The influence of domain wall substructures to the magnetization reversals... patterned Permalloy thin films

The influence of domain wall substructures to the magnetization reversals of the
patterned Permalloy thin films
S. Singh, H. Gao, and U. Hartmann
Institute of Experimental Physics, Saarland University, Campus C6 3, 66123, Saarbrücken, Germany
Understanding of the magnetization reversals is important both in fundamental magnetism
research and in industrial applications. Magnetic switching in patterned materials proceeds
through various modes. Depending on the dimensions of the patterns, switching is governed
by coherent rotation, curling and buckling of magnetic moments [1]. In the case of patterned
materials with multi-domains, magnetization reversals take place due to the nucleation and
displacement of domain walls. To reach the lowest energy levels, domain walls split into
nanometer scale substructures with dimensions less than 10 nm [2]. The self-magnetostatic
energy of the domain walls, which arises due to the magnetic free poles at the intersection of
domain walls with the material surface, divides the domain wall into periodically arranged
opposite polarized wall segments [3]. Substructures such as Bloch lines, Néel lines, vortexantivortex pairs (VAVP) significantly affect the magnetic characteristics of the patterns [4,
5]. Each part of the substructures propagates and evolves in different dynamic regimes at
different velocities [5]. This limits the maximum velocity of domain wall motions in
magnetic data storage devices. Magnetic charges emerging around the antivortex locations
stiffen the magnetization around the antivortex vicinities. This leads to low mobility of the
antivortices in cross-tie domain walls and hence, exhibits a stabilization effect against the
domain wall displacement [6]. Thus, the substructures of domain walls dominate their
characteristic behavior in applied magnetic fields. The change in the magnetic configuration
inside a magnetic field is not only limited due to the motions of domain walls but also due to
their substructures. In this work, we perform the direct observations of substructures of
domain walls using Magnetic Force Microscope (MFM) technique. Our interest is to
investigate the role of the substructures towards the magnetization reversal in patterned
Permalloy (Py) thin films.
The Py patterns were prepared in square and rectangular shapes of various aspect ratios by
means of e-beam lithography (EBL). 20 nm to 200 nm thick Py (Ni81Fe19) films were
prepared by sputtering. An in-plane static magnetic field was applied to move the domain
walls during the MFM measurement. Magnetization switching along the long and short axis
of the structures was measured and compared. Typical magnetization reversal MFM results
on a patterned structure along the both axes involving nucleation and annihilation of VAVPs
are shown in figures below. We related the shape anisotropy with the magnetic energy
landscape, in terms of the different solenoidal configurations emerged at remanence during
magnetization reversals. The experimental results were compared with micromagnetic
simulations and the contributions of the different magnetic energies were investigated for
better understanding of the magnetization reversal mechanism.
Figure 1: MFM images of magnetization reversal along the short axis of a 50 nm thick 5 x 18 µm2 Py pattern
Figure 2: MFM images of magnetization reversal along the long axis of a 50 nm thick 5 x 18 µm2 Py pattern
References:
[1] S. Chikazumi and S. H. Charap, Physics of Magnetism,Wiley, New York, 1964.
[2] A. Hubert and R. Schäfer, Magnetic Domains, the analysis of magnetic microstructure, Springer, Berlin,
1998.
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Mater., 310 (2007) e672.
[5] C. Zinoni, A. Vanhaverbeke, P. Eib, G. Salis, and R. Allenspach, Phys. Rev. Lett., 107 (2011) 207204.
[6] R. D. Gomez, T. V. Luu, A. O. Pak, I. D. Mayergoyz, K. J. Kirk and J. N. Chapman, J. Appl. Phys., 85
(1999) 4598.