Nuclear and magnetic order in Y(Ni,Mn)O_3 manganites by neutron powder diffraction

摘要

Neutron powder diffraction experiments performed on two selected compositions of the yttrium-based solid solution YNi_x Mn_(1-x)O_3 (x = 0.25 and 0.50), clearly reveal a nuclear order between the Ni~(2+) and Mn~(4+) ions in the half-substituted compound YNi_(0.50)Mn_(0.50)O_3, so that the crystal structure is no longer described in the conventional orthorhombic Pbnm space group, but in the monoclinic P2_1, all over the investigated temperature range (1.5-300 K). However, both X-rays diagrams and neutron patterns of the YNi_(0.25)Mn_(0.75)O_3 phase are indexed in the Pbnm orthorhombic-like symmetry, indicating that the Mn and Ni ions are randomly distributed on the octahedral sites. In addition, neutron diffraction points out that the nature of the magnetic ordering is strongly connected to the structural properties. Whereas no long-range 3D-magnetic ordering was detected for the Pbnm YNi_(0.25)Mn_(0.75)O_3 phase, the YNi_(0.50)Mn_(0.50)O_3 compound exhibits a magnetic transition at T_C ～ 85 K. The magnetic structure consists of two collinear Mn~(4+) and Ni~(2+) ferromagnetic layers (F_x0F_z magnetic configurations) with saturated magnetic moment values of 2.25(2) and 1.57(2) μ_B for Mn~(4+) and Ni~(2+), respectively, at 1.5 K.