Spin canting and lattice symmetry in La$_2$CuO$_4$

TL;DR

This paper investigates how lattice symmetry and weak anisotropic effects influence spin canting and magnetic phases in La$_2$CuO$_4$, revealing a noncollinear spin configuration induced by magnetic fields.

Contribution

It introduces a revised model of spin rotation in La$_2$CuO$_4$ considering lower lattice symmetry, explaining field-induced spin reorientation more accurately.

Findings

Neutron diffraction data supports noncollinear spin rotation model.

Lattice symmetry is lower than previously assumed, affecting spin canting.

Field-induced transition involves different spin rotations in neighboring planes.

Abstract

While the dominant magnetic interaction in La$_2$CuO$_4$ is superexchange between nearest-neighbor Cu moments, the pinning of the spin direction depends on weak anisotropic effects associated with spin-orbit coupling. The symmetry of the octahedral tilt pattern allows an out-of-plane canting of the Cu spins, which is compensated by an opposite canting in nearest-neighbor layers. A strong magnetic field applied perpendicular to the planes can alter the spin canting pattern to induce a weak ferromagnetic phase. In light of recent evidence that the lattice symmetry is lower than originally assumed, we take a new look at the nature of the field-induced spin-rotation transition. Comparing low-temperature neutron diffraction intensities for several magnetic Bragg peaks measured in fields of 0 and 14 T, we find that a better fit is provided by a model in which spins rotate within both neighboring planes but by different amounts, resulting in a noncollinear configuration. This model allows a more consistent relationship between lattice symmetry and spin orientation at all Cu sites.