Magnetic noise of a frozen ferrofluid

The magnetic noise of a frozen ferrofluid made of maghemite nanoparticles dispersed in glycerin is measured using a two-dimensional electron gas based quantum well Hall sensor (QWHS) with the spinning current technique. The frozen ferrofluid shows a superspin glass transition at 67 K. Below this glass transition temperature, the relation between the imaginary part of the ac susceptibility of a bulk ferrofluid sample and the magnetic noise measured with the QWHS gives an indication of a violation of the fluctuation dissipation theorem.