Magnetic Structures of Chromium-ModifiedMn2Sb

The magnetic structures of tetragonal chromium-modified ${\mathrm{Mn}}_2$Sb have been determined by neutron-diffraction studies of single crystals. Compositions of ${\mathrm{Mn}}_{2-x}{\mathrm{Cr}}_x\mathrm{Sb}$ for $x$ from 0.023 to 0.10 were examined. Three magnetic structures have been observed, depending upon composition and temperature. These all have antiferromagnetic coupling between manganese atoms Mn(I) in (a) sites of (0,0,0), (½,½,0), and Mn(II) in (c) sites of (0,½, $z$), (½,0, $\overline{z}$). The high-temperature structure is that of ferrimagnetic ${\mathrm{Mn}}_2$Sb where there is ferromagnetic coupling between successive layers of Mn(I) plus adjacent Mn(II) atoms. With decreasing temperature there is an exchange inversion to an antiferromagnetic coupling between successive layers giving a magnetic unit cell with a tetragonal axis twice that of the x-ray cell. For $x=0.03\mathrm{}$ a new weakly ferrimagnetic state exists between the ferrimagnetic and antiferromagnetic states. The magnetic unit cell of this structure has a tetragonal axis three times that of the x-ray cell. The spins are perpendicular to the $c$ axis in this intermediate ferrimagnetic structure and in the antiferromagnetic state of the compositions studied.