Low-lying magnetic excitations in amorphous and alloys

Abstract

Results of high-resolution magnetization (M) measurements performed on amorphous (a-) (x = 0, 1, 2, 4, 6, 8, 10) and (y = 0, 1) alloys over wide ranges of temperature (T) and external magnetic field are presented and discussed in the light of existing theoretical models. The magnetization at 5 K does not saturate even for fields as high as 70 kOe particularly for the alloys with and y =0, 1. The high-field differential susceptibility at , is extremely large for the alloys with x = 0, 1 and y = 0, 1, and decreases rapidly with x for such that it possesses values typical of crystalline ferromagnets such as Fe, Co, Ni for x > 6. The dominant contribution to the thermal demagnetization of the spontaneous as well as `in-field' magnetization comes from spin-wave (SW) excitations at low temperatures and from enhanced local spin-density fluctuations over a wide range of intermediate temperatures and for temperatures close to the Curie point, , for all of the alloys studied. The spin-wave stiffness, D, is independent of for all of the compositions and the ratio possesses a value characteristic of amorphous ferromagnets with competing interactions for the alloys with and y = 0, 1. For these alloys, thermomagnetic and thermoremanent effects generally associated with the cluster spin-glass behaviour have been observed in the re-entrant state which sets in at a temperature . In accordance with the predictions of the spin-fluctuation model, D renormalizes with temperature as and the spin fluctuations get strongly suppressed by Co substitution and . While the spin-fluctuation (SF) model provides a consistent theoretical basis for the observed temperature dependence of the spontaneous and `in-field' magnetization over the entire temperature range , the infinite three-dimensional (FM) matrix plus finite FM spin-clusters model extends the scope of the SF model in that it offers a straightforward explanation for the absence of SW peaks in the inelastic neutron scattering spectra taken over a certain wave-vector-transfer range, the softening of spin-wave modes for , the existence of a significant contribution due to diffusons, in addition to magnons, to the -decrease of the magnetization, and the composition dependence of D(0), M(0,0) and .