Q-band Frequencies Research Articles

Electronic and Geometrical Structure of Zn+ Ions Stabilized in the Porous Structure of Zn-Loaded Zeolite H-ZSM-5: A Multifrequency CW and Pulse EPR Study

Electron spin resonance and hyperfine sublevel correlation (HYSCORE) spectroscopy at X- and Q-band frequencies have been employed, in conjunction with X-ray absorption spectroscopy (XAS), to determine the geometric and electronic structure of Zn+ ions in H-ZSM-5 zeolite. Zn+ ions were generated by the direct exposure of dehydrated acid H-ZSM-5 to Zn vapors. The number of Zn+ ions is found to increase substantially upon UV irradiation. A single EPR active species is detected, indicating a single site of adsorption, characterized by the presence of an Al3+ site, as revealed by superhyperfine interactions. The full g (gx = 1.9951, gy = 1.9984, gz = 2.0015) and 27Al |A| (Ax = 2.8, Ay = 2.7, Az = 4.6) MHz tensors were resolved, allowing for a detailed description of the geometric and electronic structure of Zn+ ions stabilized in the cages of H-ZSM-5 zeolite. The dispersion and nuclearity of Zn species formed during the sublimation/irradiation process was assessed by means of XAS spectroscopy, which indicates ...

Tuning the spin coherence time of Cu(II)-(bis)oxamato and Cu(II)-(bis)oxamidato complexes by advanced ESR pulse protocols.

We have investigated with the pulsed ESR technique at X- and Q-band frequencies the coherence and relaxation of Cu spins S = 1/2 in single crystals of diamagnetically diluted mononuclear [n-Bu4N]2[Cu(opba)] (1%) in the host lattice of [n-Bu4N]2[Ni(opba)] (99%, opba = o-phenylenebis(oxamato)) and of diamagnetically diluted mononuclear [n-Bu4N]2[Cu(opbon-Pr2)] (1%) in the host lattice of [n-Bu4N]2[Ni(opbon-Pr2)] (99%, opbon-Pr2 = o-phenylenebis(N(propyl)oxamidato)). For that we have measured the electron spin dephasing time Tm at different temperatures with the two-pulse primary echo and with the special Carr–Purcell–Meiboom–Gill (CPMG) multiple microwave pulse sequence. Application of the CPMG protocol has led to a substantial increase of the spin coherence lifetime in both complexes as compared to the primary echo results. It shows the efficiency of the suppression of the electron spin decoherence channel in the studied complexes arising due to spectral diffusion induced by a random modulation of the hyperfine interaction with the nuclear spins. We argue that this method can be used as a test for the relevance of the spectral diffusion for the electron spin decoherence. Our results have revealed a prominent role of the opba4– and opbon-Pr24– ligands for the dephasing of the Cu spins. The presence of additional 14N nuclei and protons in [Cu(opbon-Pr2)]2– as compared to [Cu(opba)]2– yields significantly shorter Tm times. Such a detrimental effect of the opbon-Pr24− ligands has to be considered when discussing a potential application of the Cu(II)−(bis)oxamato and Cu(II)−(bis)oxamidato complexes as building blocks of more complex molecular structures in prototype spintronic devices. Furthermore, in our work we propose an improved CPMG pulse protocol that enables elimination of unwanted echoes that inevitably appear in the case of inhomogeneously broadened ESR spectra due to the selective excitation of electron spins.

