Magnetic, electrical, structural, and thermal properties of amorphous MnSi

MnSi films deposited by getter sputtering at substrate temperatures lower than 650 K are amorphous as determined by x-ray diffraction, scanning-electron-microscopy, electrical-resistivity, and magnetic-susceptibility measurements. Amorphous films display a negative temperature coefficient of resistivity (TCR), while crystalline films are characterized by a positive TCR with a resistive anomaly in the vicinity of the ferromagnetic Curie temperature ($T_C=30\mathrm{}$ K). The magnetic-susceptibility measurements clearly establish that while crystalline MnSi is a ferromagnet with a $T_C$ of 30 K, amorphous MnSi is a concentrated spin-glass with a spin-glass transition at $T_{\mathrm{SG}}=22\mathrm{}\pm 2$ K. The spin-glass transition was studied by both ac and dc susceptibility and by specific-heat measurements. The presence of amorphous ferromagnetic clusters is clearly shown by a susceptibility anomaly around 30 K, by the dependence of the susceptibility and its peak on the temperature of deposition of the films, and by the temperature dependence of the magnetic susceptibility. The independence of $T_{\mathrm{SG}}$ and of the spin-glass order parameter measured below $T_{\mathrm{SG}}$ on the degree of clustering suggests that the clusters are mediated by the same exchange interaction which governs individual spins.