  We present a study of strong intervening absorption systems in the near-IR
spectra of 31 luminous quasars at $z>5.7$. The quasar spectra were obtained
with {\it Gemini} GNIRS that provide continuous wavelength coverage from
$\sim$0.9 to $\sim$2.5 $\mu$m. We detect 32 strong Mg II doublet absorbers with
rest-frame equivalent width $W_r$ ($\lambda2796$) $>1.0$ \AA at $2.2 < z <
6.0$. Each Mg II absorber is confirmed by at least two associated Fe II
absorption lines in the rest-frame wavelength range of $\sim 1600-2600$ \AA. We
find that the comoving line density ($dN/dX$) of the strong Fe II-bearing Mg II
absorbers decreases towards higher redshift at $z>3$, consistent with previous
studies. Compared with strong Mg II absorbers detected in damped Ly$\alpha$
systems at 2 $<z<$ 4, our absorbers are potentially less saturated and show
much larger rest-frame velocity widths. This suggests that the gas traced by
our absorbers are potentially affected by galactic superwinds. We analyze the
{\it Hubble Space Telescope} near-IR images of the quasars and identify
possible associated galaxies for our strong absorbers. There are a maximum of
two galaxy candidates found within 5" radius of each absorber. The median
F105W-band magnitude of these galaxy candidates is 24.8 mag, which is fainter
than the $L^*$ galaxy luminosity at $z\sim$ 4. By using our observed $dN/dX$ of
strong Mg II absorbers and galaxy candidates median luminosity, we suggest that
at high redshift, strong Mg II absorbers tend to have a more disturbed
environment but smaller halo size than that at $z <$ 1.
