Assistant Professor
Magnetization Dynamics
Introduction: Ferromagnetic materials are capable of maintaining a permanent magnetic moment in the absence of an applied field. This property is one reason why magnetic materials are used in many data storage applications. It is possible to change the magnetic state of the material by applying a magnetic field. Each magnetic moment, m, in the material then responds according to the Landau Lifshitz equation of motion:
where the first term describes how the magnetic moment processes around the field, H, and the second term is responsible for rotating the magnetic moments into the field direction. We use computer simulation to dynamically update the magnetic structure—with nanometer spatial resolution and picoseconds temporal resolution.
Each of the sections below briefly describes an area of currently active research.
Ferromagnets and Domain Wall Motion: A ferromagnetic material typically has regions where the magnetization is the same—domains. Between oppositely aligned domains the magnetic structure goes through a rotation, this region is the domain wall. In a nanowire it is necessary to move the domain wall along the wire to change the magnetic structure—therefore the speed of a magnetic nanowire device is limited by the speed of the domain wall moving through the wire. Typically a domain wall moves quickly for small applied fields and slowly for large applied fields [6-8]. This counter-intuitive behavior is due to a change in the internal structure of the domain wall involving the nucleation of vortices within the wall. We have found techniques to maximize the wall speed for all applied field strengths [3-5].
Domain Wall Injection: A domain wall is energetically unfavorable in a nanowire so it is necessary to inject the wall. Unfortunately the injection field for a domain wall is typically in the regime where the domain wall moves the slowest. We recently reported a technique which allows for the injection of a domain wall for fields where fast domain wall motion is the norm [1]. This technique also makes it possible to quickly inject multiple domain walls into the wire. A simple topologic model helps to explain the behavior—and should also be relevant for pinning and depinning walls from notches. All of this is necessary for potential recording applications which require fast, reliable motion of multiple walls.
Domain Wall Interactions: When more than one domain wall exists in the wire the interaction of the walls is important. Typically two domain walls will find a path to annihilation when they come into contact but by controlling the domain wall structure it is possible to create states where the walls exist together [2]. The same topologic model explains this behavior. It is possible that many domain walls can be densely packed into a wire by systematically controlling the domain wall magnetization.
Publications at Marquette: denotes MU undergraduate student
- 1. A. Kunz and S.C. Reiff, “Dependence of domain wall structure for low field injection into magnetic nanowires,” Applied Physics Letters 94, 192504 (2009).
- 2. A. Kunz, “Field induced domain wall collisions in thin magnetic nanowires,” Applied Physics Letters 94, 132502 (2009). See also Nature Physics Research Highlights (Nat. Phys. 5, 313 (2009)).
- 3. A. Kunz, E. C. Breitbach, and Andrew J. Smith, “Anti-vortex dynamics in magnetic nanostripes,” Journal of Applied Physics 105, 07D502 (2009).
- 4. A. Kunz and S. C. Reiff, “Fast domain wall motion in nanostripes with out-of-plane fields,” Applied Physics Letters 93, 082503 (2008).
- 5. A. Kunz and S. C. Reiff, “Enhancing domain wall speed in nanowires with transverse magnetic fields,” Journal of Applied Physics 103, 07D903 (2008).
- 6. A. Kunz, “Micromagnetics of the domain wall mobility in permalloy nanowires,” IEEE Transactions on. Magnetics 43 (6) 2944-46 (2007).
- 7. A. Kunz, “Simulating the maximum domain wall speed in a magnetic nanowire,” IEEE Transactions on Magnetics 42 (10) 3219 (2006).
- 8. A. Kunz, “Simulated domain wall dynamics in permalloy nanowires,” Journal of Applied Physics 99 08G107 (2006).
Many talented undergraduate physics majors have contributed to this research which has been supported by the NSF, Research Corporation and Marquette University.
