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

Abstract

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,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≤ 0.28T_C (T_C=Curie point ). The spin-wave stiffness possesses a field-independent value of 69.6(14) meV Ų which conforms well with those determined earlier from small-angle and inelastic neutron-scattering experiments. In the temperature range 0.32T_C≤ T≤ 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.