  We present near-infrared H and K-band spectra of the z=6.41 quasar SDSS
J114816.64+525150.3. The spectrum reveals a broad MgII 2799 emission line with
a full-width half-maximium of 6000 km/s. From the peak wavelength of this
emission line we obtain a more accurate redshift than is possible from the
published optical spectrum and determine a redshift of z=6.41+/-0.01. If the
true peak of the Lyman alpha emission is at the same redshift, then a large
fraction of the flux blueward of the peak is absorbed. The equivalent width of
the MgII emission line is similar to that of lower redshift quasars, suggesting
that the UV continuum is not dominated by a beamed component. Making basic
assumptions about the line-emitting gas we derive an estimate for the central
black hole in this quasar of 3x10^9 solar masses. The very high luminosity of
the quasar shows that it is accreting at the maximal allowable rate for a black
hole of this mass adopting the Eddington limit criterion.
