Ultrathin regime growth of atomically flat multiferroic gallium ferrite films with perpendicular magnetic anisotropy

Room temperature magnetoelectric multiferroic thin films offer great promises for the spintronics industry. The actual development of devices, however, requires the production of ultrathin atomically smooth films of high crystalline quality in order to increase spin transfer efficiency. Using both high-resolution transmission electron microscopy and atomically resolved electron energy loss spectroscopy, we unveil the complex growth mechanism of a promising candidate, gallium ferrite. This material, with its net room-temperature magnetization of approximately $100\phantom{\rule{0.16em}{0ex}}\mathrm{emu}/\mathrm{c}{\mathrm{m}}^{3}$, is an interesting challenger to the antiferromagnetic bismuth ferrite. We obtained atomically flat gallium ferrite ultrathin films with a thickness control down to one fourth of a unit cell. Films with thicknesses as low as 7 nm are polar and show a perpendicular magnetic anisotropy of $3\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}\mathrm{J}/{\mathrm{m}}^{3}$ at 300 K, which makes them particularly attractive for spin current transmission in spintronic devices, such as spin Hall effect based heavy-metal / ferrimagnetic oxide heterostructures.