  The quantum spin liquid (QSL) state is an exotic state of matter featuring a
high degree of entanglement and lack of long-range magnetic order in the
zero-temperature limit. The triangular antiferromagnet YbMgGaO4 is a candidate
QSL host, and precise determination of the Hamiltonian parameters is critical
to understanding the nature of the possible ground states. However, the
presence of chemical disorder has made directly measuring these parameters
challenging. Here we report neutron scattering and magnetic susceptibility
measurements covering a broad range of applied magnetic field at low
temperature. Our data shows a field-induced crossover in YbMgGaO4, which we
reproduce with complementary classical Monte Carlo and Density Matrix
Renormalization Group simulations. Neutron scattering data above and below the
crossover reveal a shift in scattering intensity from M to K points and,
collectively, our measurements provide essential characteristics of the phase
crossover that we employ to strictly constrain proposed magnetic Hamiltonian
parameters despite the chemical disorder. Constrained exchange parameters
further suggest the material's proximity to the QSL state in the clean limit.
More broadly, our approach demonstrates a means of pursuing QSL candidates
where Hamiltonian parameters might otherwise be obscured by disorder.
