Abstract
W-band ({\nu} ca. 94 GHz) electron paramagnetic resonance (EPR) spectroscopy was used for a single-crystal study of a star-shaped Fe3Cr single-molecule magnet (SMM) with crystallographically imposed trigonal symmetry. The high resolution and sensitivity accessible with W-band EPR allowed us to determine accurately the axial zero-field splitting terms for the ground (S =6) and first two excited states (S =5 and S =4). Furthermore, spectra recorded by applying the magnetic field perpendicular to the trigonal axis showed a pi/6 angular modulation. This behavior is a signature of the presence of trigonal transverse magnetic anisotropy terms whose values had not been spectroscopically determined in any SMM prior to this work. Such in-plane anisotropy could only be justified by dropping the so-called 'giant spin approach' and by considering a complete multispin approach. From a detailed analysis of experimental data with the two models, it emerged that the observed trigonal anisotropy directly reflects the structural features of the cluster, i.e., the relative orientation of single-ion anisotropy tensors and the angular modulation of single-ion anisotropy components in the hard plane of the cluster. Finally, since high-order transverse anisotropy is pivotal in determining the spin dynamics in the quantum tunneling regime, we have compared the angular dependence of the tunnel splitting predicted by the two models upon application of a transverse field (Berry-phase interference).
Highlights
Polynuclear transition metal complexes provided, a couple of decades ago, unprecedented examples of individual magnetic molecules exhibiting a memory effect at low temperature.[1,2,3] Since the family of single-molecule magnets (SMMs) has grown considerably and includes complexes of lanthanides and actinides, as well as a number of mononuclear systems.[4,5,6,7] Polynuclear SMMs based on transition metals typically exhibit a large spin (S) ground state that stems from intramolecular superexchange interactions between the constituent metal ions and is accompanied by an easy-axis type anisotropy
Their relative intensity increases, suggesting that they originate from transitions within the first-excited spin multiplets with S = 5; at 20 K, the whole sets of 12 and 10 lines expected for the S = 6 and S = 5 states are observed
A first estimation of the axial component of the zfs for the different multiplets was made by plotting the resonance fields of the |MS → |MS + 1 transitions as a function of MS quantum number (Fig. 2) and using a giant-spin approach (GSA) based on the axial Hamiltonian, Hax = μB gzBzSz + DSz2 + B40O 04, (1)
Summary
Polynuclear transition metal complexes provided, a couple of decades ago, unprecedented examples of individual magnetic molecules exhibiting a memory effect at low temperature.[1,2,3] Since the family of single-molecule magnets (SMMs) has grown considerably and includes complexes of lanthanides and actinides, as well as a number of mononuclear systems.[4,5,6,7] Polynuclear SMMs based on transition metals typically exhibit a large spin (S) ground state that stems from intramolecular superexchange interactions between the constituent metal ions and is accompanied by an easy-axis type anisotropy. At fields where two levels would otherwise cross, level repulsion takes place, and the resulting tunnel splitting (TS) is directly related to the magnetization tunneling rate through the Landau-Zener-Stuckelberg formula.[12,13,14] The low-temperature magnetic properties of such systems are usually analyzed using a giant-spin approach (GSA). Within this formalism, only the ground spin multiplet is considered, and S is treated as an exact quantum number.
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