SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION Large voltage-induced netic anisotropy change in a few atomic layers of iron T. Maruyama 1, Y. Shiota 1, T. Noaki 1, K. Ohta 1, N. Toda 1, M. Miuguchi 1, A. A. Tulapurkar 1, T. Shinjo 1, M. Shiraishi 1, S. Miukami, Y. Ando 3 & Y. Suuki 1 1 Graduate school of Engineering Science, Osaka University, Toyonaka, Japan WPI Advanced Institute for Materials Research, Tohoku University, Sendai, Japan 3 Graduate school of Engineering, Tohoku University, Sendai, Japan Present Address: Institute for Materials Research, Tohoku University, Sendai, Japan 35 Magnetic anisotropy energy (kj/m 3 ) oltage () Supplementary figure 1. ysteresis behavior of the netic crystalline anisotropy energy change, induced by the application of a dc voltage.

2 The Fe layer thickness was.48 nm in this case. The arrows indicate the sequence of the voltage application. The effect exhibits a hysteresis as a function of the voltage application. This may be attributed to the electric field generated by the charges trapped in the / interface. Supplementary discussion 1. The possibility of the netostrictive being the origin of the voltage effect. As to the origin of the voltage effect, one can easily iine that a netostrictive effect causes a netic anisotropy change by the application of tensile force to a ferronetic layer, as has been shown for GaMnAs/PZT structure 1, for example. owever, the change in perpendicular anisotropy of 17 kj/m 3 requires a considerably large tensile force, on the order of 1 GPa in the case of the Fe[1] direction, whilst and polyimide layers do not exhibit such large pieoelectric effect. This pieoelectric free structure is to our advantage, since pieoelectric deformation leads to fatigue in, and short lifetime of, the device. 1. Overby, M., Chernyshov, A., Rokhinson, L. P., Liu, X. & Furdyna, J. K. GaMnAs-based hybrid multiferroic memory device. Appl. Phys. Lett. 9, 1951 (8) and references therein. Supplementary discussion. Electric field effect on band filling. We think that the applied electric field modulates the Fermi level of the ultrathin Fe layer and leads to the change in surface netic anisotropy. The Fe / / / ITO junction is considered to be a capacitor. Therefore, accumulated charges Q per one Fe atom at the surface is estimated as follows, Q d a Fe d where (= 9.8) and (= 3.4) are relative permittivity of the and polyimide layers, respectively. (= [F/m]) is permittivity of free space. And a Fe (=.86 nm ), d (= 1 nm), d (= 15 nm) are lattice constant

3 of Fe, and the thicknesses of the and polyimide layers, respectively. (= ) is the applied voltage. From this equation, the accumulated electron density, n, per Fe atom at the surface is calculated as.1-3 (electrons/atom). Due to the screening of electric fields caused by the mobile charge carriers, we can assume that the above estimated electron density change occurs only at the few topmost atomic layers in the Fe/Au electrode. From the n and the density of states (DOS) at Fermi energy, 41 (states/ry/atom) in the Fe case, the change in the Fermi level is estimated as follows. E F n DOS d Fe [ e ] ~.3 1[ me ] 41 (from1to3 ML) The change in the Fermi level depends on the Fe layer thickness. In the main t, we assumed d Fe =1 ML. ere we modified the value to include an uncertainty in the charge accumulation thickness. Although the modulation of the Fermi level seems to be very small, it is sufficient to cause the change of surface netic anisotropy, based on the first principles calculation reported by Kyuno et al.. And also, their calculations expect that the increase of the band filling causes a decrease in the perpendicular netic anisotropy in the Au / Fe(1) system, which is consistent with our experimental results.. Kyuno, K., a, J. G., Yamamoto, R. & Asano, S. First-principle calculation of the netic anisotropy energies of Ag/Fe(1) and Au/Fe(1) multilayers. J. Phys. Soc. Jpn. 65, (1996). Supplementary discussion 3. Simulation of the dynamic switching triggered by an anisotropy change. ere, we explain the method employed in the micronetic simulation shown in the t. The dynamics of the netiation of a small netic cell is well described by following Landau-Lifscht-Gilbert equation 3 : dm M dm M eff dt M dt s where and are the Gilbert damping factor and the gyronetic ratio,

4 respectively. M M, M, M x y represents the netiation of the netic cell. The Cartesian coordinates are taken as follows; x, y axes parallel to the in-plane easy axis and the in-plane hard axis, respectively, whilst the -axis is perpendicular to the film plane. ere, we assume that the spins align parallel in the netic cell and behave as a single macro-spin. This assumption is valid when the netic object is very small, as in our case, since we aim to use it for applications such as ultrahigh density solid state storage. Therefore, M is equal to the saturation netiation, M s. ere, eff is the effective field that is exerted on the netiation in the cell and is obtained from netic energy, E, per unit volume, as follows: eff 1 M s E, where /m is the netic susceptibility of the vacuum. E is expressed in terms of the Zeeman energy and the netic anisotropy energies as follows: where plane; 1 M 1 M x E // M Ku K, M s M s is the ernal netic field that is exerted perpendicular to the film K u is the uniaxial anisotropy constant of the netic cell, which depend on the applied voltage,, as described in the main t. K // is the in-plane netic anisotropy energy constant. The calculation of the netiation switching, shown in the main t was performed by taking following parameters, obtained from additional experiments and the literature. M s =1.71x1 6 A/m (From ref. [3]) =-.3x1 5 m/(a sec) ( using g=.1 from ref. [3]) =.5 (From the FMR measurement done for our ultrathin film.)

5 K u M K // M 1 ka/m:on state (From our experimental data) ka/m :Off state =1.6kA/m (Appropriate value to perform the switching) = 8 ka/m (Appropriate value to perform the switching) Although netic moment has certain -component, the resistance change caused by the in-plane switching, shown in main t, exceeds 9% of the total tunnel netoresistance effect, if we employ a counter electrode with in-plane netiation. Thus, it functions as an ideal memory/logic cell. 3. Chikaumi, S. Physics of Ferromgnetism, nd Ed., International series of monographs no physics 94, Oxford science, 1997.