Micromagnetic simulation of magnetization reversal in Co/Ni multilayers
V.D. Tsiantos, T. Schrefl, D. Suess, W. Scholz, J. Fidler and J.M. Gonzales
Journal Physica B-Condensed Matter, (2000) submitted.
Magnetic sensors and magneto-electronic devices are based on magnetic
nanostructures. Their application requires a well-defined switching
behaviour which can be tailored changing either the microstructure or
the intrinsic magnetic properties. The magnetization reversal process of
Co/Ni/Co trilayers was studied using finite element micromagnetic
simulations. A uniaxial anisotropy parallel to the film plane was
assumed within the Co layer, whereas zero anisotropy was used to mimic
the properties of Ni. This system is a perfect model to investigate the
role of anisotropy and interphase exchange on the magnetization reversal
process in exchange spring magnets. Indeed, the typical behaviour of a
nanostructured, exchange coupled system is observed. Demagnetization
curves were calculated for a multilayer with a Co layer thickness of 5 nm
and a Ni layer thickness of 15 nm. Thus the net magnetization of a Co
and a Ni layer are the same, since the spontaneous magnetization of Co
is about three times larger than that of Ni. The computational region
was a 200 nm x 200 nm with free boundary condition. Thus, in addition to
the interplay between anisotropy and exchange, the demagnetization field
from the edges significantly influences the reversal process. Figure 1
shows two distinct reversal processes depending on the strength of the
interlayer exchange. To model the influence of interlayer exchange the
exchange constant of Ni was reduced from A = 10-11 J/m
to A=10-12 J/m in a 5 nm thick region next to the Co
layer. Exchange hardening of
the Ni layer provides a one-step reversal process if
A>0.3*10-11 J/m.
Both the Ni and the Co layer reverse at the same well-defined switching
field. A two- step process occurs at interlayer exchange lower than one
third of the bulk value. The Ni layer reverses its magnetization at a
low opposite field and the magnetization of the Co keeps its initial
direction. The already reversed Ni layer stabilizes the magnetization of
the Co layer owing to magnetostatic interaction which leads to an
increase of the coercive field as compared to the high exchange coupled
systems. Figure 2 gives the magnetization pattern in the middle of the
sample at Hext = -2 kA/m for
weak interlayer exchange. The external
field causes a nonuniform magnetic state within the Ni layer whereas the
magnetization within the Co layer remains nearly parallel to the
anisotropy axis. The transition from a one-step to a two-step reversal
process occurs if the anisotropy of Co is considerably high
(Ku = 450 kJ/m3).
A completely different reversal mechanism occurs if the
uniaxial anisotropy of the Co layer approaches 1 kJ/m3. Now
demagnetizing effects override the uniaxial anisotropy. The Co layer
reversed at lower opposing field, owing to its high magnetization. The
remanence and the coercive field of the multilayer systems decrease with
increasing interlayer exchange constant between the layers. In order to
reduce the magnetostatic energy end domains are formed at zero applied
field. The end domains become more pronounced with increased interlayer
exchange which in turn facilitates magnetization reversal. In addition
to the quasi-static behaviour, the numerical integration of the
Landau-Lifshitz-Gilbert equation shows how the reversed domains are
formed as a function of time.
This work was supported by the Austrian Science Fund (Y-132 PHY).
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Oct. 18, 2001