The work is published as an Article in Nature Physics this week.
Sven Friedemann and his collaborators used detailed magnetic measurements combined with very clear-cut modelling to study what happens when the second-order transition of a ferromagnet is suppressed to zero temperature. This is of great general interest as a second order transition at zero temperature forms a so-called quantum critical point with strong quantum fluctuations and emergent behaviour. For instance, in many antiferromagnets superconductivity is found to emerge around a quantum critical point.
It was already known that ferromagnetism gives way to other types of order in many materials as pressure or chemical tuning is utilized to suppress the transition temperature whilst the ferromagnetic transition becomes first order. Thus, the ferromagnetic quantum critical point is avoided as no significant fluctuations originate from a first order transition. Dr Friedemann and his colleagues analysed the behaviour of NbFe2 – a prototypical material for the suppression of ferromagnetism – using a Landau order parameter expansion. The researchers did only locate the avoided ferromagnetic quantum critical point but they did also discover quantum tricritical points at which both the uniform (ferromagnetic) susceptibility and the finite wavevector (antiferromagnetic) susceptibility diverge.
Because of the general nature of the model used, the results are expected to apply to a whole class of ferromagnets.