Magnetic force microscopy (MFM) is a well-established technique for imaging the
magnetic structures of small magnetic particles. In cooperation with external magnetic
fields, MFM can be used to study the magnetization switching mechanism of
submicrometer-sized magnetic particles.Various MFM techniques allow the measurement
of a hysteresis curve of an individual particle, which can then be compared to
ensemble measurements. The advantage of using MFM-constructed hysteresis loops is
that one can in principle understand the origin of dispersion in switching fields. It is also
possible to directly observe the correlation between magnetic particles through careful
imaging and control of the external magnetic field. In all of these measurements,
attention needs to be paid to avoid artifacts that result from the unavoidable magnetic tip
stray field. Control can be achieved by optimizing the MFM operation mode as well as
the tip parameters. It is even possible to use the tip stray field to locally and reproducibly
manipulate the magnetic-moment state of small particles. In this article, we illustrate
these concepts and issues by studying various lithographically patterned magnetic
nanoparticles, thus demonstrating the versatility of MFM for imaging, manipulation, and
spectroscopic measurements of small particles.