Biomedical Engineering Reference
In-Depth Information
cases, the MNPs are soft magnetic materials - namely, that they have low remanent
magnetization and low coercivity. As a result, the tip or sample stray fi elds (
H
T
or
H
S
) may be mutually affected if the stray fi eld exceeds the anisotropy fi elds (
H
K
).
For nondestructive MFM imaging [27] :
HH
<
S
and
HH
<
T
T
K
S
K
One way to minimize this effect is to use a large tip-sample separation distance
(though this may limit lateral resolution) and a careful selection of the magnetic
force microscope probe.
In recent studies [2, 28], we have demonstrated the feasibility of detecting super-
paramagnetic MNPs via the MFM technique in ambient air. The SQUID studies
confi rmed the superparamagnetic nature of the MNPs, and revealed that an exter-
nal magnetic fi eld of a few tens of milliTesla is suffi cient to induce a stable mag-
netic dipole in MNPs, even at room temperature [2]. As a result, a perpendicular
or in-plane magnetic fi eld of this magnitude range was applied to the sample at
room temperature to induce a stable magnetic moment in MNPs. The MFM
experiments revealed that the presence of an external magnetic fi eld and a mag-
netic probe was essential to detect and distinguish an MFM signal from MNPs.
By applying a magnetic fi eld perpendicular to the sample (Figure 15.6a), mono-
poles of MNPs were detected as a negative phase contrast dependent on lift-height,
and by applying an in-plane magnetic fi eld (Figure 15.6b) the in-plane dipole
moment of MNPs was seen as a combination of a positive and negative phase
contrast. It is interesting to note that some particles in the AFM images did not
show any MFM phase contrast at all, suggesting that the particle composition in
Figure 15.5
MFM of ferromagnetic
nanoparticles. (A) Schematic of the domain
pattern, AFM image and MFM image of
500 nm - diameter islands on (110) - oriented
STO substrates with (a) square, (b) diamond,
and (c) circular shapes. The La
0.7
Sr
0.3
MnO
3
(LSMO) fi lms have been aligned with the
magnetically easy [001] direction pointing
horizontally [58]; (B) Single QD (
∼
50 nm)
switching in an external magnetic fi eld [62].
(a) Topography image; (b) MFM image of the
MnAs QD in an applied fi eld of 40 Oe that
opposes the QD magnetization. Thus, the QD
magnetization is stable at small opposing
fi elds; (c) MFM image of the switched QD.
The streaks in the image indicate that the
stray fi eld of the tip competes with the
applied fi eld, causing the QD to switch back
for some tip-sample confi gurations. All
images are 400
×
185 nm
2
in size;
(C) Variable-temperature MFM determination
of the QD Curie temperature [62].
(a) Topography image of MnAs/InAs-QD/
NW; (b-d) MFM phase images of the QD at
the indicated temperatures, showing that the
magnetic fi eld-dependent phase contrast is
approximately constant between 298 and
308 K, and the contrast disappears at 313 K;
(e) The phase, which is approximately
proportional to the magnetization, is plotted
as a function of temperature. The phase
drops abruptly to zero around the bulk Curie
temperature of 313 K, consistent with a
fi rst-order phase transformation, as is seen in
bulk MnAs. All images are 300
×
300 nm
2
in
size;. (D) Topography and magnetic structure
of 12-nm Co nanoparticle islands [63].
(a) AFM and (b) MFM images of scan sizes
3
×
3
μ
m. Vertical scales: 20 nm and 2 °
respectively. Reproduced with permission
from Refs [58, 62, 63], as indicated.
