Field-induced slow relaxation in a monometallic manganese(III) single-molecule magnet

High-field electron paramagnetic resonance spectroscopy shows that the structurally distorted MnIII ion in Na5[Mn(l-tart)2]·12H2O (1; l-tart = l-tartrate) has a significant negative axial zero-field splitting and a small rhombic anisotropy (∼1% of D). Alternating-current magnetic susceptibility measurements demonstrate that 1, which contains isolated MnIII centers, displays slow relaxation of its magnetization under an applied direct-current magnetic field

Electron-nuclear decoupling at a spin clock transition

The ability to design quantum systems that decouple from environmental noise sources is highly desirable for development of quantum technologies with optimal coherence. The chemical tunability of electronic states in magnetic molecules combined with advanced electron spin resonance techniques provides excellent opportunities to address this problem. Indeed, so-called clock transitions have been shown to protect molecular spin qubits from magnetic noise, giving rise to significantly enhanced coherence. Here we conduct a spectroscopic and computational investigation of this physics, focusing on the role of the nuclear bath. Away from the clock transition, linear coupling to the nuclear degrees of freedom causes a modulation and decay of electronic coherence, as quantified via electron spin echo signals generated experimentally and in silico. Meanwhile, the effective hyperfine interaction vanishes at the clock transition, resulting in electron-nuclear decoupling and an absence of quantum information leakage to the nuclear bath, providing opportunities to characterize other decoherence sources.The ability to design quantum systems that decouple from environmental noise sources is highly desirable for development of quantum technologies with optimal coherence. The chemical tunability of electronic states in magnetic molecules combined with advanced electron spin resonance techniques provides excellent opportunities to address this problem. Indeed, so-called clock transitions have been shown to protect molecular spin qubits from magnetic noise, giving rise to significantly enhanced coherence. Here we conduct a spectroscopic and computational investigation of this physics, focusing on the role of the nuclear bath. Away from the clock transition, linear coupling to the nuclear degrees of freedom causes a modulation and decay of electronic coherence, as quantified via electron spin echo signals generated experimentally and in silico. Meanwhile, the effective hyperfine interaction vanishes at the clock transition, resulting in electron-nuclear decoupling and an absence of quantum information leakage to the nuclear bath, providing opportunities to characterize other decoherence sources

Substituent effects on exchange anisotropy in single- and multiorbital organic radical magnets

The contribution of heavy-atom substituents to the overall spin-orbit interaction in two classes of organic radical molecular magnets is discussed. In “single-orbital” radicals, spin-orbit coupling (SOC) effects are well described with reference to pairwise anisotropic exchange interactions between singly occupied spin-bearing orbitals on neighboring molecules; anisotropy requires the presence of spin density on heavy-atom sites with principal quantum number n > 3. In “multiorbital” radicals, SOC involving virtual orbitals also contributes to anisotropic exchange and, as a result, the presence of heavy (n > 3) atoms in formally non-spin-bearing sites can enhance pseudodipolar ferromagnetic interaction terms. To demonstrate these effects, ferromagnetic and antiferromagnetic resonance spectroscopies have been used to probe the exchange anisotropy in two organic magnets, one a “single-orbital” ferromagnet, the other a “multiorbital” spin-canted antiferromagnet, both of which contain a heavy-atom iodine (n = 5) substituent. While the symmetry of the singly occupied molecular orbital in both radicals precludes spin-orbit contributions from iodine to the overall exchange anisotropy, the symmetry and energetically low-lying nature of the lowest unoccupied molecular orbital in the latter allows for appreciable spin density at the site of iodine substitution and, hence, a large exchange anisotropy.peerReviewe

Strong Electronic and Magnetic Coupling in M4 (M = Ni, Cu) Clusters via Direct Orbital Interactions between Low-Coordinate Metal Centers.

We present an extensive study of tetranuclear transition-metal cluster compounds M4(NPtBu3)4 and [M4(NPtBu3)4][B(C6F5)4] (M = Ni, Cu; tBu = tert-butyl), which feature low-coordinate metal centers and direct metal-metal orbital overlap. X-ray diffraction, electrochemical, magnetic, spectroscopic, and computational analysis elucidate the nature of the bonding interactions in these clusters and the impact of these interactions on the electronic and magnetic properties. Direct orbital overlap results in strongly coupled, large-spin ground states in the [Ni4(NPtBu3)4]+/0 clusters and fully delocalized, spin-correlated electrons. Correlated electronic structure calculations confirm the presence of ferromagnetic ground states that arise from direct exchange between magnetic orbitals, and, in the case of the neutral cluster, itinerant electron magnetism similar to that in metallic ferromagnets. The cationic nickel cluster also possesses large magnetic anisotropy exemplified by a large, positive axial zero-field splitting parameter of D = +7.95 or +9.2 cm-1, as determined by magnetometry or electron paramagnetic resonance spectroscopy, respectively. The [Ni4(NPtBu3)4]+ cluster is also the first molecule with easy-plane magnetic anisotropy to exhibit zero-field slow magnetic relaxation, and under a small applied field, it exhibits relaxation exclusively through an Orbach mechanism with a spin relaxation barrier of 16 cm-1. The S = 1/2 complex [Cu4(NPtBu3)4]+ exhibits slow magnetic relaxation via a Raman process on the millisecond time scale, supporting the presence of slow relaxation via an Orbach process in the nickel analogue. Overall, this work highlights the unique electronic and magnetic properties that can be realized in metal clusters featuring direct metal-metal orbital interactions between low-coordinate metal centers

Analysis of vibronic coupling in a 4f molecular magnet with FIRMS

Vibronic coupling, the interaction between molecular vibrations and electronic states, is a pervasive effect that profoundly affects chemical processes. In the case of molecular magnetic materials, vibronic, or spin-phonon, coupling leads to magnetic relaxation, which equates to loss of magnetic memory and loss of phase coherence in molecular magnets and qubits, respectively. The study of vibronic coupling is challenging, and most experimental evidence is indirect. Here we employ far-infrared magnetospectroscopy to probe vibronic transitions in in [Yb(trensal)] (where H3trensal = 2,2,2-tris(salicylideneimino)trimethylamine). We find intense signals near electronic states, which we show arise due to an “envelope effect” in the vibronic coupling Hamiltonian, and we calculate the vibronic coupling fully ab initio to simulate the spectra. We subsequently show that vibronic coupling is strongest for vibrational modes that simultaneously distort the first coordination sphere and break the C3 symmetry of the molecule. With this knowledge, vibrational modes could be identified and engineered to shift their energy towards or away from particular electronic states to alter their impact. Hence, these findings provide new insights towards developing general guidelines for the control of vibronic coupling in molecules.</p

Analysis of vibronic coupling in a 4f molecular magnet with FIRMS

OpenMOLCAS and Gaussian output files, FIRMS data (simulated and experimental), and luminescence data.For details of the methodology, see the "Methods" section of the associated paper

Self-assembly of a mixed-valence Fe^II-Fe^IIItetranuclear star

A self-assembled mixed-valence FeII–FeIII tetranuclear star is reported that shows ferromagnetic coupling, field-induced single molecule magnetism and strong magnetic anisotropy at the peripheral FeII centres.</p