Inertial terms to magnetization dynamics in ferromagnetic thin films

Inertial magnetization dynamics have been predicted at ultrahigh speeds, or frequencies approaching the energy relaxation scale of electrons, in ferromagnetic metals. Here we identify inertial terms to magnetization dynamics in thin Ni$_{79}$Fe$_{21}$ and Co films near room temperature. Effective magnetic fields measured in high-frequency ferromagnetic resonance (115-345 GHz) show an additional stiffening term which is quadratic in frequency and $\sim$ 80 mT at the high frequency limit of our experiment. Our results extend understanding of magnetization dynamics at sub-picosecond time scales.Comment: 11 pages, 3 figure

High-Frequency Electron-Spin-Resonance Study of the Octanuclear Ferric Wheel CsFe8_8

High-frequency ($f$ = 190 GHz) electron paramagnetic resonance (EPR) at magnetic fields up to 12 T as well as Q-band ($f$ = 34.1 GHz) EPR were performed on single crystals of the molecular wheel CsFe$_8$. In this molecule, eight Fe(III) ions, which are coupled by nearest-neighbor antiferromagnetic (AF) Heisenberg exchange interactions, form a nearly perfect ring. The angle-dependent EPR data allow for the accurate determination of the spin Hamiltonian parameters of the lowest spin multiplets with $S \leq$ 4. Furthermore, the data can well be reproduced by a dimer model with a uniaxial anisotropy term, with only two free parameters $J$ and $D$. A fit to the dimer model yields $J$ = -15(2) cm$^{-1}$ and $D$ = -0.3940(8) cm$^{-1}$. A rhombic anisotropy term is found to be negligibly small, $E$ = 0.000(2) cm$^{-1}$. The results are in excellent agreement with previous inelastic neutron scattering (INS) and high-field torque measurements. They confirm that the CsFe$_8$ molecule is an excellent experimental model of an AF Heisenberg ring. These findings are also important within the scope of further investigations on this molecule such as the exploration of recently observed magnetoelastic instabilities.Comment: 21 pages, 8 figures, accepted for publication in Inorganic Chemistr

Approaching the Dirac point in high mobility multi-layer epitaxial graphene

Multi-layer epitaxial graphene (MEG) is investigated using far infrared (FIR) transmission experiments in the different limits of low magnetic fields and high temperatures. The cyclotron-resonance like absorption is observed at low temperature in magnetic fields below 50 mT, allowing thus to probe the nearest vicinity of the Dirac point and to estimate the conductivity in nearly undoped graphene. The carrier mobility is found to exceed 250,000 cm$^2$/(V.s). In the limit of high temperatures, the well-defined Landau level (LL) quantization is observed up to room temperature at magnetic fields below 1 T, a phenomenon unique in solid state systems. A negligible increase in the width of the cyclotron resonance lines with increasing temperature indicates that no important scattering mechanism is thermally activated, supporting recent expectations of high room-temperature mobilities in graphene.Comment: 5 pages, 3 figure

The intricate determination of magnetic anisotropy in quasi-octahedral vanadium(III): An HF-EPR and magnetic study

We report here the synthesis and a preliminary characterization of the tetranuclear complex of formula [Ga3V(LEt)2(dpm)6], Ga3VEt, in which H3LEt = 2-Ethyl-2-(hydroxymethyl)-propane-1,3-diol and Hdpm = dipivaloylmethane, containing a single paramagnetic vanadium(III) center, from a structural, magnetic, and spectroscopic point of view. Structural characterization by X-ray diffraction evidenced that this derivative is isostructural with the star-shaped Single-Molecule Magnet [Fe3V(LEt)2(dpm)6], Fe3VEt, and can, thus, be considered a model to analyze the magnetic anisotropy of the vanadium(III) ion in that system. The observed results confirm the complexity in obtaining a rationalization of the magnetic behavior of this metal ion, with magnetization data and High Field Electron Paramagnetic Resonance (HF-EPR) spectroscopy providing apparently conflicting results. Indeed, the former were rationalized assuming a rhombic distortion of the ligand field and a dominant easy-axis type anisotropy (equivalent to D ≈ −14.1 cm−1, E ≈ 1.2 cm−1), while a simple axial Spin Hamiltonian approach could explain HF-EPR data (|D| ≈ 6.98 cm−1)

Linewidth of single photon transitions in Mn12_{12}-acetate

We use time-domain terahertz spectroscopy to measure the position and linewidth of single photon transitions in Mn$_{12}$-acetate. This linewidth is compared to the linewidth measured in tunneling experiments. We conclude that local magnetic fields (due to dipole or hyperfine interactions) cannot be responsible for the observed linewidth, and suggest that the linewidth is due to variations in the anisotropy constants for different clusters. We also calculate a lower limit on the dipole field distribution that would be expected due to random orientations of clusters and find that collective effects must narrow this distribution in tunneling measurements.Comment: 5 pages, accepted to Physical Review

