  We studied the spin-state responses to light impurity doping in low-spin
perovskite LaCoO$_{3}$ (Co^3+: d^6) through magnetization and X-ray
fluorescence measurements of single-crystal LaCo$_{0.99}$$M_{0.01}$O$_{3}$ ($M$
= Cr, Mn, Fe, Ni). In the magnetization curves measured at 1.8 K, a change in
the spin-state was not observed for Cr, Mn, or Fe doping but was observed for
Ni doping. Strong magnetic anisotropy along the [100] easy axis was also found
in the Ni-doped sample. The fluorescence measurements revealed that the
valences were roughly estimated to be Cr^3+, Mn^4+, Fe^(3+delta)+, and Ni^3+.
From the observed chemical trends, we propose that the chemical potential is a
key factor in inducing the change of the low-spin state. By expanding a model
of the ferromagnetic spin-state heptamer generated by hole doping, we discuss
the emergence of highly anisotropic ferromagnetic spin-state clusters induced
by low-spin Ni^3+ with Jahn-Teller activity. We also discuss applicability of
the present results to mantle materials and impurity-doped pyrites with Fe
(d^6).
