Quantum spin ladder with ferromagnetic rungs in Bi$_2$CuO$_3$(SO$_4$)

TL;DR

This paper characterizes Bi$_2$CuO$_3$(SO$_4$) as a unique two-leg spin-ladder magnet with ferromagnetic rungs and antiferromagnetic legs, using experimental measurements and theoretical calculations to elucidate its magnetic interactions.

Contribution

It provides the first detailed experimental and theoretical analysis of a spin-ladder system with ferromagnetic rungs and quantifies the superexchange pathways involved.

Findings

Bi$_2$CuO$_3$(SO$_4$) is a two-leg spin-ladder with ferromagnetic rungs ($J' \\approx -208$ K) and antiferromagnetic legs ($J \\approx 258$ K).

The antiferromagnetic leg coupling is the strongest oxygen-mediated superexchange in a Cu$^{2+}$ compound reported to date.

Different superexchange pathways lead to similar magnitude interactions despite different Cu--Cu distances.

Abstract

We introduce Bi$_2$CuO$_3$(SO$_4$) as a rare example of a spin-ladder magnet with ferromagnetic interactions on the rungs. Its magnetic response is studied through measurements of heat capacity, temperature-dependent magnetic susceptibility, and field-dependent magnetization, as well as electron spin resonance spectroscopy. These experiments are complemented by density-functional-theory calculations combined with the construction of maximally localized Wannier functions and an analysis of the relevant superexchange pathways. Quantum Monte Carlo simulations are employed to model thermodynamic properties and to quantitatively determine the magnetic exchange parameters. Our combined approach identifies Bi$_2$CuO$_3$(SO$_4$) as a two-leg spin-ladder system with ferromagnetic rungs ($J'$ $\approx -208$ K) and antiferromagnetic legs ($J$ $\approx 258$ K). These interactions of similar magnitude arise from remarkably different superexchange pathways, with the Cu--Cu distance along the leg being almost twice as long than the respective distance along the rung. The antiferromagnetic leg coupling represents the strongest oxygen-mediated long-range superexchange in a Cu$^{2+}$ compound reported to date and sets the benchmark for the role of complex superexchange pathways in quantum magnets.