The magnetism of $\mathrm{Pt}$-rich $\mathrm{Pt}\text{−}\mathrm{Mn}$ alloys was studied in single crystals with different $\mathrm{Mn}$ concentrations (12.5, 14.4, and $15.9\text{at}\mathrm{.}\%$) and different atomic arrangements of disordered, ${\mathrm{ABC}}_6$ type and ${\mathrm{Cu}}_3\mathrm{Au}$ 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 $\mathrm{Mn}$ 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 ${\mathrm{ABC}}_6$ type structure, fourfold splitting along $\left[011\right]$ and $\left[0\overline{1}1\right]$ with incommensurate maxima appeared in the $1∕2$, 0, 0 diffuse scattering in $\mathrm{Pt}\text{−}12.5\text{at.}\%\mathrm{Mn}$, while the $1∕2$, 0, 0 diffuse scattering completely disappeared, instead, commensurate superlattice peaks were newly observed at $1∕2,\pm 1∕4,0$ and $1∕2,0,\pm 1∕4$ in $\mathrm{Pt}\text{−}14.4\text{at.}\%\mathrm{Mn}$. Fourfold splitting and superlattice peaks were observed with the ${\mathrm{ABC}}_6$ type symmetry. In ordered alloys with the ${\mathrm{Cu}}_3\mathrm{Au}$ type structure, ferromagnetic diffuse scattering was mainly observed at the zone center of the ${\mathrm{Cu}}_3\mathrm{Au}$ 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 ${\mathrm{ABC}}_6$ type structure and a ferromagnetic interaction on the ${\mathrm{Cu}}_3\mathrm{Au}$ 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 $\mathrm{Pd}\text{−}\mathrm{M}$ and $\mathrm{Pt}\text{−}\mathrm{M}$ ($\mathrm{M}$; $3d$ element) spin-glass alloys. The persistence of the SDW fluctuations indicates the instability of these magnetic structures.

• Received 16 June 2003

DOI:https://doi.org/10.1103/PhysRevB.70.014431