Magnetic Domain Walls in Nanowires
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Magnetic Domain
Walls in Nanowires
Rolf Allenspach and Pierre-Olivier Jubert Abstract For many decades, it was assumed that the characteristics of magnetic domain walls were determined by material properties and the walls were moved by magnetic fields. In the past few years, it has been shown that domain walls behave differently on the nanometer scale. Domain walls in small elements exhibit complex spin arrangements that strongly deviate from the wall types commonly encountered in magnetic thin-film systems, and they can be modified by changing the geometry of the element. Domain walls in nanowires can also be moved by injecting electrical current pulses. Whereas wall propagation is qualitatively explained by a spin transfer from the conduction electrons to the spins of the domain wall, important aspects of the observations cannot be explained by present models. Examples include the observation of a drastic transformation of the wall structure upon current injection and domain wall velocities that tend to be orders of magnitude smaller than anticipated from theory. Keywords: magnetic properties, nanoscale.
Domain Walls Tailored by Geometry
Introduction The theory of magnetic domains as proposed 100 years ago by Weiss1 is incomplete if only the domains (i.e., the regions of different magnetization direction) are considered. Equally important are the transition zones between the domains— the domain walls. Research in domain walls spans many decades. It started with conceptual studies of wall types based on energy considerations and evolved to experimental studies to visualize walls by various methods of increasing sophistication and complexity. Domain walls were also extensively studied indirectly. They determine, to a great extent, the magnetization reversal upon applying a magnetic field. Recently, a new aspect of magnetic domain walls has been attracting attention: domain walls are considered as possible objects for high-speed logic, where each wall represents a single bit.2,3 The pioneering prediction4,5 and confirmation6–18 that domain walls can also be moved by spin-polarized electrical current offers an attractive alternative in designing novel devices such as sensors and magnetic random-access memories.19 Driven by these enormous prospects for technological applications, active studies of domain walls are underway worldwide. Along the “long and winding road”20 to applications, one encounters interesting
MRS BULLETIN • VOLUME 31 • MAY 2006
finite sample or if anisotropy is very large. In a finite system, dipolar contributions need to be taken into account. In a thin film with in-plane anisotropy, Néel proposed another type of domain wall, with the magnetization rotating entirely within the plane. This Néel wall avoids large stray field energy and hence is favored. In contrast to the Bloch wall, the narrow core region of the Néel wall is accompanied by long tails extending over micrometer distances to locally reduce internal stray fields. Both the Bloch wall and the Néel wall are limiting cases of a more
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