First-Order Phase Transition in a Quantum Hall Ferromagnet

The single-particle energy spectrum of a two-dimensional (2D) electron gas in a perpendicular magnetic field consists of equally-spaced spin-split Landau levels, whose degeneracy is proportional to the magnetic field strength. At integer and particular fractional ratios between the number of electrons and the degeneracy of a Landau level (filling factors $\nu$) quantum Hall effects occur, characterized by a vanishingly small longitudinal resistance and quantized Hall voltage. 2D electron gas in the quantum Hall regime offers unique possibilities for the study of cooperative phenomena in many-particle systems under well-controlled conditions. Among the fields that can benefit from quantum-Hall studies is two-dimensional magnetism. Both isotropic and anisotropic ferromagnetic ground states have been predicted and few of them have been experimentally studied in quantum Hall samples with different geometries and filling factors. Here we report on evidence for first-order phase transitions in $\nu=2,4$ quantum Hall states confined to a wide gallium arsenide (GaAs) quantum well. The observed anomalous temperature dependence and hysteretic behaviour in the longitudinal resistivity, complemented by detailed many-body calculations, indicate the occurrence of a transition between the two distinct ground states of an Ising quantum-Hall ferromagnet.