Magnetic anisotropy: effects
of reduced symmetry.
Marek Przybylski
Max-Planck-Institut für
Mikrostrukturphysik, Weinberg 2, 06120 Halle,
By varying the
substrate-layer combination and individual layer thickness, it is possible to
manipulate the magnetic anisotropy of thin-film and multilayer structures. A
dramatic manifestation of the varying anisotropy is the change of the
preferential direction of the magnetization from the commonly observed in-plane
to the perpendicular direction.
The physical
origin of the magnetocrystalline anisotropy is attributed to orbital moment and
spin-orbit interaction. Orbital effects are quenched in a bulk-like cubic
environment, where the coordination is highest and high symmetry reduces the
magnetic anisotropy. However, the quenching of the orbital moment can be
removed by an appropriate symmetry reduction. If the crystal field locates the dxy
and dx2-y2 states near to EF
with one state being below EF and the other above EF
with an energy separation smaller than in cubic-bulk, the orbital moment and magnetic anisotropy will
be enhanced.
There are many concepts how a strong perpendicular
magnetic anisotropy (PMA) could be obtained in the systems of reduced
coordination and symmetry. The orbital moment is not quenched at surfaces,
interfaces and monolayer thick films. In the system of bulk-like coordination orbital
moment and magnetic anisotropy can be increased by lowered symmetry e.g. due to
a tetragonal distortion [1]. No interface electronic hybridization and coordination
reduction is required in this case. A model system is provided by Fe1-xCox
alloy films which can be distorted due to their pseudomorphic growth on
substrates of mismatching lattice constant like Pd, Ir or Rh(001). The Fe1-xCox
films remain pseudomorphic and tetragonally distorted up to a thickness of more
than 10 ML when grown on Rh(001). A largely increased PMA is found
experimentally for specific compositions around x = 0.5, whereas for x < 0.3
and x > 0.65, i.e. also for pure Fe and pure Co, the films are magnetized
in-plane. This could be related to the similar composition dependence of the
orbital moment [2].
The
spacer layers of Rh support keeping the distortion and well ordered structure
up to tenses of MLs. Thus,
a fully
epitaxial
(Rh/Fe1-xCox)N/Rh(001)
exchange-coupled multilayer system is proposed. In such system every second magnetic layer (Fe1-xCox,
for
0.4 < x < 0.6)
can show an easy-magnetization
axis perpendicular to the multilayer plane, and in-plane magnetization for the
intermediate (Fe,
i.e. for x = 0) layers. The magnetic
layers are separated by Rh non-magnetic spacers mediating the exchange
coupling. The coupling
oscillates
between ferro (FM)- and antiferromagnetic (AFM) configuration
showing a first maximum of the
AFM-coupling at a thickness of 5 ML. In reality the magnetization does not
alternate between out-of-plane for the Fe1-xCox and
in-plane for the Fe layers since the bilinear interlayer exchange interaction
tends to orient the magnetization of both layers in parallel. In the case the
coupling energy is similar to the anisotropy energy, complex non-collinear
magnetization configurations could be produced.
Due to the competition between the anisotropy
energy and the coupling energy, such configurations
are expected to
be easy changed by an external magnetic field.
Since both magnetic anisotropy and interlayer
exchange coupling are temperature dependent, the magnetization configuration
varies with temperature.
From experimental
point of view, the x-ray magnetic circular dichroism (XMCD) will be emphasized
which enables to obtain element selectively spin and orbital moments and
complement a typical probe of magnetic state (and of magnetic anisotropy and
exchange coupling in particular) provided by magneto-optical Kerr effect (MOKE).
References:
[1] T. Burkert, L.
Nordström, O. Eriksson, O. Heinonen, Phys. Rev. Lett. 93, 027203 (2004).
[2]
F. Yildiz, F. Luo, C. Tieg, R. M. Abrudan, X.-L. Fu, A. Winkelmann, M.
Przybylski, J. Kirschner,
Phys. Rev. Lett. 100, 037205 (2008).