A Truly Balanced Q-Band CMOS Frequency Doubler Based on Hybrid Quadrature Coupler

In this letter, a truly balanced CMOS Q-band frequency doubler with an ultra-compact differential hybrid quadrature coupler is demonstrated using 90-nm CMOS technology. The doubler incorporates two Gilbert-cells fed by I/Q signals to compensate the time delay from transconductance stage and avoid output DC offset, providing truly balanced differential outputs. The doubler achieves a 3-dB bandwidth from 40 to 49 GHz with a minimum conversion loss of 1.6 dB at 44 GHz, while the output amplitude imbalance is measured to be less than 0.7dB. The measured I/Q mismatch of the quadrature coupler is less than 1° within 10 GHz bandwidth. The doubler chip consumes 9.6 mA from a 1.2-V supply, and the chip dimension is 0.7 × 0.9 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

An On-Chip Frequency-Reconfigurable Antenna For Q-Band Broadband Applications

This letter presents the design of a Q-band on-chip antenna with broadband characteristic using a standard 0.18- μm CMOS silicon-on-insulator process. A pair of on-chip switches is introduced in this design to implement the antenna with frequency reconfigurability, in order to enhance the bandwidth of the antenna. A small-signal equivalent circuit of this switch is illustrated in this letter, and an optimized gate width of the switch transistor is selected to achieve a tradeoff between the gain and the bandwidth of the proposed antenna. By changing the state of the on-chip switches, a measured -10-dB bandwidth from 29.5 to 51.0 GHz is achieved by this antenna, which has covered the whole Q-band frequency. A measured peak gain of 3.3 dBi is observed at 45 GHz, and a very compact chip size of 1.1 × 1.7 mm 2 is achieved by the proposed antenna in this work.

Pulse EPR Measurements of Intramolecular Distances in a TOPP-Labeled Transmembrane Peptide in Lipids

We present the performance of nanometer-range pulse electron paramagnetic resonance distance measurements (pulsed electron-electron double resonance/double electron-electron resonance, PELDOR/DEER) on a transmembrane WALP24 peptide labeled with the semirigid unnatural amino acid 4-(3,3,5,5-tetra-methyl-2,6-dioxo-4-oxylpiperazin-1-yl)-l-phenylglycine (TOPP). Distances reported by the TOPP label are compared to the ones reported by the more standard MTSSL spin label, commonly employed in protein studies. Using high-power pulse electron paramagnetic resonance spectroscopy at Q-band frequencies (34 GHz), we show that in contrast to MTSSL, our label reports one-peak, sharp (Δr ≤ 0.4 nm) intramolecular distances. Orientational selectivity is not observed. When spin-labeled WALP24 was inserted in two representative lipid bilayers with different bilayer thickness, i.e., DMPC and POPC, the intramolecular distance reported by TOPP did not change with the bilayer environment. In contrast, the distance measured with MTSSL was strongly affected by the hydrophobic thickness of the lipid. The results demonstrate that the TOPP label is well suited to study the intrinsic structure of peptides immersed in lipids.

A Q-Band Free-Space Characterization of Carbon Nanotube Composites.

We present a free-space measurement technique for non-destructive non-contact electrical and dielectric characterization of nano-carbon composites in the Q-band frequency range of 30 GHz to 50 GHz. The experimental system and error correction model accurately reconstruct the conductivity of composite materials that are either thicker than the wave penetration depth, and therefore exhibit negligible microwave transmission (less than -40 dB), or thinner than the wave penetration depth and, therefore, exhibit significant microwave transmission. This error correction model implements a fixed wave propagation distance between antennas and corrects the complex scattering parameters of the specimen from two references, an air slab having geometrical propagation length equal to that of the specimen under test, and a metallic conductor, such as an aluminum plate. Experimental results were validated by reconstructing the relative dielectric permittivity of known dielectric materials and then used to determine the conductivity of nano-carbon composite laminates. This error correction model can simplify routine characterization of thin conducting laminates to just one measurement of scattering parameters, making the method attractive for research, development, and for quality control in the manufacturing environment.

