Chemistry Reference
In-Depth Information
2
B
z
∂
Q
k
m
z
∂
d
ϕ
=−
(5.11)
z
2
The influence of the size of the magnetic tip on the measured signal will be
discussed in Sect.
5.3.3
.
As is apparent in (
5.9
), the phase of the oscillating cantilever will shift in response
to any existing force(s) between the tip and the sample. Because the magnetic force
gradients are weak, it is important to eliminate the effects of topographic and elec-
trostatic structure in imaging. To eliminate topographic interactions, phase data are
generally taken at a significant lift height from the sample. To do so, a lift mode
is used, in which the tip makes two line scans over the sample for each recorded
line. During the first scan, topography is recorded. During the second scan, the tip is
lifted at a constant height above a specific baseline calculated by the data acquisition
software and held at that level during the entire pass. To exclude phase response due
to electrostatic forces, the potential between the tip and the sample may be nulled by
an external voltage divider [
65
,
66
], thereby eliminating any electrostatic component
in the total phase response.
5.3.2 Experimental Implementation
The magnetic signal generated by current-carrying wires is much smaller than that
of magnetic samples for which early MFMs were designed. Therefore, our opti-
cal head contains a special low noise laser (Digital Instruments part no. 226-000-
0004), which is specified to have minimized mode hopping, a phenomenon that can
result in artifacts and noise in the images. The magnetic tips used are commercially
available Co/Cr-coated MESP (magnetic etched silicon probe) tips or their higher
moment counterparts (MESP-HM), magnetized along the tip axis, perpendicular to
the sample surface. Sample holders are electrically isolated from the conducting cap
on the scanner piezo. To allow current biasing, they have two or three large contact
pads that are connected by thin 40-gauge wires to external electronics. Care was
taken to protect the scanner's piezo element from overheating which can cause the
scanner's lateral distance calibration to fail by as much as 30%.
Samples were fabricated with controlled geometries to implement and measure
the effects of current stressing by standard photolithography/liftoff created on ther-
mally grown SiO
2
by photolithography, followed by thermal evaporation of 20 nm
Cr and 110 nm Au. The metallic line widths were 10-12
μ
m and the fabricated
defects were generally 1-7
m in size, as shown in Fig.
5.8
a. MFM measurements
were made with typical currents in the individual lines of about 33mA, correspond-
ing to current densities on the order of 3-4
μ
10
6
A/cm
2
. Simulated defect struc-
tures were fabricated using a combination of standard photolithography/liftoff and
focused ion beam (FIB) milling techniques. The FIB milling allowed for defect
dimensions as small as 0.5
×
μ
m with the sharpest corners and apexes having a
50 nm radius of curvature. Ion milling was performed with 50 kV Ga
+
ions using a
Micrion 2500 FIB machine with a 5 nm beam column. A serpentine beam scanning
