1. Mössbauer Spectroscopy under Magnetic Field to Explore the Low Temperature Spin Structure in a Molecular Layered Ferrimagnet A. Bhattacharjee, P. Gütlich et al. Department of Physics, Visva-Bharati University, Santiniketan 731235, India, E-mail: ashis@vbphysics.net.inashis@vbphysics.net.in Department of Chemistry, University of Mainz, 55099, Mainz, Germany, E-mail: guetlich@uni-mainz.de

2. {N(n-C 4 H 9 ) 4 [Fe II Fe III (C 2 O 4 ) 3 ]} A Molecular Ferrimagnet The oxalato-bridged layered ferrimagnet {N(n-C 4 H 9 ) 4 [Fe II Fe III (C 2 O 4 ) 3 ]} exhibits the rarely occurring negative magnetization phenomenon below 30 K. Heat capacity calorimetry of this compound under zero magnetic field detected the ferrimagnetic transition at 43.3 K and a small heat capacity anomaly around 16.3 K. Results from the magnetic field dependent heat capacity study of this material indicated the existence of different spin structures at low temperatures. An ac magnetic susceptibility measurement of this compound demonstrated the existence of a spin glass-like state below 30 K. Mössbauer spectroscopy is a powerful tool for studying magnetic systems with complicated magnetic structure, as this method offers the possibility to observe local spin configurations of different types as well as provides the quantitative estimation of different spin sites in a solid lattice. We have attempted to understand the origin of the spin glass state in the present compound at low temperatures with the help of 57 Fe Mössbauer spectroscopy under high magnetic field. For further details on this research problem see: Bhattacharjee, et al., J. Phys. Soc. Jpn. 68 (1999) 1679. Bhattacharjee, et al., J. Phys. Soc. Jpn. 69 (2000) 479. Bhattacharjee, et al., Solid State Commun. 113 (2000) 543. Bhattacharjee, et al., J. Phys. Soc. Jpn. 71 (2002) 2263. Bhattacharjee, et al., J. Phys.: Condens. Matter 15 (2003) 5103.

3. Mössbauer Spectroscopy of {N(n-C 4 H 9 ) 4 [Fe II Fe III (C 2 O 4 ) 3 ]} Bhattacharjee, et al., J. Phys.: Condens. Matter 15 (2003) 5103.

5. Observation Mössbauer spectroscopy of the molecular ferrimagnet - {N(n-C 4 H 9 ) 4 [Fe II Fe III (C 2 O 4 ) 3 ]} under high external magnetic field at 4.2 K successfully detected different types of Fe II and Fe III high spin sites in the lattices. The hyperfine field values estimated for all the Fe III high spin sites are close to the expected values, whereas those for Fe II high spin sites are substantially different than usually expected. A large fraction of magnetically disordered Fe III high spin sites were found. It is understood that the layered structure of the compound may have two kinds of sub-lattices as (Fe II A -ox-Fe III A -...) and (Fe II B -ox-Fe III B -...) along with randomly placed magnetically disordered Fe III C sites, where the Fe III A and Fe III B are antiferromagnetically aligned. Existence of the magnetically and/or structurally non-equivalent spin sites in the same lattice has been held responsible for the coexistence of different ferrimagnetic interactions leading to the spin glass state, as observed through earlier magnetic and calorimetric measurements. Schematic ferrimagnetic arrangement of spins at different sites along c-axis in the lattice. (solid arrow: Fe III spin, S = 5/2; dashed arrow: Fe II spin, S = 2).