  Ultra-fast dynamics, insensitivity to external magnetic fields, or absence of
magnetic stray fields are examples of properties that make antiferromagnets of
potential use in the development of spintronic devices. Similar to their
ferromagnetic counterparts, antiferromagnets can store information in the
orientations of the collective magnetic order vector. However, also in analogy
to ferromagnets, the readout magnetoresistivity signals in simple
antiferromagnetic films have been weak and the extension of the electrical
reorientation mechanism to optics has not been achieved. Here we report
reversible and reproducible quenching of an antiferromagnetic CuMnAs film by
either electrical or ultrashort optical pulses into nano-fragmented domain
states. The resulting resistivity changes approach 20\% at room temperature,
which is comparable to the giant magnetoresistance ratios in ferromagnetic
multilayers. We also obtain a signal readout by optical reflectivity. The
analog time-dependent switching and relaxation characteristics of our devices
can mimic functionality of spiking neural network components.
