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-) (${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathit{p}}$${)}_{80}$${\mathrm{B}}_{19}$${\mathrm{Si}}_{1}$ (0.0625\ensuremath{\le}p\ensuremath{\le}0.2), (${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathit{p}}$${)}_{80}$${\mathrm{B}}_{20}$ (0.25\ensuremath{\le}p\ensuremath{\le}1.0), and (${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathit{p}}$${)}_{80}$${\mathrm{P}}_{14}$${\mathrm{B}}_{6}$ (0.1125\ensuremath{\le}p\ensuremath{\le}1.0) alloys in the temperature range 3.8--300 K at various constant applied magnetic-field values in the interval 5\ensuremath{\le}H\ensuremath{\le}15 kOe. An elaborate data analysis reveals the following. (i) For all p in the a-(${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathit{p}}$${)}_{80}$(B,Si${)}_{20}$ alloy series, the contribution to the thermal demagnetization due to single-particle (SP) excitations of the weak itinerant type, though present at all temperatures, is completely dominated by that arising from spin-wave (SW) excitations in the temperature range 0\ensuremath{\lesssim}t (=T/${\mathit{T}}_{\mathit{C}}$)\ensuremath{\lesssim}${\mathit{t}}^{\mathrm{*}}$(p) but the reverse is true for t>${\mathit{t}}^{\mathrm{*}}$(p). However, in a-(${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathit{p}}$${)}_{80}$${\mathrm{P}}_{14}$${\mathrm{B}}_{6}$ alloys, SP excitations of the weak itinerant type give a feeble contribution, which is masked by the SW contribution, for temperatures up to 0.9${\mathit{T}}_{\mathit{C}}$ (300 K) in the concentration range 0.1125\ensuremath{\le}p\ensuremath{\le}0.25 (0.375\ensuremath{\le}p\ensuremath{\le}0.625) but for T\ensuremath{\lesssim}300 K and p\ensuremath{\gtrsim}0.75, a significant SP contribution of the strong itinerant type accompanies a dominant SW contribution.(ii) The spin-wave stiffness coefficient D in both the alloy series, as in fcc ${\mathrm{Fe}}_{\mathit{x}}$${\mathrm{Ni}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$ alloys, varies with temperature as D(T)\ensuremath{\sim}${\mathit{T}}^{5/2}$ and D(T)\ensuremath{\sim}${\mathit{T}}^{2}$ 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-(${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathit{p}}$${)}_{80}$ (B,Si${)}_{20}$ alloy series behave as weak itinerant ferromagnets while a transition from weak itinerant to strong itinerant ferromagnetism occurs at p\ensuremath{\simeq}0.75 in the a-(${\mathrm{Fe}}_{\mathit{p}}$${\mathrm{Ni}}_{1\mathrm{\ensuremath{-}}\mathit{p}}$${)}_{80}$${\mathrm{P}}_{14}$${\mathrm{B}}_{6}$ alloy series. Arguments are presented to show that the property ${\mathit{D}}_{\mathit{n}}$ (inelastic neutron scattering) \ensuremath{\gg}${\mathit{D}}_{\mathit{m}}$ (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 ${\mathit{T}}^{3/2}$ decrease of magnetization as the transverse spin fluctuations (spin waves) do, but leave ${\mathit{D}}_{\mathit{n}}$ unaltered from its ``spin-wave-only'' value.