  We report the results of near-infrared spectroscopic observations of 37
quasars in the redshift range $6.3< z\le7.64$, including 32 quasars at $z>6.5$,
forming the largest quasar near-infrared spectral sample at this redshift. The
spectra, taken with Keck, Gemini, VLT, and Magellan, allow investigations of
central black hole mass and quasar rest-frame ultraviolet spectral properties.
The black hole masses derived from the MgII emission lines are in the range
$(0.3-3.6)\times10^{9}\,M_{\odot}$, which requires massive seed black holes
with masses $\gtrsim10^{3-4}\,M_{\odot}$, assuming Eddington accretion since
$z=30$. The Eddington ratio distribution peaks at $\lambda_{\rm Edd}\sim0.8$
and has a mean of 1.08, suggesting high accretion rates for these quasars. The
CIV - MgII emission line velocity differences in our sample show an increase of
CIV blueshift towards higher redshift, but the evolutionary trend observed from
this sample is weaker than the previous results from smaller samples at similar
redshift. The FeII/MgII flux ratios derived for these quasars up to $z=7.6$,
compared with previous measurements at different redshifts, do not show any
evidence of strong redshift evolution, suggesting metal-enriched environments
in these quasars. Using this quasar sample, we create a quasar composite
spectrum for $z>6.5$ quasars and find no significant redshift evolution of
quasar broad emission lines and continuum slope, except for a blueshift of the
CIV line. Our sample yields a strong broad absorption line quasar fraction of
$\sim$24%, higher than the fractions in lower redshift quasar samples, although
this could be affected by small sample statistics and selection effects.
