The superexchange interactions and magnetic ordering in low-dimentional ludwigite Ni 5 GeB 2 O 10

The ludwigite Ni 5 Ge(BO 5 ) 2 belongs to a family of oxyborates which have low-dimensional subunits in the form of three-leg ladders unit structure. This material was studied by magnetic and thermodynamic measurements. Ni 5 Ge(BO 5 ) 2 does not show full long-range magnetic order, but one goes into a partial ordering or spin-glass state at 87 K. The superexchange interactions were calculated in the framework of a simple indirect coupling model. Different models of magnetic structure of Ni 5 Ge(BO 5 ) 2 and its unique magnetic behaviour was discussed.