MAGNETIC-STRUCTURE OF (SMMN2GE2)-SM-154 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 stat es are characterized from much clearer neutron-diffraction spectra tha n in previous works, in which natural abundance Sm was used. A tempera ture versus pressure magnetic phase diagram is proposed, consisting of six distinct magnetic states, including previously undetected incomme nsurate cone structures at low temperatures and pressures. The noncoll inear magnetic structures of these magnetic phases are such that a lar ger antiferromagnetic component appears within (001) planes in the fer romagnetic states than in the antiferromagnetic states. This observati on provides an explanation for the anisotropic changes in the lattice constant and the anomalous magnetoresistance of SmMn2Ge2. Previously u nexplained susceptibility and NMR measurements are interpreted in term s of changes to the cone structure. Changes in the Mn-Mn spacing and m agnetic coupling are consistent with previous observations on related systems.