Magnetoresistance and the spin-flop transition in single-crystal La2CuO4+y.

Abstract

Measurements are reported of the magnetoresistance (MR) for fields up to 23 T in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ single crystals, which order antiferromagnetically at ${T}_{N}$\ensuremath{\sim}240 K, and in which the conductivity at low temperature is characterized by hopping in localized states. Using the MR, the phase diagram of the spin-flop transition, observed when the magnetic field is applied parallel to the zero-field staggered magnetization, is mapped out. Two transitions of the background ${\mathrm{Cu}}^{2+}$ spins are observed, which are governed by the symmetric and antisymmetric anisotropic components of the superexchange tensor. The antiferromagnetic propagation vector changes from \ensuremath{\tau}\ensuremath{\parallel}a at zero field to \ensuremath{\tau}\ensuremath{\parallel}c at the highest fields. This subtle change in the ordering of the ${\mathrm{Cu}}^{2+}$ spins is accompanied by a large enhancement of the interlayer hopping conductivity up to a factor 2. We show that the magnetoconductance is proportional to the three-dimensional staggered moment with \ensuremath{\tau}\ensuremath{\parallel}c direction. In an appendix we discuss the possible relevance of these results to the behavior of superconducting ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$(Sr,Ba${)}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$.