Magnetic structure of 154SmMn2Ge2 as a function of temperature and pressure

Neutron scattering, ac susceptibility, and NMR measurements have been made on isotopically enriched (SmMn2Ge2)-Sm-154. All the magnetic states are characterized from much clearer neutron-diffraction spectra than in previous works, in which natural abundance Sm was used. A temperature versus pressure magnetic phase diagram is proposed, consisting of six distinct magnetic states, including previously undetected incommensurate cone structures at low temperatures and pressures. The noncollinear magnetic structures of these magnetic phases are such that a larger antiferromagnetic component appears within (001) planes in the ferromagnetic states than in the antiferromagnetic states. This observation provides an explanation for the anisotropic changes in the lattice constant and the anomalous magnetoresistance of SmMn2Ge2. Previously unexplained susceptibility and NMR measurements are interpreted in terms of changes to the cone structure. Changes in the Mn-Mn spacing and magnetic coupling are consistent with previous observations on related systems.