High-frequency two-dimensional EPR spectroscopy of Cr~(3+) and Ho~(3+) dimers in synthetic forsterite

摘要

An enormously high concentration of Cr~(3+)-Cr~(3+) and Ho~(3+)-Ho~(3+) dimers is found in synthetic forsterite doped by Cr or Ho ions, respectively. The measurements were performed by means of high-frequency two-dimensional EPR spectroscopy in the frequency region of 65 - 250 GHz at 4.2 K in magnetic fields up to 0.8 T. The analysis of the spin Hamiltonian parameters and rotational diagrams showed that the Cr~(3+)-Cr~(3+) dimers are formed by a pair of Cr~(3+) ions with an Mg~(2+) vacancy between them replacing three Mg~(2+) ions in Ml positions situated in a quasi-one-dimensional chain aligned parallel to the crystal c-axis. It is found that the interaction between Cr~(3+) ions in the dimer is the ferromagnetic exchange with parameters J_z =0.47 GHz and J_t = 0.79 GHz. Most probable structures of Ho~(3+)-Ho~(3+) dimers consist of two Ho~(3+) ions coupled by the dipole-dipole interaction (J_z= 11.4 GHz) and situated in the M2 octahedral positions in the same bc-layer with a Mg~(2+) vacancy in the M1 position between them. The mechanism which favors the formation of dimers of trivalent impurity ions in forsterite is proposed. It is connected with the conservation of the overall charge balance under the substitution of three nearest neighboring Mg~(2+) cations by the two impurity ions with a Mg vacancy between them.