  Exciting new fundamental scientific questions are currently being raised
regarding nonequilibrium dynamics in spin systems, as this directly relates to
low power and low loss energy transport for spintronics. Inelastic neutron
scattering (INS) is an indispensable tool to study spin excitations in complex
magnetic materials. However, conventional INS spectrometers currently only
perform steady-state measurements and probe averaged properties over many
collision events between spin excitations in thermodynamic equilibrium, while
the exact picture of re-equilibration of these excitations remains unknown. In
this work, we designed and implemented a time-resolved laser-neutron pump-probe
capability at HYSPEC (Hybrid Spectrometer, beamline 14-B) at the Spallation
Neutron Source (SNS) at Oak Ridge National Laboratory. This capability allows
us to excite out-of-equilibrium magnons with a nanosecond pulsed laser source
and probe the resulting dynamics using INS. Here, we discussed technical
aspects to implement such a capability in a neutron beamline, including choices
of suitable neutron instrumentation and material systems, laser excitation
scheme, experimental configurations, and relevant firmware and software
development to allow for time-synchronized pump-probe measurements. We
demonstrated that the laser-induced nonequilibrium structural factor is able to
be resolved by INS in a quantum magnet. The method developed in this work will
provide SNS with advanced capabilities for performing out-of-equilibrium
measurements, opening up an entirely new research direction to study
out-of-equilibrium phenomena using neutrons.
