Activity Report 2010 - CNRS

DETECTING SINGLE
NANOPARTICLE
MAGNETIZATION
REVERSAL WITH A
NANOTUBE
Nanospintronics benefits from advances
in quantum transport and molecular
electronics. Combining these concepts
provides new devices highly sensitive to
the local electromagnetic environment
such as carbon nanotube quantum dots.
Moreover molecular objects offer great
versatility in their functionnalization with
magnetic systems such as nanoparticles
or molecular magnets, offering new
routes towards nanoscale spin detection.
Magneto-Coulomb effect in
nanotube quantum dots
filled with magnetic
nanoparticles
“Fil de l’eau” PhD student 2007:
Subhadeep DATTA
Carbon nanotubes at low temperature
behave as quantum dots (QD) for which
charging processes become quantized,
giving rise to Coulomb blockade. Any
small change in the electrostatic
environment (tuned by the gate
electrode, see Fig. 1, top) can induce a
shift of the energy of the QD, leading to
conductivity variation. A carbon nanotube
can therefore be a very accurate
electrometer. If a magnetic system is
electronically coupled to a nanotube, its
spin state can influence sequential
tunneling through the nanotube (socalled
magneto-Coulomb effect).
The context in which the magneto-
Coulomb effect (MCE) was first observed
was that of single-electron transistors
connected with two ferromagnetic leads.
This effect was characterized by an
enhanced magnetoresistance (MR) of the
island in the Coulomb blockade regime.
This feature originated from the Zeeman
energy of the ferromagnetic contacts,
inducing a shift between the majority and
minority spin energy bands. It resulted in
a modification of the island chemical
potential, which is equivalent to effective
electrostatic gating. The MCE thus
enabled the single electron transistor to
be driven by the magnetic field.
In our case, the hollow center of a double
wall carbon nanotube is filled with
magnetic nanoparticles such as iron
(collaboration with Institut Carnot
CIRIMAT, University of Toulouse). We
observe unprecedented high MR reaching
up to 53% at 40 mK with a hysteretic
behaviour and sharp jumps at specific
magnetic fields corresponding to the
magnetization reversal of the
encapsulated particle (see Fig. 1,
bottom). Moreover these features are
strongly gate dependent and reflect
directly the features from Coulomb
blockade in the QD. Indeed, the spin flip
of the iron island at non-zero magnetic
field causes a sharp change in the
nanoparticle chemical potential due to
the Zeeman energy, which is acting on
the nanotube like an effective offset
charge. Such coupling allows the
detection of a single reversal event, with
high accuracy considering its strong
influence on the magnetoresistance.
This effect is thus a new gate dependent
MR, which differs from that described in
previous reports in which the MCE
originated from the contact between
nanostructures and ferromagnetic leads.
Here the MCE is induced by the local
coupling of a nanotube quantum dot with
low-dimensional magnets. This coupling
allows differentiating the sensor from the
probed magnetic object. It opens up new
possibilities for exploiting the versatility
of nanotube QD functionnalization with
different nanomagnets, using double wall
nanotube (DWNTs) filled with various
materials, functionalized on their surface
with molecular magnets.
Fig. 1: Top: transistor based on a double wall
carbon nanotube, the inner tube of which is
filled with magnetic iron nanomagnet; Bottom:
strong resistance variations R and strong MR
hysteresis observed for different applied gate
voltages Vg. Quantum transport and
magnetization reversal are directly correlated
and appear as color changes.
CONTACTS
4
HIGHLIGHT : NANOMAGNETISM AND SPINTRONICS
laetitia.marty@grenoble.cnrs.fr
wolfgang.wernsdorfer@grenoble.cnrs.fr
FURTHER READING
L. Bogani et al., Nature Mater. 7, p179,
(2008)