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In the present study we have demonstrated epitaxial stabilization of themetastable magnetically-hard $\varepsilon$-Fe$_2$O$_3$ phase on top of a thinMgO(111) buffer layer grown onto the GaN (0001) surface. The primary purpose tointroduce a 4\,nm-thick buffer layer of MgO in between Fe$_2$O$_3$ and GaN wasto stop thermal migration of Ga into the iron oxide layer. Though suchmigration and successive formation of the orthorhombic GaFeO$_3$ was supposedearlier to be a potential trigger of the nucleation of the isostructural$\varepsilon$-Fe$_2$O$_3$, the present work demonstrates that the growth ofsingle crystalline uniform films of epsilon ferrite by pulsed laser depositionis possible even on the MgO capped GaN. The structural properties of the 60\,nmthick Fe$_2$O$_3$ layer on MgO / GaN were probed by electron and x-raydiffraction, both suggesting that the growth of $\varepsilon$-Fe$_2$O$_3$ ispreceded by formation of a thin layer of $\gamma$-Fe$_2$O$_3$. The presence ofthe magnetically hard epsilon ferrite was independently confirmed bytemperature dependent magnetometry measurements. The depth-resolved x-ray andpolarized neutron reflectometry reveal that the 10\,nm iron oxide layer at theinterface has a lower density and a higher magnetization than the main volumeof the $\varepsilon$-Fe$_2$O$_3$ film. The density and magnetic moment depthprofiles derived from fitting the reflectometry data are in a good agreementwith the presence of the magnetically degraded $\gamma$-Fe$_2$O$_3$ transitionlayer between MgO and $\varepsilon$-Fe$_2$O$_3$. The natural occurrence of theinterface between magnetoelectric $\varepsilon$- and spin caloritronic$\gamma$- iron oxide phases can enable further opportunities to design novelall-oxide-on-semiconductor devices.

Stabilization of $\varepsilon$-Fe$_2$O$_3$ epitaxial layer on MgO(111)/GaN via an intermediate $\gamma$-Fe$_2$O$_3$-phase