Magnetoelastic effects in multiferroic HoMnO3

We have investigated magnetoelastic effects in multiferroic HoMnO3 below the antiferromagnetic phase transition T N ≈ 75 K by neutron powder diffraction. The lattice parameter a of the hexagonal unit cell of HoMnO3 decrease in the usual way at lower temperatures and then shows abrupt contraction below TN whereas the lattice parameter c increases continuously with decreasing temperature and shows an anomalous increase below TN. The unit cell volume decreases continuously with decreasing temperature and undergoes abrupt contraction below TN. By fitting the background thermal expansion for a nonmagnetic lattice with the Einstein–Grüneisen equation we determined the lattice strains Δ a , Δ c and Δ V due to the magnetoelastic effects as a function of temperature. We have also determined the temperature variation of the ordered magnetic moment of Mn ion by fitting the measured Bragg intensities of the nuclear and magnetic reflections with the known crystal and magnetic structure models.