The influence of Ni and Ga doping on orbital and charge ordering (CO) in ${\text{La}}_{1/3}{\text{Ca}}_{2/3}{\text{Mn}}_{1-y}{M}_y{\text{O}}_3$ ($M$: Ni, Ga, and $0\le y\le 0.1$) has been investigated by combining magnetic and transport measurements with observation and analysis in transmission electron microscopy (TEM). Both types of doping cause a gradual decrease of $T_{\text{CO}}$—the temperature of the charge ordering transition—while the temperature dependences of the magnetization and electric resistivity remain typical of the CO state. In the undoped parent sample, direct observations by TEM reveal the extended commensurate striped superstructure with $\mathit{q}$-vector $1/3{\mathit{a}}^{\ast }$ over the whole temperature range $<200\text{K}$. In Ni-doped samples, the long-range superstructure is kept but has an incommensurate character with reduced $\mathit{q}$-vector by 18%–20% for Ni concentration $y=0.03$. The extent of the superstructure parameter modulation is in agreement with the resulting decrease in the concentration of ${\text{Mn}}^{3+}$ ions due to their substitution by ${\text{Ni}}^{2+}$. Unlike doping by Ni, the incorporation of Ga on the Mn site completely suppresses the formation of the extended striped superstructure. Analysis of the obtained data allows us to conclude about the crucial role of the orbital-orbital (super)exchange interaction (rather than the Jahn-Teller interaction) in the stabilization of striped superstructure in the Mn-site-doped ${\text{La}}_{1/3}{\text{Ca}}_{2/3}{\text{MnO}}_3$ system.

We unify the method of exchange perturbation theory for multicenter systems. For the case of exchange degeneracy in the total spin of the system we give a secular equation that is more compact and con

The formalism of exchange perturbation theory is presented with regard to the general principles of constructing an antisymmetric vector with the use of the Young diagrams and tableaux in which the coordinate and spin parts are not separated. The form of the energy and wave function corrections coincides with earlier obtained expressions, which are reduced in the present paper to a simpler form of a symmetry-adapted perturbation operator, which preserves all intercenter exchange contributions. The exchange perturbation theory (EPT) formalism itself is presented in the standard form of invariant perturbation theory that takes into account intercenter electron permutations between overlapping nonorthogonal states. As an example of application of the formalism of invariant perturbation theory, we consider the magnetic properties of perovskite manganites La_1/3Ca_2/3MnO₃ that are associated with the charge and spin ordering in magnetic chains of manganese. We try to interpret the experimental results obtained from the study of the effect of doping the above alloys by the model of superexchange interaction in manganite chains that is constructed on the basis of the exchange perturbation theory (EPT) formalism. The model proposed makes it possible to carry out a quantitative analysis of the effect of substitution of manganese atoms by doping elements with different electron configurations on the electronic structure and short-range order in a magnetic chain of manganites.

The first analytic study of superexchange ion interaction in the cuprate layer of high-Tc superconductors is reported. It is shown that the superexchange nonadditive contributions are dominant in the