  The observed long-term spin-down evolution of isolated radio pulsars cannot
be explained by the standard magnetic dipole radiation with a constant braking
torque. However how and why the torque varies still remains controversial,
which is an outstanding problem in our understanding of neutron stars. We have
constructed a phenomenological model of the evolution of surface magnetic
fields of pulsars, which contains a long-term decay modulated by short-term
oscillations; a pulsar's spin is thus modified by its magnetic field evolution.
The predictions of this model agree with the precisely measured spin evolutions
of several individual pulsars; the derived parameters suggest that the Hall
drift and Hall waves in the NS crusts are probably responsible for the
long-term change and short-term quasi-periodical oscillations, respectively.
Many statistical properties of the timing noise of pulsars can be well
re-produced with this model, including correlations and the distributions of
the observed braking indices of the pulsars, which span over a range of more
than 100 millions. We have also presented a phenomenological model for the
recovery processes of classical and slow glitches, which can successfully model
the observed slow and classical glitch events without biases.
