  Thermodynamic properties of strange quark matter (SQM) in a nonuniform
magnetic field are studied within the phenomenological MIT bag model under the
charge neutrality and beta equilibrium conditions, relevant to the interior of
strange quark stars. The spatial dependence of the magnetic field strength is
modeled by the dependence on the baryon chemical potential in the exponential
and power forms. The total energy density, longitudinal and transverse
pressures in magnetized SQM are found as functions of the baryon chemical
potential. It is clarified that the central magnetic field strength in a
strange quark star is bound from above by the critical value at which the
derivative of the longitudinal pressure with respect to the baryon chemical
potential vanishes first somewhere in the interior of a star under varying the
central field. Above this upper bound, the instability along the magnetic field
is developed in magnetized SQM. The change in the form of the dependence of the
magnetic field strength on the baryon chemical potential between the
exponential and power ones has a nonnegligible effect on the critical magnetic
field strength while the variation of the bag pressure within the absolute
stability window for magnetized SQM has a little effect on the critical field.
