Activity Report 2010 - CNRS

SCIENTIFIC REPORT
FURTHER READING:
[3] Phys. Rev. B 81, 134408 (2010)
Effect of crystalline defects on domain wall
motion under field and current in
nanowires with perpendicular
magnetization
[4] New J. Phys. 12, 103040 (2010)
Ultrathin epitaxial cobalt films on graphene
for spintronic investigations and applications
[5] Appl. Phys. Lett. 96, 262509 (2010)
Effect of structural relaxation and oxidation
conditions on interlayer exchange coupling
in Fe|MgO|Fe tunnel junctions
[6] Phys. Rev. B 81, 220407 (2010)
Oscillatory interlayer exchange coupling in
MgO tunnel junctions with perpendicular
magnetic anisotropy
[7] Science 315 349 (2007)
Electric Field-Induced Modification of
Magnetism in Thin-Film Ferromagnets
8
the depinning of the domain wall from a
single pinning centre using spin polarized
current injection [3]. Combining
structural and magnetic contrast in
Lorentz imaging it was possible to show a
clear correlation between a micromacle
and the domain wall pinning in a FePt
nanowire.
Please read the corresponding Highlight
at the end of this report for further
information
Perpendicular Anisotropy
and exchange interactions
Chair of Excellence 2007: Mairbek
CHSHIEV
PhD student: Hongxin YANG
(INAC/SPINTEC).
Perpendicular anisotropy materials play a
more and more important role for the
design of spintronics devices. Here,
theoretical electronic band structure
calculations provide an important mean
to study the effect and the role of the
structural properties on the electronic
and magnetic properties and thus a guide
for the development of spintronics
devices. Confirming recent experiments
relevant to strong perpendicular
magnetic anisotropy (PMA), it is shown
by first principles calculation that the PMA
in Fe/MgO magnetic tunnel junctions
(MTJ) can be as large as 3 erg/cm 2 . The
nature of the PMA has been clarified and
is attributed to the hybridization between
Fe and oxygen orbitals via spin orbit
interaction (SOI). Additional oxygen or
oxygen vacancies at the interface,
destroys the hybridization between Fe
and oxygen and leads to a reduction of
the PMA. Hence good crystalline quality
of the Fe/MgO interface is of importance
to obtain a large PMA.
Similarly, in order to explain recent
experiments realized at Institut Néel, it
has been confirmed by calculation that a
strong PMA is induced at Co/graphene
interfaces [4].
As a further result of the band structure
calculations it has been revealed that
structural relaxation influences the
interlayer exchange coupling (IEC)
between the Fe layers in Fe/MgO/Fe
tunnel barriers. In particular oxygen
vacancies increase the antiferromagnetic
coupling strength, while oxygen rich
interfaces decrease the coupling or
induce a ferromagnetic interaction [5].
Finally, oscillations of the IEC as a
function of the ferromagnetic layer
thickness in magnetic tunnel junctions of
PMA ferromagnetic layers has been
observed experimentally and explained in
the frame of a free electron model [6].
An experimental study (“fil de l’eau” PhD
student 2007: Marcio MEIDEROS-
SOARES) of the exchange coupling
between antiferromagnetic PtMn and
chemically ordered ferromagnetic FePt
with strong PMA has been performed to
better understand the exchange bias
mechanism. XMCD measurements
(ID08/ESRF beamline) at the Mn and Fe
L 2 , L 3 -edges show that the major part of
the Mn spins reverse along with the Fe
spins and thus only 10% contribute to
the exchange-bias shift. This outcome
shows that interfacial Mn spins are
strongly coupled to the ferromagnetic
FePt layer and less coupled to the spins
inside the antiferromagnetic layer.
Electric field control of
anisotropy
RTRA Project 2007: POMME
Coordinator: Dominique GIVORD (Institut
Néel).
As a new approach to control the intrinsic
magnetic properties in metals, a large
electric field is used to appreciably
change the electron density at surfaces
or interfaces in ultrathin ferromagnetic
metal films. This has been demonstrated
in 2007 by the Institut Néel for FePt and
FePd immersed in an electrolyte where
the magneto-crystalline anisotropy can
be reversibly modified by an applied
electric field [7]. The aim of the current
studies is to replace the electrolyte with
an insulating material to charge the
ferromagnetic surface (Fig. 2). The
challenge is to develop oxide overlayers
of high breakdown voltage.
Fig. 2: FEM simulation of the electric field and
surface charge induced on a nanocontact. The
charge distribution is maximum at the apex of
the nanocontact
Al 2 0 3 and HfO 2 barriers have been
prepared by Atomic Layer Deposition
(ALD) that permits to grow extremely
homogeneous and compact layers. The
mean voltage breakdown shows that the
goal of 10 8 V/m has been reached
allowing the application of strong electric
fields to the surface. The next step will be
to characterize the effect on a 2 nm FePt
thin film under electric fields of more