Biomedical Engineering Reference
In-Depth Information
of a laser beam from the reverse side of the cantilever. This is a particularly attrac-
tive approach when the MFM instrument is operated in a vacuum environment,
especially at low temperatures, where optical adjustments are diffi cult. Among the
non-optical methods for MFM are piezoresistive cantilevers [74, 75], piezoelectric
cantilevers [76, 77], and tuning fork-based cantilevers. In the latter case, a com-
mercial magnetic cantilever tip can be attached to one prong of the tuning fork to
realize shear-mode MFM operation [78]. While the resonance frequency of a
tuning fork is similar to that of a cantilever, the quality factor
Q
and the spring
constant
k
of a tuning fork are typically 102- and 104-fold larger than those of the
cantilever, respectively. Hence, the force sensitivity of tuning fork-based MFM is
less than that of cantilever-based MFM by a factor of 10. In another advancement,
a scanning magnetoresistance microscope (SMRM) has been developed, that is
equipped with a microfabricated cantilever on which a spin-valve (SV) -type MR
sensor element is mounted [79]. These non-optical cantilevers lend themselves
well to multiple cantilever applications. However, further challenges lie ahead with
the need to servo control all of the cantilevers independently.
15.12.2
Application of External Magnetic Fields
When examining MNPs with MFM, innovative techniques must be selected and
developed to apply external magnetic fi elds to the sample, while minimally affect-
ing the magnetic moment of the magnetic force microscope probe. One confi gura-
tion for application of magnetic fi eld is by placing an electromagnet (e.g., a
Helmholtz coil) very close to the sample. This technique can be used to apply fi elds
of up to a few hundred Oe. Increasing the fi eld beyond this value is known to heat
the sample and cause drifts in the probe. Another technique for applying a higher
magnetic fi eld to the sample, without sample heating, relies on a rare-earth magnet
mounted on a rotating cylinder [80]. In this case, fl ux from the magnet is shunted
through either the sample or the cylinder faces, depending on the angular position
of the rotating armature. In recent years, hardware accessories to enable the appli-
cation of external magnetic fi elds have become available for commercial AFM
instrumentation.
15.12.3
Technique Developments for MFM
High-frequency MFM (HF-MFM) is one of the most promising current develop-
ments in MFM. In this procedure, the cantilever is oscillated by the piezoelement
and
the HF magnetic fi eld applied to the sample, leading to an overlap of the two
oscillations. This technique ensures HF-MFM images also in situations where the
standard amplitude modulation technique fails [81], and offers several parameters
to increase the achieved HF-MFM signal strength. Moreover, recent model calcula-
tions [82] have shown that the HF-MFM technique indeed measures the emerging
stray fi elds, and not the fi eld gradients as in conventional MFM. The HF-MFM
