Neutron diffraction investigation of the magnetic structure and magnetoelastic effects in NdMnO3

We investigated the evolution of the magnetic structure and magnetoelastic effects of NdMnO3 by neutron powder diffraction. We confirmed the A-type antiferromagnetic (AF) structure of NdMnO3 below TN approximate to 82 K with magnetic moments parallel to the b axis of the orthorhombic crystal structure (space group Pbnm). We found that the magnetic moments of Nd order below about T-N (Nd) approximate to 20 K in a ferromagnetic structure with moments parallel to the c axis. At the same temperature the magnetic moments of Mn develop a ferromagnetic component parallel to the c axis. We found strong magnetoelastic effects associated with the AF transition at T-N approximate to 82 K. The effect is very prominent for the b lattice parameter. All three lattice parameters and therefore the unit cell volume contracts at T-N approximate to 82 K. We treated quantitatively the magnetoelastic effect of the b lattice parameter and found that the extra change in the lattice parameter Delta b due to the magnetoelastic effect is proportional to the magnetic moment of the Mn ion. We also determined the critical exponent of the AF phase transition to be beta = 0.296 +/- 0.008.