Spin polarized scanning tunneling microscopy
Encyclopedia
Spin polarized scanning tunneling microscopy (SP-STM) is a specialized application of scanning tunneling microscopy (STM) that can provide detailed information of magnetic phenomena on the single-atom scale additional to the atomic topology gained with STM. SP-STM opened a novel approach to static and dynamic magnetic processes as precise investigations of domain walls in ferromagnetic and antiferromagnetic systems, as well as thermal and current-induced switching of nanomagnetic particles.

Method of Operation

An extremely sharp tip coated with a thin layer of magnetic material is moved systematically over a sample. A voltage is applied between the tip and the sample allowing electrons to tunnel between the two, resulting in a current. In the absence of magnetic phenomena, the strength of this current is indicative for local electronic properties.

If the tip is magnetized, electrons with spins matching the tip's magnetization will have a higher chance of tunneling. This is essentially the effect of tunnel magnetoresistance
Tunnel magnetoresistance
The Tunnel magnetoresistance is a magnetoresistive effect that occurs in magnetic tunnel junctions . This is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough , electrons can tunnel from one ferromagnet into the other...

 and the tip/surface essentially acts as a spin valve
Spin valve
A spin valve is a device consisting of two or more conducting magnetic materials, that alternates its electrical resistance depending on the alignment of the magnetic layers, in order to exploit the Giant Magnetoresistive effect. The magnetic layers of the device align "up" or "down" depending on...

.

Since a scan using only a magnetized tip cannot distinguish between current changes due to magnetization or space separation, multi-domain structures and/or topological information from another source (frequently conventional STM) must be utilized. This makes possible magnetic imaging down to the atomic scale, for example, in antiferromagnetic system.

Nowadays, antiferromagnetic tips are preferred since ferromagnetic tips induce magnetization reversal
Magnetization reversal
Magnetization reversal, or switching, represents the process that leads to a 180° reorientation of the magnetization vector with respect to its initial direction, from one stable orientation to the opposite one. Technologically, this is one of the most important processes in magnetism that is...

 in the systems to be studied due to stray fields. In STM tunneling electrons come from the very same atom most close to the surface. Therefore, with the magnetization of this atom well defined, the tunneling current remains polarized for antiferromagnetic tips but without stray field.

Alternate method

Another way to obtain the magnetization distribution is to have the tip provide a strong stream of spin polarized electrons. One method to achieve this is to shine polarization laser light onto a GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....

 tip, which produces spin polarized electrons due to spin-orbit coupling. The tip is then scanned along the sample much like conventional STM. One limitation of this method is that the most effective source of spin polarized electrons is obtained by having the incident laser light shine directly opposite of the tip, i.e. through the sample itself. This restricts the method to measuring thin samples.

See also

  • Scanning tunneling microscopy
  • Microscopy
    Microscopy
    Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...

  • Scanning probe microscopy
    Scanning probe microscopy
    Scanning Probe Microscopy is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. An image of the surface is obtained by mechanically moving the probe in a raster scan of the specimen, line by line, and recording the probe-surface interaction as a...

  • spin valve
    Spin valve
    A spin valve is a device consisting of two or more conducting magnetic materials, that alternates its electrical resistance depending on the alignment of the magnetic layers, in order to exploit the Giant Magnetoresistive effect. The magnetic layers of the device align "up" or "down" depending on...

  • tunnel magnetoresistance
    Tunnel magnetoresistance
    The Tunnel magnetoresistance is a magnetoresistive effect that occurs in magnetic tunnel junctions . This is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough , electrons can tunnel from one ferromagnet into the other...

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