Temperature- and Field-Induced Transformation of the Magnetic State in Co 2.5 Ge 0.5 BO 5 .

A tetravalent-substituted cobalt ludwigite Co 2.5 Ge 0.5 BO 5 has been synthesized using the flux method. The compound undergoes two magnetic transitions: a long-range antiferromagnetic transition at T N1 = 84 K and a metamagnetic one at T N2 = 36 K. The sample-oriented magnetization measurements revealed a fully compensated magnetic moment along the a - and c -axes and an uncompensated one along the b -axis leading to high uniaxial anisotropy. A field-induced enhancement of the ferromagnetic correlations at T N2 is observed in specific heat measurements. The DFT+GGA calculation predicts the spin configuration of (↑↓↓↑) as a ground state with a magnetic moment of 1.37 μ B /f.u. The strong hybridization of Ge(4s, 4p) with O (2p) orbitals resulting from the high electronegativity of Ge 4+ is assumed to cause an increase in the interlayer interaction, contributing to the long-range magnetic order. The effect of two super-superexchange pathways Co 2+ -O-B-O-Co 2+ and Co 2+ -O-M4-O-Co 2+ on the magnetic state is discussed.