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In this Talk, I will describe our theoretical investigations regarding the microscopic origin of the technologically important issue of the spin orientation i.e., the direction favoured by spins, on a correlated magnetic surface and how to manipulate it [1], and observed strain-driven spin-reorientation transitions (SRT) as investigated by experiments earlier [2-4]. Considering the prototypical example of a strongly correlated system, thin films of NiO grown on MgO [2] or on Ag [3], we show that a single particle approach with anisotropic hoppings, or even a many-electron model with a scalar Hubbard U and Hund’s J, fails to explain the strain-driven spin-reorientation transition (SRT). We set up a model treating both anisotropic single-particle effects and orbital-dependent electron-electron interactions on an equal footing. Our results clearly demonstrate that the symmetry and ordering of the many-body excited states that arise due to the inclusion of multipole (anisotropic) Coulomb interactions (ligand field multiplets), as against those in the presence of a scalar Hubbard-like U, or only anisotropic hoppings, decide the direction favoured by spins on thin films of correlated oxides. The model is also able to successfully reproduce the experimental trends, relating to both the easy axis of magnetization, as well as those in the X-ray magnetic linear dichroism (XMLD) spectra on the two substrates [2,3]. These results, augmented by a detailed symmetry analysis, support and further elucidate the novel possibility of using an electric field to control SRT in magnetic films grown on piezoelectric substrates.
References:
[1] S. Sen Gupta et al., arXiv:2507.14598 [cond-mat.str-e] (2025).
[2] D. Alders et al., Phys. Rev. B 57, 11623 (1998).
[3] D. Spanke et al., Phys. Rev. B 58, 5201 (1998).
[4] S. R. Krishnakumar et al., J. Magn. Magn. Mater 310, 8 (2007). |