Electric-field-induced changes of magnetic moments and magnetocrystalline anisotropy in ultrathin cobalt films

In this study, the microscopic origins of the voltage-controlled magnetic anisotropy (VCMA) in $3d$-ferromagnetic metals are revealed. Using in situ x-ray fluorescence spectroscopy that provides a high quantum efficiency, electric-field-induced changes in orbital magnetic moment and magnetic dipole $T_{\mathrm{z}}$ terms in ultrathin Co films are demonstrated. An orbital magnetic moment difference of $0.013{\mu }_{\mathrm{B}}$ was generated in the presence of electric fields of $\pm 0.2\mathrm{V}/\mathrm{nm}$. The VCMA of Co was properly estimated by the induced change in orbital magnetic moment, according to the perturbation theory model. The induced change in the magnetic dipole $T_{\mathrm{z}}$ term only slightly contributed to the VCMA in $3d$-ferromagnetic metals.

Figure 1

Schematic of the sample structure and measurement configuration. An external electric field was applied to the ferromagnetic ultrathin Co film with a two-dimensional square lattice via a dielectric consisting of MgO and ${\mathrm{SiO}}_2$. XAS/XMCD measurements were performed using the PFY method with an SDD.

Figure 2

(a) Polarization-averaged x-ray absorption $\left({\mu }_{+}+{\mu }_{-}\right)$/2 and (b) its XMCD $\left({\mu }_{+}-{\mu }_{-}\right)$ spectra obtained using the PFY method around the Co-absorption edge. An external magnetic field of $\pm 1.9\mathrm{T}$ was applied to saturate the magnetization of the Fe/Co layer in the magnetic-field direction. The inset shows the magnetization as a function of the external magnetic field $H$ measured by XMCD at the Co-$L_3$ edge (778.4 eV). The measurements were conducted in a magnetic field with $\theta ={20}^{\circ }$.

Figure 3

External-voltage-induced changes in XMCD at the (a) Co-$L_3$ (778.4 eV) and (b) Co-$L_2$ (793.8 eV) edges obtained using the PFY method. External-voltage-induced changes in the (c) orbital magnetic moment $m_{\mathrm{L}}$ and (d) effective spin magnetic moment $m_S-7m_{\mathrm{T}}$ of Co. An external voltage of $\pm 3\mathrm{V}$ corresponds to an external electric field of $\pm 0.2\mathrm{V}/\mathrm{nm}$ in the 2-nm-MgO dielectric. An external magnetic field of $\pm 1.9\mathrm{T}$ was applied to saturate the magnetization of the Fe/Co layer in the magnetic-field direction. A negative bias voltage, where holes accumulate at the Co-MgO interface, increases both the $m_{\mathrm{L}}$ and $m_S-7m_{\mathrm{T}}$.

Figure 4

(a) Computational model with the induced change of charge density. The induced change of the charge density at an electric field of +0.2 V/nm in the MgO is subtracted from that at −0.2 V/nm. The blue and red areas represent the hole accumulation and depletion, respectively. (b) Electric-field-induced changes to the perpendicular MAE of Co and Fe atoms at the MgO interface calculated with Eq. (2). The MAE at +0.2 V/nm in the MgO is subtracted from that at −0.2 V/nm. The spin-conserved-term-induced values of the MAE change $\left(\delta E_{\uparrow \uparrow }+\delta E_{\downarrow \downarrow }\right)$ in Co provide the dominant contribution to the VCMA.