Some materials exhibit antiferromagnetism, where atoms or molecules behave as small magnets, and align in a regular pattern pointing in opposite directions (left). Other materials are ferromagnetic, and all their magnetic moments point in the same direction (right). Michael Schmid CC BY-SA 3.0
In the same way that a compass, which consists of a ferromagnetic needle, tracks the earth's magnetic field, so will a thin film of ferromagnetic material. If this film is being used to store or read your digital data this is not ideal as the data can be corrupted by a small magnetic field.
Although at an atomic scale antiferromagnetic materials are magnetic, when you add up all the moments that point in opposite directions you get a zero net moment. Because of this, they are not as susceptible to disruption by a magnetic field (except in the case of typically very large fields).
By placing layers of these two types of material next to each other, the additional antiferromagnetic layer locks the ferromagnet to point in a particular direction: this effect is known as exchange bias, as shown below. Exchange bias is used to make smaller devices to read data or to increase the stability and storage density of recording media.
To test this, there have been investigations into systems with two of these interfaces, forming a ferromagnetic / antiferromagnetic / ferromagnetic sandwich. In these systems, the exchange bias at one interface can influence the interface on the other side of the antiferromagnetic layer. This could have applications in spintronics, but the mechanism behind it is still not very well understood.
A group of researchers have used ISIS to investigate such a system to help improve the understanding of how these processes work. The group created a multi-layer system in which each layer was about 10,000 times thinner than a human hair. They varied the distance between the layers by increasing the thickness of the antiferromagnetic “filling". This enabled them to test the effect of the thickness of this layer in the middle on the magnetic properties of the interfaces of the layers on either side.
Polarised neutron reflectometry (PNR) is very useful for studying these kinds of systems, as the technique studies the effect of the sample on a stream of neutrons with their spins aligned in one direction. By analysing the direction of the neutron spins after they have interacted with the sample, the scientists are able to measure the magnitude and direction of the magnetism inside the sample.
Using PNR on Polref and Offspec at ISIS, they were able to conclude that there must be a competition between the coupling between the interfaces, and the disorder inside the layer in between them. A thicker “filling" is more disordered, making the coupling of the layers on either side of it more difficult. By using computer modelling, they were able to simulate this system, and found that their simulations agreed with their experimental results.
The full paper can be viewed on the Scientific Reports website.
Other science highlights can be viewed for other experiments on Offspec and POLREF.