Tunneling splitting of magnetic levels in Fe8 detected by 1H NMR cross relaxation

TL;DR

This study uses proton NMR to detect tunneling splitting in Fe8 molecules, revealing how quantum tunneling influences nuclear spin relaxation under varying magnetic fields.

Contribution

It provides the first experimental evidence of tunneling splitting effects on proton spin relaxation in Fe8, combining NMR measurements with theoretical modeling.

Findings

Enhanced 1/T1 relaxation rate over 2.5-5 T fields

Tunneling splitting matches proton Larmor frequency at certain fields

Distribution of tunneling splittings explains relaxation behavior

Abstract

Measurements of proton NMR and the spin lattice relaxation rate 1/T1 in the octanuclear iron (III) cluster [Fe8(N3C6H15)6O2(OH)12][Br8 9H2O], in short Fe8, have been performed at 1.5 K in a powder sample aligned along the main anisotropy z axis, as a function of a transverse magnetic field (i.e., perpendicular to the main easy axis z). A big enhancement of 1/T1 is observed over a wide range of fields (2.5-5 T), which can be attributed to the tunneling dynamics; in fact, when the tunneling splitting of the pairwise degenerate m=+-10 states of the Fe8 molecule becomes equal to the proton Larmor frequency a very effective spin lattice relaxation channel for the nuclei is opened. The experimental results are explained satisfactorily by considering the distribution of tunneling splitting resulting from the distribution of the angles in the hard xy plane for the aligned powder, and the results of the direct diagonalization of the model Hamiltonian.