeB layers. The p-MTJs have the key structure of Co40Fe40B20(1.2-1.3nm)/MgO(1.2-2nm)/Co40Fe40B20(1.6nm) exhibiting at room temperature tunneling magnetoresistance in excess of 100{\%}. The perpendicular magnetic anisotropy (PMA) in this system is believed to be stabilized by hybridization between the out-of-plane 3d orbitals of the FM and oxygen 2p orbitals. We show that both the magnitude and the direction of the electric field can systematically alter the PMA of the thin CoFeB layers interfaced with the MgO barrier. Furthermore, under a given electric field, the two CoFeB layers on either side of the MgO barrier respond in the opposite manner as expected. By exploiting the combined effect of spin transfer torque and VCMA in CoFeB/MgO/CoFeB nanopillars, we have accomplished voltage controlled spintronic devices, where the MTJ can be manipulated by a unipolar switching process using consecutive negative voltages less than 1.5 V in magnitude. In this manner, voltage can access the high resistance or the low resistance state of an MTJ with very small current densities. Wang, W.-G., Li, M., Hageman, S. {\&} Chien, C. L. Electric-field-assisted switching in magnetic tunnel junctions. Nature Materials 11, 64 (2012). [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 9:12AM |
T47.00002: Voltage-Induced Ferromagnetic Resonance in Magnetic Tunnel Junctions Invited Speaker: Ilya Krivorotov Excitation of sub-nanosecond magnetic dynamics by an electric field is a grand challenge in the field of spintronics. The ability to perform high-speed manipulation of magnetization by electric fields rather than by current-induced spin torques or magnetic fields would greatly improve energy efficiency of spintronic devices such as nonvolatile magnetic memory and logic. In this talk, I will discuss our experiments on excitation of ferromagnetic resonance in CoFeB/ MgO/ CoFeB magnetic tunnel junctions by the combined action of voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque [1]. Our measurements reveal that GHz-frequency VCMA torque and spin torque in low resistance (resistance-area product of a few Ohm $\cdot$ $\mu$m$^{2})$ CoFeB-based magnetic tunnel junctions have similar magnitudes, and thus that both torques are equally important for understanding high-speed voltage-driven magnetization dynamics in CoFeB magnetic tunnel junctions such as magnetization switching and auto-oscillations induced by spin torque. As an example, we show that VCMA can increase the sensitivity of a microwave signal detector based on a magnetic tunnel junction to the sensitivity level of semiconductor Schottky diodes. Our measurements also demonstrate that ferromagnetic resonance in high resistance magnetic tunnel junctions can be excited by VCMA alone without a significant contribution from the spin torque drive. I will conclude this talk with a discussion on how voltage-induced ferromagnetic resonance can be used for quantitative measurements of various voltage-dependent torques in magnetic tunnel junctions: in-plane and perpendicular spin torques as well as VCMA torque. \\[4pt] [1] J. Zhu \textit{et al.}, Phys. Rev. Lett. \textbf{108}, 197203 (2012) [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:48AM |
T47.00003: Dynamic magnetization switching and spin wave excitations by voltage-induced torque Invited Speaker: Yoichi Shiota The effect of electric fields on ultrathin ferromagnetic metal layer is one of the promising approaches for manipulating the spin direction with low-energy consumption, localization, and coherent behavior. Several experimental approaches to realize it have been investigated using ferromagnetic semiconductors [1], magnetostriction together with piezo-electric materials [2], multiferroic materials [3], and ultrathin ferromagnetic layer [4-9]. In this talk, we will present a dynamic control of spins by voltage-induced torque. We used the magnetic tunnel junctions with ultrathin ferromagnetic layer, which shows voltage-induced perpendicular magnetic anisotropy change. By applying the voltage to the junction, the magnetic easy-axis in the ultrathin ferromagnetic layer changes from in-plane to out-of-plane, which causes a precession of the spins. This precession resulted in a two-way toggle switching by determining an appropriate pulse length [8]. On the other hand, an application of rf-voltage causes an excitation of a uniform spin-wave [9]. Since the precession of spin associates with an oscillation in the resistance of the junction, the applied rf-signal is rectified and produces a dc-voltage. From the spectrum of the dc-voltage as a function of frequency, we could estimate the voltage-induced torque.