Magnetism is one of the most familiar properties of materials that we come across in our everyday life but it can also be one of the most counterintuitive. From electric motors to computer hard disks, we rely on magnetic materials to underpin much of modern technology. Advances in such technology require new materials and greater understanding of existing ones. Magnetism is also quantum mechanics writ large because the quantum interactions between the electrons in a material control its magnetic properties, even at the bulk scale. ISIS neutrons and muons are used to investigate a broad range of topics in magnetism, facilitating discoveries of both theoretical and practical significance.
Most magnetic materials cooled below a certain temperature enter a state where their magnetic moments order into a pattern controlled by the symmetry of the material and the interactions between the atoms. Neutron diffraction allows this pattern to be determined, providing information about the interactions within the material. This can be done for samples centimetres across or thin films ten thousand times thinner than a human hair.
Magnetic moments within materials also fluctuate and this gives a further window on how they are interacting. The fluctuations can be driven by the effects of temperature or, as the material is cooled towards absolute zero, by quantum mechanics. Inelastic neutron scattering allows these fluctuations to be measured and compared to detailed theoretical predictions.
Some materials avoid magnetic ordering down to the lowest temperatures available in experiments, hundredths of a degree above absolute zero. This happens because the interactions between their magnetic moments frustrate each other and they cannot find a state which satisfies all the interactions. Even the tiniest opportunity to satisfy their interactions will eventually be seized upon by magnetic moments and this can lead to weakly magnetic states that only appear at very low temperatures. Muons are used to search for the tiny magnetic fields associated with such ordering.
Magnetism can also couple to other properties of materials. Magnetostructural interactions can allow materials to display both magnetic and ferroelectric moments, allowing electric fields to control magnetism and vice versa, in multiferroic compounds. One of the central questions in high-temperature superconductivity is why small changes in the composition of certain compounds turn them from magnetic insulators into superconductors. Pairing magnetic and semiconducting materials offers the possibility of using the spin degree-of-freedom of the electron to carry information and providing ways of improving the properties of semiconductor devices. These areas are all actively studied at ISIS.