$d^0$ Ferromagnetic Interface between Nonmagnetic Perovskites

We use computational and experimental methods to study $d^0$ ferromagnetism at a charge-imbalanced interface between two perovskites. In ${\mathrm{SrTiO}}_3/{\mathrm{KTaO}}_3$ superlattice calculations, the charge imbalance introduces holes in the ${\mathrm{SrTiO}}_3$ layer, inducing a $d^0$ ferromagnetic half-metallic 2D hole gas at the interface oxygen $2p$ orbitals. The charge imbalance overrides doping by vacancies at realistic concentrations. Varying the constituent materials shows ferromagnetism to be a general property of hole-type $d^0$ perovskite interfaces. Atomically sharp epitaxial $d^0$ ${\mathrm{SrTiO}}_3/{\mathrm{KTaO}}_3$, ${\mathrm{SrTiO}}_3/{\mathrm{KNbO}}_3$, and ${\mathrm{SrTiO}}_3/{\mathrm{NaNbO}}_3$ interfaces are found to exhibit ferromagnetic hysteresis at room temperature. We suggest that the behavior is due to the high density of states and exchange coupling at the oxygen $t_{1g}$ band in comparison with the more studied $d$ band $t_{2g}$ symmetry electron gas.

Figure 1

Calculated ${\mathrm{SrTiO}}_3/{\mathrm{KTaO}}_3$ $n$-type (left) and $p$-type (right) superlattices, with nominal ionic charges of the layers shown.

Figure 2

Calculated (a) $p$-type and (b) $n$-type ${\mathrm{SrTiO}}_3/{\mathrm{KTaO}}_3$ 1.5-1.5 superlattice magnetization (squares, black) and energies of the FM (triangles, blue) and AFM (upside down triangles, red) phases relative to the PM solution, as a function of $U_{\mathrm{eff}}=U-J$ on oxygen $2p$ and $\mathrm{Ti}/\mathrm{Ta}$ $3d/5d$ orbitals. The values are per $p(2\times 2)$ supercell in $\mathrm{GGA}+U$. The lines are guides for the eye. FM phase HSEsol energy and magnetization are marked with dashed arrows.

Figure 3

(a) High-resolution HAADF image of the ${\mathrm{KTaO}}_3/{\mathrm{SrTiO}}_3$ interface along the [100] zone axis. The inset shows the local fast Fourier transform pattern. The different elemental atoms close to the interface are marked by colored open circles. (b) Intensity profile of atom columns averaged over the indicated area from $A$ to $B$ in (a).

Figure 4

(a) Absorption coefficient $\alpha$ as a function of photon energy $E$ in the ${\mathrm{SrTiO}}_3$ substrate (blue curve), ${\mathrm{KTaO}}_3$ film (black curve), and interfacial nanolayer (red curve). The spectra are obtained from ellipsometric data for the model stack shown schematically. (b) SQUID measured total magnetization as a function of external magnetic field determined at room temperature in as-deposited $10\times 5{\mathrm{mm}}^2$ films of ${\mathrm{KTaO}}_3/{\mathrm{SrTiO}}_3$ (red curve), ${\mathrm{KNbO}}_3/{\mathrm{SrTiO}}_3$ (blue curve), and ${\mathrm{NaNbO}}_3/{\mathrm{SrTiO}}_3$ (black curve).

Figure 5

Densities of states for $1.5/1.5$ ${\mathrm{SrTiO}}_3/{\mathrm{KTaO}}_3$ $p$-type superlattice with (a) HSEsol hybrid functional, (b) GGA, and (c) $\mathrm{GGA}+U$, $U_{\mathrm{eff}}=2\mathrm{eV}$.