Quantum Phase Interference and Parity Effects in Magnetic Molecular Clusters

Abstract

An experimental method based on the Landau-Zener model was developed to measure very small tunnel splittings in molecular clusters of eight iron atoms, which at low temperature behave like a nanomagnet with a spin ground state of S = 10. The observed oscillations of the tunnel splittings as a function of the magnetic field applied along the hard anisotropy axis are due to topological quantum interference of two tunnel paths of opposite windings. Transitions between quantum numbers M = -S and (S - n), with n even or odd, revealed a parity effect that is analogous to the suppression of tunneling predicted for half-integer spins. This observation is direct evidence of the topological part of the quantum spin phase (Berry phase) in a magnetic system.

Cyanide-bridged Fe(iii)–Co(ii) bis double zigzag chains with a slow relaxation of the magnetisation

“…each pair of adjacent M atoms is connected through iron [Fe (2)]. A cyano group bridges each Fe-M pair.…”

Many-spin effects and tunneling splittings in Mn12 magnetic molecules

“…A number of impressive experimental results have been obtained recently, such as thermally assisted tunneling [2,3], ground state-to-ground state tunneling [4,6] and topological phase effects in spin tunneling [4]. Among others, the molecular magnet 12 ) has received special attention.…”

“…The basic prerequisite for solving this problem is our ability to evaluate accurately and reliably the energy splittings occurring as a result of tunneling between two (quasi) degenerate levels [7]. At present, carefully designed magnetic relaxation experiments at low and ultralow temperatures (tens or hundreds of millikelvin) can detect [2,3] the changes in relaxation time caused by the splittings of order 10 À2 -10 À4 K; and even smaller [4], of order 10 À6 -10 À7 K: The relaxation time data obtained in these experiments give information (although indirect) about the splitting values, so that predictions of the theoretical models can be compared with experimental results.…”

Calculations of the onset temperature for tunneling in multispin systems

“…However, this method is not general and can only be applied to simple model systems. High-order anisotropy terms, such as (Ŝ 2 + +Ŝ 2 − ) and (Ŝ 4 + +Ŝ 4 − ), have not been incorporated in the mapping approach [15], but can significantly affect the tunneling rate [5,16]. So far, systems with high-order anisotropy have only been studied numerically by direct diagonalization of the Hamiltonian [17,18].…”