Chemical disorder reinforces magnetic order in ludwigite (Ni,Mn)^sub 3^BO^sub 5^ with Mn^sup 4+^ inclusion

Crystals of ludwigite Ni2.14Mn0.86BO5 were synthesized by flux growth technique. We show in the paper that it contains Mn3+ and Mn4+. A possible mechanism of the manganese valence states stabilization has been proposed. The structural and magnetic characterization of the synthesized samples has been carried out in detail. The cations composition and Mn valence states of the crystal were determined using X-ray diffraction and EXAFS technique. The comparative analysis was carried out between the studied crystal and Ni2MnBO5 synthesized previously. Magnetic susceptibility measurements were carried out. The magnetic transition in the studied composition occurs at the 100 K that is higher than in Ni2MnBO5 although the studied composition is more disordered. The calculations of the exchange integrals in the frameworks of indirect coupling model revealed strong antiferromagnetic interactions. The several magnetic subsystems existence hypothesis was supposed. The possible magnetic structure was suggested from the energies estimation for different ordering variants.