Magnetic phase diagram for Fe3−xMnxBO5

The heterometallic ludwigite system Fe3−xMnxBO5, recently investigated for its ferroelectric properties, has been studied using synchrotron and neutron diffraction, combined with x-ray absorption near-edge structure and magnetization measurements. The results show that the Pbam crystal structure is preserved with little structural distortions up to x=1.5, and that divalent Mn is substituted preferentially on the 3LL2 sublattice unit. As x increases, and up to Fe2MnBO5, the decoupled magnetic sublattice character of Fe3BO5 is preserved: magnetic order on 3LL1 [k1=(0012), moments along b] survives with reduced magnetic moments, while the correlation length of the magnetic order on 3LL2 [k2=(000), moments along a] decreases. In contrast, for x=1.5, a k = (0 0 0) magnetic ordering, coupling both sublattices, is observed, with all moments aligned along c. These results provide insight on the physical properties of the system, which are discussed in terms of three main parameters : (i) nonlinear evolution of the substitution on each sublattice, (ii) changes in the direct-exchange and superexchange couplings as Mn2+ (3d5, isoelectronic with Fe3+, is introduced in the structure), and (iii) competing easy-axis anisotropy and magnetic exchanges along the 3LL legs in the decoupled sub-lattice regime. These three parameters are at the origin of an extremely rich (x, T) magnetic phase diagram in the Fe3−xMnxBO5 system.