Four-spin exchange in Bi2CuO4 (abstract)

Abstract

Magnetic properties of Bi2CuO4 single crystals are of interest owing to their relationship to the high-Tc superconductor problem. Earlier we carried out the magnetic and neutron scattering investigations on high quality large single crystals. The Bi2CuO4 crystal belongs to the space group P4/ncc and the magnetic group is the collinear plane antiferromagnet with a Neel temperature of 45.8 K. The three-dimensional character of the antiferromagnetic ordering is reflected by the value of the critical magnetization exponent 0.352. The sublattice magnetization depends very little on temperature up to about 15 K. The spin-wave spectrum contains two branches with energy gaps of 2.1 and 3.4 meV. The theoretical interpretation of the experimental data shows the necessity of introducing the tensor four-site exchange in the S=1/2 spin Hamiltonian with the tetragonal anisotropic bilinear exchange. This provides the gap characteristics of the acoustic branch of spin waves and the correct temperature dependence of the sublattice magnetization. This result shows the importance of the four-site exchange for the formation of Bi2CuO4 magnetic properties. From the similarity of magnetic subsystems, Bi2CuO4, and high-Tc superconductors it is suggested that the four-site exchange is responsible for magnetic ordering of the square spin S=1/2 lattice. We also suggest a thermodynamic theory of the temperature dependence of magnetic properties of Bi2CuO4.