Experimenting with Theory – Rare Earth Nitrides
Dr Ben Ruck and Joe Trodahl were already working on metallic nitrides when they came across a paper predicting that related compounds—the rare earth nitrides—would not only have interesting electronic properties, but magnetic properties as well.
“That gave us the impetus to start making and measuring them. Our first and perhaps our most significant breakthrough was proving that, at low temperature, gadolinium nitride is both a semi-conductor and a magnet at the same time,” says Ben.
The group grows thin films (100–200 nm) of the nitrides on a support under ultra-high vacuum. Analysis of the magnetic, physical and electronic properties of the material follows, using various spectroscopic and cryogenic techniques overseas, and more recently synchrotrons. Changing the ratios of the metal and nitrogen, as well as mixing and putting layers of different metallic nitrides next to each other, subtly changes the properties of the material. The challenge is to work out why and how.
“We’ve spent a lot of time working with theorists,” says Ben, “taking their calculations and using them to suggest experiments, then using our experiments to help refine their calculations. Some key collaborations with several overseas groups have been established.”
The new materials may be useful in the development of technologies such as MRAM (magnetic RAM) that uses electron spin, not charge, to store data. Because data is retained when the power is switched off, a device can be faster, more versatile and use less energy.
“MacDiarmid Institute funding got this research area kicked off in the first place and has supported it all the way with equipment for making and analysing our compounds. The new vacuum system, for example, enables us to make one sample a day rather than one a week, so progress is much faster”
The team’s latest work with europium nitride has received significant worldwide attention. The material is not usually magnetic, but has been ‘tricked’ into behaving like a magnet by being produced with slightly too few nitrogen atoms.
“Now we have to propose some theory to work out why. New results are fantastic, but understanding them is the best.”