  Magnetic transitions in AlAs two-dimensional electrons give rise to sharp
resistance spikes within the quantum Hall effect. Such spikes are likely caused
by carrier scattering at magnetic domain walls below the Curie temperature. We
report a critical behavior in the temperature dependence of the spike width and
amplitude, from which we deduce the Curie temperature of the quantum Hall
ferromagnet. Our data also reveal that the Curie temperature increases
monotonically with carrier density.