Multi-Frequency Pulsed Overhauser DNP at 1.2 Tesla

Abstract Dynamic nuclear polarization (DNP) is a methodology to increase the sensitivity of nuclear magnetic resonance (NMR) spectroscopy. It relies on the transfer of the electron spin polarization from a radical to coupled nuclear spins, driven by microwave excitation resonant with the electron spin transitions. In this work we explore the potential of pulsed multi-frequency microwave excitation in liquids. Here, the relevant DNP mechanism is the Overhauser effect. The experiments were performed with TEMPOL radicals in aqueous solution at room temperature using a Q-band frequency (1.2 T) electron paramagnetic resonance (EPR) spectrometer combined with a Minispec NMR spectrometer. A fast arbitrary waveform generator (AWG) enabled the generation of multi-frequency pulses used to either sequentially or simultaneously excite all three 14N-hyperfine lines of the nitroxide radical. The multi-frequency excitation resulted in a doubling of the observed DNP enhancements compared to single-frequency microwave excitation. Q-band free induction decay (FID) signals of TEMPOL were measured as a function of the excitation pulse length allowing the efficiency of the electron spin manipulation by the microwave pulses to be extracted. Based on this knowledge we could quantitatively model our pulsed DNP enhancements at 1.2 T by numerical solution of the Bloch equations, including electron spin relaxation and experimental parameters. Our results are in good agreement with theoretical predictions. Whereas for a narrow and homogeneous single EPR line continuous wave excitation leads to more efficient DNP enhancements compared to pulsed excitation for the same amount of averaged microwave power. The situation is different for radicals with several hyperfine lines or in the presence of inhomogeneous line broadening. In such cases pulsed single/multi-frequency excitation can lead to larger DNP enhancements.

Peculiar Electron Spin Resonance of Mn2+ in Kerogen: the Zvonce Graptolitic Black Shale (Serbia)

Mn2+ ions incorporated into kerogen isolated from the Zvonce black shale (Silur, Eastern Serbia) were detected by electron spin resonance (ESR) spectroscopy. Similar species were found in dark shale (Silur/Devon, central Morocco). No Mn2+ ions were previously detected by ESR spectroscopy in any geoorganic material. The Mn2+ ions in question were investigated at X- and Q-band frequencies at temperatures of liquid helium (ca. 4.2 K) to 250 K. The spectra exhibit the usual six-line pattern with a high g factor of 2.154 ± 0.005 and an isotropic hyperfine constant (A) of 7.56 ± 0.1 mT. These six lines are identical but asymmetric at all temperatures and microwave bands (X and Q). A computer simulation revealed that the line asymmetry is probably due to the superposition of the two (or possibly more) very similar Lorentzians for different Mn2+ sites. The line width [ΔHp–p(T)] of the Mn2+ ESR lines shows a strong dependence on temperature with a T–2 variation of the relaxation time. It is suggested that the Rama...

Aminoxyl Radicals of B/P Frustrated Lewis Pairs: Refinement of the Spin-Hamiltonian Parameters by Field- and Temperature-Dependent Pulsed EPR Spectroscopy.

Q-band and X-band pulsed electron paramagnetic resonance spectroscopic methods (EPR) in the solid state were employed to refine the parameters characterizing the anisotropic interactions present in six nitroxide radicals prepared by N,N addition of NO to various borane-phosphane frustrated Lewis pairs (FLPs). The EPR spectra are characterized by the g-anisotropy as well as by nuclear hyperfine coupling between the unpaired electron and the 11B/10B, 14N and 31P nuclear magnetic moments. It was previously shown that continuous-wave spectra measured at X-band frequency (9.5 GHz) are dominated by the magnetic hyperfine coupling to 14N and 31P, whereas the g-tensor values and the 11B hyperfine coupling parameters cannot be refined with high precision from lineshape fitting. On the other hand, the X-band electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) spectra are completely dominated by the nuclear hyperfine coupling to the 11B nuclei, allowing a selective determination of their interaction parameters. In the present work this analysis has been further validated by temperature dependent ESEEM measurements. In addition, pulsed EPR data measured in the Q-band (34 GHz) are reported, which present an entirely different situation: the g-tensor components can be measured with much higher precision, and the ESEEM and HYSCORE spectra contain information about all of the 10B, 11B, 14N and 31P hyperfine interaction parameters. Based on these new results, we report here high-accuracy and precision data of the EPR spin Hamiltonian parameters measured on six FLP-NO radical species embedded in their corresponding hydroxylamine host structures. While the ESEEM spectra at Q-band frequency turn out to be very complex (due to the multinuclear contribution to the overall signal) in the HYSCORE experiment the extension over two dimensions renders a better discrimination between the different nuclear species, and the signals arising from hyperfine coupling to 10B, 11B, 14N and 31P nuclei can be individually analyzed.

