Evolution of the Magnetic Structure of Hexagonal HoMnO₃ from Neutron Powder Diffraction Data

Hexagonal, nonperovskite HoMnO 3 oxide, containing a triangular arrangement of Mn 3+ cations, has been prepared in polycrystalline form by the thermal decomposition of metal citrates. The crystal structure has been refined from neutron powder diffraction data. Magnetic and specific-heat measurements anticipate a complex phase diagram: HoMnO 3 becomes magnetically ordered at T N ≈ 72 K, and another two magnetic transitions take place at lower temperatures. Neutron powder diffraction measurements demonstrate that, below the ordering temperature, the moments of the Mn 3+ cations adopt a triangular spin arrangement, the magnetic moments lying in the basal plane and parallel to the [100] axis. At T = 44.6 K, the moments suddenly reorientate within the basal plane and become aligned perpendicularly to the initial direction. Below T = 25.4 K, an ordered magnetic moment is observed on the Ho atoms at the 4b sites of the crystal structure, whereas those of the 4a site remain in a paramagnetic state. The Ho atoms adopt an antiferromagnetic structure with the moments parallel to the c axis. At 1.7 K, the ordered moment on the Mn 3+ cations is 3.05(2) μ B, and that on the Ho 3+ cations is 2.97(3) MB.