Origin of Interface Magnetism in ${\mathrm{BiMnO}}_3/{\mathrm{SrTiO}}_3$ and ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ Heterostructures

Possible ferromagnetism induced in otherwise nonmagnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between ${\mathrm{SrTiO}}_3$ and two insulating polar oxides, ${\mathrm{BiMnO}}_3$ and ${\mathrm{LaAlO}}_3$. By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetism can be stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated with magnetic ${\mathrm{Ti}}^{3+}$ ions at the interface itself for ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ and to ${\mathrm{Mn}}^{3+}$ ions in the overlayer for ${\mathrm{BiMnO}}_3/{\mathrm{SrTiO}}_3$. In ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ the induced magnetism is quenched by annealing in oxygen, suggesting a decisive role of oxygen vacancies in this phenomenon.

Figure 1

Linear and circular dichroism in the XAS of ${\mathrm{SrTiO}}_3$ interfaces. (a) Schematics of the experimental setup: by absorption of a photon (zigzag red arrow) of appropriate energy and known polarization, a Ti or Mn $2p$ electron is promoted to the $3d$ states. The external magnetic field $\mathbf{B}$ is always parallel to the beam direction and the sample can be oriented at normal (0°) or 70° incidence. (b) The crystal structure of an ideal 4 unit cell polar insulating oxide film (${\mathrm{LaAlO}}_3$ or ${\mathrm{BiMnO}}_3$) deposited on ${\mathrm{TiO}}_2$ terminated ${\mathrm{SrTiO}}_3$ single crystal, and a pictorial view of the outcomes of Ti $L_{2,3}$ XMCD and XLD, which provide insight on the symmetry and occupation of lowest-lying $3d$ states ($\mathrm{Ti}\mathrm{\text{−}}3d_{xy}$ orbitals in the picture), and on the consequent magnetic moments (yellow arrow).

Figure 2

(a) Ti XAS spectra of $\mathrm{BMO}(4\mathrm{nm})/\mathrm{STO}$ (black circles), undoped STO crystal (red circles), and bulk ${\mathrm{LaTiO}}_3$ (blue circles); (b) XLD spectra at grazing incidence (black circles) and integrated XLD (blue line) of $\mathrm{BMO}/\mathrm{STO}$, compared to the spectrum calculated in single ion crystal field model (red line) [splitting $(t_{2g})=-50\mathrm{meV}$ and $(e_g)=-100\mathrm{meV}$]; (c) same for XMCD at 4 T, where the theoretical spectra include also charge transfer and a magnetic exchange energy ($-10\mathrm{meV}$, see text). (d),(e) Ti and Mn XMCD spectra of $\mathrm{BMO}/\mathrm{STO}$ as a function of magnetic field (bottom panels, offset for clarity) and the typical XAS spectra at 5 T and 8 K (upper panels, red and black curves). (f) Schematics of the charge-transfer process from ${\mathrm{Mn}}^{3+}$ to a ${\mathrm{Ti}}^{4+}$ and arrangement of their magnetic moments at the interface.

Figure 3

X-ray linear and circular dichroism of $\mathrm{LAO}/\mathrm{STO}$ interfaces. XAS spectra of similar $\mathrm{LAO}(10\mathrm{\text{−}}\mathrm{uc})/\mathrm{STO}$ samples, one (a) containing oxygen vacancies (nonannealed, blue circles) and one (b) annealed in oxygen (standard $\mathrm{LAO}/\mathrm{STO}$, black circles); insulating STO (red line) is shown as reference. Corresponding XLD (c),(d) and XMCD (e),(f) spectra. Stars in XLD and red sticks in XMCD indicate the energy of the main ${\mathrm{Ti}}^{4+}$ XAS peaks, while blue sticks indicate those of the ${\mathrm{Ti}}^{3+}$ XAS. The XMCD spectra measured at lowest $T$ and highest $B$ have been integrated leading to a sizable orbital moment in the vacancy rich sample (g) and to an almost vanishing value for the defect free sample (h).

Figure 4

Magnetism in $\mathrm{LAO}/\mathrm{STO}$ heterostructures: (a) Normalized Ti XMCD of $\mathrm{BMO}/\mathrm{STO}$ (black circles, scaled by a factor 0.1), of standard $\mathrm{LAO}/\mathrm{STO}$ (red circles) and of oxygen-vacancy rich $\mathrm{LAO}/\mathrm{STO}$ (blue open circles). (b) Single ion calculations of the XMCD spectra for ${\mathrm{Ti}}^{4+}$ (black line) and ${\mathrm{Ti}}^{3+}$ (blue line), using an exchange field of $-10\mathrm{meV}$ and $+10\mathrm{meV}$, respectively. (c) The field dependence of the orbital moment at 8 K (black circles) and at 3 K (red squares) as obtained from the XMCD integral for oxygen-vacancy-rich $\mathrm{LAO}/\mathrm{STO}$ (some of the corresponding spectra in the inset). Cartoon: in $\mathrm{LAO}/\mathrm{STO}$ oxygen vacancies induce spin and orbital moments at the ${\mathrm{Ti}}^{3+}$ sites; the Q2DEG gets polarized by these magnetic impurities and mediates the magnetic interaction.