Deviations from Curie-Weiss behavior in spin-glasses

The magnetic susceptibility in the spin-glass concentration regime of amorphous alloys (FeₓNi_1-x)₇₅P₁₆B₆Al₃ with x=0.04, 0.06, and 0.08 has been measured using a superconducting quantum interference detector magnetometer with an external field of 5 Oe for temperatures $5<~T<~60.5$ K, and a Faraday balance with an external field of 5 kOe for $4.3<~T<~300$ K. We observe a high-temperature (T>20Tf) Curie-Weiss law with positive values for the paramagnetic Curie temperature, indicating predominantly ferromagnetic exchange couplings. At temperatures approaching the spin-glass freezing temperature, however, there are strong deviations from Curie-Weiss behavior. A simple theory is developed which allows a determination of the angular spin correlations in the spin-glass ground state from these deviations. The number of such correlated spins (1.5 to 4.4) determined from our data compares well with computer-simulation studies by Walker and Walstedt as well as data obtained by other authors for Au-Fe, Cu-Mn, and Co- and Mn-aluminosilicate spin-glasses.