High-frequency Electron Paramagnetic Resonance Spectroscopy Research Articles

Engineering Clock Transitions in Molecular Lanthanide Complexes.

Molecular lanthanide (Ln) complexes are promising candidates for the development of next-generation quantum technologies. High-symmetry structures incorporating integer spin Ln ions can give rise to well-isolated crystal field quasi-doublet ground states, i.e., quantum two-level systems that may serve as the basis for magnetic qubits. Recent work has shown that symmetry lowering of the coordination environment around the Ln ion can produce an avoided crossing or clock transition within the ground doublet, leading to significantly enhanced coherence. Here, we employ single-crystal high-frequency electron paramagnetic resonance spectroscopy and high-level ab initio calculations to carry out a detailed investigation of the nine-coordinate complexes, [HoIIIL1L2], where L1 = 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraaza-cyclododecane and L2 = F- (1) or [MeCN]0 (2). The pseudo-4-fold symmetry imposed by the neutral organic ligand scaffold (L1) and the apical anionic fluoride ion generates a strong axial anisotropy with an mJ = ±8 ground-state quasi-doublet in 1, where mJ denotes the projection of the J = 8 spin-orbital moment onto the ∼C4 axis. Meanwhile, off-diagonal crystal field interactions give rise to a giant 116.4 ± 1.0 GHz clock transition within this doublet. We then demonstrate targeted crystal field engineering of the clock transition by replacing F- with neutral MeCN (2), resulting in an increase in the clock transition frequency by a factor of 2.2. The experimental results are in broad agreement with quantum chemical calculations. This tunability is highly desirable because decoherence caused by second-order sensitivity to magnetic noise scales inversely with the clock transition frequency.

Magneto-optical and high-frequency electron paramagnetic resonance spectroscopy of Er3+ ions in CaMoO4 single crystal

We present the optical and magneto-optical spectroscopy and electron paramagnetic resonance (EPR) investigations of CaMoO4 single crystals doped with the erbium ions. Telecom-wavelength resonance transition inhomogeneous line width of Er3+ is relatively narrow for oxide crystals which makes this material promising for quantum technologies applications. The hyperfine structure in optical spectra of 167Er3+ isotope is well resolved. Energies and symmetries of wavefunctions of 39 energy levels of Er3+ ions in the crystal-field (CF) of S4 symmetry and g-factors of some CF Kramers doublets were measured and successfully simulated on the basis of CF calculations. The obtained set of CF parameters was used for modeling the hyperfine structure profiles observed in the optical absorption spectra.

Synergy of Magnetic Anisotropy and Ferromagnetic Interaction Triggering a Dimeric Cr(II) Zero-Field Single-Molecule Magnet

A novel CrII-dimeric complex, [CrIIN(SiiPr3)2(μ-Cl)(THF)]2 (1), has been successfully constructed using a bulky silyl-amide ligand. Single-crystal structure analysis reveals that complex 1 exhibits a binuclear motif, with a Cr2Cl2 rhombus core, where two equivalent tetra-coordinate CrII centers in the centrosymmetric unit display quasi-square planar geometry. The crystal structure has been well simulated and explored by density functional theory calculations. The axial zero-field splitting parameter (D < 0) with a small rhombic (E) value is unambiguously determined by systematic investigations of magnetic measurements, high-frequency electron paramagnetic resonance spectroscopy, and ab initio calculations. Remarkably, ac magnetic susceptibility data unveil that 1 features slow dynamic magnetic relaxation typical of single-molecule magnet behavior with Ueff = 22 K in the absence of a dc field. This increases up to 35 K under a corresponding static field. Moreover, magnetic studies and theoretical calculations point out that a non-negligible ferromagnetic coupling (FMC) exists in the dimeric Cr-Cr units of 1. The coexistence of magnetic anisotropy and FMC contributes to the first case of CrII-based single-molecule magnets (SMMs) under zero dc field.

High-Frequency EPR and ENDOR Spectroscopy of Mn2+ Ions in CdSe/CdMnS Nanoplatelets.

