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The various paramagnetic and metal-insulator transitions in LaCoO₃ make it among the most intriguing of the perovskite oxides, and the discovery of ferromagnetism in epitaxial LaCoO₃ further intensified the drive to understand the nature of its magnetic and electronic properties [1]. Key to achieving this goal is the elucidation of the atomic and electronic structural responses of LaCoO₃ to epitaxial strain. X-ray diffraction and extended x-ray absorption fine structure (EXAFS) spectroscopy reveal that tensile strain lengthens the in-plane Co-O bonds, thereby distorting the cobalt containing oxygen octahedra, and lowers the symmetry from rhombohedral in bulk to monoclinic [2]. These structural distortions reduce the Co-O orbital hybridization as observed in the x-ray absorption near edge structure (XANES) and stabilize an ordered arrangement of Co sites. In turn, concurrent order of Co charge and spin states arises, as shown by a combination of resonant x-ray scattering and computational density functional theory (DFT) calculations [3]. In essence, spin state order is frozen by epitaxial strain, precipitating the unique transition from paramagnet to ferromagnet found in LaCoO₃ films.

[1] D. Fuchs, C. Pinta, T. Schwarz, P. Schweiss, P. Nagel, S. Schuppler, R. Schneider, M. Merz, G. Roth, and H. v. Lohneysen, Phys. Rev. B 75, 144402 (2007).

[2] G. E. Sterbinsky, P. J. Ryan, J.-W. Kim, E. Karapetrova, J. X. Ma, J. Shi, and J. C. Woicik, Phys. Rev. B 85, 020403(R) (2012)

[3] G. E. Sterbinsky, R. Nanguneri, J. X. Ma, J. Shi, E. Karapetrova, J. C. Woicik, H. Park, J.-W. Kim, and P. J. Ryan, Phys. Rev. Lett. 120, 197201 (2018)