  Magnetars are proposed to be peculiar neutron stars which could power their
X-ray radiation by super-strong magnetic fields as high as $\gtrsim 10^{14}$ G.
However, no direct evidence for such strong fields is obtained till now, and
the recent discovery of low magnetic field magnetars even indicates that some
more efficient radiation mechanism than magnetic dipole radiation should be
included. In this paper, quantum vacuum friction (QVF) is suggested to be a
direct consequence of super-strong {\em surface} fields, therefore the magnetar
model could then be tested further through the QVF braking. Pulsars' high
surface magnetic field interacting with the quantum vacuum result in a
significantly high spindown rate ($\dot{P}$). It is found that QVF dominates
the energy loss of pulsars when pulsar's rotation period and its first
derivative satisfy the relationship $P^3\cdot
\dot{P}>0.63\times10^{-16}\xi^{-4}$s$^2$, where $\xi$ is the ratio of the
surface magnetic field over diploe magnetic field. In the "QVF $+$
magnetodipole" joint braking scenario, the spindown behavior of magnetars
should be quite different from that in the pure magnetodipole model. We are
expecting these results could be tested by magnetar candidates, especially the
low magnetic field ones, in the future.
