Low-lying magnetic excitations in Ni3Al and their suppression by a magnetic field

Results of high-resolution magnetization (M) measurements performed on well-characterized polycrystalline Ni3Al 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,O), at low temperatures, we find that propagating transverse spin-density fluctuations (spin waves) almost entirely account for the thermal demagnetization of both M(T,O) and "in-field" magnetization M(T,H), at temperatures T less than or similar to 0.28T(C) (T-C=Curie point). The spin-wave stiffness possesses a field-independent value of 69.6(14) meV Angstrom(2) which conforms well with those determined earlier from small-angle and inelastic neutron-scattering experiments. In the temperature range 0.32T(C) less than or similar to T less than or similar to 0.92T(C), enhanced nonpropagating spin-density fluctuations (SF) give a contribution to M(T,O) 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. [S0163-1829(99)01142-X].