Exploring the potential of graphene topography for nanomagnetism Magnetic materials offer a broad range of possibilities for the development of sensors, memories and logic elements. We spoke to Dr Sebastian Gliga about his work in creating functional magnetic materials that can be reconfigured through shape change, which could open up new possibilities for the manipulation of spin waves with the aim of creating logical devices. A material formed of an atomthick layer of carbon atoms, graphene has attracted a lot of attention in research, now scientists are seeking to harness its mechanical features in the development of functional magnetic nanomaterials. While a graphene sheet is commonly thought of as being perfectly flat, wrinkles as well as quasiparticles called wrinklons emerge when it is placed under stress. “We can think of wrinkles as sinusoidal variations in the height and width of a graphene sheet,” explains Dr Sebastian Gliga, a Scientist in the Microspectroscopy Group at the Paul Scherrer Institute. When a graphene sheet is suspended over a trench then wrinkles develop, in a way similar to the effect of pulling on a curtain. “We get wrinklons when wrinkles come together and merge with each other. Certain topological properties are associated with this,” says Dr Gliga. “I’m interested in this junction where two wrinkles meet.” Spin waves While Dr Gliga is working with graphene, this is primarily a medium to support thin magnetic films, which is his main area of expertise. “I’m investigating the possibility
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of creating magnetic films whose topography can be changed. The aim is to create functional magnetic materials through shape change. I am looking to manipulate spin waves using these materials - spin waves are essentially collective excitations of the electronic spins,” he outlines. A number of proposals have been put forward for creating logic elements based on spin waves; Dr Gliga says reconfigurability is an important issue in this respect. “Magnetic systems have been
domain wall in the film will have a favourable direction of propagation. Spin waves can also display asymmetric propagation, with waves traveling in opposite directions having different frequencies,” explains Dr Gliga. The intention is to build on these findings to create a material with properties that can be changed through bending, which Dr Gliga says has to be done on very small length scales, ideally of a few tens of nanometres. “These scales are associated
The aim is to create functional magnetic materials through shape change. I am looking to manipulate spin waves using these materials - spin waves are essentially
collective excitations of the electronic spins.
developed which can propagate spin waves and do interesting things, such as define logical gates through spin wave interference, but they’re not reconfigurable. Other systems are reconfigurable, but you have to change their magnetic state,” he says. An alternative approach would be to achieve functionality through shape change, a topic at the heart of Dr Gliga’s research. There has been a lot of research over the last few years on curved magnetic films, and it has been found that when a thin film is curved, novel properties emerge. “This means for example that a given type of
with what is called the exchange length of the material. It’s a characteristic length of a magnetic material which determines the extension of spin inhomogeneities in the magnetic structure, such as domain walls for example,” he continues. The wrinkles in graphene can extend between a few nanometres to a few hundreds of nanometres, in principle matching these scales, so have the ideal dimensions to enable reconfigurability. The wrinkles appear when graphene is placed under stress, but they are also affected by the temperature of the graphene sheet. “Graphene has a negative thermal expansion coefficient. This means that when you heat up the graphene, together
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with the substrate, the substrate will expand, while the graphene sheet will actually shrink, so the wrinkles disappear. As you cool down the graphene, the substrate will shrink but the graphene sheet will expand, thus creating wrinkles,” says Dr Gliga. This opens up interesting possibilities in terms of controlling these wrinkles. “One of my hopes is to control the size and shape of these wrinkles, through different thermal cooling and heating cycles,” continues Dr Gliga. A technique called scanning electron microscopy is used to assess the size of the wrinkles, with researchers investigating how this can be controlled. From this point, Dr Gliga can then investigate whether there is a relationship between the properties of the wrinkles and the magnetic properties of films grown on top of the graphene sheets. “This is the hard part of the project. The graphene is the underlayer, the mechanical support, and the idea is then to grow a magnetic layer on top,” he outlines. Researchers are now investigating how magnetic layers grow. “Ideally the magnetic layer will then take on the wrinkle shape of the graphene. Then, once we manipulate the graphene and the wrinkles change in size, this will hopefully be transferred to the magnetic film, and then we can really tune the magnetic film properties,” continues Dr Gliga. “However, I’m not at that point yet, and there is a danger that the magnetic film might just ‘crush’ these wrinkles.”
Size and shape There are a number of other potential issues, so this is a high-risk, exploratory project at this stage, rather than focusing on any specific applied goals. The primary aim of the project is to control the size and shape of these wrinkles, with respect to amplitude and wavelength. “The ideal shape would be something like a half circle. To get the properties I’m interested in, the width and the height would need to be proportional,”
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says Dr Gliga. While an iron-nickel alloy called Permalloy is being used in the project, Dr Gliga expects other materials like nickel, cobalt or iron would be equally interesting. “One of the differences might be in the way the magnetic films grow on graphene, which is another area of debate,” he says. “The literature suggests that it’s not easy to grow magnetic thin films on graphene. A solution to this is to have a thin layer of a different material in-between the graphene and the magnetic film. This of course makes the system less flexible.” The project itself is relatively short in duration, but if the results are positive then this could provide the foundations for further research into spin wave manipulation. There are many ideas around about the possible applications of functional magnetic materials in logical devices; one of the main issues in this respect is the energy consumption of computing chips. “As they get smaller, they dissipate more energy, thus generating large amounts of heat,” explains Dr Gliga. One problem with conventional computers, which are based on the von Neumann architecture, is that the memory and processing units are separate, and data needs to constantly be transported between them; this is fundamentally inefficient, an issue that Dr Gliga believes these materials could address. “One application could be in neuromorphic computing. This is the idea that you would simultaneously store data and perform logical operations,” he outlines. “The beauty of this system is that it opens new possibilities to achieve that through its structural reconfigurability.” This hinges however on the ability to modify and control the size of these wrinkles, to move the wrinklons along the sheet, and to nucleate new wrinklons when required. The main priority at this stage for Dr Gliga is to achieve this level of control, yet he is very much aware of the wider possibilities. “If this turns out to be possible, then you would be able to define a material through which spin waves can propagate and be simultaneously processed in reprogrammable ways,” he says.
WRINKLES AND WRINKLONS Wrinkles and wrinklons: magnetic films with tuneable topographies Project Objectives
The aim of this project is to grow magnetic thin films on top of wrinkled graphene sheets. A first objective is to determine if the graphene wrinkles can be used as a template to create wrinkled magnetic films. A second objective is to determine if, by actively changing the properties (e.g. size) of the graphene wrinkles, the topography of the deposited magnetic film can be modified. Concretely, this would mean that it is possible to actuate the magnetic film using the graphene underlayer.
Project Funding
Spark project funded by the Swiss National Science Foundation (SNF).
Contact Details
Project Coordinator, Dr Sebastian Gliga Paul Scherrer Institute WSLA/126 Forschungsstrasse 111 5232 Villigen PSI Switzerland T: +41 56 310 54 81 E: sebastian.gliga@psi.ch W: www.psi.ch/en/microspec/scientifichighlights/wrinkles W: http://p3.snf.ch/project-190736
Dr Sebastian Gliga
Dr Sebastian Gliga is a Scientist in the Microspectroscopy Group at the Paul Scherrer Institute. His research focuses on nanomagnetism, in particular the investigation of emergent phenomena in two- (2D) and three-dimensional (3D) artificial spin systems, aiming to create energy efficient functional materials. Sebastian Gliga has published over 60 peer-reviewed papers that have been cited more than 2300 times.
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