Dr William Griggs is an ISIS User who recently completed his PhD, titled ‘Disorder-and-Strain-mediated Magnetic Phase Modifications in FeRh thin films’, at the University of Manchester. He is part of the nano engineering and spintronic technologies (NEST) group at Manchester which focusses primarily on magnetic materials at the nanoscale and their applications for data storage and spintronics. “After completing my undergraduate at [the University of] Manchester, I had the opportunity to carry on the work I was doing with my masters project, now I have just finished my PhD and I’m now looking forward to taking the work further.”
At the heart of William’s work has been a quite strange material, a 50/50 binary alloy of Iron and Rhodium. “Most magnetic materials when heated go through a certain process that results in them becoming non-magnetic, Ion-Rhodium has some really peculiar properties in that at room temperature it's actually what we call an antiferromagnet, which means that its internal magnetic structure oscillates so that if you zoom out far enough, it sort of cancels itself out, and so it appears nonmagnetic. But when you heat up iron rhodium up to just 80 degrees Celsius it undergoes this quite unique phase transition where it becomes spontaneously ferromagnetic, and whilst several materials do this, FeRh is the only one that does it close to room temperature.”
It is this pre-programmed ‘switchability’ that William is interested in. Developments in thin film fabrication techniques have enabled the production of ultra-thin nanoscale films of this material. These developments have allowed the further study of FeRh which could have potential applications in data storage or spintronics as it can be used to help store and manipulate information.
This was approached by incorporating FeRh with other materials and then exploiting the coupling of this phase transition to some other property of some other material.
“The focus of my PhD was to take a couple of these previously identified systems that incorporate FeRh which might be useful for various information technologies and try to understand the length scale over which interactions in these systems occur”
An example of this would be the use of FeRh to help overcome the instability of each bit on a hard drive. As more data is crammed onto a hard drive the size of each bit of information becomes smaller and more unstable, meaning that it could flip between a 1 or a 0 due to the magnetic field flipping. One approach to solving this problem would be to use very stable materials that are unaffected by the fluctuations of the magnetic field, however as these materials are so magnetically stable, they are difficult to write to due to the large magnetic field required to turn a 1 into a 0. This is where the incorporation of FeRh is useful. As FeRh is magnetic at high temperatures the system can be heated to 80 degrees and then the magnetic properties of FeRh will become active allowing data to be written, then when the system is cooled, the new data stays written but in a very stable state.
The issue with this process is that it can only occur over a finite length scale, a value that had been theorized but not directly observed. As the materials being used are of great cost the need to reduce material redundancy is high. One method of helping to determine this length is by using neutrons. “I was very fortunate during the course of my PhD to have several visits to the POLREF instrument at ISIS where we did polarized neutron reflectometry. We reflected neutrons from the surface of a sample and measured the way in which the reflected intensity varies with the angle of incidence.”
As neutrons have a magnetic dipole moment they interact with the sample through both normal nuclear scattering as well as magnetically. It is then possible to reconstruct what the magnetism inside the film looks like by inference from this reflection profile.
Alongside his work on using FeRh in bilayer structures, William’s PhD saw him also perform research on FeRh thin films that have been subject to Ne+ ion irradiation. This irradiation causes non-uniform damage to the film, creating ‘pockets’ of depth varying antiferromagnetic, ferromagnetic, and paramagnetic regions. William and his team wanted to take this and find a useful application for it. This was done by exploring the effect of changing the fluence, the total number of particles crossing over a sphere of unit cross section which surrounds a point Source of ionising radiation, of the radioactive Ne+ ions on the changes induced in the FeRh film. The technique of PNR was employed to accomplish this analysis due to its unique ability to analyse depth dependent magnetism. It was found that there is a direct correspondence between the existence of the three magnetic phases and lattice defects. As well as this, by careful selection of fluence it was shown that it is possible to produce simple and thermally stable magnetic configurations, such as uniform magnetization or a bilayer phase structure. These results are able to provide a new insight into magnetic ordering in alloy lattices, which in turn expands the scope for its potential applications.
The last system that William looked at for his PhD was a multiferroic system. The objective in this system was to apply a voltage and get a change in the system’s magnetism without applying any magnetic fields. FeRh has been identified as potentially being very useful in this domain when incorporated onto a substrate has piezoelectric properties; its lattice structure expands when a voltage is passed through it.
“When FeRh becomes very magnetic it slightly stretches causing a change in its lattice. By growing a film of FeRh on a film of a Piezoelectric substrate, a substrate that undergoes deformation when a voltage is applied, then you can expect to apply a voltage over that system, the piezoelectric substrate will stretch and that in turn should stretch the FeRh which should affect its magnetic properties.”
William again used PNR to help deduce how the magnetism varies as a function of depth. However, the sample environment for this experiment was more advanced than before; the experiment had to be performed at a suitable temperature with applied electric fields/voltages and applied magnetic fields.
“As this type of experiment hadn’t been done before a large portion of this work was working really closely with the beamline scientists Christy Kinane and Andrew Caruana on the POLREF instrument to try to create and experiment where we can do these 3 things at once.”
One challenged faced was the fact that as the experiment was using neutron scattering, a technique that relies on reflection, and therefore whatever was implemented to obtain the required sample environment couldn’t obstruct the beamline or surface of the film.
“I had several visits to ISIS throughout my PhD and it really wound up forming the backbone of my thesis, everything that I wanted to look at needed this really powerful technique”