Separation of intra- and intermolecular contributions to the PELDOR signal

Pulsed Electron–electron Double Resonance (PELDOR) is commonly used to measure distances between native paramagnetic centers or spin labels attached to complex biological macromolecules. In PELDOR the energies of electron magnetic dipolar interactions are measured by analyzing the oscillation frequencies of the recorded time resolved signal. Since PELDOR is an ensemble method, the detected signal contains contributions from intramolecular, as well as intermolecular electron spin interactions. The intramolecular part of the signal contains the information about the structure of the studied molecules, thus it is very important to accurately separate intra- and intermolecular contributions to the total signal. This separation can become ambiguous, when the length of the PELDOR signal is not much longer than twice the oscillation period of the signal. In this work we suggest a modulation depth scaling method, which can use short PELDOR signals in order to extract the intermolecular contribution. Using synthetic data we demonstrate the advantages of the new approach and analyze its stability with regard to signal noise. The method was also successfully tested on experimental data of three systems measured at Q-Band frequencies, two model compounds in deuterated and protonated solvents and one biological sample, namely BetP. The application of the new method with an assigned value of the signal modulation depth enables us to determine the interspin distances in all cases. This is especially interesting for the model compound with an interspin distance of 5.2nm in the protonated solvent and the biological sample, since an accurate separation of the intra- and intermolecular PELDOR signal contributions would be difficult with the standard approach in those cases.

Magnetic properties of Co3O4 polycrystal powder

The results of investigations of magnetic properties of Co3O4 polycrystals in powder morphology with average crystallite size of 10µm are presented. The temperature dependence of the magnetic susceptibility, measured in a wide temperature interval (1.5≤T≤400K) using SQUID magnetometer, as well as electron spin resonance (ESR) spectra measured at X-band and Q-band frequency ranges have been studied. Antiferromagnetism with a Néel temperature TN at about 39K was observed from the analysis of evolution of molar magnetic susceptibility and ESR intensity as a function of temperature. ESR parameters namely g-factor, intensity (IESR), g-value, and linewidth ΔH for powder Co3O4 have been obtained. Some deviations from the expected values of some magnetic properties were determined and discussed.

Quantifying the Electron Donor and Acceptor Abilities of the Ketimide Ligands in M(N═C(t)Bu2)4 (M = V, Nb, Ta).

Addition of 4 equiv of Li(N═C(t)Bu2) to VCl3 in THF, followed by addition of 0.5 equiv of I2, generates the homoleptic V(IV) ketimide complex, V(N═C(t)Bu2)4 (1), in 42% yield. Similarly, reaction of 4 equiv of Li(N═C(t)Bu2) with NbCl4(THF)2 in THF affords the homoleptic Nb(IV) ketimide complex, Nb(N═C(t)Bu2)4 (2), in 55% yield. Seeking to extend the series to the tantalum congener, a new Ta(IV) starting material, TaCl4(TMEDA) (3), was prepared via reduction of TaCl5 with Et3SiH, followed by addition of TMEDA. Reaction of 3 with 4 equiv of Li(N═C(t)Bu2) in THF results in the isolation of a Ta(V) ketimide complex, Ta(Cl)(N═C(t)Bu2)4 (5), which can be isolated in 32% yield. Reaction of 5 with Tl(OTf) yields Ta(OTf)(N═C(t)Bu2)4 (6) in 44% yield. Subsequent reduction of 6 with Cp*2Co in toluene generates the homoleptic Ta(IV) congener Ta(N═C(t)Bu2)4 (7), although the yields are poor. All three homoleptic group 5 ketimide complexes exhibit squashed tetrahedral geometries in the solid state, as determined by X-ray crystallography. This geometry leads to a d(x(2)-y(2))(1) ((2)B1 in D(2d)) ground state, as supported by DFT calculations. EPR spectroscopic analysis of 1 and 2, performed at X- and Q-band frequencies (∼9 and 35 GHz, respectively), further supports the (2)B1 ground-state assignment, whereas comparison of 1, 2, and 7 with related group 5 tetra(aryl), tetra(amido), and tetra(alkoxo) complexes shows a higher M-L covalency in the ketimide-metal interaction. In addition, a ligand field analysis of 1 and 2 demonstrates that the ketimide ligand is both a strong π-donor and strong π-acceptor, an unusual combination found in very few organometallic ligands.

