Magnetic phase diagram for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>Fe</mml:mi><mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">Mn</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi mathvariant="normal">BO</mml:mi><mml:mn>5</mml:mn></mml:msub></mml:math>
F. Damay, Jonas Sottmann, F. Lainé, L. Chaix, Maria Poienar, Přemysl Beran, E. Elkaïm, François Fauth, Lucie Nataf, A. Guesdon, A. Maignan, C. Martin
Abstract
The heterometallic ludwigite system ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{BO}}_{5}$, 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 [${\mathbf{k}}_{2}=(000)$, 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 ${\mathrm{Mn}}^{2+}$ ($3{d}^{5}$, 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 ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{BO}}_{5}$ system.