Dynamic magnetization switching and spin wave excitations by voltage-induced torque*

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)