SCANNING HALL PROBE MICROSCOPY - SHPM
fundamentals

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The Scanning Hall Probe Microscope (SHPM) is one of the few scanning probe microscopes which do not originate from the Scanning Tunneling Microscope (STM) as invented by Binnig and Rohrer in 1981. In contrast to its companions MFM and scanning-SQUID, scanning Hall effect sensors were already used in the late 1960s to image superconducting materials with a spatial resolution as high as 4µm and a field sensitivity of 100µT. It was not until the late 1970s, however, that semiconductor Hall sensors with a two dimensional electron gas layer (2DEG) could be manufactured by modulation doping (Dingle 1978). This invention increased electron carrier mobilities to values far greater than in any other existing compound, allowing the combination of high field sensitivity with high spatial resolution – even at low temperature. Compared to other magnetic imaging techniques such as MFM and scanning-SQUID, the SHPM is the only microscope capable of providing a non-invasive, quantitative information of the local magnetic field of a sample while yielding a sub-µm lateral resolution.
Today, the SHPM is a standard tool for the investigation of magnetic properties of a sample at both room- and low temperature and is particularly, but not only, used for the investigation of superconducting materials. In a typical experiment, the Hall sensor is approached to close proximity with the sample surface and then rastered across the sample with the help of a dedicated scanner. Measuring the Hall-Voltage during this process directly yields the local magnetic field which can be recorded and displayed in two dimensions using conventional scanning probe microscopy electronics and software. There are typically two modes of operation applied, which are tracking mode and constant height mode. In tracking mode, the Hall sensor is kept in close proximity to the sample while rastering the latter, whereas the Hall sensor is retracted from the sample by several 100nm before initiating the scan in constant height mode. For both cases, a mechanism to detect the location of the sample surface with respect to the Hall sensor is necessary, which is typically achieved by either measuring a tunneling current (STM-tracking SHPM) or by measuring long-range attractive forces between Hall sensor and sample (Tuning Fork-tracking SHPM).

Hall Sensors
The highest-quality Hall Sensors for low temperature operation existing today are made from a GaAs/AlGaAs heterostructure, created by a molecular-beam-epitaxy (MBE) growth process. These sensors typically provide an electron mobility exceeding 100000cm²/Vs and a sensitivity of 1500V/AT at low temperature. attocube systems currently offers these kind of sensors with high and ultra-high resolution technology, yielding 500nm and 300nm spatial resolution. The thermodynamic noise limit of attocubes’ sensors is typically 15nT/Hz1/2 at 4K and 80nT/Hz1/2 at 77K, while the practically attainable magnetic field resolution is limited to 1x10-4 F0 in a typical experiment, where F0 is the magnetic flux quantum.
At room temperature, GaAs/AlGaAs-sensors are intrinsically limited by 1/f and generation-recombination noise contributions, compromising the field detection limit. This is typically compensated for by either choosing a larger Hall sensor or by using a different hall sensor material. For this purpose, attocube systems also offers Bi-based Hall sensors which are dedicated for room temperature application (on request).

attocube systems SHPM

Similar to all other scanning probe microscopes offered by attocube systems, the attoSHPM is compatible with cryogenic and vacuum environments as well as high magnetic fields. attocubes scanning hall probe microscope is available as both 2” and 1” version, being compatible with both standard 2” cryogenic systems such as attocubes LTSYS-series and all 1” cryostats, including the QD PPMS.

attoSHPM / xs:
This STM-tracking, compact SHPM system ensures outstanding stability and enables non-invasive, quantitative magnetic field measurements on the nanometer scale. The adjustment of the Hall sensors angel with respect to the sample surface is performed outside of the cryostat prior to cooling down the microscope.