Detailed magnetization study of quenched random ferromagnets. I. Low-lying magnetic excitations

Abstract

Magnetization as a function of temperature has been measured for amorphous (a-) (Fe_pNi_1-p)₈₀B₁₉Si₁ (0.0625≤ p≤ 0.2), (Fe_pNi_1-p)₈₀B₂₀ (0.25≤ p≤ 1.0), and (Fe_pNi_1-p)₈₀P₁₄B₆ (0.1125≤p≤1.0) alloys in the temperature range 3.8-300 K at various constant applied magnetic-field values in the interval 5≤H≤15 kOe. An elaborate data analysis reveals the following. (i) For all p in the a-(Fe_pNi_1-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≤t (=T/T_C)≤t^∗(p) but the reverse is true for t>t^∗(p). However, in a-(Fe_pNi_1-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≤p≤0.25 (0.375≤p≤0.625) but for T≤300 K and p≥ 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_xNi_100-x alloys, varies with temperature as D(T)~T^5/2 and D(T)~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_pNi_1-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≈ 0.75 in the a-(Fe_pNi_1-p)₈₀P₁₄B₆ alloy series. Arguments are presented to show that the property D_n (inelastic neutron scattering) »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 T^3/2 decrease of magnetization as the transverse spin fluctuations (spin waves) do, but leave D_n unaltered from its "spin-wave-only" value.