Magnetoresistance and the spin-flop transition in single-crystalLa2CuO4+y

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 $T_N$∼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 τ∥a at zero field to τ∥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 τ∥c direction. In an appendix we discuss the possible relevance of these results to the behavior of superconducting ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$(Sr,Ba${)}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$.