Parasitic small-moment-antiferromagnetism and non-linear coupling of hidden order and antiferromagnetism in URu2Si2 observed by Larmor diffraction

TL;DR

This study uses neutron diffraction to reveal that the tiny magnetic moment in URu2Si2's hidden order state is parasitic, stemming from lattice constant variations, and uncovers a non-linear coupling between hidden order and antiferromagnetism.

Contribution

It provides direct experimental evidence that the hidden order and antiferromagnetism are distinct, coupled phases with different symmetries, advancing understanding of URu2Si2's complex order parameters.

Findings

The tiny moment in the hidden order state is parasitic and originates from lattice constant distribution.

Hidden order and large-moment antiferromagnetism are separated by a first-order transition line.

The coupling between hidden order and antiferromagnetism is non-linear, implying different symmetries.

Abstract

We report simultaneous measurements of the distribution of lattice constants and the antiferromagnetic moment in high-purity URu2Si2, using both Larmor and conventional neutron diffraction, as a function of temperature and pressure up to 18 kbar. We establish that the tiny moment in the hidden order (HO) state is purely parasitic and quantitatively originates from the distribution of lattice constants. Moreover, the HO and large-moment antiferromagnetism (LMAF) at high pressure are separated by a line of first-order phase transitions, which ends in a bicritical point. Thus the HO and LMAF are coupled non-linearly and must have different symmetry, as expected of the HO being, e.g., incommensurate orbital currents, helicity order, or multipolar order.