Microscopic Investigation into the Electric Field Effect on Proximity-Induced Magnetism in Pt

Electric field effects on magnetism in metals have attracted widespread attention, but the microscopic mechanism is still controversial. We experimentally show the relevancy between the electric field effect on magnetism and on the electronic structure in Pt in a ferromagnetic state using element-specific measurements: x-ray magnetic circular dichroism (XMCD) and x-ray absorption spectroscopy (XAS). Electric fields are applied to the surface of ultrathin metallic Pt, in which a magnetic moment is induced by the ferromagnetic proximity effect resulting from a Co underlayer. XMCD and XAS measurements performed under the application of electric fields reveal that both the spin and orbital magnetic moments of Pt atoms are electrically modulated, which can be explained not only by the electric-field-induced shift of the Fermi level but also by the change in the orbital hybridizations.

Figure 1

(a) Schematic cross section of the electric double-layer capacitor. The ionic liquid is used to apply the electric field to the surface of the ferromagnetic Pt layer. (b) Schematic image of the measurement configuration for XMCD and XAS.

Figure 2

(a) The XMCD intensities $I_{\mathrm{XMCD}}$ at Pt $L_3$ and $L_2$ edges for $V_G=+6$ and $-4\mathrm{V}$ (upper panels) and their difference $\mathrm{\Delta }{I}_{\mathrm{XMCD}}[=I_{\mathrm{XMCD}}(+6\mathrm{V})-I_{\mathrm{XMCD}}(-4\mathrm{V})]$ (bottom panels). (b) The XAS intensities $I_{\mathrm{XAS}}$ at Pt $L_3$ and $L_2$ edges for $V_G=+6$ and $-4\mathrm{V}$ (upper panels) and their difference $\mathrm{\Delta }{I}_{\mathrm{XAS}}[=I_{\mathrm{XAS}}(+6\mathrm{V})-I_{\mathrm{XMCD}}(-4\mathrm{V})]$ (bottom panels). (c) The orbital and the effective spin magnetic moments ($m_o^{\perp }$ and $m_{s,\mathrm{eff}}^{\perp }$, shown in the right and left panels, respectively) determined using the sum rules [32, 33]. The horizontal axis corresponds to the number of measurements, where $V_G$ is changed in the order of $+6\to -4\to +6\to -4\mathrm{V}$.

Figure 3

(a) The calculated $I_{\mathrm{XMCD}}$ spectra around Pt $L_3$ and $L_2$ edges for the electric field of $\pm 10\mathrm{V}/\mathrm{nm}$ and their difference $\mathrm{\Delta }{I}_{\mathrm{XMCD}}[=I_{\mathrm{XMCD}}(+10\mathrm{V}/\mathrm{nm})-I_{\mathrm{XMCD}}(-10\mathrm{V}/\mathrm{nm})]$. $\mathrm{\Delta }{I}_{\mathrm{XMCD}}$ is magnified by a factor of 10. The inset shows the slab model used that consisted of a Pd (four MLs)/Co (two MLs)/Pt (two MLs) from the bottom side (ML, monolayer). The electric field is applied to the surface of the Pt layer. (b) The calculated $I_{\mathrm{XAS}}$ spectra around Pt $L_3$ and $L_2$ edges for the electric field of $\pm 10\mathrm{V}/\mathrm{nm}$ and their difference $\mathrm{\Delta }{I}_{\mathrm{XAS}}[=I_{\mathrm{XAS}}(+10\mathrm{V}/\mathrm{nm})-I_{\mathrm{XAS}}(-10\mathrm{V}/\mathrm{nm})$]. $\mathrm{\Delta }{I}_{\mathrm{XAS}}$ is magnified by a factor of 50. (c) The differences of the densities of states ($\mathrm{\Delta }\mathrm{DOS}$) calculated for the positive and negative electric fields ($\pm 10\mathrm{V}/\mathrm{nm}$). The lines indicate the $\mathrm{\Delta }\mathrm{DOS}$ for $6s$, $6p$, and $5d$ orbitals, and those for $s$ and $p$ orbitals are magnified by a factor of 3.