Nanostructures and Nanotechnology by Douglas Natelson

Author:Douglas Natelson [Natelson, Douglas]
Language: eng
Format: epub, azw3, pdf
Publisher: Cambridge University Press
Published: 2015-06-29T21:00:00+00:00

7.4Magnetism at small scales

The interplay of magnetic energy scales and the intrinsic size scales inherent in magnetic phenomena make for very interesting magnetic phenomenology at the micro- and nanoscale. A question that arises immediately is, how do we assess and characterize the magnetic response of materials on these scales? There are a number of techniques, each with their own advantages and drawbacks. Here is a representative sampling.

Bitter decoration As we all learned in elementary school, one way to trace the spatial arrangement of magnetic field is to sprinkle a region of interest with iron filings, or, more generally, small magnetically polarizable particles. Those particles will feel forces as described in Eq. (7.9) and will concentrate in regions of strong fields. One can use the same approach to examine the field distribution caused by stray fields near domain boundaries on the surfaces of magnetic materials. This process, pioneered by Bitter [490] using colloid suspensions of magnetite, is not well suited for quantitative analysis or micro/nanoscale magnetism. However, it has been of major importance in understanding magnetic phenomena over the past century.

Hall sensors We considered the Hall effect back in Section 6.3 as a means of extracting knowledge of a conducting system’s parameters (2d carrier density and mobility) from longitudinal and Hall measurements in a known magnetic field. It is possible to come at this from the other direction, and use a well characterized semiconductor system’s Hall response to infer the stray magnetic field at some location. For a fixed Hall measurement geometry, the Hall response provides a measure of the component of the magnetic field normal to the plane of the 2d electronic system. Operating temperatures can be constrained by the type of Hall bar material. For example, GaAs/AlGaAs heterojunction electron gases typically need to be cooled well below room temperature to optimize their transport characteristics.

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