Stochastic resonance in paramagnetic resonance systems

Experimental evidence of the stochastic resonance phenomenon in an electron paramagnetic resonance (EPR) system is reported. The amplitude and phase response of the EPR system operating in bistable conditions are measured for increasing values of the noise intensity. Theoretical predictions based on a simple dynamical model for the relevant system observables are shown to be in good agreement with experimental results.     

On the analysis of broad Dysonian electron paramagnetic resonance spectra

We analyze the equation used for simulating the lineshapes of broad electron paramagnetic resonance spectra in conducting samples (i.e., broad Dysonian lineshapes) where it becomes necessary to include the effects of both clockwise and counterclockwise rotating components of the microwave magnetic field. Using symmetry arguments, we propose a modification to the equation. We show that the modified equation fits the experimental results better than the equation used in literature.     

Nuclear polarization by electron paramagnetic resonance in paramagnetic crystals

The possibility of the polarization of nuclei in paramagnetic salts by saturation of electron paramagnetic resonance is theoretically analyzed. The proposed method assumes saturation of the forbidden transition of the typeM=±1, m=±1, ±2, for mutually perpendicular external magnetic and high-frequency fields. The analysis is carried out for the case of a large quadrupole moment of the nucleus. The degree of orientation attained is comparable in order of magnitude with Overhauser's method. This method is particularly suitable for the polarization of nuclei of transuranium elements.

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. M=± 1, m==±1, ±2 . . . .

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In conclusion the author would like to thank J. Burget, J. ajko, M. Kolá and M. ott for helping in the laborious solution of system (16).     

Tunable-frequency high-field electron paramagnetic resonance

A tunable-frequency methodology based on backward wave oscillator sources in high-frequency and -field EPR (HFEPR) is described. This methodology is illustrated by an application to three non-Kramers transition metal ion complexes and one Kramers ion complex. The complexes are of: Ni(II) (S=1) as found in dichlorobistriphenylphosphanenickel(II), Mn(III) (S=2) as found in mesotetrasulfonatoporphyrinatomanganese(III) chloride, Fe(II) (S=2) as found in ferrous sulfate tetrahydrate, and Co(II) (S=3/2) as found in azido(tris(3-tert-butylpyrazol-1-yl)hydroborate)cobalt(II). The above Ni(II) and Mn(III) complexes have been studied before by HFEPR using the multifrequency methodology based on Gunn oscillator sources, but not by the present method, while the Fe(II) and Co(II) complexes presented here have not been studied by any form of HFEPR. Highly accurate spin Hamiltonian parameters can be obtained by the experimental methodology described here, in combination with automated fitting procedures. This method is particularly successful in determining g-matrix parameters, which are very difficult to extract for high-spin systems from single frequency (or a very limited set of multi-frequency) HFEPR spectra, but is also able to deliver equally accurate values of the zero-field splitting tensor. The experimental methods involve either conventional magnetic field modulation or an optical modulation of the sub-THz wave beam. The relative merits of these and other experimental methods are discussed.     

Electron paramagnetic resonance of radiation-induced paramagnetic centers in an aerogel

Silicon oxide aerogel samples irradiated with x rays at room temperature have been analyzed using the electron paramagnetic resonance method. It has been found that three types of paramagnetic centers appear: paramagnetic centers with a g factor of 2.0035, centers associated with the presence of protons in SiO₂ globules, and centers in the adsorbed film on the aerogel surface. The fast (T _fast = 30 h) and slow (T _slow = 70 d) processes have been revealed in the recombination of these centers.     

A quantum approach to paramagnetic resonance

In the semi-classical theory of the problem of interaction of radio-frequency (r.f.) field with electron or nuclear spin, the application of quantisation is restricted to the spin system only, the r.f. field being treated classically. In this paper, a purely quantum approach is discussed for a system with spin > 1/2 using a Glauber vector to describe coherent excitations of the r.f. field. It is illustrated here for the case of spin 3/2. One finds that this quantum approach contains the classical one. An erratum to this article is available at .     

Far infrared paramagnetic resonance in dag

The g-value of the lowest and first higher Kramer's doublets of Dy³⁺ ions in an undiluted Dysprosium Aluminum Garnet sample are measured, in a magnetic field along the [001] direction. This is done in a high field EPR experiment with far infrared lasers.     

