Figure 2. Three-dimensional ( 3D ) magnetic field and temperature dependence of the real part of the ac susceptibility 4 times pi times chi ' ( T ). Note that two values of ac fields were used in the measurements. For H is less than 0.3 T, H sub a c = 0.17 mT, and for H is greater than 0.3 T, H sub a c = 0.7 mT, f = 1.0 kHz. The solid and dashed lines are guides to eyes. For the ac fields used, T sub p is independent of the a c field amplitude.