Semiconductor colloidal nanoplatelets based of CdSe have excellent optical properties. Their magneto-optical and spin-dependent properties can be greatly modified by implementing magnetic Mn2+ ions, using concepts well established for diluted magnetic semiconductors. A variety of magnetic resonance techniques based on high-frequency (94 GHz) electron paramagnetic resonance in continuous wave and pulsed mode were used to get detailed information on the spin structure and spin dynamics of Mn2+ ions in core/shell CdSe/(Cd,Mn)S nanoplatelets. We observed two sets of resonances assigned to the Mn2+ ions inside the shell and at the nanoplatelet surface. The surface Mn demonstrates a considerably longer spin dynamics than the inner Mn due to lower amount of surrounding Mn2+ ions. The interaction between surface Mn2+ ions and 1H nuclei belonging to oleic acid ligands is measured by means of electron nuclear double resonance. This allowed us to estimate the distances between the Mn2+ ions and 1H nuclei, which equal to 0.31 ± 0.04, 0.44 ± 0.09, and more than 0.53 nm. This study shows that the Mn2+ ions can serve as atomic-size probes for studying the ligand attachment to the nanoplatelet surface.

High-frequency EPR spectroscopy of paramagnetic manganese centers in GaAs : Mn crystals

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs : Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+-SH complex, which is a manganese ion with spin S=5/2, which replaces gallium (MnGa2+) and an ionized acceptor (A-) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J=3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2- replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the &amp;lt;111&amp;gt; axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+-SH complex. Keywords: high-frequency EPR, GaAs crystal, Mn acceptor, shallow hole, exchange interaction.

High-frequency EPR spectroscopy of paramagnetic manganese centers in GaAs : Mn crystals

High-frequency electron paramagnetic resonance (EPR) is used to study the unique properties of manganese centers in a GaAs : Mn crystal in strong magnetic fields at low temperatures. At frequencies of 94 and 130 GHz, EPR transitions were recorded in the MnGa2+-SH complex, which is a manganese ion with spin S=5/2, which replaces gallium (MnGa2+) and an ionized acceptor (A-) associated via an isotropic antiferromagnetic exchange interaction with a shallow hole (SH) with angular momentum J=3/2. A complex system of energy levels of this complex in a magnetic field and the possibility of accurately determining exchange interactions from EPR spectra are analyzed. Another complex was investigated, in which an ionized acceptor MnGa2+ interacts with a localized hole center in the form of a diamagnetic ion O2- replacing As. This complex, MnGa2+-OAs2-, is characterized by axial symmetry along the &lt;111&gt; axis of the cubic GaAs crystal and an anisotropic EPR spectrum. Due to the high Boltzmann factor, in our studies, the order of the fine structure spin levels of this complex was determined. The effect of the Boltzmann populations of the energy levels on the high-frequency EPR spectra was also demonstrated for the MnGa2+-SH complex. Keywords: high-frequency EPR, GaAs crystal, Mn acceptor, shallow hole, exchange interaction.

Wide-band EPR spectroscopy and relaxation study of Tm3+ ions in PbGa2S4 crystal

The PbGa2S4: Tm3+ crystal was investigated by tunable high-frequency EPR spectroscopy. Two thulium centers located in positions with different local symmetry were observed in the frequency range of 9.8–170 GHz. Both spectra with the resolved hyperfine structure correspond to the transition between sublevels of the main quasi-doublet Tm3+ with crystal field splitting of 6.93 GHz and 1.42 GHz. The times of phase memory and spin-lattice relaxation of thulium ions were measured by the spin-echo method for various values of the magnetic field.

Iron(II), Cobalt(II), and Nickel(II) Complexes of Bis(sulfonamido)benzenes: Redox Properties, Large Zero-Field Splittings, and Single-Ion Magnets.

