[ Up ]    [ Home ]   [ Projects ]    [ Publications ]    [ Gallery ]   [ Teaching ]   


by Hermann Forster

The magnetization reversal process of Co-nanowires was investigated using a moving mesh technique. The nucleation and expansion of reversed domains is calculated by solving the Gilbert equation of motion for different damping constants.

The magnetic Co-nanowire has a total length of 600 nm. The nanowire has two stable states: all magnetic moments are parallel to the long axis, pointing either in one direction or in the other.  We apply an external field parallel to the long axis in the opposite direction of the magnetization. If the field is large enough, a reversed domain will be formed, a head-to-head domain wall will be built and it will propagate through the whole nanowire, until it arrives at the other end. There it is annihilated, leaving the system in the second possible stable state.

We found that the domain wall velocity and the structure of the domain wall strongly depend on the diameter d of the nanowire. Two types of domain walls can be formed during their motion through the nanowire: transverse walls and vortex walls. In the first case the magnetization in the centre of the wall points perpendicular to the long axis of the wire. As the wall moves, the transverse component of the magnetization circles around driven by gyromagnetic precession. In the second case a vortex is formed in a plane normal to the long axis of the wire. The magnetization aligns parallel to the wire surface. Within the domain wall the magnetization rotates around the long axis which leads to the formation of a Bloch point in the middle of the wire. The formation of the vortex decreases the magnetostatic energy at the expense of the exchange energy. For small diameters, it is energetically preferable to form a transverse wall which minimizes the exchange energy due to the parallel alignment of the magnetization in the domain wall. On the other hand there are no flux closure states in the wire and so the magnetostatic energy is not reduced. However, for small diameters, the exchange energy is the most dominant energy contribution to the total Gibbs' free energy, and so its minimization determines the wall configuration. With increasing diameter the magnetostatic energy becomes more important in the minimization process. Although the vortex wall increases the exchange energy, the minimization of the magnetostatic energy due to the lower demagnetizing field makes it energetically preferable. Since the difference in the total Gibbs' free energy is less than 1  % at a critical diameter of d = 20 nm, it is possible to obtain both domain wall structures. For d < 20 nm only transverse walls are observed and for d > 20 nm only vortex walls are observed.

The dependence on the damping constant  is not the same for both structures. For d = 10 nm just transverse walls are formed. With increasing damping constant the velocity increases from 50 m/s for a damping constant of 0.05 to 520 m/s  for a damping constant of 1 at an applied field of 500 kA/m. For higher damping constants the domain wall velocity increases faster for higher external fields. In the transverse wall gyromagnetic precession plays a major role during the wall motion. On the contrary, for the vortex walls the domain wall velocity increases with decreasing damping constant, reaching 2000 m/s for d = 40, a damping constant of 0.05 and an applied field of 250 kA/m.

[ References ]

H. Forster, T. Schrefl, D. Suess, W. Scholz, V. Tsiantos, R. Dittrich, and J. Fidler, ``Domain wall motion in nano-wires using moving grids,'' J. Appl. Phys., vol. 91, pp. 6914-6919, 2002. 
H. Forster, T. Schrefl, W. Scholz, D. Suess, V. Tsiantos, and J. Fidler, ``Micromagnetic simulation of domain wall motion in magnetic nano-wires,'' J. Magn. Magn. Mater., vol. 249, pp. 181-186, 2002. 
W. Scholz, H. Forster, D. Suess, T. Schrefl, and J. Fidler, ``Micromagnetic simulation of domain wall pinning and domain wall motion,''
Computational Materials Science, vol. 25, pp. 554-561, 2002. 

H. Forster, ``Reversal modes in mesoscopic structures,'' PhD-Thesis, TU Vienna, 2003. 

[ Up ]    [ Home ]   [ Projects ]    [ Publications ]    [ Gallery ]   [ Teaching ]   

webmaster: fidler (at) tuwien.ac.at
Mar. 05, 2003