Magnetic Order And Disorder In A Quasi-Two-Dimensional Quantum Heisenberg Antiferromagnet With Randomized Exchange

We present an investigation of the effect of randomizing exchange coupling strengths in the S = 1/2 square lattice quasi-two-dimensional quantum Heisenberg antiferromagnet (QHAF) (QuinH)(2)Cu(ClxBr1-x)(4)center dot 2H(2)O (QuinH = Quinolinium, C9H8N+), with 0 <= x <= 1. Pulsed-field magnetization measurements allow us to estimate an effective in-plane exchange strength J in a regime where exchange fosters short-range order, while the temperature T-N at which long-range order (LRO) occurs is found using muon-spin relaxation, allowing us to construct a phase diagram for the series. We evaluate the effectiveness of disorder in suppressing T-N and the ordered moment size, and we find an extended disordered phase in the region 0.4 less than or similar to x less than or similar to 0.8 where no magnetic order occurs. The observed critical substitution levels are accounted for by an energetics-based competition between different local magnetic orders. Furthermore, we demonstrate experimentally that the ground-state disorder is driven by quantum effects of the exchange randomness, which is a feature that has been predicted theoretically and has implications for other disordered quasi-two-dimensional QHAFs.