CONCERNING THE 2 MAGNETIC CORRELATION LENGTHS ABOVE T-C IN AN INVAR IRON-NICKEL ALLOY

The magnetic phase transition in the Invar alloy Fe70Ni30 is investiga ted by means of small-angle neutron scattering over a wide range of mo mentum transfer. This method was used to measure two magnetic correlat ion lengths R-c1 and R-c2 which coexist in the alloy above the phase t ransition temperature T-c. The critical correlations with correlation length R,, are described well by an Ornstein-Zernicke expression, and the critical correlations with the second correlation length, an order of magnitude larger than R-c2, are described well by a squared Ornste in-Zernicke expression, The temperature dependences obtained for the c orrelation lengths R-c1 and R-c2 satisfy the power law R-c similar to( (T-T-c)/T-c)(-v) with critical exponents v(1)=0.65 +/- 0.05 and v(2)=1 .3 +/- 0.1 for the shorter and longer scales, respectively. (C) 1997 A merican Institute of Physics.