Phase relations, crystal chemistry, and physical properties of MgZn2-type Laves phases in the Mn-Cu-Si and Mn-Ni-Si systems

Abstract

In this study, we present the phase relations, crystal structure, and physical properties of the MgZn${}_{2}$-type Laves phases in the Mn-Cu-Si and Mn-Ni-Si systems. Our results evidence that the homogeneous regime of the Laves phase in the Mn-Cu-Si system at $800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ ranges from 32.5 to 36.7 at.% Mn and from 11.5 to 13.5 at.% Si, indicating the Laves phase in this system having an ideal stoichiometry AB${}_{2}$, inconsistent with previous reports. For structural and physical property investigations, two alloys with compositions MnCu${}_{1.65}$Si${}_{0.35}$ and MnNi${}_{1.25}$Si${}_{0.75}$ are considered. In both alloys, Mn atoms are preferably situated at the $4f$ site and Cu(Ni) and Si share the $2a$ and $6h$ sites. Both are antiferromagnets with ${T}_{N}\ensuremath{\approx}800$ K for MnCu${}_{1.65}$Si${}_{0.35}$ and ${T}_{N}\ensuremath{\approx}630$ K for MnNi${}_{1.25}$Si${}_{0.75}$. Rietveld refinements of the room-temperature neutron diffraction data demonstrate that in both cases only the Mn atoms carry magnetic moments, which are aligned antiferromagnetically along the $c$ axis. The magnetic moments at room temperature are $2.7{\ensuremath{\mu}}_{B}$/Mn for MnCu${}_{1.65}$Si${}_{0.35}$ and $2.9{\ensuremath{\mu}}_{B}$/Mn for MnNi${}_{1.25}$Si${}_{0.75}$, respectively. The magnetic properties are confirmed by transport, magnetization, NMR measurements, and band-structure calculations.