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Magnetic fields of micro- and nanostructures

Together with Göttingen University, PTB has developed a procedure which will allow the magnetization and the magnetic field of smallest structures to be determined quantitatively and in a traceable way. In combination with a magneto-optical method, this will for the first time allow traceable magnetic measurements to be performed with magnetic force microscopy which provides an extremely high spatial resolution.

Optically determined magnetization distribution of FePt blocks in false colour representation and magnetization profile of a structure approx. 10 µm in size.

Magnetic stray fields of microscopic structures larger than approx. 300 nm can be determined by means of the magneto-optic indicator films (MOIF). Thereby, traceability is guaranteed by calibrating the measurement signal by means of known macro-scale fields. To determine the magnetization distribution of the samples from it, the measured stray field must be mathematically inverted. For this problem, which usually has no unambigious solution, a solution has so far only been known for the special case of a magnetization vertical to the sample plane. Now, a definite inversion procedure has for the first time also been developed for magnetizations in the sample plane.

If the structures are smaller than 300 nm, a stray-field-sensitive magnetic force microscope (MFM) with a resolution of a few 10 nm is used. However, this microscope furnishes quantitative stray field values only if its transfer function has been determined with reference samples of known stray field. Such samples, based on hard magnetic FePt films, were manufactured at PTB. By means of electron beam lithography, test structures of different form and size were patterned. The new inversion method allows the magnetization to be determined from the stray field distribution of these structures measured by means of MOIF on correspondingly large structures. This, in turn, allows the stray field to be calculated at the measurement height of the MFM stylus tip and the MFM to be calibrated.

With the transfer function now known, which is independent of the structure size, the MFM can also be used to measure the stray fields of smaller structures in a quantitative and traceable way. By applying the developed inversion procedures once again, the magnetization distributions are reconstructed; this time, however, with an MFM-typical spatial solution in the nanometer range.

Contact at PTB:

Phone: +49-531-592-0