Scanning Hall probe microscope

Scanning Hall probe microscope (SHPM) is a variety of a scanning probe microscope which incorporates accurate sample approach and positioning of the scanning tunnelling microscope with a semiconductor Hall sensor. This combination allows to map the magnetic induction associated with a sample. Current state of the art SHPM systems utilize 2D electron gas materials (e.g. GaAs/AlGaAs) to provide high spatial resolution (~300 nm) imaging with high magnetic field sensitivity. Unlike the magnetic force microscope the SHPM provides direct quantitative information on the magnetic state of a material. The SHPM can also image magnetic induction under applied fields up to ~1 tesla and over a wide range of temperatures (millikelvins to 300 K).

The SHPM can be used to image many types of magnetic structures. A few structures are listed below.

• Thin films
• Permanent Magnets.
• MEMS structures.
• Current carrying traces on PCBs
• Permalloy disks
• Recording media

Advantages to other magnetic raster scanning methods

SHPM is a superior magnetic imaging technique due to many reasons including:

• Unlike the MFM technique, the Hall probe exerts negligible force on the underlying magnetic structure and is noninvasive.
• Unlike the magnetic decoration technique, the same area can be scanned over and over again.
• Magnetic field caused by hall probe is so minimal it has a negligible effect on sample it is measuring.
• Sample does not need to be an electrical conductor, unless using STM for height control.
• Measurement can be performed from 5 – 500 K.
• Measurement can be performed in ultra high vacuum (UHV).
• Measurement is nondestructive to the crystal lattice or structure.
• Tests requires no special surface preparation or coating.
• Detectable magnetic field sensitivity, is approximately 0.1 uT – 10 T.
• SHPM can be combined with other scanning methods like STM.

Limitations

There are some shortcomings or difficulties when working with an SHPM, and some of these are as follows:

• High resolution scans become difficult due to the thermal noise of extremely small hall probes.
• There is a minimum scanning height distance due to the construction of the hall probe. (This is especially significant with 2DEG semi-conductor probes due to their multi-layer design).
• Scanning (lift) height affects obtained image.
• Scanning large areas takes a significant amount of time.
• Relatively short practical scanning range (order of 1000's micrometer) along any direction.
• Housing is important to shield electromagnetic noise (Faraday cage), acoustic noise (anti-vibrating tables), air flow (air isolation cupboard), and static charge on the sample (ionizing units).

See also

• Scanning tunneling microscope