Large easy-plane anisotropy induced spin reorientation in magnetoelectric materials (Co4-xMnx)Nb2O9

Neutron powder diffraction experiments were carried out on the magnetoelectric compound series (Co4-xMnx)Nb2O9 (x = 0, 1, 2, 3, 3.5, 3.9, 3.95 and 4) from base temperature to above their Neel temperatures. Their magnetic structures were analysed by using the irreducible representation analysis and Rietveld refinement method. Similar to Co4Nb2O9, the compounds with x <= 3.9 have noncollinear in-plane magnetic structures (Gamma 6) with magnetic moments lying purely in the ab plane with certain canting angles. Mn4Nb2O9 has a collinear antiferromagnetic structure (Gamma 2) with magnetic moments aligning along the c axis. The compound of x = 3.95 shows two magnetic phases in the magnetization, which was confirmed to have the Gamma 2 magnetic structure above 60 K and develop a second Gamma 6 local phase in addition to the main Gamma 2 phase due to doping. This study indicates 2.5 at% Co2+ doping is sufficient to alter the collinear easy-axis magnetic structure of Mn4Nb2O9 into the noncollinear easy-plane magnetic structure, which is attributed to the large easy-plane anisotropy of Co2+ and relative small Ising-like anisotropy of Mn2+. The doping effects on the Neel temperature and occupancy are also discussed.