Noise studies of magnetization dynamics in dilute magnetic semiconductor heterostructures

We study theoretically and experimentally the frequency and temperature dependence of resistivity noise in semiconductor heterostructures delta-doped by Mn. The resistivity noise is observed to be non-monotonous as a function of frequency. As a function of temperature, the noise increases by two orders of magnitude for a resistivity increase of about 50%. We study two possible sources of resistivity noise -- dynamic spin fluctuations and charge fluctuations, and find that dynamic spin fluctuations are more relevant for the observed noise data. The frequency and temperature dependence of resistivity noise provide important information on the nature of the magnetic interactions. In particular, we show how noise measurements can help resolve a long standing debate on whether the Mn-doped GaAs is an p-d Zener/RKKY or double exchange ferromagnet. Our analysis includes the effect of different kinds of disorder such as spin-glass type of interactions and a site-dilution type of disorder. We find that the resistivity noise in these structures is well described by a disordered RKKY ferromagnet model dynamics with a conserved order parameter.