Magnetic structures and spin-glass-like behavior in ordered and disordered Pt-rich Pt-Mn alloys

The magnetism of Pt-rich Pt-Mn alloys was studied in single crystals with different Mn concentrations (12.5, 14.4, and 15.9 at.%) and different atomic arrangements of disordered, ABC₆ type and Cu₃Au type structures by using neutron scattering and susceptibility measurements. The neutron scattering measurements showed drastic changes in the magnetic diffraction pattern with a slight difference in the atomic arrangement and Mn concentration. In disordered alloys, magnetic diffuse peaks were observed at 1∕2, 0, 0 and equivalent symmetry positions of the fcc fundamental structure. In ordered alloys with the ABC₆ type structure, fourfold splitting along [011] and [01̅ 1] with incommensurate maxima appeared in the 1∕2, 0, 0 diffuse scattering in Pt-12.5 at.%Mn, while the 1∕2, 0, 0 diffuse scattering completely disappeared, instead, commensurate superlattice peaks were newly observed at 1∕2,±1∕4,0 and 1∕2,0,±1∕4 in Pt-14.4 at.%Mn. Fourfold splitting and superlattice peaks were observed with the ABC₆ type symmetry. In ordered alloys with the Cu₃Au type structure, ferromagnetic diffuse scattering was mainly observed at the zone center of the Cu₃Au type structure. The origin of these magnetic scatterings is discussed in terms of three different magnetic interactions: a spin-density wave (SDW) originated by a nesting Fermi surface, an antiferromagnetic interaction on the ABC₆ type structure and a ferromagnetic interaction on the Cu₃Au type structure. The dominant interaction largely depends on the atomic arrangement, indicating a strong coupling between the magnetic and atomic structures in the alloys. Susceptibility measurements showed a spin-glass-like behavior in all of the samples, which is attributed to the dynamics of the fluctuating SDW clusters, as observed in other Pd-M and Pt-M (M; 3d element) spin-glass alloys. The persistence of the SDW fluctuations indicates the instability of these magnetic structures.