Understanding Delinquent Domain Walls

Dr Tom Hayward is an EPSRC Career Acceleration Research Fellow at the University of Sheffield. His recent article “Beyond the quasi-particle: stochastic domain wall dynamics in soft ferromagnetic nanowires” was published as part of the Emerging Leaders special issue in JPhysD. Dr Hayward introduces his group’s latest work in understanding the behaviour of magnetic domain walls in memory devices.

The vast majority of the 2.7 zettabytes (2.7 x 1021) of data currently stored worldwide are on conventional hard-disk drives. In these devices data is held on spinning magnetic discs, with data being written and read by a mechanically actuated sensor. Unfortunately, the moving parts in these devices limit both the speed at which they operate and their durability, problems that have led a drive towards creating solid-state magnetic data storage devices (i.e. those with no moving parts). One such technology is racetrack memory, in which data is magnetically encoded as strings of ones and zeros along magnetic nanowires that are hundreds of times thinner than a human hair. Magnetic fields or electric currents are then used to “flow” this data through the device to perform reading and writing operations.

At a physical level, the flow of information in racetrack memory is enabled by the movement of particle-like magnetic “domain walls” that separate regions of magnetism pointing in opposite directions, and hence the ones and zeros that form the stored data. This seems like a simple operation, akin to rolling tennis balls down a drainpipe, but experiments have shown that domain walls react differently each time we try to move them. Ultimately, this has stopped the commercialisation of racetrack memory devices.

Computer simulations showing the complex motion of a domain wall as it is driven through a Ni80Fe20 nanowire by an applied magnetic field. The black arrows indicate the direction of the magnetisation in the two magnetic domains the domain wall separates, while the colour contrast indicates the local direction of the magnetisation at each point in the nanowire. Image taken from T J Hayward and K A Omari 2017 J. Phys. D: Appl. Phys. 50 084006, © IOP Publishing, All Rights Reserved.

Computer simulations showing the complex motion of a domain wall as it is driven through a Ni80Fe20 nanowire by an applied magnetic field. The black arrows indicate the direction of the magnetisation in the two magnetic domains the domain wall separates, while the colour contrast indicates the local direction of the magnetisation at each point in the nanowire. Image taken from T J Hayward and K A Omari 2017 J. Phys. D: Appl. Phys. 50 084006, © IOP Publishing, All Rights Reserved.

In our paper, “Beyond the Quasi-Particle: Stochastic Domain Wall Dynamics in Soft Ferromagnetic Nanowires” we use a series of experiments and computer simulations to explain why these apparently simple systems produce such complicated behaviour. Our results show that, far from behaving as simple particles, the domain walls change shape constantly as they pass through the nanowires, and that this causes them to “snag” on imperfections in the nanowires, making their motion complex and unreliable.

My group at the University of Sheffield is now trying to explore both how can we overcome the limitations imposed by complex domain wall motion, and how we can exploit its complexity to create entirely new types of devices. For example, our current research projects include using materials engineering to create nanowires in which domain wall propagation is greatly simplified and thus much more reliable. We are also developing devices that embrace the complex nature of domain wall motion to create devices that process data in a manner similar to the human brain.


CC-BY logoThis work is licensed under a Creative Commons Attribution 3.0 Unported License. Image taken from T J Hayward and K A Omari 2017 J. Phys. D: Appl. Phys. 50 084006, © IOP Publishing, All Rights Reserved.



Categories: Journal of Physics D: Applied Physics

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