Magnetic structure of ¹⁵⁴SmMn₂(Ge_0.9Si_0.1)₂ as a function of temperature and pressure

Neutron scattering and AC susceptibility measurements have been made on isotopically enriched ¹⁵⁴SmMn₂(Ge_0.9Si_0.1)₂. A temperature versus pressure magnetic phase diagram is proposed. The incommensurate cone structures detected in SmMn₂Ge₂ at low temperatures and pressures do not appear in the phase diagram of the Si-substituted system. There is no evidence of a change in the magnetic structure associated with a small peak in susceptibility observed at around 145 K which has previously been interpreted in terms of changes to the cone structure. However, as with SmMn₂Ge₂, the non-collinear magnetic structures of these magnetic phases are such that a larger antiferromagnetic component appears within (0 0 1) ab-planes in the ferromagnetic states than in the antiferromagnetic state. With increasing Si substitution, the antiferromagnetic phase becomes the magnetic equilibrium state over a wider temperature range. The observed changes in the Mn-Mn spacing and magnetic coupling are largely consistent with previous related observations, although under a pressure of approximately 8 kbar, low-temperature magnetic structures are found below the critical lattice spacing of a = 4.031 angstroms which are similar to those observed at ambient pressure.