De Almeida-Thouless Line in the Reentrant Spin-Glass Phase of a FeZr Alloy

Differences in the T-dependence of the zero-field- (ZFC) and field-cooled (FC) magnetizations (M) measured for different values of H is a signature of spin-glass (SG) phases. The magnetic field dependent irreversibility transition is given by the de Almeida-Thouless line (AT-line): H = H₀ (1-T/T_G)^Φ/2, where T_G is the glass temperature and Φ a critical exponent. Determining the AT-line in a reentrant SG is complicated by the ferromagnetism (FM). Fe₉₀Zr₁₀ is FM below 225 K and enters a SG-phase at T_G = 15.5 K. The AT-line was obtained from the T-dependence of M measured for 2.5 ≤ H ≤ 60 Oe in the temperature range 2 ≤ T ≤ 30 K after cooling in -85 kOe. Two regimes were observed: (1) near T_G a fit to the data yielded Φ = 3.6 while (2) below about 11 K one obtains Φ = 5.2. Exchange-bias (EB) was also observed in the hysteresis loops measured after cooling the sample in a field of -85 kOe for 2 ≤ T ≤ 300 K. The EB was found to rapidly increase below T_G following the trend of the coercivity data. The overall results are in good agreement with the χac and GMI data used for probing the dynamics of the spins in the reentrant SG-phase both below 10 kHz and in the MHz regime.