Model for the low-temperature magnetic phases observed in doped YBa_2Cu_3O_{6+x}

TL;DR

This paper presents a classical statistical model explaining the low-temperature magnetic phases in doped YBa_2Cu_3O_{6+x}, capturing various observed antiferromagnetic phases through effective couplings.

Contribution

It introduces a new classical model that accounts for multiple magnetic phases in doped YBa_2Cu_3O_{6+x} and explains phase transitions via effective ferromagnetic interactions.

Findings

Model reproduces experimental magnetic phase diagram

Effective ferromagnetic coupling explains low-temperature phases

Good agreement with experimental data for realistic parameters

Abstract

A classical statistical model for the antiferromagnetic (AFM) ordering of the Cu-spins in the CuO_2 planes of reduced YBa_2Cu_3O_{6+x} type materials is presented. The magnetic phases considered are the experimentally observed high-temperature AFI phase with ordering vector Q_I=(1/2,1/2,0), and the low-temperature phases: AFII with Q_II=(1/2,1/2,1/2) and intermediate TA (Turn Angle) phases TAI, TAII and TAIII with components of both ordering vectors. It is shown that the AFII and TA phases result from an effective ferromagnetic (FM) type coupling mediated by free spins in the CuO_x basal plane. Good agreement with experimental data is obtained for realistic model parameters.