A. Bhattacharjee, P. Gütlich et al.

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

2. {N(n-C4H9)4[FeIIFeIII(C2O4)3]}A Molecular Ferrimagnet The oxalato-bridged layered ferrimagnet {N(n-C4H9)4[FeIIFeIII(C2O4)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 compounddemonstrated 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 compoundat low temperatures with the help of 57Fe 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-C4H9)4[FeIIFeIII(C2O4)3]} Bhattacharjee, et al., J. Phys.: Condens. Matter 15 (2003) 5103.

5. Observation Mössbauer spectroscopy of the molecular ferrimagnet - {N(n-C4H9)4[FeIIFeIII(C2O4)3]} under high external magnetic field at 4.2 K successfully detected different types of FeII and FeIII high spin sites in the lattices. The hyperfine field values estimated for all the FeIII high spin sites are close to the expected values, whereas those for FeII high spin sites are substantially different than usually expected. A large fraction of magnetically disordered FeIII high spin sites were found. It is understood that the layered structure of the compound may have two kinds of sub-lattices as (FeIIA-ox-FeIIIA-...) and (FeIIB-ox-FeIIIB-...) along with randomly placed magnetically disordered FeIIIC sites, where the FeIIIA and FeIIIB 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: FeIII spin, S = 5/2; dashed arrow: FeII spin, S = 2).