Iron-filled carbon nanotubes as magnetic force microscopy (MFM) probes
CNTs are already known to be suitable for high quality scanning force microscopy probes. Magnetic force microscopy (MFM) is a technique based on scanning force microscopy. Here, the magnetic moment of the sensor interacts with the sample’s magnetic stray field. A couple of experimental studies introduce CNT based MFM sensors using, e.g., metal coated CNTs, metal capped CNTs, or metal-filled CNTs.
Ideally, an appropriate MFM probe would consist of an elongated single-domain needle made from high remanence material. These requirements can be explained as follows. A high aspect ratio supports the magnetic shape anisotropy and thus stabilizes the probe’s magnetization. The MFM signal strength is proportional to the probe’s magnetic moment or its remanent magnetization. A single domain configuration maximizes the overall magnetic moment of the probe. In a broad range of imaging conditions, a probe with an elongated cylinder shape might be regarded as a magnetic monopole of which only the monopole close to the sample surface contributes to the magnetic interaction. In the framework of Fourier transfer functions (Hug et al., J. Appl. Phys. 83, 5609 (1998)), a probe’s monopole behavior is equivalent to a region of a constant force transfer function. The monopole approach is attractive because it leads to a simple proportionality between the first derivative of the sample’s stray field and the MFM signal in dynamic MFM.
The preparation of MFM probes using filled carbon nanotubes is still a delicate issue. Yet the in-situ preparation of filled CNTs on cantilever probes needs to be implemented. We bypassed the latter by mechanically attaching FeCNTs to cantilever probes. Such procedures may include the following steps. First, FeCNTs are grown by chemical vapor deposition on catalyst-coated silicon substrates. Selected nanotubes are then attached to conventional scanning force microscopy tips using a SEM equipped with a micromanipulator. After this, the fabricated probes are inspected and can, if necessary, be tailored by etching off unwanted carbon parts by localized electron-beam induced oxidation in an SEM or by FIB treatment. FeCNTs proved to be high-resolution long-lasting MFM probes. These sensors are expected to meet the requirements of ideal MFM probes as outlined above. In particular the verification of the monopole approach allows for an easy implementation of quantitative MFM approaches. In addition, the magnetic stability of FeCNT probes has been demonstrated by MFM measurements in in-plane fields up to 250 mT.
(a) SEM image of an FeCNT attached to a conventional silicon AFM cantilever. (b) – (d) Top part of the FeCNT. (b) SEM image. (c) Backscattered electron contrast (the bright region is iron filled). (d) Schematic sketch. The iron filling reaches to the very end of the nanotube.
Calibration of FeCNT MFM probes
Top: Schematic diagram showing the calibration structure and the coordinate system in use. (1) 70 nm gold layer to shield electrostatic potentials. (2) 200 nm silicon nitride insulation layer. (3) Silicon substrate. (4) 70 nm thick and 1 μm wide parallel gold lines carrying opposing current ±I. Bottom: SEM image of parallel gold lines with varying distance b.
Calibration of an FeCNT probe. Top: Phase shift as a function of the height z for different values of the current line separation b (see above) measured with an FeCNT tip. The solid lines represent the best fit to the point monopole approximation, yielding the displayed values for the probe’s monopole moment. Bottom: Monopole moment q and its position d of a conventional coated MFM probe (squares) and an FeCNT probe (circles) as a function of b. The solid lines should just guide the eye but do not reflect the underlying law.
The performed calibration measurements show that the simple point probe approximation can adequately describe the behavior of the FeCNT MFM sensor. They confirm experimentally that the effective monopole moment of an extended dipole with a length bigger than the sample stray field’s decay length stays constant for magnetic samples with different feature sizes. In addition, the measured moment corresponds to the calculated moment according to the FeCNT geometry, thus this new type of sensor has very predictable magnetic properties.
Further reading:
- Andreas Winkler, Thomas Mühl, Siegfried Menzel, Radinka Kozhuharova-Koseva, Silke Hampel, Albrecht Leonhardt, and Bernd Büchner Magnetic force microscopy sensors using iron-filled carbon nanotubes, J. Appl. Phys. 99, 104905 (2006) URL
- Franziska Wolny, Uhland Weissker, Thomas Mühl, Albrecht Leonhardt, Siegfried Menzel, Andreas Winkler, and Bernd Büchner Iron-filled carbon nanotubes as probes for magnetic force microscopy, J. Appl. Phys. 104, 064908 (2008) URL
- F. Wolny, T. Mühl, U. Weissker, A. Leonhardt, U. Wolff, D. Givord, and B. Büchner Magnetic force microscopy measurements in external magnetic fields-comparison between coated probes and an iron filled carbon nanotube probe, J. Appl. Phys. 108, 01398 (2010) URL
- F. Wolny, T. Mühl, U. Weissker, K. Lipert, J. Schumann, A. Leonhardt, and B. Büchner Iron filled carbon nanotubes as novel monopole-like sensors for quantitative magnetic force microscopy, Nanotechnology 21, 435501 (2010) URL
- S. Vock, F. Wolny, T. Mühl, R. Kaltofen, L. Schultz, B. Büchner, C. Hassel, J. Lindner, and V. Neu Monopolelike probes for quantitative magnetic force microscopy: Calibration and application, Appl. Phys. Lett. 97, 252505 (2010) URL
Contact:
Dr. Thomas Mühl
Department 'Chemical Vapor Deposition'
Institute for Solid State Research (IFF)
IFW Dresden
D-01171 Dresden, Germany
Phone: +49-351-4659-496
Fax: +49-351-4659-9496
Email: t.muehl(et)ifw-dresden.de