Iron-filled carbon nanotubes (Fe-CNTs) were used to prepare probes for magnetic force microscopy (MFM) by attaching them to the tips of conventional atomic force microscopy cantilevers. An optimized chemical vapor deposition process, employing a two stage furnace and ferrocene as a precursor, supplied the homogeneously filled Fe-CNTs required for the MFM probes. These can be regarded as cylindrically shaped single-domain nanomagnets that are protected from oxidation by a carbon shell. Carbon nanotubes are known to possess both great mechanical stability and elasticity, which lead to a much longer lifetime of these probes compared to conventional magnetically coated probes. It is shown that the prepared probes are suitable for magnetic imaging and so far show no sign of deterioration. Even very long nanotubes can be used as probes, which implies that they are extraordinarily stiff. It is also shown that attached Fe-CNTs can subsequently be tailored by electron-beam induced oxidation (e.g., to remove disturbing empty carbon shell parts) to better fit the requirements of an MFM tip.

We present a novel ultrahigh stability sensor for quantitative magnetic force microscopy (MFM) based on an iron filled carbon nanotube. In contrast to the complex magnetic structure of conventional MFM probes, this sensor constitutes a nanomagnet with defined properties. The long iron nanowire can be regarded as an extended dipole of which only the monopole close to the sample surface is involved in the imaging process. We demonstrate its potential for high resolution imaging. Moreover, we present an easy routine to determine its monopole moment and prove that this calibration, unlike other approaches, is universally applicable. For the first time this enables straightforward quantitative MFM measurements.

The magnetization reversal and switching behavior of an individual Fe-filled carbon nanotube has been measured using vibrating cantilever magnetometry. We report measurements of the magnetic field at which the 25 nanometer diameter iron core inside the nanotube reverses. The fields at which reversal occurs, characterized by an exceptionally narrow distribution (σH≤1 G at 6.3 K), are determined by thermally activated excitation over a field dependent barrier. The high precision achievable by virtue of measuring individual nanowires allows detailed quantitative understanding of magnetization reversal.

Single crystal iron carbide nanowires contained in multiwalled carbon nanotubes have been prepared by aerosol-based thermal chemical vapor deposition. Investigations by transmission electron microscopy reveal the crystallographic [010] axis of the orthorhombic Fe3C nanowires to be predominantly aligned along the nanotube axis. Despite the high aspect ratio of the Fe3C nanowires, magnetic force microscopy measurements imply single domain behavior with the easy magnetic axis of the nanowires perpendicular to the wire axis. In agreement with the structural results, these findings show that the magnetic behavior is dominated by the magnetocrystalline anisotropy contribution, causing the easy axis to be along the [001] direction.

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