Quantum Hall ferromagnetism of AlAs 2D electrons

We present magneto-transport measurements in tilted magnetic fields on high mobility View the MathML source AlAs two-dimensional electron systems. Our samples show integer and fractional quantum Hall effects. In titled magnetic fields, at the crossings of Landau levels with opposite spin, the resistance shows a spike if the crossing occurs near integer filling or is significantly enhanced otherwise. The position of the spike displays a hysteresis below View the MathML source that reaches more than View the MathML source at View the MathML source. From the position of the crossing in the (Btot,Bperp)-plane the difference in exchange energy of the crossing levels is determined and found to be a non-linear function of the perpendicular magnetic field.