Abstract
Bimetallic oxalates are a class of layered molecule-based magnets with transition-metal ions M(II) and M'(III) coupled by oxalate molecules (C2O4)-2 in an open honeycomb structure. Magnetic compensation (MC) has been observed in ferrimagnetic Fe(II)Fe(III) compounds with certain cations between the bimetallic layers. This behavior can be explained [1] by considering the C3-symmetric crystal field produced by the six oxygen atoms surrounding each Fe ion, which splits the L = 2, 3d6 multiplet on the Fe(II) sites into two doublets and one singlet. MC occurs when the doublet lies lowest in energy and carries an orbital angular momentum Lz between about 0.25 and 1.0. Because the low-energy doublet is half-filled, a Jahn-Teller (JT) distortion may break the C3 symmetry near the ferrimagnetic transition temperature. In the absence of spin-orbit coupling on the Fe(II) sites, the JT distortion would always occur at T = 0. However, due to the competition between the spin-orbit coupling and JT energies, the JT distortion disappears at low temperatures in compounds that display MC [2]. Comparison is made with recent experiments and predictions are made for controlling the MC and JT critical temperatures. Research sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy under contract with UT-Battelle, LLC.
[1] R.S. Fishman and F. Reboredo, “Giant Negative Magnetization in a Layered Organic Magnet,” Physical Review Letters 99 (2007) 217203.
[2] R.S. Fishman, S. Okamoto, and F.A. Reboredo, “Inverse Jahn-Teller Transition in Bimetallic Oxalates,” Physical Review Letters 101, (2008) 116402; “Spin-orbit Coupling and Jahn-Teller Distortion in Bimetallic Oxalates,” Polyhedron 28 (2009) 1740-1745.