Decoherence-free molecular spin qubits with chemically designed frequencies

Resumen del trabajo presentado a la XII Reunión del grupo de física de la materia condensada de la RSEF (GEFES), celebrada en Salamanca del 1 al 3 de febrero de 2023.We report a sizeable quantum tunnelling splitting for the mononuclear Ni(II) molecular complexes [Ni(Me6tren)Cl](ClO4) (1) and [Ni(2-Imdipa)(NCS)](NCS) (2). With their S = 1 ground state and strong anisotropy, these molecules provide a realization of the simplest non-Kramers system (integer spin). The “clock transition” between levels associated with superpositions of mS = ±1 spin states, with its characteristic non-linear magnetic field dependence, has been directly monitored by heat capacity experiments. The comparison of complex 1 with a Co derivative (S = 3/2), for which tunnelling is forbidden, shows that the clock transition leads to an effective suppression of intermolecular spin–spin interactions. We also show that the splitting admits a chemical tuning via the modification of the ligand shell that determines the magnetic anisotropy. In particular, the weaker magnetic anisotropy of complex 2 makes its qubit frequency compatible with superconducting microwave circuits, and has allowed its direct detection by on-chip broadband transmission experiments.Peer reviewe

On the possibility of magneto-structural correlations: detailed studies of di-nickel carboxylate complexes

A series of water-bridged dinickel complexes of the general formula [Ni<sub>2</sub>(μ<sub>2</sub>-OH<sub>2</sub>)(μ2- O<sub>2</sub>C<sup>t</sup>Bu)<sub>2</sub>(O<sub>2</sub>C<sup>t</sup>Bu)2(L)(L0)] (L = HO<sub>2</sub>C<sup>t</sup>Bu, L0 = HO<sub>2</sub>C<sup>t</sup>Bu (1), pyridine (2), 3-methylpyridine (4); L = L0 = pyridine (3), 3-methylpyridine (5)) has been synthesized and structurally characterized by X-ray crystallography. The magnetic properties have been probed by magnetometry and EPR spectroscopy, and detailed measurements show that the axial zero-field splitting, D, of the nickel(ii) ions is on the same order as the isotropic exchange interaction, J, between the nickel sites. The isotropic exchange interaction can be related to the angle between the nickel centers and the bridging water molecule, while the magnitude of D can be related to the coordination sphere at the nickel sites

High resolution spectroscopy of methyltrioxorhenium: towards the observation of parity violation in chiral molecules

Originating from the weak interaction, parity violation in chiral molecules has been considered as a possible origin of the biohomochirality. It was predicted in 1974 but has never been observed so far. Parity violation should lead to a very tiny frequency difference in the rovibrational spectra of the enantiomers of a chiral molecule. We have proposed to observe this predicted frequency difference using the two photon Ramsey fringes technique on a supersonic beam. Promising candidates for this experiment are chiral oxorhenium complexes, which present a large effect, can be synthesized in large quantity and enantiopure form, and can be seeded in a molecular beam. As a first step towards our objective, a detailed spectroscopic study of methyltrioxorhenium (MTO) has been undertaken. It is an ideal test molecule as the achiral parent molecule of chiral candidates for the parity violation experiment. For the 187Re MTO isotopologue, a combined analysis of Fourier transform microwave and infrared spectra as well as ultra-high resolution CO2 laser absorption spectra enabled the assignment of 28 rotational lines and 71 rovibrational lines, some of them with a resolved hyperfine structure. A set of spectroscopic parameters in the ground and first excited state, including hyperfine structure constants, was obtained for the antisymmetric Re=O stretching mode of this molecule. This result validates the experimental approach to be followed once a chiral derivative of MTO will be synthesized, and shows the benefit of the combination of several spectroscopic techniques in different spectral regions, with different set-ups and resolutions. First high resolution spectra of jet-cooled MTO, obtained on the set-up being developed for the observation of molecular parity violation, are shown, which constitutes a major step towards the targeted objective.Comment: 20 pages, 6 figure

HF-EPR, Raman, UV/VIS Light Spectroscopic, and DFT Studies of the Ribonucleotide Reductase R2 Tyrosyl Radical from Epstein-Barr Virus

Epstein-Barr virus (EBV) belongs to the gamma subfamily of herpes viruses, among the most common pathogenic viruses in humans worldwide. The viral ribonucleotide reductase small subunit (RNR R2) is involved in the biosynthesis of nucleotides, the DNA precursors necessary for viral replication, and is an important drug target for EBV. RNR R2 generates a stable tyrosyl radical required for enzymatic turnover. Here, the electronic and magnetic properties of the tyrosyl radical in EBV R2 have been determined by X-band and high-field/high-frequency electron paramagnetic resonance (EPR) spectroscopy recorded at cryogenic temperatures. The radical exhibits an unusually low g1-tensor component at 2.0080, indicative of a positive charge in the vicinity of the radical. Consistent with these EPR results a relatively high C-O stretching frequency associated with the phenoxyl radical (at 1508 cm−1) is observed with resonance Raman spectroscopy. In contrast to mouse R2, EBV R2 does not show a deuterium shift in the resonance Raman spectra. Thus, the presence of a water molecule as a hydrogen bond donor moiety could not be identified unequivocally. Theoretical simulations showed that a water molecule placed at a distance of 2.6 Å from the tyrosyl-oxygen does not result in a detectable deuterium shift in the calculated Raman spectra. UV/VIS light spectroscopic studies with metal chelators and tyrosyl radical scavengers are consistent with a more accessible dimetal binding/radical site and a lower affinity for Fe2+ in EBV R2 than in Escherichia coli R2. Comparison with previous studies of RNR R2s from mouse, bacteria, and herpes viruses, demonstrates that finely tuned electronic properties of the radical exist within the same RNR R2 Ia class