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The magnetic properties and the low-temperature magnetic structures of the orthorhombic perovskite HoMnO3 (space group Pnma) have been studied on polycrystalline samples by magnetization, specific heat, and neutron diffraction measurements. By cooling, HoMnO3 exhibits three singularities at T N = 41 K, T ≈ 26 K, and T ≈ 6.5 K, suggesting a rich magnetic phase diagram. The neutron diffraction data show that below T N = 41 K, the Mn3+ magnetic moments become ordered in an antiferromagnetic arrangement, adopting a modulated sinusoidal magnetic structure characterized by the wave vector k = (k x ,0,0) (k x = 0.40 at 41 K) and defined by the magnetic mode (C x ,0,0). When the temperature is decreased, the propagation vector varies and at T ≈ 29 K a transition to a commensurate magnetic structure defined by k = (1/2,0,0) takes place. Below T ≈ 22 K, a small ordered magnetic moment appears on the Ho3+ cations, strongly increasing below 9 K and reaching 7.3(1) μB at T = 1.8 K. The magnetic structure of the Ho3+ moments is defined by a (A x ,0,C z ) mode. The (H,T) phase diagram has been mapped out and the different magnetic structures interpreted on the basis of competing superexchange interactions.

The magnetic structure of the orthorhombic perovskite YMnO3 has been investigated. A study on a polycrystalline sample based on neutron diffraction data and magnetization measurements has shown that YMnO3 becomes magnetically ordered below TN = 42 K. In the space group Pnma, the sinusoidal magnetic structure is defined by a (Cx,0,0) mode and characterized by the propagation vector k = (kx,0,0). The kx-component increases from 0.420(4), immediately below the ordering temperature, to 0.435(2) at T = 28.7 K. Below 28 K the kx-component remains unchanged. The sinusoidal spin arrangement remains stable down to 1.7 K; at this temperature the amplitude of the sinusoid is Ak = 3.89(6) µB. YMnO3 is the most distorted perovskite of the RMnO3 series (R = rare earths); the observed sinusoidal magnetic structure is in contrast with those exhibited by the less-distorted members (i.e. LaMnO3), which are commensurate-type antiferromagnetic structures.

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