Gd(III)–Gd(III) distance measurements with chirp pump pulses

The broad EPR spectrum of Gd(III) spin labels restricts the dipolar modulation depth in distance measurements between Gd(III) pairs to a few percent. To overcome this limitation, frequency-swept chirp pulses are utilized as pump pulses in the DEER experiment. Using a model system with 3.4nm Gd–Gd distance, application of one single chirp pump pulse at Q-band frequencies leads to modulation depths beyond 10%. However, the larger modulation depth is counteracted by a reduction of the absolute echo intensity due to the pump pulse. As supported by spin dynamics simulations, this effect is primarily driven by signal loss to double-quantum coherence and specific to the Gd(III) high spin state of S=7/2. In order to balance modulation depth and echo intensity for optimum sensitivity, a simple experimental procedure is proposed.An additional improvement by 25% in DEER sensitivity is achieved with two consecutive chirp pump pulses. These pulses pump the Gd(III) spectrum symmetrically around the observation position, therefore mutually compensating for dynamical Bloch–Siegert phase shifts at the observer spins. The improved sensitivity of the DEER data with modulation depths on the order of 20% is due to mitigation of the echo reduction effects by the consecutive pump pulses. In particular, the second pump pulse does not lead to additional signal loss if perfect inversion is assumed. Moreover, the compensation of the dynamical Bloch–Siegert phase prevents signal loss due to spatial dependence of the dynamical phase, which is caused by inhomogeneities in the driving field.The new methodology is combined with pre-polarization techniques to measure long distances up to 8.6nm, where signal intensity and modulation depth become attenuated by long dipolar evolution windows. In addition, the influence of the zero-field splitting parameters on the echo intensity is studied with simulations. Herein, larger sensitivity is anticipated for Gd(III) complexes with zero-field splitting that is smaller than for the employed Gd-PyMTA complex.

Synthesis, Structure, and Electron Paramagnetic Resonance Study of a Mixed Valent Metal–Organic Framework Containing Cu2 Paddle-Wheel Units

We report synthesis and composite study of a novel metal–organic framework (MOF) compound of chemical formula ∞3[Cu2ICu2II{H2O}2{(Me–trz–mba)2thio}2]Cl2, where (Me–trz–mba)2thio2– stands for 3,3-(5,5-(thiophene-2,5-diyl)bis(3-methyl-4H-1,2,4-triazole-5,4-diyl))dibenzoate. This coordination polymer was synthesized by solvothermal synthesis. The crystal structure was determined using single crystal X-ray diffraction. The main building block of this compound is a so-called Cu2 paddle-wheel (PW) unit, which contains two Cu2+ ions connected via four carboxylate groups. Magnetic properties of the investigated MOF were studied by continuous-wave electron paramagnetic resonance (EPR) spectroscopy at X- and Q-band frequencies in a wide temperature range. Mononuclear Cu2+ ions were observed in the EPR spectra and characterized by spectral simulations. In addition, the obtained EPR data allowed us to detect and investigate three distinct magnetic interactions related to the Cu2+ pairs. At higher temperatures the fine structure pattern was observed in the EPR spectra and the spin–spin interaction tensor D was determined. The origin of this pattern was assigned to the thermally populated excited triplet states of the Cu2+ pairs. It was found that two Cu2+ ions within a single PW unit couple antiferromagnetically with the exchange coupling constant J = −258 cm–1. Moreover, the EPR spectra of dehydrated MOF samples show a broad, poorly resolved spectral feature, the origin of which is an exchange of the spin triplets between neighboring Cu2 PW units. By simulating the powder pattern of this interdinuclear exchange line, we estimated the exchange coupling between neighboring PW units (|J′| = 4.9 cm–1). It was also found that the interdinuclear exchange gradually disappears, if the dehydrated samples are allowed to interact with air, demonstrating that this exchange can be rather easily manipulated in the investigated MOF.

