Rapid and precise determination of zero-field splittings by terahertz time-domain electron paramagnetic resonance spectroscopy

Tracking nuclear motion in single-molecule magnets using femtosecond X-ray absorption spectroscopy

The development of new data storage solutions is crucial for emerging digital technologies. Recently, all-optical magnetic switching has been achieved in dielectrics, proving to be faster than traditional methods. Despite this, single-molecule magnets (SMMs), which are an important class of magnetic materials due to their nanometre size, remain underexplored for ultrafast photomagnetic switching. Herein, we report femtosecond time-resolved K-edge X-ray absorption spectroscopy (TR-XAS) on a Mn(III)-based trinuclear SMM. Exploiting the elemental specificity of XAS, we directly track nuclear dynamics around the metal ions and show that the ultrafast dynamics upon excitation of a crystal-field transition are dominated by a magnetically active Jahn-Teller mode. Our results, supported by simulations, reveal minute bond length changes from 0.01 to 0.05 Å demonstrating the sensitivity of the method. These geometrical changes are discussed in terms of magneto-structural relationships and consequently our results illustrate the importance of TR-XAS for the emerging area of ultrafast molecular magnetism.

Optical-pump-terahertz-probe spectroscopy in high magnetic fields with kHz single-shot detection

We demonstrate optical pump-THz probe (OPTP) spectroscopy with a variable external magnetic field (0-9 T), in which the time-dependent THz signal is measured by echelon-based single-shot detection at a repetition rate of 1 kHz. The method reduces data acquisition times by more than an order of magnitude compared to conventional electro-optic sampling using a scanning delay stage. The approach illustrates the wide applicability of the single-shot measurement approach to non-equilibrium systems that are studied through OPTP spectroscopy, especially in cases where parameters such as magnetic field strength (B) or other experimental parameters are varied. We demonstrate the capabilities of our measurement by performing cyclotron resonance experiments in bulk silicon, where we observe B-field-dependent carrier relaxation and distinct relaxation rates for different carrier types. We use a pair of economical linear array detectors to measure 500 time points on each shot, offering an equivalent performance to camera-based detection with possibilities for higher repetition rates.

Limitations on the D-Parameter in Ni(II) Complexes

A number of hexacoordinate, pentacoordinate, and tetracoordinate Ni(II) complexes have been investigated by applying ab initio CASSCF + NEVPT2 + SOC calculations and Generalized Crystal Field Theory. The geometry of the coordination polyhedron covers D4h, D3h, D2h, D2d, C4v, C3v, and C2v symmetry. The calculated spin-Hamiltonian parameters D and E were compared to the available experimental data. The limiting values of the D-parameter in the class of Ni(II) complexes are identified. Magnetic anisotropy in Ni(II) complexes, expressed by the axial zero-field splitting parameter D, seriously depends upon the ground and first excited electronic states. In hexacoordinate complexes, the ground electronic term is nondegenerate 3B1g for the D4h symmetry; D is slightly positive or negative. In tetracoordinate systems, D is only positive when the electronic ground state is nondegenerate 3A or 3B; this diverges on the τ4 path when oblate bisphenoid approaches the prolate geometry and a level crossing with 3E occurs. In pentacoordinate systems, D could be extremely negative when approaching a trigonal bipyramid (Addison index τ5 ∼ 1, ground state 3E″). In pentacoordinate Ni(II) complexes with the D3h and C3v symmetry of the coordination polyhedron, the ground electronic term is orbitally doubly degenerate which causes the D-parameter stays undefined. It is emphasized that one has to inspect compositions of the spin-orbit multiplets from the spin states |MS⟩ and check whether the weights confirm the expected spin-Hamiltonian picture: with D > 0, the ground state contains a dominant part of |0⟩ (close to 100%) whereas with D < 0 the spin-orbit doublet is formed of |±1⟩ with high weights (approaching 50 + 50%). The calculations show that the situations are not black and white, and the mixing of the states might be more complex especially when the rhombic zero-field splitting parameter E is in the play. In the case of the 3E ground term, six spin-orbit multiplets are formed by mixing six |MS⟩ states from the ground and quasi-degenerate excited states.

Applying Unconventional Spectroscopies to the Single-Molecule Magnets, Co(PPh3 )2 X2 (X=Cl, Br, I): Unveiling Magnetic Transitions and Spin-Phonon Coupling

Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single-molecule magnets (SMMs). Spin-phonon coupling (interactions of magnetic levels with phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin-phonon coupling in molecules is challenging. We have found that far-IR magnetic spectra (FIRMS) of Co(PPh₃ )₂ X₂ (Co-X; X=Cl, Br, I) reveal rarely observed spin-phonon coupling as avoided crossings between magnetic and u-symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero-field split (ZFS) levels of the S=3/2 electronic ground state were probed by INS, high-frequency and -field EPR (HFEPR), FIRMS, and frequency-domain FT terahertz EPR (FD-FT THz-EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) and g values. Ligand-field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities in Co-X, showing that the larger Co(II) spin density in a molecule, the larger its ZFS magnitude. The current work reveals dynamics of magnetic and phonon excitations in SMMs. Studies of such couplings in the future would help to understand how spin-phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling.

Broadband Tunable Electron Paramagnetic Resonance Spectroscopy of Dilute Metal Complexes

Analysis of the electron paramagnetic resonance (EPR) of transition ion complexes requires data taken at different microwave frequencies because the spin Hamiltonian contains operators linear in the frequency as well as operators independent of the frequency. In practice, data collection is hampered by the fact that conventional EPR spectrometers have always been designed to operate at a single frequency. Here, a broadband instrument is described and tested that operates from 0.5 to 12 GHz and whose sensitivity approaches that of single-frequency spectrometers. Multifrequency EPR from triclinic substitutional (0.5%) Cu(II) in ZnSO₄ is globally analyzed to illustrate a novel approach to reliable determination of the molecular electronic structure of transition ion complexes from field-frequency 2D data sets.

A Trigonal Prismatic Cobalt(II) Complex as a Single Molecule Magnet with a Reduced Contribution from Quantum Tunneling

Herein, we report a new trigonal prismatic cobalt(II) complex that behaves as a single molecule magnet. The obtained zero-field splitting, which is also directly accessed by THz-EPR spectroscopy (-102.5 cm^-1 ), results in a large magnetization reversal barrier U of 205 cm^-1 . Its effective value, however, is much lower (101 cm^-1 ), even though there is practically no contribution from quantum tunneling to magnetization relaxation.

Ultra-broadband EPR spectroscopy in field and frequency domains

Electron paramagnetic resonance (EPR) is a powerful technique to investigate the electronic and magnetic properties of a wide range of materials.

A new series of tetrahedral Co(ii) complexes [CoLX2] (X = NCS, Cl, Br, I) manifesting single-ion magnet features

The influence of ligand field strength on the magnetic anisotropy of a series of isostructural tetrahedral Co^II complexes has been investigated by using a combined experimental and theoretical approach.