  Physical reservoir computing is a type of recurrent neural network that
applies the dynamical response from physical systems to information processing.
However, the relation between computation performance and physical
parameters/phenomena still remains unclear. This study reports our progress
regarding the role of current-dependent magnetic damping in the computational
performance of reservoir computing. The current-dependent relaxation dynamics
of a magnetic vortex core results in an asymmetric memory function with respect
to binary inputs. A fast relaxation caused by a large input leads to a fast
fading of the input memory, whereas a slow relaxation by a small input enables
the reservoir to keep the input memory for a relatively long time. As a result,
a step-like dependence is found for the short-term memory and parity-check
capacities on the pulse width of input data, where the capacities remain at 1.5
for a certain range of the pulse width, and drop to 1.0 for a long pulse-width
limit. Both analytical and numerical analyses clarify that the step-like
behavior can be attributed to the current-dependent relaxation time of the
vortex core to a limit-cycle state. }
