  The normal state in iron chalcogenides is metallic but highly unusual, with
orbital and spin degrees of freedom partially itinerant or localized depending
on temperature, leading to many unusual features. In this work, we report on
the observations of two of such features, the orbital selective Mott phase
(OSMP) and spin nematicity, evidenced in magnetization and magnetotransport
[resistivity, Hall effect, anisotropic magnetoresistance (AMR)] of
Fe$_{1-y}$Ni$_y$Te$_{0.65}$Se$_{0.35}$ single crystals, with $0 < y < 0.21$.
Substitution of Ni dopes crystals with electrons, what eliminates some of the
hole pockets from Fermi level, leaving only one, originating from $d_{xy}$
orbital. This leads to electron-dominated conduction at low $T$ for $y \gtrsim
0.06$. However, at high temperatures, $T \gtrsim 125 \div 178$ K, the
conduction reverses to hole-dominated. Anomalies in magnetization and
resistivity are observed at temperatures which approach high-$T$ boundary of
the electron-dominated region. Analysis of these effects suggests a link with
the appearance of the $d_{z^2}$ hole pockets at X points of the Brillouin zone
in the OSMP phase, facilitated by the localization of $d_{xy}$ orbital, as
recently reported by angular resolved photoemission experiments ($\textit{J.
Huang et al., Commun. Phys. 5, 29 (2022)}$). The low-$T$ AMR shows mixed 4-fold
and 2-fold rotational symmetry of in-plane magnetocrystalline anisotropy, with
the 4-fold term the largest at small $y$, and suppressed at intermediate $y$.
These results are consistent with the mixed stripe/bicollinear magnetic
correlations at small $y$, and suppression of stripe correlations at
intermediate $y$, indicating development of spin nematicity with increasing Ni
doping, which possibly contributes to the suppression of superconductivity.