Pulsed EPR dipolar spectroscopy at Q- and G-band on a trityl biradical.

Pulsed electron paramagnetic resonance (EPR) spectroscopy is a valuable technique for the precise determination of distances between paramagnetic spin labels that are covalently attached to macromolecules. Nitroxides have commonly been utilised as paramagnetic tags for biomolecules, but trityl radicals have recently been developed as alternative spin labels. Trityls exhibit longer electron spin relaxation times and higher stability than nitroxides under in vivo conditions. So far, trityl radicals have only been used in pulsed EPR dipolar spectroscopy (PDS) at X-band (9.5 GHz), Ku-band (17.2 GHz) and Q-band (34 GHz) frequencies. In this study we investigated a trityl biradical by PDS at Q-band (34 GHz) and G-band (180 GHz) frequencies. Due to the small spectral width of the trityl (30 MHz) at Q-band frequencies, single frequency PDS techniques, like double-quantum coherence (DQC) and single frequency technique for refocusing dipolar couplings (SIFTER), work very efficiently. Hence, Q-band DQC and SIFTER experiments were performed and the results were compared; yielding a signal to noise ratio for SIFTER four times higher than that for DQC. At G-band frequencies the resolved axially symmetric g-tensor anisotropy of the trityl exhibited a spectral width of 130 MHz. Thus, pulsed electron electron double resonance (PELDOR/DEER) obtained at different pump-probe positions across the spectrum was used to reveal distances. Such a multi-frequency approach should also be applicable to determine structural information on biological macromolecules tagged with trityl spin labels.

Sensitivity enhancement by population transfer in Gd(III) spin labels.

In order to enhance echo signals observed with selective pulses, equilibrium populations of the energy levels of S = 7/2 Gd(III) spin labels are rearranged with frequency-swept passage pulses. To transfer population from as many energy levels as possible, the 2 μs long passage pulses range over more than 1 GHz. Application of this technique at Q-band frequencies to three different Gd(III) complexes and spin dynamics simulations reveal large signal enhancements beyond 100% for Gd(III) complexes with zero-field splitting parameters below 1 GHz. For complexes with larger splittings, experimental enhancements are on the order of 90%. Moreover, population transfer is combined with distance measurements on a model system with a pair of Gd(III) ions. As a result, a signal enhancement of 85% is achieved without inducing changes in the obtained distance information. Besides this enhancement by population transfer, a dipolar modulation depth of 9% is demonstrated, which results in a total enhancement of 3.3 with respect to data obtained with monochromatic rectangular pulses. The limitations of the population transfer technique are discussed. In particular, the extraordinary broad pulse bandwidths caused heating effects and pulse distortions, which constrain the pulse length and thus the achievable signal enhancement.

