Magnetic order and disorder in a quasi-two-dimensional quantum Heisenberg antiferromagnet with randomized exchange

Abstract

We present an investigation of the effect of randomizing exchange strengths in the $S=1/2$ square lattice quasi-two-dimensional quantum Heisenberg antiferromagnet (QuinH)${}_2$Cu(Cl${}_x$Br${}_{1-x}$)${}_4\cdot$2H${}_2$O (QuinH$=$Quinolinium, C${}_9$H${}_8$N${}^{+}$), with $0\le x\le 1$. 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 $T_{\mathrm{N}}$ 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 $T_{\mathrm{N}}$ and the ordered moment size and find an extended disordered phase in the region $0.4\lesssim x\lesssim 0.8$ where no magnetic order occurs, driven by quantum effects of the exchange randomness.

6 pages, 3 figures