  Pulsars are stars that emit electromagnetic radiation in well-defined time
intervals. The frequency of such pulses decays with time as is quantified by
the {\it braking index} ($n$). In the canonical model $n = 3$ for all pulsars,
but observational data show that $n \neq 3$, indicating a limitation of the
model. In this work we present a new approach to study the frequency decay of
the rotation of a pulsar, based on an adaptation of the canonical one. We
consider the pulsar a star that rotates in vacuum and has a strong magnetic
field but, differently from the canonical model, we assume that its moment of
inertia changes in time due to a uniform variation of a displacement parameter
in time. We found that the braking index results smaller than the canonical
value as a consequence of an increase in the star's displacement parameter,
whose variation is small enough to allow plausible physical considerations that
can be applied to a more complex model for pulsars in the future. In
particular, this variation is of the order of neutron vortices' creep in
rotating superfluids. When applied to pulsar data our model yielded values for
the stars' braking indices close to the observational ones. The application of
this approach to a more complex star model, where pulsars are assumed to have
superfluid interiors, is the next step in probing it. We hypothesize that the
slow expansion of the displacement parameter might mimic the motion of core
superfluid neutron vortices in realistic models.