Metal complexes of 1,2-diamidobenzenes have been long studied because of their intriguing redox properties and electronic structures. We present here a series of such complexes with 1,2-bis(sulfonamido)benzene ligands to probe the utility of these ligands for generating a large zero-field splitting (ZFS, D) in metal complexes that possibly act as single-ion magnets. To this end, we have synthesized a series of homoleptic ate complexes of the form (X)n[M{bis(sulfonamido)benzene}2] (n equals 4 minus the oxidation state of the metal), where M (Fe/Co/Ni), X [K+/(K-18-c-6)+/(HNEt3)+, with 18-c-6 = 18-crown ether 6], and the substituents (methyl and tolyl) on the ligand [bmsab = 1,2-bis(methanesulfonamido)benzene; btsab = 1,2-bis(toluenesulfonamido)benzene] were varied to analyze their effect on the ZFS, possible single-ion-magnet properties, and redox behavior of these metal complexes. A combination of X-ray crystallography, (spectro)electrochemistry, superconducting quantum interference device magnetometry, high-frequency electron paramagnetic resonance spectroscopy, and Mössbauer spectroscopy was used to investigate the electronic/geometric structures of these complexes and the aforementioned properties. These investigations show that the cobalt(II) complexes display very high negative D values in the range of -100 to -130 cm-1, and the nickel(II) complexes display very high positive D values of 76 and 58 cm-1. In addition, the cobalt(II) complexes shows barriers of 200-260 cm-1 and slow relaxation of the magnetization in the absence of an external magnetic field, underscoring the robustness of this class of complexes. The iron(II) complex exhibits a D value of -3.29 cm-1 and can be chemically oxidized to an iron(III) complex that has D = -1.96 cm-1. These findings clearly show that bis(sulfonamido)benzenes are ideally suited to stabilize ate complexes, to generate very high ZFSs at the metal centers with single-ion-magnet properties, and to induce exclusive oxidation at the metal center (for iron) despite the presence of ligands that are potentially noninnocent. Our results therefore substantially enhance the scope for this class of redox-active ligands.

Playing with Magnetic Anisotropy in Hexacoordinated Mononuclear Ni(II) Complexes, An Interplay Between Symmetry and Geometry

The magnetic anisotropy parameters of a hexacoordinate trigonally elongated Ni(II) complex with symmetry close to D3d are measured using field-dependent magnetization and High-Field and High-Frequency EPR spectroscopy (D = + 2.95 cm−1, |E/D| = 0.08 from EPR). Wave function based theoretical calculations reproduce fairly well the EPR experimental data and allows analysing the origin of the magnetic anisotropy of the complex. Calculations on model complexes allows getting insight into the origin of the large increase in the axial magnetic anisotropy (D) when the complex is brought to a prismatic geometry with a symmetry close to D3h.

Zero-Field Splitting Parameters of Hemin Investigated by High-Frequency and High-Pressure Electron Paramagnetic Resonance Spectroscopy

We systematically studied the zero-field splitting (ZFS) parameters of Fe(III) protoporphyrin IX chloride, or hemin, using the terahertz electron paramagnetic resonance (EPR) spectroscopy technique at ambient and high pressures. Although hemin is known as a model substance of hemoproteins, the pressure effect on the electronic structure has not yet been explored owing to the large ZFS. In this study, high-field and high-frequency EPR measurements were carried out in the frequency range up to 700 GHz and at hydrostatic pressures up to 2 GPa. At ambient pressure, multiple EPR branches were clearly observed, and the axial and rhombic components of ZFS were determined as $$D = 6.90\pm 0.01\hbox { cm}^{-1}$$ and $$E = 0.055 \pm 0.005\hbox { cm}^{-1}$$ , respectively. Upon pressure application, we observed a systematic shift of the resonance field, indicating a monotonous increase of the axial component from D = 6.9 to 7.9 $$\hbox {cm}^{-1}$$ at 2 GPa. The origin of this unusually large shift was discussed from a microscopic viewpoint of the electronic structure of iron under pressure.

Reduction of surface spin-induced electron spin relaxations in nanodiamonds

Nanodiamonds (NDs) hosting nitrogen-vacancy (NV) centers are promising for applications of quantum sensing. Long spin relaxation times (T1 and T2) are critical for high sensitivity in quantum applications. It has been shown that fluctuations of magnetic fields due to surface spins strongly influence T1 and T2 in NDs. However, their relaxation mechanisms have yet to be fully understood. In this paper, we investigate the relation between surface spins and T1 and T2 of single-substitutional nitrogen impurity (P1) centers in NDs. The P1 centers located typically in the vicinity of NV centers are a great model system to study the spin relaxation processes of the NV centers. By employing high-frequency electron paramagnetic resonance spectroscopy, we verify that air annealing removes surface spins efficiently and significantly reduces their contribution to T1.