Low-power electron paramagnetic resonance spin-echo spectroscopy

Electron spin-echo experiments generally require microwave power levels of hundreds of watts to produce the 5–10 G of RF field to generate 90° and 180° pulses in 10 ns. A low-power (i.e., less than I W) EPR spectrometer using a loop-gap resonator can generate the full range of time-domain experiments on samples with submicrosecond recovery times; 90° pulses are generated in 40 ns, and relaxation times as short as 22 ns are measured. Appropriate time-domain experiments were performed to independently measure the spinspin relaxation time, phase memory time, and spin-lattice relaxation time; the results were compared with CW saturation. It was found that the spin-spin and spin-lattice relaxation rates do differ by about 5%. The entire CW signal of PADS is reconstructed from a pulse experiment at a single field position. Small differences in linewidths among the three lines were seen in accordance with theory.     

Double quantum transitions in paramagnetic resonance

The theory of double quantum transitions of the M=±2 type, with regard to inhomogeneously broadened spin systems is studied in this paper with the approximation ²T₂T₃ 1. We suppose that the inhomogeneous broadening is formed by an inhomogeneous crystal field. The obtained results describe the magnitude of absorption as a function of the h.f. power and also describe the shape of the absorption curve. It is demonstrated that in inhomogeneously broadened spin systems the absorption curve of double quantum transitions has the form of the difference of two different Lorentz's curves and that at the saturation ( ²T₂T₁ 1) the absorption increases with the cube of the h.f. field intensity. The shape of the curves is expressed by means of phenomenological relaxation constants of the system.

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M=±2 ²T₂T₃ 1. , - . . , ( ²T₂T₁ 1) . .

     

Electron paramagnetic resonance in Kondo insulators

This review is devoted to the application of electron paramagnetic resonance (EPR) in the study of fluctuating-valence materials, which are characterized by a narrow gap in the electron energy spectrum (Kondo insulators or Kondo semiconductors). The authors’ papers on studying classical objects of this field of solid-state physics, SmB₆ and YbB₁₂, are considered as an illustration of the potentiality of the EPR method. Temperature dependences of the gap width in these materials were obtained, the static and dynamic Jahn-Teller effects on Sm³⁺ ions in SmB₆ were detected, and the formation of Yb³⁺ ion pairs and the spontaneous breaking of cubic symmetry in YbB₁₂ were observed. The results obtained indicate that preference should be given to the exciton-polaron model developed by Kikoin et al. for the ground state of Kondo insulators.     

Electron paramagnetic resonance radiation dosimetry in fingernails

There is now an increased need for accident dosimetry due to the increased risk of significant exposure to ionizing radiation from terrorism or accidents. In such scenarios, dose measurements should be made in individuals rapidly and with sufficient accuracy to enable effective triage. Electron paramagnetic resonance (EPR) is a physical method of high potential for meeting this need, providing direct measurements of the radiation-induced radicals, which are unambiguous signatures of exposure to ionizing radiation. For individual retrospective dosimetry, EPR in tooth enamel is a proven and effective technique when isolated teeth can be obtained. There are some promising developments that may make these measurements feasible without the need to remove the teeth, but their field applicability remains to be demonstrated. However, currently it is difficult under emergency conditions to obtain tooth enamel in sufficient amounts for accurate dose measurements. Since fingernails are much easier to sample, they can be used in potentially exposed populations to determine if they were exposed to life-threatening radiation doses. Unfortunately, only a few studies have been carried out on EPR radiation-induced signals in fingernails, and, while there are some promising aspects, the reported results were generally inconclusive. In this present paper, we report the results of a systematic investigation of the potential use of fingernails as retrospective radiation dosimeters.     

Electron paramagnetic resonance in deuterated nickel fluosilicate

Electron paramagnetic resonance measurements in single crystals of NiSiF₆. 6D₂O were made at K, Ku and Ka bands at 4.2 K and between 77 K and 300 K. The measured g values were in the range 2.23–2.26, while the zero-field splitting parameter D varied from ?(0.185 ± 0.005) cm^?1 at 4.2 K to ?(0.53 ± 0.01) cm^?1 at 298 K. The parameters of the trimolecular hexagonal unit cell were determined to be approximately a = 9.28 Å, c = 9.58 Å from powder X-ray diffraction measurements at room temperature.     

Multifrequency electron paramagnetic resonance of ultramarine blue

Electron paramagnetic resonance spectra of the S^3/? radical center in ultramarine blue over a factor of about 2500 in frequency (258 MHz to 670 GHz) reveal a substantially Lorentzian shape, without resolution of g anisotropy. Variable temperature measurements found that the line width is independent of temperature, within experimental uncertainty, up to about 90 K at 9.5 GHz and between ca. 5 K and room temperature at 95 and 217 GHz, as expected for an exchange-narrowed signal. Analysis of the increase in the low-temperature line width as a function of frequency above 9 GHz is consistent with an exchange interaction of about 2· 10^?2 K. The line width increases as frequency is decreased from 2.7 GHz to 258 MHz which is attributed to the contribution from nonsecular terms that has been denoted the “10/3” effect.