Magneto-Thermal Heating Effects in Magnetic Nanoparticles

Interview conducted by Alexander ChiltonSep 3 2015

Professor Kevin O'Grady, head of the Magnetic Materials Research Group in the Physics Department at The University of York, talks to AZoNano about measuring magneto-thermal heating effects in magnetic nanoparticle research.

Please could you give our readers a brief overview of your research at the University of York?

My principle area of interest is in magnetic materials for information storage such as hard drives.

We have a fairly extensive programme associated with materials for recording heads. That work is supported by government studentships but is topped up mainly by Seagate Technology from Northern Ireland.

We also have a very large fully-funded contract from Seagate Media Research in Fremont, California, associated with the development of new structures to enable a new form of recording called heat-assisted magnetic recording.

These projects are largely associated with a phenomenon known as exchange bias, which occurs when a ferromagnetic material is grown in intimate contact with an antiferromagnetic material and you get some very strange and complicated effects, which to a physicist are called interesting!

There is a second strand to my work. I have a young colleague called Professor Atsufumi Hirohata who works in our Electronics department. He is interested in materials for spintronic applications. The term spintronics means that you make electronic devices that, instead of relying on the flow of charge when an electron moves, manipulates the spin of the electron, because electrons spin about their own axis much like an ice skater. You can use that property to create smaller, faster and lower energy consumption devices.

Image Credit: Vit Kovalcik/Shutterstock.com

We have two major projects, both of which are associated with collaborations with Japan, one of which is looking at a new class of materials with unusual magnetic properties called Heusler alloys. These are complicated alloys consisting of metallic, often magnetic elements and a semiconductor. Again, they have unusual electrical properties which make them suitable for spintronic applications.

The second project is a major collaboration with Tohoku University in Sendai, Japan. Tohoku is the world’s leading university of technological magnetism and specifically we collaborate with a Professor Hideo Ohno, who was short-listed for the Nobel Prize a couple years ago.

That is funded by the EPSRC, the Engineering and Physical Sciences Research Council, the UK Science Foundation and the Japan Society for the Promotion of Science. This is a five-year program looking at spintronic devices, but also providing funds for a significant programme of exchange between the UK and Japan and vice versa, and also an annual program of high-level symposia involving not only York and Tohoku but also the Technical University of Kaiserslautern in Germany.

What are some of the potential "real-world" applications of the magnetic materials you are working on?

Everything we do has real-world applications. We are not making devices. It is a bit like if you consider the building industry; we are not architects or bricklayers building an edifice, we are interested in how to make better bricks, or better materials.

When you get to the level we are at, you enter the quantum world, which means that there is unknown or poorly understood physics that needs to be explored or researched because you cannot design a better brick until you know how the existing brick works.

So what we are doing is looking at new materials, Heusler alloys in particular, for new applications in electronics: faster, quicker and maybe even cheaper. We are looking at the fundamental physics of exchange bias because exchange bias is already included in every magnetic hard drive read head in the world.

We made a major step forward five years ago, and I even have letters from all the major companies saying that they are using our work in their design of their read heads. That is particularly true in Seagate Technology in Northern Ireland, which is Britain’s highest technology company. If you want to know what a billion pounds look like, go on have a look through the window of their lab and you can see a billion pounds worth of equipment.

It is all real-world, but perhaps real-world in a five to 10-year time scale, not real-world ‘the day after tomorrow’.

Image Credit: Inozemtsev Konstantin/Shutterstock.com

Can you share any recent results which you are excited about?

There are two recent results we are excited about. One is the potential new heat-assisted magnetic recording system. The world of the hard drives has been working on this for ten years, and they have made progress, but they have not yet got it to a product.

That is because the material they are using, which is a 50/50 alloy of iron and platinum, is very difficult to crystallize. What you want to have happen is you want to the material to give you a magnetic signal but you want its properties to vary with heat. So when you warm it up the properties weaken, you can switch it, when you cool it down the properties strengthen and you cannot lose the data that you have printed. So you have got a bi-functional material.

What we dreamt up was to use this strange exchange bias phenomenon so that you have the temperature-dependent properties coming from an antiferromagnet and the magnetic signal coming from a ferromagnet lying next to it. The beauty of that is that you can reorient the antiferromagnet and the only way you can lose the piece of data you have just printed is by using heat.

So it’s completely stable, and because you are using two separate materials you can optimize the functionality of each, as opposed to trying to optimize two functions in one material, where inevitably you are going to have a compromise between properties. That is funded by a very large grant from Seagate Media Research in Fremont. We have just achieved proof of principle so we are quite excited about that.

I guess the other thing that we are pleased about, is that almost exactly two years ago in collaboration with another young colleague, ironically whose post is funded from Seagate money, he and I worked out a completely new description of the mechanisms by which magnetic nanoparticles generate heat. This is where my work in the company and my work in university get (too) close together.

We published a paper in July 2013, and when it hit the press there was a huge level of interest in this. This paper in the Journal of Physics D, the Institute of Physics (IOP) Applied Physics Journal, now has in excess of 10 thousand electronic downloads. That is extraordinary for a physics paper and it continues to generate interest.

How does your research tie in to your company, Liquids Research, and how was the company founded?

Liquids Research Limited is not a spinout company and never has been. It was founded in 1991 using private money which derived from a UK subsidiary of a major American oil exploration company. At that time we were doing a project trying to work out how you could measure magnetic liquids, out of which came a request from this company that we supply them with these liquids.

In 1991 universities were all registered charities and were not allowed to trade for a profit, so this company funded me to establish a small private company in collaboration with a colleague who has subsequently retired, to manufacture ferrofluids or magnetic liquids.