Application of High-Frequency EPR Spectroscopy for the Identification and Separation of Nitrogen and Vanadium Sites in Silicon Carbide Crystals and Heterostructures

The advantage of the high-frequency spectroscopy of electron paramagnetic resonance (EPR) for the identification of nitrogen donors and a deep compensating vanadium impurity in various crystallographic positions of the silicon-carbide crystal is shown. Measurements are performed using a new generation EPR spectrometer operating in the continuous wave and pulsed modes at frequencies of 94 and 130 GHz in a wide range of magnetic fields (–7–7 T) and temperatures (1.5–300 K). A magneto-optical closed-cycle cryogenic system (Spectormag PT), highly stable generators (94 and 130 GHz), and a cavity-free system for supplying microwave power to the sample are used.

Orientation dependence of phase memory relaxation in the V(IV) ion at high frequencies

Understanding how magnetic relaxation depends on molecular orientation is a fundamental parameter for designing magnetic molecules for application. Herein we report the first use of pulsed high-frequency electron paramagnetic resonance spectroscopy (HFEPR, 120 and 240 GHz) to define the orientation dependence of phase memory relaxation in the S = 1/2 V(IV) complex (n-Bu3NH)2[V(C6H4O2)3]. We demonstrate a variation of 20% of the phase memory relaxation time (Tm) as a function of the orientation of the [V(C6H4O2)3]2– molecule in the applied magnetic field. Ultimately, this work underlines an important design strategy for molecule-based quantum computing architectures.

High frequency EPR spectroscopy of Er3+ ions in LiYF4 and LiLuF4: a case study of crystal fields

High frequency EPR spectroscopy of Er3+ ions in LiYF4 and LiLuF4: a case study of crystal fields

Force detection of high-frequency electron paramagnetic resonance spectroscopy of microliter solution sample

Force detection of magnetic resonance is now able to attain extremely high spin sensitivity. In these setups, microcantilevers were usually used as a sensitive force sensor and, in most cases, have been applied to solid-state samples such as paramagnetic impurities in solids. On the other hand, there are now growing demands for their applications to liquid-state samples in the research areas of life science because many proteins and enzymes are biofunctionally active only in solutions, where they interact with the surrounding water molecules. In this letter, we present an electron paramagnetic resonance (EPR) technique for solution samples using a SiNx nanomembrane and report high-frequency EPR spectroscopy of a microliter-volume frozen solution sample of hemin and myoglobin at multiple frequencies up to 350 GHz. This technique would be particularly useful to obtain more detailed insight into the electronic structure of metalloproteins/metalloenzymes under biologically active conditions.

Elementary excitations in single-chain magnets

Single-chain magnets (SCMs) are one-dimensional coordination polymers or spin chains that display slow relaxation of the magnetization. Typically their static magnetic properties are described by the Heisenberg model, while the description of their dynamic magnetic properties is based on an Ising-like model. The types of excitations predicted by these models (collective vs localized) are quite different. Therefore we probed the nature of the elementary excitations for two SCMs abbreviated ${\mathrm{Mn}}_{2}\mathrm{Ni}$ and ${\mathrm{Mn}}_{2}\mathrm{Fe}$, as well as a mononuclear derivative of the ${\mathrm{Mn}}_{2}\mathrm{Fe}$ chain, by means of high-frequency electron paramagnetic resonance spectroscopy (HFEPR) and inelastic neutron scattering (INS). We find that the HFEPR spectra of the chains are clearly distinct from those of the monomer. The momentum transfer dependence of the INS intensity did not reveal significant dispersion, indicating an essentially localized nature of the excitations. At the lowest temperatures these are modified by the occurrence of short-range correlations.

High-Frequency Electron Paramagnetic Resonance Spectroscopy of Nitroxide-Functionalized Nanodiamonds in Aqueous Solution.

Nanodiamond (ND) is an attractive class of nanomaterial for fluorescent labeling, magnetic sensing of biological molecules, and targeted drug delivery. Many of those applications require tethering of target biological molecules on the ND surface. Even though many approaches have been developed to attach macromolecules to the ND surface, it remains challenging to characterize dynamics of tethered molecule. Here, we show high-frequency electron paramagnetic resonance (HF EPR) spectroscopy of nitroxide-functionalized NDs. Nitroxide radical is a commonly used spin label to investigate dynamics of biological molecules. In the investigation, we developed a sample holder to overcome water absorption of HF microwave. Then, we demonstrated HF EPR spectroscopy of nitroxide-functionalized NDs in aqueous solution and showed clear spectral distinction of ND and nitroxide EPR signals. Moreover, through EPR spectral analysis, we investigate dynamics of nitroxide radicals on the ND surface. The demonstration sheds light on the use of HF EPR spectroscopy to investigate biological molecule-functionalized nanoparticles.