Spectroscopic and Biochemical Studies of TRIP8b Regulation of HCN Channels

TRIP8b, an accessory subunit of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, alters both cell surface expression and cyclic nucleotide dependence of these channels. The mechanism through which TRIP8b exerts these dual effects is still poorly understood. Besides binding the terminal three residues of HCN channels, TRIP8b also binds directly to the cyclic nucleotide-binding domain (CNBD). A small central portion of TRIP8b, termed TRIP8bcore, is involved in this interaction. Binding of TRIP8bcore to the CNBD dramatically reduces the effects of cAMP on the channel. Using spectroscopic and biochemical techniques, we sought to understand how and where TRIP8b binds to the CNBD and how it reduces the cyclic nucleotide dependence of HCN channels. To closely examine the binding of TRIP8bcore to the CNBD, we used double electron-electron resonance (DEER) at Q-band frequencies to study conformational changes in the soluble CNBD of HCN2 in the presence of TRIP8b and cAMP. We show that the overall structure of the TRIP8b bound conformation of the CNBD closely resembles the apo state. In addition, we use DEER between the CNBD and TRIP8bcore and nuclear magnetic resonance (NMR) to localize the binding site for TRIP8b on the CNBD. Finally, to understand the mechanism of TRIP8b inhibition of cAMP regulation of HCN channels, we performed binding studies of cAMP and TRIP8b on the CNBD and developed a multi-state mathematical model to explore 1) whether cAMP and TRIP8b can bind simultaneously to HCN channels, 2) whether TRIP8b reduces the affinity for cAMP binding, and 3) whether TRIP8b reduces the effect of cAMP on HCN channels by preventing the structural rearrangements associated with channel opening.

Broadband spin echoes and broadband SIFTER in EPR

Applications of broadband pulses for EPR have been reported for FID, echo detection and inversion pulses recently. Here we present a broadband Hahn, stimulated and refocused echo sequence derived from adiabatic pulses. The formation of echoes is accomplished by using variable chirp rates and pulse lengths. In all three broadband echo experiments the complete spectral shape of a nitroxide (about 70 Gauss at X-band frequency) could be recovered by Fourier transformation of the quadrature detected echo signals. Such broadband echoes provide an exciting opportunity to optimize pulse sequences where a full excitation of the spectrum is mandatory for an optimum performance. We applied our pulses to the SIFTER (single frequency technique for refocusing dipolar couplings) experiment, a solid echo based pulse sequence to measure the dipolar coupling between two unpaired electron spins. By employing our broadband Hahn echo sequence on a nitroxide biradical we could achieve an artifact free dipolar evolution time trace in the SIFTER experiment with 95% modulation depth at X-band frequency and of 10% modulation depth at Q-band frequency.

Structure of the Catalytic Active Sites in Vanadium-Doped Aluminophosphate Microporous Materials. New Evidence from Spin Density Studies

Electron spin resonance and hyperfine sublevel correlation (HYSCORE) spectroscopy at X- and Q-band frequencies have been employed, in conjunction with DFT modeling, to determine the location of V(IV) ions in AlPO-5 zeotype materials. Two EPR-active species are detected, whose spin Hamiltonian parameters are in accord with vanadyl ions (VO2+) experiencing slightly different local environments. Interactions of the unpaired electrons of the paramagnetic VO2+ species with all relevant nuclei (1H, 31P, 27Al, and 51V) could be resolved, allowing for the first detailed structural analysis of the VO2+ paramagnetic ions in AlPO materials. Dehydration treatments indicate that the observed 1H hyperfine couplings stem from structural protons in the first coordination sphere of the VO2+ species, strongly pointing to charge compensating mechanisms associated with isomorphous framework substitution at Al3+ sites, in good agreement with the large 31P hyperfine couplings. Detection of fairly large 27Al couplings point to ...

Parallel ferromagnetic resonance and spin-wave excitation in exchange-biased NiFe/IrMn bilayers

Ferromagnetic Resonance study of sputtered Ru(7nm)/NiFe(tFM)/IrMn(6nm)/Ru(5nm) exchange-biased bilayers at X and Q-band microwave frequencies reveals the excitation of spin-wave and NiFe resonance modes. Angular variations of the in-plane resonance fields of spin-wave and NiFe resonance modes show the effect of the unidirectional anisotropy, which is about twice larger for the spin-wave mode due to spin pinning at the NiFe/IrMn interface. At Q-band frequency the angular variations of in-plane resonance fields also reveal the symmetry of a uniaxial anisotropy. A modified theoretical model which also includes the contribution of a rotatable anisotropy provides a good description of the experimental results.