A theoretical study is made of the spin excitations in two-dimensional three-sublattice Heisenberg antiferromagnets containing an impurity atom in one of the sublattices. It is assumed that the coupling parameter between the impurity and matrix is positive, so that the impurity atom and the atom of the sublattice containing the impurity have the same spin directions in the ground state. In the Green’s function method those functions are chosen that consist of combinations of sums and differences of the spin deviation operators. From the poles of these functions expressions are obtained for the states and of the group and these expressions are used to determine their energies.
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