Low-lying magnetic excitations inNi3Aland their suppression by a magnetic field

Abstract

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 $M(T,0),$ at low temperatures, we find that propagating transverse spin-density fluctuations (spin waves) almost entirely account for the thermal demagnetization of both $M(T,0)$ and ``in-field'' magnetization $M(T,H),$ at temperatures $T\ensuremath{\lesssim}{0.28T}_{C}$ ${(T}_{C}=\mathrm{Curie}\mathrm{point}).$ The spin-wave stiffness possesses a field-independent value of $69.6(14) \mathrm{meV} {\mathrm{\AA{}}\mathrm{}}^{2}$ which conforms well with those determined earlier from small-angle and inelastic neutron-scattering experiments. In the temperature range ${0.32T}_{C}\ensuremath{\lesssim}T\ensuremath{\lesssim}{0.92T}_{C},$ enhanced nonpropagating spin-density fluctuations (SF) give a contribution to $M(T,0)$ and $M(T,H)$ 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.