  Black holes are spun up by accreting matter and possibly spun-down by
magnetic fields. In our work we consider the effect on black hole rotation of
the two electromagnetic processes, Blandford-Znajek and Direct Magnetic Link,
that differ in their magnetic field configuration. The efficiency of these
processes varies with mass accretion rate and accretion regime and generally
result in an equilibrium spin parameter in the range from 0.35 to ~0.98.
Magnetic field loses its energy while being accreted that may lead to an
increase in equilibrium Kerr parameter for the case of advection-dominated
disc. We find magnetic field decay decay can decrease electromagnetic term
significantly thus increasing the Kerr parameter. We have performed Monte-Carlo
simulations for a supermassive black hole population. Our simulations show
broad distributions in Kerr parameter (0.1< a <0.98) with a peak at a~0.6. To
explain the high observational Kerr parameter values of a>0.9, episodes of
supercritical accretion are required. This implication does not however take
into account black hole mergers (that play an important role for supermassive
black hole evolution).
