Charge Extraction by Linearly Increasing Voltage

When linearly polarized light is reflected from a magnetic film, its polarization becomes elliptic (Kerr ellipticity) and the polarization principal axis is rotated (Kerr rotation). This rotation is proportional to a component (depending on the geometry of experiment) of the magnetization of the film. For this reason the MOKE magnetometry is a very popular tool for the characterization of the magnetic properties of materials.

Since the laser spot can be focused down to a few microns, the magnetic properties (e.g. coercivity, squareness ratio) of single micrometer or submicrometer structures can be evaluated. With a suitable set-up the sensitivity of the MOKE apparatus can be pushed to detect the signal coming from very thin films (ideally down to the atomic plane) making the technique suitable for the study of magnetic structure of thin films, interfaces and nanostructures.

Note that although this technique can be very sensitive, no quantitative measure of the magnetization is provided. Nevertheless, from the shape of the hysteresis loop, both static and dynamic magnetization reversal processes can be investigated in detail. Longitudinal and polar configurations are used routinely to detect the in-plane or the out-of-plane magnetization components, respectively. This technique is well suited to characterize either flat or micron- and submicron-patterned films.

The facility consists of a magneto-transport equipment for electrical characterization integrated in a cryogen-free measurement system at variable temperature and magnetic fields. It has capabilities for 2-point or 4-point measurements, for Hall effect measurements in the Van der Pauw and Hall bar geometries, as well as I-V and C-V characteristics in DC and AC modes. This allows study of electrical transport (both semiclassical, i.e. conductivity and electric carrier density and mobility, and quantum transport effects in nano- and mesoscopic devices) on material systems such as semiconductor low-D structures (2DEGs, nanowires…), 2D and 1D materials (graphene and graphene-like nanomaterials, nanotubes, transition metal dichalcogenides, ultrathin 2D-oxides…), topological insulators and superconductors.  Additional electrical contacts allow application of gate biases for sample polarization. Electric Transport-Optical measurements in Electro-Optical and Magneto-optical configurations are possible through a tuneable Xenon light source and fibre-optic sample illumination.

Samples are fitted in a 16-pin dual-in-line socket, which can be placed perpendicular or parallel to the magnetic field. All 16 electrical connections can be independently and automatically switched through a matrix switch unit under software control. Samples with a wide range of resistivities can be measured, ranging from diluted 2D electron gases in semiconductors to metals. It is possible to record sweeps of gate voltages and magnetic fields (for, e.g., the observation of Shubnikov–de Haas oscillations / Quantum Hall effect in semiconductor 2D systems), as well as of temperature.