Fluctuation-induced ferrimagnetism in sublattice-imbalanced antiferromagnets with application to SrCu$_2$(BO$_3$)$_2$ under pressure

TL;DR

This paper demonstrates that sublattice symmetry breaking in certain antiferromagnets induces fluctuation-driven ferrimagnetism at finite temperatures, with implications for materials like SrCu$_2$(BO$_3$)$_2$ under pressure.

Contribution

It introduces a mechanism for fluctuation-induced ferrimagnetism in sublattice-imbalanced antiferromagnets and applies it to a layered Shastry-Sutherland model relevant to SrCu$_2$(BO$_3$)$_2$.

Findings

Sublattice symmetry breaking leads to ferrimagnetism below T_N.

Layered models show temperature-dependent magnon spectrum changes.

Predicted non-monotonic magnetization behavior in pressurized SrCu$_2$(BO$_3$)$_2$.

Abstract

We show that a collinear Heisenberg antiferromagnet, whose sublattice symmetry is broken at the Hamiltonian level, becomes a fluctuation-induced ferrimagnet at any finite temperature $T$ below the N\'eel temperature $T_{\rm N}$. We demonstrate this using a layered variant of a square-lattice $J_1$-$J_2$ model. Linear spin-wave theory is used to determine the low-temperature behavior of the uniform magnetization, and non-linear corrections are argued to yield a temperature-induced qualitative change of the magnon spectrum. We then consider a layered Shastry-Sutherland model, describing a frustrated arrangement of orthogonal dimers. This model displays an antiferromagnetic phase for large intra-dimer couplings. A lattice distortion which breaks the glide symmetry between the two types of dimers corresponds to broken sublattice symmetry and hence gives rise to ferrimagnetism. Given indications that such a distortion is present in the material SrCu$_2$(BO$_3$)$_2$ under hydrostatic pressure, we suggest the existence of a fluctuation-induced ferrimagnetic phase in pressurized SrCu$_2$(BO$_3$)$_2$. We predict a non-monotonic behavior of the uniform magnetization as function of temperature.