Tetranuclear <i>s</i> = 1/2 single-molecular magnets with the fourfold rotational symmetry

Exchange anisotropy effects in equal-spin s = 1/2 tetrameric single-molecule magnets with C4 and S4 point symmetry groups are investigated. The spin Hamiltonian, written in a four-spin 16 × 16 representation of the Pauli matrixes in the appropriate non-collinear local coordinate systems (LCS) of magnetic ions, is transformed into a general coordinate representation. Restrictions on the angles of deviation of the LCS from general axes are found to be a result of the symmetry coupling of the spins. Numerical exact diagonalization is employed for a Hamiltonian matrix obtained due to symmetry restrictions to derive its eigenvalues and the eigenvectors. The appearance of new transitions of frequency-dependent EPR at the divergence of initially collinear coordinate axes and suppression of initial transitions is revealed. The angle boundaries of the presence of EPR are determined for the deflections of the local ion axes on polar angles for all possible transitions between the energy levels for the four-spin clusters obeying the C4 or S4 symmetry operations.