Abstract
The low-temperature magnetic structures of the orthorhombic perovskite HoMnO 3 have been studied on a polycrystalline sample from neutron diffraction, specific heat and susceptibility data. With cooling, HoMnO 3 exhibits three singularities at T N=41 K, T≈26 K and T≈6.5 K, which suggests a rich magnetic phase diagram. At T N, the Mn 3+ magnetic moments become ordered in an incommensurate antiferromagnetic (AFM) arrangement, defined by a (C x ,0,0) mode, adopting a modulated sinusoidal magnetic structure which becomes commensurate with the unit cell below 29 K. Finally, below 22 K, an ordered magnetic moment appears on Ho 3+ cations in an antiferromagnetic arrangement defined by a (A x ,0,C z ) mode; this moment significantly increases below 6.5 K, reaching a value of 7.27(11) μ B at 1.8 K. The different magnetic structures are interpreted on the basis of competing superexchange interactions of opposite signs.
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