A compact concentric STM for point contact

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A compact concentric STM for point contact
investigations of magnetic nanostructures M.Saxe
gaard, A. Kapelrud, D.Yang, B.M. Førre and E.
Wahlström Department of physics, Norwegian
University of Science and Technology, 7491
Trondheim, Norway. Corresponding author
magne.saxegaard_at_ntnu.no
Motivation The compact concentric scanning
tunneling microscope (CCSTM) is a new instrument
for local measurements of transport properties in
nanostructures. By combining the imaging mode of
the scanning tunneling microscope (STM) with
point contact measurements (PCM), transport
properties may be probed with high lateral
resolution at current density levels suitable for
spin transport related phenomena. Method The
tip is positioned with the STM imaging mode. An
electrical point contact is created by indenting
the tip into the sample surface. Purpose built
electronics allow large currents to be injected
while simultaneously measuring voltage drops
across the sample. A conventional electromagnet
provides external fields for studies of magnetic
systems. Design specifications Scan
ning tunneling microscope Range X,Y 1.3 13
µm _at_ 0.022 Å Range Z 0.2 1.7 µm _at_ 0.0050.5
Å Speed X,Y 0.012 80 000 nm/s Point contact
measurements Applied current 1 µA 30 mA _at_ 0
1 kHz Magnetic field 0 50 mT _at_ 0 10 Hz Max.
resistance 10M 0.3 kO _at_ 1uA 30mA Sampling
speed 0.5 15 MS/s _at_ 24 16 bit
Duration Infinite Temp. Pres. ambient
Custom electronics A custom pre-amplifier,
voltage controlled current sender and signal
splitter board was made in order to combine PCM
with STM operation Scanner Tw
o concentrically mounted piezoelectric tubes
provide low thermal drift and compact design,
suitable for placing in the narrow gap of a
conventional electromagnet. Coarse
motion is realized with an intertial slider
mechanism, where the tip and tip holder acts as
sliding electrical contacts.
Preliminary results Stable and reproducible
point contact measurements were made on an
exchange biased spin valve system of FeMn (8 nm)
/ FeNi (8 nm) / Cu (3 nm) / FeNi (3 nm) / Cu (10
nm) / Si. Giant magnetoresistance curves were
measured by applying a constant sense current of
10 mA and measuring changes in voltage drop as
function of an applied magnetic field ( 30 mT _at_
4 Hz). The above
figures show changes in sample resistance of ?R
0.3 0.02 O as the free (3 nm NiFe) and
exchange biased (8 nm NiFe) layers are reversed
during repeated field sweeps. Contact radius was
estimated from the Sharwin resistance to r 5
nm, providing a current density of J 5108
A/cm2 at the FeMn/NiFe interface. Outlook A new
version of the CCSTM is being prepared for use
within an ultra high vacuum system. In addition,
this will be mounted on a helium cryostat,
allowing for temperature control in the range 4.2
300 K. Noise levels on the measured change in
resistance is expected to decrease with further
developement of the custom made electronics