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Quantitative Magnetic Force Microscopy and its Application for Assessing Thin Film Magnetism

Kolloquium der Abteilung 2

Hans J. Hug1,2

1 Empa, Nanoscale Materials Science, Duebendorf, Switzerland.

2 University of Basel, Department of Physics, Basel, Switzerland.

Magnetic Force Microscopy is a versatile technique for imaging the stray field emanating

from magnetic samples. Because stray fields decay exponentially with distance from the

sample surface, the imaging of high-spatial frequencies requires a high force (derivative)

sensitivity and thus high quality factor of the cantilever. The latter is achieved by operation

under vacuum conditions and by a masked coating of the cantilever tip area. High-quality

factor cantilevers do not permit the use of the typically implied MFM double passage

operation mode [1,2] that maps the sample topography with intermittent tip-sample contact.

Several new tip-sample distance control modes suitable for operation in vacuum were

developed [3,4,5]. The capacitive frequency modulated feedback operation mode [5] can

keep the tip-sample distance with sub-Nanometer precision over days and allows MFM

operation in variable magnetic fields up to 7T and different temperatures.

Precise tip-sample distance control is also needed for differential imaging: The magnetic thin film sample is scanned with opposite tip magnetization and also after saturation of the

sample. Such a data set then allows the separation of the frequency-shift signals arising from the van der Waals force from those generated by the stray magnetic field of the sample’s domain pattern and local sample thickness variations.

 

Reproducible imaging of magnetic stray field is further key for the calibration of the tip

response to different spatial wavelengths of the sample stray field [6]. Once the tip is

calibrated, the measured MFM data can be compared to MFM data calculated from stray

fields of candidate magnetization structures, and thus to assess which magnetic moment

distribution of the sample fits best to the observed MFM image. Conversely, at least under

some limiting conditions, it is possible to deconvolve all vector components of the magnetic

field at the sample surface from the measured MFM frequency shift image. Recently we have extensively used quantitative magnetic force microscopy techniques to image domain patterns and skyrmions in multilayer thin film samples with interfacial Dzyaloshinskii-Moriya interaction (DMI). The average DMI and its sign can be determined from MFM measurements of the domain structure [7,8]. Local DMI, anisotropy, and other

parameters can be obtained from high-resolution data of skyrmions. Skyrmions with radii

below 10nm have been observed.

 

References:

[1] S. Hosaka, et al., Jpn. J. Appl. Phys. 31 (1992) 908

[2] R. Giles et al., Appl. Phys. Lett. 63 (1993) 617

[3] J. Schwenk et al. Appl. Phys. Lett. 104 (2014) 112412

[4] J. Schwenk et al. Appl. Phys. Lett. 107 (2015) 132407

[5] X. Zhao et al., New J. Phys. 20 (2018) 013018

[6] P.J.A. van Schendel et al., J. Appl. Phys. 88 (2000) 35–45

[7] M. Bacani et al., arXiv: 1609.01615

[8] M. Marioni et al., Nano Lett. (2018) DOI: 10.1021/acs.nanolett.7b04802