Structure and magnetism of the β-Mn–Co solid-solution phase

The crystal structure of the β-Mn1−tCot solid-solution phase (0 ≤ t ≤ 0.40) has been studied with powder neutron (10 and 298 K) and single-crystal X-ray (150 K) diffraction methods. The lattice-constant (a) isotherms at 10, 150, and 298 K go through flat maxima between t = 0.10 and 0.25. Up to t = 0.25 all Co is found to substitute at the T1 (T1 = Mn and/or Co) site of the β-Mn crystal structure (position 8c of space group P4132) whereas for t = 0.40 also the T2 site (position 12d) is partly occupied by Co (some 0.2 Mn + 0.8 Co occupancy of the T2 site). The variable positional parameters x (for T1) and y (for T2) exhibit remarkably small variations with composition (t) and temperature. The present low-temperature powder neutron-diffraction data confirm the earlier finding that the β-Mn1−tCot phase does not exhibit conventional co-operative magnetic ordering. However, the appearance of diffuse scattering in the low-temperature diffraction patterns is clearly generated by short-range ordering of magnetic moments, which owing to the atomic arrangement of the β-Mn-type structure becomes geometric frustrated. The temperature dependence of the magnetic susceptibility for β-Mn1−tCot is re-measured. Neglecting β-Mn itself (which exhibits virtually temperature-independent paramagnetism), our magnetic susceptibility curves above some 80 K for t = 0.15, 0.25, and 0.40 can with good-will be described by the Curie–Weiss relation, indicating antiferromagnetic correlations at low temperatures. However, the thus involved paramagnetic moments and Weiss constants must indeed be stamped as unphysically large.