  Quantum triangular-lattice antiferromagnets are important prototype systems
to investigate phenomena of the geometrical frustration in condensed matter.
Apart from highly unusual magnetic properties, they possess a rich phase
diagram (ranging from an unfrustrated square lattice to a quantum spin liquid),
yet to be confirmed experimentally. One major obstacle in this area of research
is the lack of materials with appropriate (ideally tuned) magnetic parameters.
Using Cs$_2$CuCl$_4$ as a model system, we demonstrate an alternative approach,
where, instead of the chemical composition, the spin Hamiltonian is altered by
hydrostatic pressure. The approach combines high-pressure electron spin
resonance and magnetization measurements, allowing us not only to
quasi-continuously tune the exchange parameters, but also to accurately monitor
them. Our experiments indicate a substantial increase of the exchange coupling
ratio from 0.3 to 0.42 at a pressure of 1.8 GPa, revealing a number of emergent
field-induced phases.
