Ferromagnetic boundary magnetization properties of epitaxial Cr$_{\mathrm{(2-x)}}$Al$_{\mathrm{(x)}}$O$_{3}$ thin films

The existence of an equilibrium net magnetization at (0001) surfaces is enabled by symmetry constrains for the magnetoelectric antiferromagnet $\alpha $-Cr$_{2}$O$_{3}$. The occurrence of this boundary magnetization (BM) is furthermore roughness insensitive [1]. The BM is hereby fully coupled to the bulk antiferromagnetic order parameter and can be reversed together with it by a combination of E and H fields in bulk materials [2], or solely by magnetic means for single crystal (0001) oriented thin chromia films [3]. In order to understand whether the BM can be extended to alloys containing different oxide materials, we investigated the effect of Al$_{2}$O$_{3}$ doping onto the structural and magnetic properties of $\alpha $-Cr$_{2}$O$_{3}$. We grew, using a hybrid growth procedure, 100 nm thick high-quality epitaxial Cr$_{\mathrm{2-x}}$Al$_{\mathrm{x}}$O$_{3}$(0001) thin films in the concentration range between x$=$0 to x$=$0.6, preserving the original corundum crystal structure and symmetry. Using SQUID magnetometry, we showed that the critical temperature T$_{\mathrm{N}}$ can be tuned by alloying with $\alpha $-Al$_{2}$O$_{3}$ using the BM as a probe to study the magnetic transition. Furthermore, we were able to evaluate the critical exponent and the absolute BM values for different samples. Both properties are consistent with the expected values, corroborating the BM nature of the observed magnetic signals. References: [1] K. D. Belashchenko, Phys. Rev. Lett. \textbf{105}, 147204 (2010); [2] X. He et al., Nat. Mater. \textbf{9},579 (2010); [3] L. Fallarino et al., Appl. Phys. Lett. \textbf{104}, 022403 (2014)