Eur. Phys. J. B 21, 313-329 (2001)
https://doi.org/10.1007/BF01313313

Quantum effects in magnetic structures on the fcc lattice

¹ Condensed Matter Physics and Chemistry Department, Risø National Laboratory, 4000 Roskilde, Denmark
² Department of Mathematics and Physics, The Royal Veterinary and Agricultural University, 1871 Frederiksberg C, Denmark and Niels Bohr Institute, University of Copenhagen, 2100 København Ø, Denmark

Corresponding author: ^a kim.lefmann@risoe.dk

Received: 4 July 2000
Revised: 23 February 2001
Published online: 15 June 2001

Abstract

We present the results of a numerical diagonalization study of the next-nearest neighbour Heisenberg model on the fcc lattice. We have studied clusters of s=1/2 spins of sizes N=32 and 64 and obtained ground state energy, magnetization, susceptibility, and static structure factor for various values of applied fields and interaction parameters. From the magnetic structure factors, we find that the antiferromagnetic type-II order penetrates into the ferromagnetic area of the mean-field model. The magnetisation of the type-II state in this area jumps from zero to totally magnetised at a certain applied field. Other areas containing type-II or type-III order, displays "offsets" in the magnetisation curves. Another anomaly is that the phase boundary between the type-I and the type-III states moves in favour of the type-I state around half magnetisation. Apart from this, the phase boundaries are located at their mean field values at zero field, and the positions of the phase boundaries do not depend on the field value. By comparing the longitudinal and transverse structure factors, it is found that the spin orientation in the type-I structure at intermediate fields shows a transition between a canted spin-flop-like structure and a collinear "up-up-up-down" structure. This confirms earlier theoretical predictions. However, in the type-II and type-III phases only the spin-flop structures are observed.