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.