Shortly thereafter we realized that the majority of ferrofluids in this world were used for vacuum-sealing applications, so we developed a range of fluids for engineering applications of magnetic fluids.

In my university work, and in accordance with my employment contract, any work I do associated with magnetic liquids is the property of Liquids Research Ltd, and anything I do associated with magnetic solids, is the property of the university.  That keeps things nice and clear.

There are a few areas where my work can overlap. One of these is this subject of magnetic hypothermia involving magnetic nanoparticles, which under the correct conditions can generate heat which can be used to destroy or damage tumors.

My company did have a relationship with the University of York, but that came to an end a year ago. I continue to support and help a young colleague who is continuing to work in this area.

What types of instrumentation do you use in your research?

In the university it’s all physics equipment. X-ray diffractometers for establishing crystal structures, electromicroscopy for looking at the physical shape and other properties of materials, we do magnetic measurements using an instrument called vibrating sample magnetometer. We have other facilities for doing resistance measurements and we can measure magnetic properties optically.

So basically we are a magnetic measurements group, but we also grow thing films, we have the capability to splutter thin films, where you bombard a target with Ar+ ions so as to erode atoms on the surface which you then catch on a substrate.

We can also do this by molecular beam epitaxy (MBE), where you evaporate materials but you do it so slowly and in such a high vacuum that you grow single crystals of the material.

How does the magneTherm fit into this toolset?

Liquid Research Ltd owns one of the magneTherm systems and we use it to measure magneto-thermal heating effects. A little while ago we realized that this measurement procedure, not the magneTherm system itself were inaccurate.  

If you want to measure the thermal properties of a piece of copper or something, then that’s fine – you can suspend it in a cryostat or a calorimeter with a fine piece of wire or piece of nylon or something, then you could measure its properties. But with a liquid you can’t do that because a liquid needs to sit in a bottle. So when the liquid gets warm the bottle gets warm. There are heat losses. None of the measurement systems were taking into account these heat losses.

So using a summer student, we did an evaluation of how these heating systems work and we have redesigned the measurement cell. This work is associated with liquids, so even though the work is done here in the university, the IP in this goes to Liquid Research Ltd. We have offered this system to Nanotherics because the system is designed specifically to go into their coil system. They are evaluating it with a view to offering the new measurement system as an upgrade to the existing tools they have in the field.

Fundamentally we’ve worked out how to make sure the temperature of the system is uniform and that the uniformity of the magnetic field is known. By a clever technique we’ve managed to completely eliminate the effect of heat losses. That sounds implausible because you can never eliminate the effect of heat losses when you heat a liquid in a bottle. What you can do is to measure the heating effect and then extrapolate the data to an imaginary point where the heat losses are zero.

You can do exactly the same heating measurement, but this time using a resistive heater dipped into the sample. You can directly compare the heating rate from a heater where the heating rate is known very accurately, to heat generated by the magnetic field effect on the nanoparticles.

What have you found to be the main benefits of using the magneTherm?

The magneTherm system is much cheaper than the competitor systems. It’s also simpler, but none of the systems currently give you the correct heating value. Our work was recently presented at the recent Intellectual Conference on Magnetism in Barcelona and has been written up and submitted to Applied Physics Letters, which is the top applied physics publication vehicle in the world.

The magneTherm™ system

What direction do you think your work, and the field in general, will move in over the next few years?

I am due to retire in four years’ time, so my future is that I am slowing down, thankfully, but I’m spending more of my time supporting and helping my young colleagues to get going, and for sure the Heusler alloy story will run and run.

I plan to get funding from Seagate to continue the work on heat-assisted magnetic recording. There is also a new project on recording heads, but again because of my age I’m taking a backseat on that, or as much of a backseat as I ever do, and letting some of the youngsters get on with things.

In terms of Liquids Research Limited, we are a manufacturing company. We exist to make and sell magnetic liquids. But there are new, and in some cases, quite interesting, things to look at. Also there is the case that the world in which we live constantly demands improvements in materials, and those improvements in materials can only be of value if the under-pinning physics is understood.

So in the next four years I plan to carry on doing what I’m doing. I cannot go off in any new directions because there simply isn’t time. I will continue to do my teaching and support these youngsters who are around. And when I retire, and that will depend on economic circumstances and whatever happens in the public spending review, I intend to spend about a year writing a monograph.

During the last 30 years I’ve written about 300 peer reviewed papers or more, of which maybe 40 or 50 are quite good and 10 of them were significant advances. So what I plan to do is to write a monograph so that people don’t have to go scrambling through all the journals looking for 300 papers. They can find it all in one place. And that I will leave behind me. I guess, if you like, that’s my life’s work.

About Professor Kevin O'Grady

Professor Kevin O'Grady was educated at The University of Wales in Bangor, studying for a degree and a PhD in Physics. Subsequently he was employed as a junior professor in Bangor, Loughborough University of Technology and subsequently in the School of Electronic Engineering at Bangor where he achieved a full professorship.

In 2000, Kevin relocated to the Physics Department at The University of York. Kevin leads the Magnetics Materials Research Group and his work concentrates on magnetisation reversal in a wide range of materials but particularly those finding application in the information storage industry such as magnetic hard disk drives.

In his early career, Kevin studied fundamental fine particle magnetism utilizing colloidal dispersions or ferrofluids as the medium for study. His current research interests are in the field of materials for hard disks and in exchange bias materials.

Kevin has published over 250 refereed works on the subject of magnetisation reversal. He is a former President of the IEEE Magnetics Society and was the associate editor of J.Phys.D:Appl.Phys. from 1995 to 2012.