Low-lying magnetic excitations in amorphous Fe_90-xCo_xZr₁₀ and Fe_90+yZr_10-y alloys

Abstract

Results of high-resolution magnetization (M) measurements performed on amorphous (a-) Fe_90-xCo_xZr₁₀ (x = 0, 1, 2, 4, 6, 8, 10) and Fe_90+yZr_10-y (y = 0, 1) alloys over wide ranges of temperature (T) and external magnetic field ( H_ext) 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 x≤6 and y =0, 1. The high-field differential susceptibility at T=5K, χ_hf (0), is extremely large for the alloys with x ≤ 0, 1 and y = 0, 1, and decreases rapidly with x for x ≤4 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 ( T ≤ 0.4T_c) and from enhanced local spin-density fluctuations over a wide range of intermediate temperatures (0.45T_c ≤ T ≤ 0.75T_c) and for temperatures close to the Curie point,T_c (0.75T_c ≤ T ≤ 0.95T_c) , for all of the alloys studied. The spin-wave stiffness, D, is independent of H_ext for all of the compositions and the D/T_c ratio possesses a value ≈ 0.14characteristic of amorphous ferromagnets with competing interactions for the alloys with x ≤ 6 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 T_RE. In accordance with the predictions of the spin-fluctuation model, D renormalizes with temperature as D(T) = D(0)(1-D₂T²)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 0 ≤T ≤ T_c, 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 T < T_RE, the existence of a significant contribution due to diffusons, in addition to magnons, to the T^3/2-decrease of the magnetization, and the composition dependence of D(0), M(0,0) and T_c.