Spin dynamics study of magnetic molecular clusters by means of Mössbauer spectroscopy

Abstract

Spin dynamics of the two magnetic molecular clusters Fe4 and Fe8, with four and eight Fe͑III͒ ions, respectively, was studied by means of Mössbauer spectroscopy. The transition probabilities W's between the spin states of the ground multiplet were obtained from the fitting of the spectra. For the Fe4 cluster we found that, in the range from 1.38 to 77 K, the trend of W's versus the temperature corresponds to an Orbach's process involving an excited state with energy of about 160 K. For the Fe8, which, due to the presence of a low-energy excited state, could not be studied at temperatures greater than 20 K, the trend of W's in the range from 4 to 18 K seems to correspond to a direct process. The correlation functions of the magnetization were then calculated in terms of the W's. They have an exponential trend for the Fe4 cluster, while a small oscillating component is also present for the Fe8 cluster. For the first of the clusters, vs T ( is the decay time of the magnetization͒ has a trend which, at low temperatures (TϽ15 K), corresponds to an Arrhenius law with a potential barrier of the order of the energy difference between the states ͉M ͉ϭ5 and M ϭ0 (Ϸ7 K). Instead, for TϾ15 K, follows the trend of W Ϫ1 . For the Fe8, follows an Arrhenius law, but with a prefactor which is smaller than the one obtained susceptibility measurements.

Field-induced internal Fe and Ln spin reorientation in butterfly{Fe3LnO2}(LnBadía-Romano1, Bartolomé2, Bartolomé3 et al. 2013Phys. Rev. B222140View full textAdd to dashboardCite

“…We aim in this work to characterize its strength and anisotropy in two compounds in the series [Fe 3 Ln(μ 3 -O) 2 (CCl 3 COO) 8 (H 2 O)(THF) 3 ] (henceforth denoted {Fe 3 LnO 2 } here), which also show SMM behavior, when synthesized both with nonmagnetic lanthanides, such as Lu and the pseudolanthanide Y(III), and with magnetic Ln = Gd, Tb, Dy, and Ho. 20 SMM compounds, such as the Mn 12 acetate cluster [21][22][23][24][25][26][27][28] and the Fe 8 cluster, [29][30][31] have been exhaustively studied using the giant spin model. This model is based on a strong exchange approximation which consists of considering only the total spin of the molecule, i.e., coupling the spins of all the magnetic atoms of the core.…”

Spin dynamics and quantum tunneling in Fe8 nanomagnet and in AFM rings by NMR

“…The small values of AQ 0.13, -0.11 and 0.057 mm/», respectively, in Ref. [124] leads to the conclusion that the dipolar term would be too small to contribute to the main reduction of the local hyperGne fields but It is noted that even by taking the upper limit values for z, our estimation for the 3d derealization are still smaller than the values calculated with density functional theory (DFT)…”

NMR in oriented powder of Fe8 in zero and applied field

“…Let us describe the configuration of the magnetic electron in the covalent bond as 3d 5Àx 4s y : We can estimate the amount y of 4s hybridization from the isomer shift in M . ossbauer effect [3]. We obtain a value of y less than 0.05 which is too small to contribute a sizeable positive contact term.…”

“…The internal fields are consistent with the results of M . ossbauer spectroscopy [3]. The narrow NMR lines allow for the observation of a fine structure of the three groups of lines which could not be resolved in the M .…”