Magnetic-field-induced ordering in a spin-1/2 chiral chain

TL;DR

This study reveals a magnetic-field-induced transition to long-range order in a spin-1/2 chiral chain, challenging previous models and highlighting the role of internal staggered fields in magnetic ordering.

Contribution

It provides the first experimental evidence of field-induced magnetic order in a quantum chiral chain and proposes a new magnetic structure model based on internal staggered fields.

Findings

Field-induced magnetic order observed above 3 T

Magnetic structure explained by internal two-fold staggered g tensor

Contradicts previous sine-Gordon model predictions

Abstract

We present neutron diffraction, muon spin rotation and pulsed-field magnetometry measurements on the Heisenberg quantum chiral chain [Cu(pym)(H2O)4]SiF6.H2O, which displays a four-fold-periodic rotation of the local environment around the Cu(II) S = 1/2 ions from site to site along the chain. Previous measurements on this material have shown the absence of magnetic order down to surprisingly low temperatures >= 20 mK, as well as the presence of an energy gap for magnetic excitations that grows linearly with magnetic field. Here we find evidence at dilution refrigerator temperatures for a field-induced transition to long-range magnetic order above an applied magnetic field of 3 T. From the polarization of magnetic moments observed in applied fields we can identify the static magnetic structure that best accounts for the data. The proposed model is supported microscopically by the presence of an alternating component of the g tensor, which produces an internal two-fold staggered field that dictates both the direction of the ordered moments and the effective coupling between adjacent chains. The observed magnetic structure is contrary to previous proposals for the departure of the magnitude and field dependence of the energy gap from the predictions of the sine-Gordon model.