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We report the results of near-infrared spectroscopic observations of 37quasars 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. Thespectra, taken with Keck, Gemini, VLT, and Magellan, allow investigations ofcentral 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 holeswith 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. TheCIV - MgII emission line velocity differences in our sample show an increase ofCIV blueshift towards higher redshift, but the evolutionary trend observed fromthis sample is weaker than the previous results from smaller samples at similarredshift. The FeII/MgII flux ratios derived for these quasars up to $z=7.6$,compared with previous measurements at different redshifts, do not show anyevidence of strong redshift evolution, suggesting metal-enriched environmentsin these quasars. Using this quasar sample, we create a quasar compositespectrum for $z>6.5$ quasars and find no significant redshift evolution ofquasar broad emission lines and continuum slope, except for a blueshift of theCIV line. Our sample yields a strong broad absorption line quasar fraction of$\sim$24%, higher than the fractions in lower redshift quasar samples, althoughthis could be affected by small sample statistics and selection effects.

Probing Early Super-massive Black Hole Growth and Quasar Evolution with Near-infrared Spectroscopy of 37 Reionization-era Quasars at 6.3 < z <= 7.64