Low-lying magnetic excitations in ${\mathrm{Ni}}_3\mathrm{Al}$ and their suppression by a magnetic field

Results of high-resolution magnetization (M) measurements performed on well-characterized polycrystalline ${\mathrm{Ni}}_3\mathrm{Al}$ sample over wide ranges of temperature and external magnetic field are presented and discussed in the light of existing theoretical models. Contrary to the earlier claims that either Stoner single-particle excitations or nonpropagating spin fluctuations solely determine the temperature dependence of spontaneous magnetization $M(T,0\mathrm{}),$ at low temperatures, we find that propagating transverse spin-density fluctuations (spin waves) almost entirely account for the thermal demagnetization of both $M(T,0\mathrm{})\mathrm{}$ and “in-field” magnetization $M(T,H),$ at temperatures $T\lesssim {0.28T}_C$ ${(T}_C=\mathrm{Curie}\mathrm{point}).\mathrm{}$ The spin-wave stiffness possesses a field-independent value of $69.6\left(14\right)\mathrm{}\hspace{0.5em}\mathrm{meV}\hspace{0.5em}{\mathrm{\AA }\mathrm{}}^2$ which conforms well with those determined earlier from small-angle and inelastic neutron-scattering experiments. In the temperature range ${0.32T}_C\lesssim T\lesssim {0.92T}_C,$ enhanced nonpropagating spin-density fluctuations (SF) give a contribution to $M(T,0\mathrm{})\mathrm{}$ and $M(T,H)\mathrm{}$ that completely overshadows the one arising from spin waves. In accordance with the predictions of a modified spin-fluctuation theory, proposed by the authors recently, the thermally excited SF’s get strongly suppressed by magnetic field H while the zero-point SF’s are relatively insensitive to H.