Overview
The current-driven motion of domain walls is the basis of several new concepts of radically new magnetic memory and logic devices. Although domain wall devices have been under consideration for more than 50 years, there are fundamental difficulties in manipulating domain walls with magnetic field: Neighboring walls, for instance, move in opposite directions. This is entirely different for current-driven wall motion. The current in the ferromagnetic nanowire becomes spin-polarized, and the spins exert a torque on the domain wall, which can be large enough to move the wall. Adjacent walls in this case move in the same direction, determined by the current.
We have studied the propagation of domain walls in NiFe nanowires with electric pulses in the microsecond range for both the vortex type [1,2] and the transverse type, demonstrating domain propagation up to 15 m/s. Furthermore, our unique spin-SEM setup makes it possible to study the transformation of the domain wall after current pulses. We found that vortex walls transformed after several pulses into the transverse state and then stopped propagating [1,2].
We also observed that transverse domain walls, which are encountered in narrower nanowires, kept their characteristic shape after a current pulse. A new effect is observed in bilayer wires, consisting of NiFe and Fe. After propagation of transverse domain walls, the wall polarity is entirely determined by the polarity of the current pulse, an effect that is not reconciled by present theories even when taking into account the nonuniform Oersted field generated by the current [3].
In part of our efforts in this field, we developed an extension for the OOMMF micromagnetic simulations framework, which allows the simulation of current-induced effects on magnetic domain wall.
References
[1] M. Kläui,
P.-O. Jubert, R. Allenspach, A. Bischof, J. A. C. Bland,
G. Faini, U. Rüdiger, C. A. F. Vaz, L. Vila and C.
Vouille,
Direct observation of domain-wall configurations transformed
by spin currents,
Phys. Rev. Lett. 95, 026601 (2005).
[2] P.-O. Jubert, M. Kläui,
A. Bischof, U. Rüdiger and R. Allenspach,
Velocity of vortex walls moved by current, J.
Appl. Phys. 99, 08G523 (2006).
[3] A. Vanhaverbeke, A. Bischof, and R. Allenspach,
Control of domain wall polarity by current pulses, Phys. Rev.
Lett. 101, 107202 (2008).
Figure 1. (a) Topographic image of the device structure, showing the four
Fe20Ni80 wires. (b) Spin-SEM image
and magnetic configuration in a wire after magnetization
with the field pulse. (c) After injection of a single pulse
of 10µs, both domain walls moved in the direction of
the electron flow.
Figure 2. High-resolution experimental image of a domain wall obtained with
spin-SEM measurements. (a) After a first current injection,
the domain wall has kept its vortex structure. (b) After
three current injections, the domain wall has transformed
into the transverse state.
