Magnetism is extremely important to our everyday lives. Without it, we would not have computers, credit cards, or the inevitable fridge magnets, to name a few. But our understanding of magnetism remains limited. When a material that is normally magnetic becomes too thin, or even at the boundary between two different materials, the magnetic properties can be lost – and this area of physics is not very well understood.
Dr Mikel Holcomb, from West Virginia University, has dedicated her career to better understanding magnetism, specifically looking at one interesting class of materials, complex oxides. A complex oxide is any compound that contains oxygen along with typically at least two other elements. They make up some of the most abundant minerals on Earth, and complex oxides show a huge variety of interesting properties, from superconductivity to dielectrics.
In particular, Dr Holcomb’s group (and many collaborators including Romero, LeBeau, Stanescu and several national facility scientists) has made progress looking into thin films of a particular complex oxide called lanthanum strontium manganite, or LSMO. LSMO is an important material because it has the potential to be used in a variety of applications, including computers and sensors. It exhibits a range of interesting properties, including ferromagnetism – when a material maintains its magnetism even after a magnetic field has been removed. Dr Holcomb has developed and combined new methods to study the interfaces of these thin films, with an aim to learn more about magnetism and other types of interface properties.
In thin films in general, the properties at the boundary of two materials are often extremely different from the bulk of the materials. Many devices use boundaries, which can be the surface of a material – the boundary between it and the air – or the interface between layers of two different materials. This is because many properties like spin, charge, and orbital degrees of freedom, depend on the ions’ surroundings within a material.
While all of these properties can be affected in different ways, they all link together to cause different magnetic effects in a ‘complicated web’ according to Dr Holcomb, who tries to shed light on these effects by measuring as much as possible, to compare all factors. For example, she examines the different ways strain, thickness of materials and choice of materials can affect the magnetic properties. She uses a variety of methods including X-ray absorption spectroscopy, which uses synchrotron radiation to excite core electrons, to probe the inner structure of the materials. Dr Holcomb’s group combines this kind of technique with others like neutron reflectivity – which uses the reflection of neutrons (at the National Institute of Standards and Technology, NIST) to determine the strength and direction of the magnetism at every layer of the material.
Dr Holcomb examines the different ways strain, thickness of materials and the conditions under which materials are grown can affect the magnetic and other properties
Ferroelectric and ferromagnetic materials
In recent years, Dr Holcomb and her team have studied what happens when LSMO, a ferromagnetic material, is placed adjacent to a ferroelectric layer. Ferroelectric materials can become electrically polarised, with one side positively charged and the other negatively, which can switch under an electric field. When placing a thin film of LSMO on this kind of material, Dr Holcomb discovered some curious properties.
At the interface between ferroelectric and ferromagnetic materials, something called magnetoelectric coupling occurs. This means there is a link between the magnetic and electric properties of the material. For example, the magnetic properties of LSMO thin films with a layer of lead zirconate titanate (PZT) on top can be controlled by the presence of an external electric field. Electrical control of magnetism could revolutionise computing (and other devices) by removing the constant need for current in transistors.
By varying the thickness of the LSMO film, Dr Holcomb and her team found that the thicker the layers of LSMO and PZT, the higher the valence (the capacity of the ion to combine with others) of manganese ions within the LSMO. The valence of these ions is linked to the magnetic properties of LSMO, and understanding what affects it can help to create more efficient devices in the future.
A materials database
Although Dr Holcomb has focussed much of her work on LSMO, there are a huge number of materials that exhibit interesting properties on their own and at the interface with other materials. Because of this, keeping on top of the possibilities is a difficult task, especially when the data about each material is not always readily available. Dr Holcomb and her team are in the process of developing a unique kind of database that will give researchers a simple way to compare different materials, using machine learning.
When a material that is normally magnetic becomes too thin, or even at the boundary between two different materials, the magnetic properties can be lost
At the moment, the database focuses only on the materials the group’s expertise lies with, like LSMO, but they are hopeful they can expand it to include many others. To do this, however, they will need the help of colleagues who will eventually benefit from the database. ‘Our database currently includes our own samples, but we’d like to expand it to include others’ samples’, says Dr Holcomb. ‘We are probably going to pull a lot from published literature, but that takes a lot of time and people don’t publish all data on a sample. The more help we can get from others, the better.’
Dr Holcomb’s vision for the database is that anyone using it will be able to plot various properties of a material and see how they change for a variety of samples that meet a given search criterion. Everything from the dependence of magnetism with temperature to the chemical spectrum could be included. ‘I’ve seen something similar done for simple semiconductors, but I’ve not seen it for complex oxides,’ she says.
Why are you interested in the interfaces between materials?
How thick are these interfaces, is it just one layer of atoms thick or do the layers merge in a more messy way?
Why did you choose to focus on LSMO?
How do you decide what parameters you change, e.g. thickness?
Where did the idea for the materials database come from?
Will people have to pay to use your database once it is up and running?
- Just entering data (either from your own work or pulling from published work). I have trained high school students how to do this so even non-scientists could help. If you know how to automate this process, please let me know! This is the biggest hurdle right now.
- Helping us create a nice website for people to use. I am a physicist, so it might not surprise you that organised programming that is really easy for anyone to understand and use is not my speciality.
- Aid in implementing machine learning. There is going to be a LOT of data in this database and a lot of it will be graphs. What will be the best things to have the computers compare? I expect there will be a lot of trial and error in the initial analysis.