Eur. Phys. J. B 23, 153-158 (2001)
https://doi.org/10.1007/s100510170062

Magnetic properties of undoped YBa₂Cu₃O_6+x system

¹ Theory Group, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India
² Department of Physics, G.B. Pant University of Ag. & Tech. Pantnagar 263145, India

Corresponding author: ^a govind@csnpl.ren.nic.in

Received: 13 July 2000
Revised: 14 May 2001
Published online: 15 September 2001

Abstract

In the present paper, we study the magnetic properties of bilayer cuprate antiferromagnets. In order to evaluate the expressions for spin-wave dispersion, sublattice magnetization, Néel temperature and the magnetic contribution to the specific heat, the double time Green's function technique has been employed in the random phase approximation (RPA). The spin wave dispersion curve for a bilayer antiferromagnetic system is found to consist of one acoustic and one optic branch. The "optical magnon gap" has been attributed solely to the intra-bilayer exchange coupling (J⊥) as its magnitude does not change significantly with the inter-bilayer exchange coupling (J_z). However J_z is essential to obtain the acoustic mode contribution to the magnetization. The numerical calculations show that the Néel temperature (T_N) of the bilayer antiferromagnetic system increases with the J_z and a small change in J_z gives rise to a large change in the Néel temperature of the system. The magnetic specific heat of the system follows a T² behaviour but in the presence of J_z it varies faster than T².