Methods and Facilities / Methoden und Ausstattung
Experimental Methods
Overview
Hall-Magnetometry
The fundamental principle of micro-Hall magnetometry is depicted in the figure below. A magnetic sample on the sensor surface creates a magnetic stray field which penetrates the sensor material and, in particular, a two-dimensional electron gas (2DEG) located about 100 nm below the surface. When a current is applied across this electrically conducting cross-shaped structure, the perpendicular component of the magnetic stray field arising from the sample will lead to a deflection of the electrons. Therefore, perpendicular to the current flow direction, a Hall-voltage proportional to the stray field component can be measured. Thanks to the possibility of patterning the Hall devices micro- and nanoscopically, and, due to the high mobility of the 2DEG, the Hall sensors based on GaAs/AlGaAs are extremely sensitive. Consequently, high-resolution measurements of individual magnetic micro- and nanoparticles are facilitated.
Figure shows the basic principles of micro-Hall magnetometry (left) and an electron microscope image of a CrO2 microcrystal placed on the surface of a Hall sensor (right).
Fluctuation Spectroscopy
Why is it interesting and instructive to study the fluctuations of a physical quantity and, in particular, the electronic noise in solid matter? The anwer lies in the fact that the time-dependent signal (i.e., the fluctuations of a physical variable) contains hidden valuable information about the intrinsic dynamic properties of charge carriers. Fluctuations are a purely random (statistical) phenomenon, but, however, the power spectral density S of the fluctuations is a non-random stationary quantity which is related to the so-called autocorrelation function of the fluctuating variable via a Fourier transform. By measuring the power spectral density of the sample's conductivity fluctuations one can gain access to the current-current correlation function which describes the microscopic charge carrier kinetics.
Figure shows an example for a fluctuating voltage signal (left) and the corresponding calculated noise spectrum which is of a 1/f type (right).
Experimental Facilities
