DETAILED MAGNETIZATION STUDY OF QUENCHED RANDOM FERROMAGNETS .1. LOW-LYING MAGNETIC EXCITATIONS

Magnetization as a function of temperature has been measured for amorphous (a-) (Fe(p)Ni1-p)80B19Si1 (0.0625 less-than-or-equal-to p less-than-or-equal-to 0.2), (Fe(p)Ni1-p)80B20 (0.25 less-than-or-equal-to p less-than-or-equal-to 1.0), and (Fe(p)Ni1-p)80P14B6 (0.1125 less-than-or-equal-to p less-than-or-equal-to 1.0) alloys in the temperature range 3.8-300 K at various constant applied magnetic-field values in the interval 5 less-than-or-equal-to H less-than-or-equal-to 15 kOe. An elaborate data analysis reveals the following. (i) For all p in the a-(Fe(p)Ni1-p)80(B,Si)20 alloy series, the contribution to the thermal demagnetization due to single-particle (Sp) excitations of the weak itinerant type, though present at all temperatures, is completely dominated by that arising from spin-wave (SW) excitations in the temperature range 0 less-than-or-similar t (= T/T(C)) less-than-or-similar-to t* (p) but the reverse is true for t > t*(p). However, in a-(Fe(p)Ni1-p)80P14B6 alloys, SP excitations of the weak itinerant type give a feeble contribution, which is masked by the SW contribution, for temperatures up to 0.9T(C) (300 K) in the concentration range 0.1125 less-than-or-equal-to p less-than-or-equal-to 0.25 (0.375 less-than-or-equal-to p less-than-or-equal-to 0.625) but for T less-than-or-similar-to 300 K and p greater-than-or-similar-to 300 K and p greater-than-or-equal-to 0.75, a significant SP contribution of the strong itinerant type accompanies a dominant SW contribution. (ii) The spin-wave stiffness coefficient D in both the alloy series, as in fcc Fe(x)Ni100-x alloys, varies with temperature as D(T) approximately T5/2 and D(T) approximately T2 for Fe concentrations below and above x = 80p = 60 at. %, respectively. (iii) The direct exchange interactions extend beyond the second nearest-neighbor distance for compositions close to, but above, the critical concentration for the appearance of long-range ferromagnetic order whereas the competing interactions in the alloy with p > 0.75 confine the direct exchange to the nearest neighbors only. The observation (i) above is shown to imply that all the compositions in the a-(Fe(p)Ni1-p)80 (B,Si)20 alloy series behave as weak itinerant ferromagnets while a transition from weak itinerant to strong itinerant ferromagnetism occurs at p congruent-to 0.75 in the a-(Fe(p)Ni1-p)80P14B6 alloy series. Arguments are presented to show that the property D(n) (inelastic neutron scattering) much-less-than D(m) (magnetization) of the alloys with p > 0.75 in both the alloy series studied is a consequence of the fact that the longitudinal spin fluctuations make as significant a contribution to the T3/2 decrease of magnetization as the transverse spin fluctuations (spin waves) do, but leave D(n) unaltered from its ''spin-wave-only'' value.