\\[4pt] [1] H. Ohno, \textit{et al., Nature} \textbf{408}, 944-946 (2000), D. Chiba, \textit{et al, Science} \textbf{301}, 943-945 (2003). \newline [2] V. Novosad, \textit{et al., J. Appl. Phys.} \textbf{87}, 6400-6402 (2000), J. --W. Lee, \textit{et al., Appl. Phys. Lett.} \textbf{82}, 2458-2460 (2003). \newline [3] W. Eerenstein, \textit{et al., Nature} \textbf{442}, 759-765 (2006), Y. --H. Chu, \textit{et al., Nature Materials} \textbf{7}, 478-482 (2008). \newline [4] M. Weisheit, \textit{et al., Science} \textbf{315}, 349-351 (2007). \newline [5] T. Maruyama, \textit{et al., Nature Nanotechnology} \textbf{4}, 158-161 (2009). \newline [6] M. Endo, \textit{et al., Appl. Phys. Lett.} \textbf{96}, 212503 (2010). \newline [7] D. Chiba, \textit{et al., Nature Materials} \textbf{10}, 853 (2011). \newline [8]Y. Shiota, \textit{et al., Nature Materials} \textbf{11}, 39 (2012) \newline [9]T. Nozaki, \textit{et al., Nat. Phys}. \textbf{8}, 491 (2012) [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:24AM |
T47.00004: Electric Field as Switching Tool for Magnetic States in Atomic-Scale-Nanostructures Invited Speaker: Valeri Stepanyuk We present the state of the art ab initio studies of the effect of the external electric field on electronic, magnetic and transport properties of atomic-scale nanostructures on metal surfaces. We demonstrate a possibility of a local control and switching of magnetism in such nanostructures [1]. The effect of the electric field on surface-state electrons is discussed [2]. Our results reveal that the local spin-polarization of electrons and the local magnetoresistance on nanoislands can be tuned by the electric field [3,4]. Our studies give a clear evidence that an external surface charging can strongly affect substrate-mediated exchange interactions [5].\\[4pt] [1] N. N. Negulyaev, V.S. Stepanyuk, W. Hergert, J. Kirschner, Phys. Rev. Lett. {\bf106}, 037202 (2011)\\[0pt] [2] P.A. Ignatiev and V.S. Stepanyuk, Phys. Rev. B {\bf84}, 075421 (2011)\\[0pt] [3] H. Oka, P.A. Ignatiev, S. Wedekind, G. Rodary, L. Niebergall, V.S. Stepanyuk, D. Sander, J. Kirschner, Science {\bf327}, 843 (2010)\\[0pt] [4] P.A. Ignatiev, O.O. Brovko, V.S. Stepanyuk, Phys. Rev.B {\bf 86}, 045409 (2012) \\[0pt] [5] L. Juarez-Reyes, G.M. Pastor, V.S. Stepanyuk, Phys. Rev. B, in press [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 11:00AM |
T47.00005: Electrically-induced ferromagnetism at room temperature in (Ti,Co)O$_{2}$: carrier-mediated ferromagnetism Invited Speaker: Tomoteru Fukumura Oxide-diluted magnetic semiconductors (DMS) is expected to have high Curie temperature via carrier-mediated ferromagnetism through heavy electron mass and large electron carrier density. We have studied various oxide-DMS such as (Zn,Mn)O [1], and discovered room temperature ferromagnetism in (Ti,Co)O$_{2}$ [2]. The origin of ferromagnetism has been discussed for a decade. Previously, the control of ferromagnetism was demonstrated through carrier control by chemical doping [3]. But it was difficult to exclude the defect-mediated ferromagnetism, since the electron donor was the oxygen vacancy [4]. In order to evidence the carrier-mediated ferromagnetism, the electric field control of ferromagnetism is useful [5]. The control of ferromagnetism at room temperature is also important for implementation of spintronic devices. By gating with electric double layer transistor, the ferromagnetism was induced at room temperature, representing electron carrier-mediated ferromagnetism [6]. Chemical doping study in (Ti,Co)O$_{2}$ for wider range of carrier density exhibited clearer paramagnetic insulator to ferromagnetic metal transition with increasing carrier density [7]. At a medium carrier density, a ferromagnetic insulator phase appeared possibly related with a phase separation between ferromagnetic and paramagnetic phases. Also, a superparamagnetic phase appeared for excessively reduced sample. Taking all these results into account, previously proposed extrinsic mechanisms such as oxygen vacancy-mediated mechanism [4], metal segregation [8], and superparamagnetism [9] are not correct picture of the ferromagnetism. This study was in collaboration with Y. Yamada, K. Ueno, M. Kawasaki, H. T. Yuan, H. Shimotani, Y. Iwasa, L. Gu, S. Tsukimoto, Y. Ikuhara, A. Fujimori, and T. Mizokawa.\\[4pt] [1] T. Fukumura et al., APL 75, 3366 (1999); [2] Y. Matsumoto et al., Science 291, 854 (2001); [3] H. Toyosaki et al., Nature Mater. 3, 221 (2004); [4] K. A. Griffin et al., PRL 94, 157204 (2005); [5] H. Ohno et al., Nature 408, 944 (2000); [6] Y. Yamada et al., Science 332, 1065 (2011); [7] Y. Yamada et al., APL 99, 242502 (2011); [8] J.-Y. Kim et al., PRL 90, 017401 (2003); [9] S. R. Shinde et al., PRL 92, 166601 (2004). [Preview Abstract] |
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