Arguably the most fundamental collective physics problem in magnetism is that of interacting dipoles. Yet investigations of antiferromagnetic dipolar-coupled magnets have been scarce due to lack of materials exhibiting this property and due to the low energy scale involved.

We have established the material LiErF4 as a dipolar antiferromagnet exhibiting both classical and quantum phase transitions. The excellent resolution of the new Let instrument on the ISIS Second Target Station has allowed direct observation of the full pattern of magnetic interactions in the material, including accurate measurement of a tiny 25 μeV energy gap. The excellent signal and low background allowed full 4-dimensional datasets (3 momentum directions and one energy axis) to be recorded. While detailed analysis and a follow-up experiment are still ongoing, the results so far are providing detailed information on the nature of the phase transitions and how the behaviour of LiErF4 is linked to that of other materials. The excellent instrument resolution and high flux of Let open a new range of low energy scales that can be exploited by neutron spectroscopy to address fundamental questions in quantum physics.

N Nikseresht (EPFL, Switzerland), B Dalla Piazza, HM Rønnow (Ecole Polytechnique Federale de Lausanne), R Bewley, J Taylor (ISIS), C Kraemer (ETH-Zurich)

Research date: August 2011