Single-Crystal Polarized Optical Absorption Spectroscopy of the One-Dimensional Ferrimagnet Mn^IICu^II(pba)(H₂O)₃·2H₂O (pba = 1,3-Propylenebis(oxamato))

Powder and single-crystal optical absorption of the ferrimagnet Mn^IICu^II(pba)(H₂O)₃·2H₂O (denoted MnCu) and the Mn-doped compound Mn_0.1Mg_0.9Cu(pba)(H₂O)₃·2H₂O (denoted Mn_0.1Mg_0.9Cu) with pba standing for 1,3-propylenebis(oxamato) was investigated in the 10−300 K range. The crystal structure of MnCu was previously reported, and consists of bimetallic chains with octahedral Mn^II and square pyramidal Cu^II ions linked by oxamato bridges, MnCu and Mn_0.1Mg_0.9Cu being isostructural. The spectra of both MnCu and Mn_0.1Mg_0.9Cu show an important dichroism along the chain direction, due to the strong polarization of the Cu^II band at around 16 000 cm^-1 in this direction. They exhibit narrow and intense spin-forbidden Mn^II transitions in the 24000−25000 cm^-1 range, which are activated by an exchange mechanism. The polarization and thermal dependence of the ⁶A_1g → ⁴A_1g, ⁴E_g(G) Mn^II transitions were recorded. The polarization along the chain axis was interpreted in the framework of the pair mechanism first introduced by Tanabe and co-workers. A theoretical expression for the thermal dependence of the intensity was derived by considering the Cu^II spin as a quantum spin and the Mn^II spin as a classical spin, and compared with the experimental data. The interaction parameter between the local ground states has been found to be J = −25 cm^-1 using the spin Hamiltonian H = −J∑_i(S_Mn,is_Cu,i + S_Mn,i+1s_Cu,i). The spectra of Mn_0.1Mg_0.9Cu showed cold and hot bands, whose energy difference is directly related to J and the interaction parameter J* between the Cu^II ion in its ground state and the Mn^II ion in its spin-flip excited state. J* has been estimated to be +40 cm^-1. These results have been compared to those obtained with other Mn^IICu^II compounds. The complementarity between optical and magnetic properties has been discussed.