Optical, magneto-optical, and magnetic properties of stoichiometric and off-stoichiometricγ′−phaseNi3Alalloys

Abstract

The optical and magneto-optical properties of two ${\ensuremath{\gamma}}^{\ensuremath{'}}\ensuremath{-}\mathrm{p}\mathrm{h}\mathrm{a}\mathrm{s}\mathrm{e}$${\mathrm{Ni}}_{3}\mathrm{Al}$ alloys $({\mathrm{Ni}}_{0.752}{\mathrm{Al}}_{0.248}$ and ${\mathrm{Ni}}_{0.771}{\mathrm{Al}}_{0.229})$ have been investigated experimentally and theoretically. In the optical-conductivity spectra of the investigated alloys two intense interband absorption features at $\ensuremath{\sim}0.8\mathrm{eV}$ and $\ensuremath{\sim}4.2\mathrm{eV}$ were experimentally observed. The peak at $\ensuremath{\sim}0.8\mathrm{eV}$ was theoretically predicted, but not experimentally confirmed in the previous investigations. The low-energy peak exhibits a multiple-peak structure for ${\mathrm{Ni}}_{0.771}{\mathrm{Al}}_{0.229}$ alloy. It is also theoretically shown that the fine structure of the low-energy absorption features originates from the extra spin ordering due to the appearance of the so-called antistructure Ni atoms and their clusters in the ${\mathrm{Ni}}_{0.771}{\mathrm{Al}}_{0.229}$ alloy, and it is experimentally shown that this fine structure persists up to room temperature. The temperature dependencies of the optical and magneto-optical properties of ${\mathrm{Ni}}_{0.771}{\mathrm{Al}}_{0.229}$ alloy can be ascribed to the presence of magnetic inhomogeneity in the sample. The existence of magnetic inhomogeneity in the alloy is further confirmed by the ac magnetic-susceptibility measurements in which the susceptibility curve exhibits two magnetic transitions, one at 87.5 K and the other at 252 K. The spin ordering (paramagnetic-to-ferromagnetic transition) and Ni enrichment play a key role for the physical properties.