Anisotropic spin freezing in the S=1/2 zigzag ladder compound SrCuO2

TL;DR

This study reveals an anisotropic spin freezing in SrCuO2, where magnetic moments freeze at low temperatures with long-range order along some directions but only short-range correlations along frustrated axes, indicating complex magnetic dynamics.

Contribution

It provides the first detailed characterization of anisotropic spin freezing in a zigzag ladder compound, highlighting the role of frustrated interactions and slow magnetic defect dynamics.

Findings

Spin freezing occurs below 5 K with long-range order along chains.

Correlation length is anisotropic, with minimal correlation along the frustrated axis.

Frozen magnetic moments are very small and temperature-dependent.

Abstract

Using magnetic neutron scattering we characterize an unusual low temperature phase in orthorhombic SrCuO2. The material contains zigzag spin ladders formed by pairs of S=1/2 chains (J=180 meV) coupled through a weak frustrated interaction |J'|<0.1J. At T<Tc1=5.0(4)K an elastic peak develops in a gapless magnetic excitation spectrum indicating spin freezing on a time scale larger than 200 picoseconds. While the frozen state has long range commensurate antiferromagnetic order along the chains with the correlation length exceeding 200 lattice periods along the c-axis and a substantial correlation length of 60(25) spacings along the a-axis perpendicular to the zigzag plane, only 2 lattice units are correlated along the b-axis which is the direction of the frustrated interactions. The frozen magnetic moment of each Cu ion is very small, 0.033(7) Bohr magneton even at T=0.35K, and has unusual temperature dependence with a cusp at Tc2=1.5K reminiscent of a phase transition. We argue that slow dynamics of stripe-like cooperative magnetic defects in tetragonal a-c planes yield this anisotropic frozen state.