The heterometallic ludwigite system ${\mathrm{Fe}}_{3-x}{\mathrm{Mn}}_x{\mathrm{BO}}_5$, 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 ${\mathrm{Fe}}_2{\mathrm{MnBO}}_5$, the decoupled magnetic sublattice character of ${\mathrm{Fe}}_3{\mathrm{BO}}_5$ is preserved: magnetic order on 3LL1 [${\mathbf{k}}_1=(00\frac{1}{2}$), moments along $b$] survives with reduced magnetic moments, while the correlation length of the magnetic order on 3LL2 [${\mathbf{k}}_2=\left(000\right)$, moments along $a$] decreases. In contrast, for $x=1.5$, a $\mathbf{k}$ = (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 ${\mathrm{Mn}}^{2+}$ ($3d^5$, isoelectronic with ${\mathrm{Fe}}^{3+}$, 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 ${\mathrm{Fe}}_{3-x}{\mathrm{Mn}}_x{\mathrm{BO}}_5$ system.

• Received 30 August 2019
• Revised 6 January 2020
• Accepted 18 February 2020

DOI:https://doi.org/10.1103/PhysRevB.101.094418