Fyzikální ústav Akademie věd ČR

Magnetic Force Microscopy (MFM) is the method derived from the Atomic Force Microscopy (AFM), allowing visualization of the magnetic contrast (different orientation of magnetic moments) and is based on the measurement of the magnetic probe-sample interaction. Imaging of magnetic nanoparticles (NPs) is non-trivial because of the principal complications [1-3] (contribution of Van der Waals forces into the contrast, Fig. 1; visualization of NPs with sufficiently large magnetic moment, μ_m and their orientation in the anti/parallel direction with respect to the direction of the probe magnetization; thermal fluctuation stability; interparticle distances larger than the tip radius etc.).

Possibility of detection of NPs by MFM is mainly important in the materials that do not allow direct imaging of NPs (NPs in matrix, covered with some diamagnetic layer etc.).

We are currently studying methods of optimization of single NPs imaging for NPs dispersed on substrates, aggregates of NPs in thin films and NPs in matrix.

Fig. 1: (a) Basic scheme of the creation of MFM, the last figure illustrates contribution of the VdW forces to the contrast (tip in the LSH1 distance). (b) TEM, topography and the phase scans for the CoFe₂O₄ NPs with μ_m ~ 1.4 ×10³ μ_B. (c) Illustration of the tested alignment of NPs devoted for imaging, (d) 3D topography (left) and topography with the magnetic contrast as the skin (right) for 13 nm CoFe₂O₄ NPS with μ_m ~ 4 ×10³ μ_B. It is obvious that magnetic contrast of single NPs is screened by the signal from aggregates.

References:
[1] S. Schreiber, M. Savla, D. V Pelekhov, D. F. Iscru, C. Selcu, P. C. Hammel, and G. Agarwal, Small 4 (2008) 270.
[2] V. L. Mironov, D. S. Nikitushkin, C. Bins, A. B. Shubin, and P. Zhdan, IEEE Transactions on Magnetics, 43 (2007) 3961.
[3] V. L. Mironov, B. Gribkov, D. S. Nikitushkin, S. Gusev, S. V. Gaponov, B. Shubin, P. Zhdan, and C. Binns, IEEE Transactions on Magnetics, 44 (2008) 2296.

Research team: B.Pacakova