Abstract
Epitaxial orthorhombic HoMnO${}_{3}$ films were grown on Nb-doped SrTiO${}_{3}$ $(001)$ single-crystal substrates. X-ray diffractometry showed a uniform crystallographic orientation with the $c$ axis along the substrate normal and an anisotropic compressive stress along the $b$ axis of the Pbnm structure. The magnetization of the films was dominated by the paramagnetism of the Ho${}^{3+}$ ions; the latter showed a strong anisotropy with respect to the in-plane and perpendicular-to-plane magnetic field direction. The rotational anisotropy of the magnetoelectric effect was measured for magnetic field rotation in the ${(110)}_{o}$, ${(1\overline{1}0)}_{o}$, and ${(001)}_{o}$ planes. Whereas magnetic field rotation in the ${(110)}_{o}$ and ${(1\overline{1}0)}_{o}$ planes showed a twofold pattern with the smallest magnetoelectric effect observed in magnetic fields along the $c$ axis, in-plane ${(001)}_{o}$ magnetic field rotations revealed an intricate rotational symmetry. A magnetic-field-induced crossover was observed from a low-field region with fourfold rotation patterns to a high-field region with rotation patterns up to the 12th order. This complex rotational symmetry arises from spin-orbit coupling of the Ho${}^{3+}$ moments that induces a modulation of the magnetoconductance as well as a magnetoelectric effect through the Maxwell-Wagner mechanism.
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