Ferromagnetism in transparent thin films of MgO

We show both theoretical and experimental evidences of the appearance of ferromagnetism in MgO thin films. First-principles calculations allow predicting the possibility of the formation of a local moment in MgO, provided the existence of Mg vacancies which create holes on acceptor levels near the $\text{O}2p$-dominated valence band. Magnetic measurements evidence of the existence of room-temperature ferromagnetism in MgO thin films. High-resolution transmission electron microscopy demonstrates the existence of cation vacancies in our samples. Finally, by applying the element specificity of the x-ray magnetic circular dichroism technique, we also demonstrate that the magnetic moments of the system arise from the spin polarization of the $2p$ electrons of oxygen atoms surrounding Mg vacancies.

Figure 1

(Color online) Theoretical results on the magnetization dependence on cation vacancies. (a) Perspective view of the atomic layout in a $2\times 2\times 2$ times MgO unit cell, or sc 64 atoms, with the unit cell highlighted in bold dashed lines. The green (big) and red (small) circles correspond to Mg and O atoms, respectively. The vectors characterizing the different lattices, used for the construction of the periodic $V_{\text{Mg}}$ network, are depicted by lines: continuous for sc, dashed for bcc, and dotted for fcc. (b) Spin density for the sc 32 atoms supercell along the two different axis with short $\left(4.25\text{Å}\right)$ and long $\left(8.50\text{Å}\right)$ lengths. The lobe shape distribution resembles the $2p$ orbitals of the O atom. Inset: spin density plot at a constant value of $1.2\times {10}^{-3}\text{electrons}/\left({\text{Å}}^3\right)$ for the sc 32 atoms supercell with one $V_{\text{Mg}}$ located in the vertexes of the prism. (c) PDOS on the atoms of the fcc 54 atoms supercell lying within $3.7\text{Å}$ from the $V_{\text{Mg}}$ are shown by a continuous line. That PDOS is spin decomposed in major (positive) and minor (negative) components and is compared to the nonspin decomposed PDOS of the equivalent atoms in MgO absent of $V_{\text{Mg}}$ (dashed line). The Mulliken population $\left(Q\right)$ in $\left|e\right|$ and the magnetic moment (mm) in Bohr magneton of the selected atoms are also given compared to the bulk value in between parentheses.

Figure 2

(Color online) (a) HRTEM general view of the MgO layer (10 nm) grown on the STO substrate. Both layers grow along their respective [001] axis. (b) HRTEM detail of the MgO layer corresponding to the red squared area in (a). (c) HRTEM computer simulation of the same area in (b), simulation parameters: beam $\text{energy}=200\text{kV}$, $\text{Cs}=0.5$, $\text{Cc}=1.1$, energy $\text{spread}=0.8\text{eV}$, $\text{defocus}=-53.4\text{nm}$, and thickness 21.1 nm. (d) 3D intensity map of the experimental HRTEM image shown in (b). (e) 3D intensity map of the nine central atomic columns squared (cyan) in the experimental detail (b). (f) 3D intensity map of the nine central atomic columns squared (green) in the HRTEM computer simulation in (c). (g) Frontal view of the MgO supercell used to obtain the simulated image in (c). (h) Side view of the same supercell in (g).

Figure 3

(Color online) Hysteresis $M\left(H\right)$ curves of MgO(001) films of different thicknesses on ${\text{SrTiO}}_3$ taken at room temperature and 10 K. Magnetic field was applied parallel to the film surface. The films are transparent enough in the visible region to make the background text readable (inset on the right).

Figure 4

(Color online) Top: $\text{O}K$ edge x-ray scattering structure. Bottom: XRMS spectra collected at 120 K. The sample was magnetically saturated applying a magnetic field of 2 T nearly parallel to its surface plane. The angle of incidence of the radiation on the sample, with respect to the surface plane was ${\theta }_{\text{i}}=18°$. The degree of polarization of the incoming radiation was $P=0.9$. Spectra were collected by a photodiode set at angle $2{\theta }_{\text{i}}$.