Strain control of Morin temperature in epitaxial α-Fe₂O₃(0001) film

¹ Department of Physics, Pohang University of Science and Technology - Pohang 790-784, Korea
² Pohang Acceleration Laboratory, Pohang University of Science and Technology - Pohang 790-784, Korea
³ Division of Advanced Materials Science, Pohang University of Science and Technology - Pohang 790-784, Korea

^(a) masson@postech.ac.kr
^(b) jhp@postech.ac.kr

Received: 21 May 2013
Accepted: 21 July 2013

Abstract

We present comprehensive studies of strain effects on the spin reorientation transition (SRT), the so-called Morin transition, in α-Fe₂O₃(0001) films. The α-Fe₂O₃(0001) epitaxial films were grown with a Cr₂O₃ buffer layer on Al₂O₃(0001) substrates through an oxide molecular beam epitaxy. The antiferromagnetic spin axis was monitored by using the Fe L₂-edge X-ray magnetic linear dichroism. The buffer layer was found to introduce compressive strain into the α-Fe₂O₃(0001) film due to its 1.6% smaller in-plane lattice constant. The degree of strain is monotonically reduced with the increase of the α-Fe₂O₃ film thickness and becomes relaxed in the thick region (> 20 nm). The transition temperature T_M, which increases up to ≃360 K, well above the bulk T_M = 263 K, for the film thickness ≃3 nm, gradually decreases as the film thickness increases. We also examined the Néel temperature T_N in the ultra-thin region (< 3 nm), which rapidly drops with the decrease of the film thickness. The correlation between T_M and the strain in the α-Fe₂O₃(0001) epitaxial films was found to be well explained in terms of two competing energies of magnetic dipole anisotropy and single-ion magnetocrystalline anisotropy except for the ultra-thin region, in which T_N is dominated by the finite-size effects.