  The magneto-rotational instability (MRI) is the most likely mechanism for
transportation of angular momentum and dissipation of energy within hot,
ionized accretion discs. This instability is produced through the interactions
of a differentially rotating plasma with an embedded magnetic field. Like all
substances in nature, the plasma in an accretion disc has the potential to
become magnetically polarized when it interacts with the magnetic field. In
this paper, we study the effect of this magnetic susceptibility, parameterized
by $\chi_m$, on the MRI, specifically within the context of black hole
accretion. We find from a linear analysis within the Newtonian limit that the
minimum wavelength of the first unstable mode and the wavelength of the fastest
growing mode are shorter in paramagnetic ($\chi_m>0$) than in diamagnetic
($\chi_m<0$) discs, all other parameters being equal. Furthermore, the
magnetization parameter (ratio of gas to magnetic pressure) in the saturated
state should be smaller when the magnetic susceptibility is positive than when
it is negative. We confirm this latter prediction through a set of numerical
simulations of magnetically polarized black hole accretion discs. We
additionally find that the vertically integrated stress and mass accretion rate
are somewhat larger when the disc is paramagnetic than when it is diamagnetic.
If astrophysical discs are able to become magnetically polarized to any
significant degree, then our results would be relevant to properly interpreting
observations.
