Stability of Mn moments and exchange interactions in cobalt substituted Mn2Sb

The magnetic moment distributions in the ferrimagnetic (FI) and antiferromagnetic (AF) phases of Co substituted Mn2Sb (Mn1.9Co0.1Sb) have been determined using both polarized and unpolarized neutron diffraction. In Mn1.9Co0.1Sb a transition from a ferrimagnetic to an antiferromagnetic state takes place on cooling through Tms = 138 K. The antiferromagnetic structure has propagation vector , and contains (001) layers of Mn atoms with ferromagnetically coupled Mn moments aligned perpendicular to [001]. The two crystallographically distinct Mn atoms have moments 3.98 and 1.71 µB at 4 K and are oppositely aligned within the unit cell. At 200 K the moments determined from polarized neutron scattering are 3.86 and 1.84 µB. In both the AF and FI phases the magnetization associated with the high moment (Mn2) site is nearly spherically symmetric; that around the Mn1 site has predominantly t2g character which is stronger in the AF than in the FI phase. At Tms the only bonds whose lengths change significantly are those between Mn and Sb establishing the 125° Mn1–Sb–Mn2 interlayer exchange path as the dominant exchange interaction determining the type of long range magnetic order.