Activity Report 2010 - CNRS

SCIENTIFIC REPORT
Magnetic Microsystems
“Fil de l’eau” PhD student 2007: Mikhail
KUSTOV
Coordinators: Nora DEMPSEY (Institut
Néel), Orphée CUGAT et Gilbert REYNE
(G2ELab)
Due to the scalability of magnetic forces
upon reducing size, they are successfully
used in microsystems. A close
collaboration covering materials sciences
and electrical engineering, and potential
users (such as biologists) allowed us to
optimize arrays of micro-patterned hard
magnets (NdFeB). The stray fields
produced are fully characterized thanks
to the development of new tools such as
3D measurements using a singlecomponent
Hall probe and quantitative
magneto-optic imaging using a uniaxial
magneto-optic indicator film in a bias
field [9]. New micro-systems have been
demonstrated, such as the "flying carpet"
shown in Fig.5, or micromagnet arrays
for the manipulation of biological objects.
QUANTUM
SPINTRONICS
Downscaling spintronics devices one
reaches the limit of single magnetic
molecules and atomic spins in magnetic
quantum dots. A major challenge here is
to read and manipulate the spin states
and to perform basic quantum
operations. On the one hand, Carbon
nanotubes (CNTs) are very good
candidates to study these effects due to
their high sensitivity to small changes in
the electrostatic environment. On the
other hand, inserting a single Mn atom
into a quantum dot provides the ultimate
tool to manipulate individual spin states.
Finally, for developing beyond “CMOS”
nanoelectronics, graphene as a new
material has emerged recently. The
conditions under which the magnetic
order can be obtained is explored
theoretically by atomic scale
‘nanopatterning’.
Magnetic order in graphene
Chair of Excellence 2007: Mairbek
CHSHIEV
PhD student: Hongxin YANG
(INAC/SPINTEC).
FURTHER READING:
[9] J. Appl. Phys. 108, 063914 (2010)
Magnetic characterization of
micropatterned Nd-Fe-B hard magnetic films
using scanning Hall probe microscopy
10
Fig. 5: The "flying carpet": the design of the
assembly of micromagnets ensures stability in
both directions and can replace complex
systems involving superconductors and
associated cryogenics.
Spin transfer torque in
nanoparticles
“Fil de l’eau” PhD student 2008: Irina
GROZA
Coordinator: Alain MARTY (INAC/SP2M).
Aggregates of magnetic Co/CoO coreshell
nanoparticles (5 nm diameter)
provide a test system to study the effect
of the spin polarized current on the
antiferromagnetic exchange bias from the
CoO to the Co. As a first step a few tens
of nm thick films of nanoparticles were
prepared and the correlation between the
particles was established using the
magneto-resistance effect. At room
temperature, the CoO is unblocked and
the correlation comes from the dipolar
interaction between particles.
Huge values of the charge mobility in
addition to a weak intrinsic spin-orbit
coupling in carbon-based sp 2 structures
could potentially allow for very large
(micron long) spin diffusion lengths.
These features, together with the other
”semi-conductor like” properties of
graphene, make graphene-based
spintronic devices highly promising and
have triggered a quest for controlling
spin injection in graphene. Many routes
have been attempted to induce
magnetism by proximity effect or inject
spins from magnetic electrodes. Here we
graphene nanomeshes investigate by
first-principles calculations to address the
important question of whether and under
what conditions graphene can exhibit
correlated (ordered) magnetic properties.
By removing an equal number of A and B
sites of the graphene bipartite lattice, a
regular network of atomic scale vacancies
is obtained. Such a nanomesh (Fig. 6)
made mostly of zigzag (armchair) type
edges exhibits antiferromagnetic (spin
unpolarized) states. In contrast, in a
situation of sublattice symmetry
breaking, stable ferrimagnetic states are
obtained. For a hydrogen-passivated
nanomesh, the ground state is found to
strongly depend on the vacancies shape
and size. The obtained net magnetic
moments increase with the difference
between the number of removed A and B