Spin-wave, Stoner single-particle and correlated particle-hole pair contributions to thermal demagnetization in amorphous Fe_90+xZr_10-x

Abstract

Detailed magnetization (M) measurements have been performed on amorphous (a-)Fe_90+xZr_10-x alloys with x=0 and 1 at temperatures ranging from 4.2 to 300 K in external magnetic fields (H) up to 15 kOe. Arrott plot isotherms of M²(H,T) against H/M(H,T) are nearly linear at high magnetic fields for temperatures well outside the critical region. Spontaneous magnetization varies with temperature as M(0,T)/M(0,0)=1-BT³2/-AT², (M(0,T)/M(0,0))²=a-bT² and (M(O,T)/M(0,0))²=a'-b'T⁴3/ in the temperature intervals 0< or approximately=T< or approximately=0.38T_c (Curie temperature), 0.39T_c< or approximately=T< or approximately=0.9T_c and 0.91 T_c< or approximately=T< or approximately=0.98 T_c, respectively. These results can be adequately described by the theory that includes corrections to the conventional Stoner model arising from enhanced fluctuations in the local spin density. An elaborate analysis of highly accurate 'in-field' magnetization data for the first time permits an unambiguous separation of spin-wave ( Delta m_sw) and single-particle ( Delta m_sp) contributions to the thermal demagnetization ( Delta m) and reveals that: (i) contrary to the previous finding, the spin-wave stiffness coefficient D is independent of H; (ii) the spin-density fluctuations get strongly suppressed by the external field: (iii) spin-wave modes soften at low temperatures where the ferromagnetic state gives way to a 'mixed magnetic state'; (iv) the D/T_c ratio possesses a value of about 0.14 typical of amorphous ferromagnets with competing interactions while Delta m_sp has a sizable value characteristic of Invar systems; and (v) the competing interactions confine the direct Heisenberg exchange interaction to the nearest neighbours only. A straightforward explanation has been provided for the absence of spin-wave peaks in the inelastic neutron scattering spectra taken on a-Fe₉₁Zr₉ in the wavevector transfer range of 0.05 AA^-1< or=q< or=0.12 AA^-1 in terms of a model previously proposed by the author in connection with the static critical phenomena in amorphous alloys including those investigated in this work.