Large easy-plane anisotropy induced spin reorientation in magnetoelectric materials (Co_4-x Mn _x )Nb₂O₉

Neutron powder diffraction experiments were carried out on the magnetoelectric compound series (Co_4-x Mn _x )Nb₂O₉ (x  =  0, 1, 2, 3, 3.5, 3.9, 3.95 and 4) from base temperature to above their Néel 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 (Γ6) with magnetic moments lying purely in the ab plane with certain canting angles. Mn₄Nb₂O₉ has a collinear antiferromagnetic structure (Γ2) 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 Γ2 magnetic structure above 60 K and develop a second Γ6 local phase in addition to the main Γ2 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 Néel temperature and occupancy are also discussed.