Criticality in a disordered quantum antiferromagnet studied by neutron diffraction

Field-induced magnetic ordering in the structurally disordered quantum magnets Ni(Cl_1-xBrₓ)₂{{0.16em}{0ex}}·{}{{0.16em}{0ex}}$4SC(NH${}₂$)${}₂$, $x=8$% and 13%, is studied by means of neutron diffraction. The order-parameter critical exponent is found to be very close to its value ${β}=0.5$ expected for magnetic Bose-Einstein condensation in the absence of disorder. This result applies to temperatures down to 40 mK, and a 1 T range in magnetic field. The crossover exponent is found to be φ~0.4 for temperature ranges as small as 300 mK. Possible reasons for a discrepancy with recent numerical simulations and bulk measurements are discussed.