Enhancing the Magnetic Anisotropy of Linear Cr(II) Chain Compounds Using Heavy Metal Substitutions.

Magnetic properties of the series of three linear, trimetallic chain compounds Cr2Cr(dpa)4Cl2, 1, Mo2Cr(dpa)4Cl2, 2, and W2Cr(dpa)4Cl2, 3 (dpa = 2,2'-dipyridylamido), have been studied using variable-temperature dc and ac magnetometry and high-frequency EPR spectroscopy. All three compounds possess an S = 2 electronic ground state arising from the terminal Cr(2+) ion, which exhibits slow magnetic relaxation under an applied magnetic field, as evidenced by ac magnetic susceptibility and magnetization measurements. The slow relaxation stems from the existence of an easy-axis magnetic anisotropy, which is bolstered by the axial symmetry of the compounds and has been quantified through rigorous high-frequency EPR measurements. The magnitude of D in these compounds increases when heavier ions are substituted into the trimetallic chain; thus D = -1.640, -2.187, and -3.617 cm(-1) for Cr2Cr(dpa)4Cl2, Mo2Cr(dpa)4Cl2, and W2Cr(dpa)4Cl2, respectively. Additionally, the D value measured for W2Cr(dpa)4Cl2 is the largest yet reported for a high-spin Cr(2+) system. While earlier studies have demonstrated that ligands containing heavy atoms can enhance magnetic anisotropy, this is the first report of this phenomenon using heavy metal atoms as "ligands".

Electronic Structure of ZnO Quantum Dots Studied by High-Frequency EPR, ESE, ENDOR and ODMR Spectroscopy

High-frequency electron paramagnetic resonance (EPR), electron spin echo (ESE), electron-nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) were applied for the investigation of the electronic properties of ZnO colloidal quantum dots (QDs) which consist of a ZnO nanocrystal core and Zn(OH)2 shell. Shallow donors (SDs) in the form of interstitial atoms of lithium and sodium, as well as substituent of the aluminum have been identified and the spatial distribution of their electronic wave functions has been determined in the regime of quantum confinement. Hyperfine interactions as monitored by ENDOR quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. We studied the process of charge separation in the quantum dot when excited by UV light resulting in the formation of shallow donors and deep acceptors in the paramagnetic state, which recombine when heated, accompanied by thermoluminescence go back again in the non-paramagnetic state. It was shown that in all studied quantum dots deep acceptors are present positioned near the interface and including one sodium atom. ODMR techniques, which is based on EPR detection via photoluminescence or via tunneling afterglow that can be observed after preliminary X-ray or UV irradiation proved to be very useful to study colloidal ZnO NCs. The higher sensitivity of ODMR via afterglow allowed characterization of ZnO QDs dispersed in transparent media, which is an important advantage, since these systems are more relevant for a number of practical applications. A direct evidence of Co (Mn) interaction with SD in the core and hyperfine coupling with 1H in the shell of QDs has been demonstrated in Mn (Co)-doped ZnO QDs, which are promising classes of diluted magnetic semiconductors.

Magnetic Interactions in a Series of Homodinuclear Lanthanide Complexes.

A series of seven isostructural homodinuclear lanthanide complexes are reported. The magnetic properties (ac and dc SQUID measurements) are discussed on the basis of the X-ray structural properties which show that the two lanthanide sites are structurally different. MCD spectroscopy of the dysprosium(III) and neodymium(III) complexes ([Dy(III)2(L)(OAc)4](+) and [Nd(III)2(L)(OAc)4](+)) allowed us to thoroughly analyze the ligand field, and high-frequency EPR spectroscopy of the gadolinium(III) species ([Gd(III)2(L)(OAc)4](+)) showed the importance of dipolar coupling in these systems. An extensive quantum-chemical analysis of the dysprosium(III) complex ([Dy(III)2(L)(OAc)4](+)), involving an ab initio (CASSCF) wave function, explicit spin-orbit coupling (RASSI-SO), and a ligand field analysis (Lines model and Stevens operators), is in full agreement with all experimental data (SQUID, HF-EPR, MCD) and specifically allowed us to accurately simulate the experimental χT versus T data, which therefore allowed us to establish a qualitative model for all relaxation pathways.