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We study spin wave relaxation in quantum Hall ferromagnet regimes. Spin-orbit coupling is considered as a factor determining spin nonconservation, and external random potential as a cause of energy dissipation making spin-flip processes irreversible. We compare this relaxation mechanism with other relaxation channels existing in a quantum Hall ferromagnet.
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Electron spin relaxation in a spin-polarized quantum Hall state is studied. Long spin relaxation times that are at least an order of magnitude longer than those measured in previous experiments were observed and explained within the spin-exciton relaxation formalism. Absence of any dependence of the spin relaxation time on the electron temperature and on the spin-exciton density, and specific dependence on the magnetic field indicate the definite relaxation mechanism -- spin-exciton annihilation mediated by spin-orbit coupling and smooth random potential.
Time-dependent capacitance measurements reveal an unstable phase of electrons in gallium arsenide quantum well that occurs when two Landau levels with opposite spin are brought close to degeneracy by applying a gate voltage. This phase emerges below a critical temperature and displays a peculiar non-equilibrium dynamical evolution. The relaxation dynamics is found to follow a stretched exponential behavior and correlates with hysteresis loops observed by sweeping the magnetic field. These experiments indicate that metastable randomly-distributed magnetic domains are involved in the relaxation process in a way that is equivalently tunable by a change in gate voltage or temperature.
Resistively Detected Nuclear Magnetic Resonance (RD-NMR) has been used to investigate a two-subband electron system in a regime where quantum Hall pseudo-spin ferromagnetic (QHPF) states are prominently developed. It reveals that the easy-axis QHPF state around the total filling factor $ u =4 $ can be detected by the RD-NMR measurement. Approaching one of the Landau level (LL) crossing points, the RD-NMR signal strength and the nuclear spin relaxation rate $1/T_{1}$ enhance significantly, a signature of low energy spin excitations. However, the RD-NMR signal at another identical LL crossing point is surprisingly missing which presents a puzzle.
A theory of collective states in a magnetically quantized two-dimensional electron gas (2DEG) with half-filled Landau level (quantized Hall ferromagnet) in the presence of magnetic 3d impurities is developed. The spectrum of bound and delocalized spin-excitons as well as the renormalization of Zeeman splitting of the impurity 3d levels due to the indirect exchange interaction with the 2DEG are studied for the specific case of n-type GaAs doped with Mn where the Lande` g-factors of impurity and 2DEG have opposite signs. If the sign of the 2DEG g-factor is changed due to external influences, then impurity related transitions to new ground state phases, presenting various spin-flip and skyrmion-like textures, are possible. Conditions for existence of these phases are discussed. PACS: 73.43.Lp, 73.21.Fg, 72.15.Rn
It is shown that the collective spin rotation of a single Skyrmion in quantum Hall ferromagnet can be regarded as precession of the entire spin texture in the external magnetic field, with an effective moment of inertia which becomes infinite in the zero g-factor limit. This low-lying spin excitation may dramatically enhance the nuclear spin relaxation rate via the hyperfine interaction in the quantum well slightly away from filling factor equal one.
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