Uniform Magnetic Order in a Ferromagnetic-Antiferromagnetic Random Alternating Quantum Spin Chain

TL;DR

This study uses quantum Monte Carlo simulations to show that a ferromagnetic-antiferromagnetic random alternating quantum spin chain exhibits a uniform antiferromagnetic order that persists despite randomness, explaining experimental magnetic transitions.

Contribution

It reveals that uniform antiferromagnetic order can survive in a disordered ferromagnetic-antiferromagnetic quantum spin chain, a novel quantum effect not previously understood.

Findings

Excitation energy gap closes at intermediate concentrations.

Uniform AF order becomes critical in the intermediate region.

Results explain experimental magnetic transition observations.

Abstract

An $S=1/2$ ferromagnetic (F) - antiferromagnetic (AF) random alternating Heisenberg quantum spin chain model is investigated in connection to its realization compound: (CH$_3$)$_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x}$)$_3$. The exchange interaction bonds have alternating strong F-AF random bonds and weak uniform AF bonds. Using the quantum Monte Carlo method we have found that the excitation energy gap closes and the uniform AF order becomes critical in the intermediate concentration region. This finding explains the experimental observation of the magnetic transition by considering weak interchain interactions. Present results suggest that the uniform AF order survives even in the presence of randomly located ferromagnetic bonds. This may be a new quantum effect.