Superposition of ferromagnetic and antiferromagnetic spin chains in the quantum magnet BaAg2Cu[VO4]2

TL;DR

This study reveals that BaAg2Cu[VO4]2's magnetic behavior is governed by a superposition of ferromagnetic and antiferromagnetic spin chains, challenging the previous triangular lattice model and emphasizing detailed microscopic analysis.

Contribution

It demonstrates that the magnetic properties of BaAg2Cu[VO4]2 are due to a superposition of ferromagnetic and antiferromagnetic chains, not a triangular lattice, highlighting the importance of structural details.

Findings

Magnetic behavior explained by superposed spin chains

Structural details determine exchange interactions

Magnetization and specific heat mimic different models

Abstract

Based on density functional theory band structure calculations, quantum Monte-Carlo simulations, and high-field magnetization measurements, we address the microscopic magnetic model of BaAg2Cu[VO4]2 that was recently proposed as a spin-1/2 anisotropic triangular lattice system. We show that the actual physics of this compound is determined by a peculiar superposition of ferromagnetic and antiferromagnetic uniform spin chains with nearest-neighbor exchange couplings of Ja(1) ~ -19 K and Ja(2) ~ 9.5 K, respectively. The two chains featuring different types of the magnetic exchange perfectly mimic the specific heat of a triangular spin lattice, while leaving a clear imprint on the magnetization curve that is incompatible with the triangular-lattice model. Both ferromagnetic and antiferromagnetic spin chains run along the crystallographic 'a' direction, and slightly differ in the mutual arrangement of the magnetic CuO4 plaquettes and non-magnetic VO4 tetrahedra. These subtle structural details are, therefore, crucial for the ferromagnetic or antiferromagnetic nature of the exchange couplings, and put forward the importance of comprehensive microscopic modeling for a proper understanding of quantum spin systems in transition-metal compounds.