“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Mössbauer Spectroscopy underMagnetic 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.in

Department of Chemistry, University of Mainz, 55099, Mainz, Germany, E-mail: guetlich@uni-mainz.de

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

{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 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 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.

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Mössbauer Spectroscopy of {N(n-C4H9)4[FeIIFeIII(C2O4)3]}

-12 -8 -4 0 4 8 12

92

94

96

98

100

FeII

Rel

. Tra

nsm

issi

on (

%)

v (mm s-1)

FeIII

H = 0 TeslaT = 4.2 K

-12 -8 -4 0 4 8 1292

94

96

98

100

FeIII

B

FeIII

A

FeIII

C FeII

B

FeIIA

Rel

. Tra

nsm

issi

on (

%)

v (mm s-1)

H = 5 TeslaT = 4.2 K

Bhattacharjee, et al., J. Phys.: Condens. Matter 15 (2003) 5103.

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

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 (FeII

A-ox-FeIIIA-...)

and (FeIIB-ox-FeIII

B-...) along with randomly placed magnetically disordered FeIII

C sites, where the FeIII

A 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).