The magnetoelectric antiferromagnet $\alpha \text{−}\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3$ (chromia) is known to possess a roughness insensitive net equilibrium magnetization at the (0001) surface, called boundary magnetization (BM), which is coupled to the bulk antiferromagnetic order parameter. In order to verify whether this symmetry sensitive BM persists in alloys, we investigate the impact of diamagnetic dilution on chromia thin films alloyed with the isostructural $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ (alumina). Single-crystalline $\mathrm{C}{\mathrm{r}}_{2-x}\mathrm{A}{\mathrm{l}}_x{\mathrm{O}}_3$ thin films with (0001) surface orientation and varying stoichiometry have been grown by sputter codeposition in the concentration range between $x=0$ and $x=0.6$. For these samples, we find the corundum crystal structure, the antiferromagnetic ordering, and the boundary magnetization to be preserved. We also find that the critical temperature $T_{\mathrm{N}}$ can be tuned by alloying with $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, using the BM as a probe to study the magnetic phase transition. Furthermore, we were able to evaluate the critical exponent and the absolute BM values for different samples. Both properties corroborate that the observed magnetic signals originate from the BM rather than the bulk of the samples.

• Received 2 February 2015
• Revised 2 May 2015

DOI:https://doi.org/10.1103/PhysRevB.91.214403