Aug 21, 2013
Ion irradiation improves magnetic domain wall conduction
Ga+ ions produced by focused ion beam (FIB) tools are typically used to pattern materials down to the nanoscale. In this work, we have used them differently: by gently irradiating magnetic nanowires, we can substantially improve the conduction of magnetic domain walls along them.
Domain walls are areas in magnetic nanowires with a typical size of tens of nanometres that separate regions of different magnetic orientation. In the last decade, the use of magnetic domain walls to encode binary information and to perform logic operations has been proposed; this is why researchers look for their better control and integration.
When domain walls are created inside nanowires, these act as magnetic roads with the walls driven by magnetic fields or electrical currents. One of the problems with this technology is that the small difference between the magnetic field needed to propagate an existing domain wall and the field needed to nucleate a new one can result in the loss of the encoded information.
In the present work, this problem is circumvented by Ga+ ion irradiation. Surprisingly, we have found that a shower of highly energetic Ga+ ions in the appropriate dose enlarges the operating margin of these devices up to a factor of 9. This is due to an increase of the domain wall nucleation field whilst the propagation field is hardly affected.
This has been observed in cobalt nanowires grown by focused electron beam induced deposition (FEBID) and the proposed origin of the effect is twofold. On the one hand, the profile of the wires becomes steeper; on the other hand, the microstructure of the top part of the wire is strongly affected. Global ion irradiation is thus found to be a good way to improve the performance of domain wall-based devices. This discovery could have potential applications in magnetic racetrack memories.
Fabrication and characterization
We have used a FIB-SEM system located at the Laboratory of Advanced Microscopies (LMA) at the University of Zaragoza to grow the cobalt nanowires and irradiate them. The growth and subsequent irradiation were performed without breaking the chamber vacuum, which made it possible to systematically study the influence of ion irradiation on the magnetic properties of the wires.
The behaviour of the magnetic domain walls was examined using a Magneto-optical Kerr effect system at the Cavendish Laboratory, University of Cambridge, designed to measure magnetic nanostructures.
The important changes to the magnetic properties of the wires observed after ion irradiation has aroused our curiosity. In order to fully understand the mechanisms behind these effects further experiments are in progress to measure the magnetic behavior of the wires by means of Lorentz Microscopy.
This technique uses electrons to image magnetic configurations at the nanoscale. These experiments will be done by applying in situ magnetic fields inside the TEM, which will permit the direct observation of nucleation and propagation of domain walls in cobalt wires.
More information can be found in the journal Nanotechnology 24 345703.
About the author
This work has been performed via a collaboration of laboratories in Zaragoza (Spain) and Cambridge (UK). The work was supervised by Prof. José M De Teresa from ICMA (CSIC-University of Zaragoza) and Dr Amalio Fernández-Pacheco from the TFM Group, Cavendish Laboratory at the University of Cambridge. The PhD student Luis Serrano-Ramón carried out most of the experimental work, using extensively the premises of the LMA at INA (University of Zaragoza). Other co-authors of the work are Dr Rosa Córdoba, Dr César Magen, MSc, Luis A Rodríguez and Prof. Ricardo Ibarra from Zaragoza, and Dr Dorothée Pétit and Prof. Russell Cowburn from Cambridge.