Field-induced parameters of re-entrant magnets and concentrated spin glasses

Abstract

We have used electron spin resonance measurements to derive the temperature and frequency dependences of the field-induced magnetization [ M( T, f)] and anisotropy field [ H an ( T)] in a number of amorphous alloys belonging to the series (Fe p Ni 1− p ) 75P 16B 6Al 3. In re-entrant ( p > p c, the critical concentration for ferromagnetism) alloys at hi gh frequencies ( f = 35 GHz, field ≈ 12 kOe) M reduces as T 3/2 at high T and as T below ≈ 40 K, the deviation from T 3/2 becoming more marked as p → p + c. For p close to p c, lowering the frequency first causes the T 3 2 term to increase and ultimately ( ≈ 4 GHz) changes the variation of M with T to that discovered previously for concentrated spin glasses, namely M is constant at low T and drops linearly at high temperatures. For the re-entrants, the results are interpreted on the basis of a model which invokes an energy gap in the spin-wave spectrum, introduces a non-zero density of states of the gap energy and takes into consideration a low- q cut-off in the spin-wave integral in thelow- T (≲ T) regime.In the concentrated spin glasses [ M (0) - M ( T)]/ M (0) is well represented by the function [exp (Δ / T) - 1] -1, where Δ has values close to the corresponding Curie-Weiss temperatures θ p but much larger than the respective spin glass transition temperatures T SG. The temperature dependence of H an is largely given by the function (1 - T/ T ∗), where T ∗ is equal to the zero-field freezing temperature for the re-entrants and T SG for the spin glasses, respectively.