Mössbauer spectroscopic study of the thermal spin crossover in [Fe(II)(isoxazole)6](ClO4)2

The ⁵⁷Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)₆](ClO₄)₂ has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with T_C(↓)=223 and T_C(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below T_C. The present study reveals an increase in high-spin fraction upon heating in the temperature range below T_C, and an explanation is provided.     

Magnetic behaviour of NpAs2 from Mössbauer spectroscopy

The Mössbauer hyperfine spectra of the 60 keV resonance of ²³⁷Np in powder and single crystal absorbers of NpAs₂ were measured between 4.2 and 60 K. Below 18 K a simple magnetic plus quadrupole pattern is seen in accordance with a ferromagnetic spin structure in tetragonal NpAs₂. The isomer shift favors the 4+ charge state, the hyperfine field of 288 T implies a moment of 1.5μ_B at the Np ion. The large reduction compared to the free ion values points towards a strong mixing of the electronic ground state by crystalline field interactions. Above 18 K the spectrum changes into a complex hyperfine pattern indicating a sinusoidally modulated spin structure. Near 54 K a transition into the paramagnetic state is observed. Both magnetic transitions (18 and 54 K) exhibit a feature typical for a first-order character.     

151,153Eu electron paramagnetic resonance in SrMoO4 and determination of signs of spin Hamiltonian parameters at different temperatures

The electron paramagnetic resonance (EPR) spectra of Eu²⁺ impurity centers in SrMoO₄ crystals have been studied in the temperature range of 1.8, 100–300 K. A hyperfine structure has been simulated for ^151,153Eu of different EPR transitions observed experimentally at different temperatures and external magnetic field orientations. A unique set of all parameters of the spin Hamiltonian for the known sign of the hyperfine interaction parameters A _i has been determined. It has been found that the diagonal parameters |b _n ⁰ | of the spin Hamiltonian decrease with increasing temperature; however, the parameter b ₄ ⁴ increases. The results of the study have demonstrated that |b ₂ ⁰ (T)/P ₂ ⁰ (T)| ～ const for ^151,153Eu in this single crystal.     

High-Spin Nitrene Fine-Structure ESR Spectroscopy in Frozen Rigid Glasses: Exact Analytical Expressions for the Canonical Peaks and A D-Tensor Gradient Method for Line Broadening

In high-spin chemistry, random-orientation fine-structure electron paramagnetic resonance (FS ESR) spectroscopy holds the advantages of the most facile and convenient method to identify high-spin systems. The FS ESR spectroscopy for high spins in frozen rigid glasses has seemingly been well established since the first spin-quintet m-dicarbene and m-dinitrene appeared in 1967. The FS ESR spectra of organic quintet entities generated by photolysis in the 2-methyltetrahydrofuran (2-MTHF) glass, however, have never been fully analyzed due to a peculiar line broadening appearing at many canonical peaks. The line broadening has been a notorious obstacle that masks key FS transitions of many cases in organic glasses or argon matrices. We examine the origin of the line broadening, illustrating the comprehensive spectral analysis for m-dinitrenes and other types of typical quintet-state dinitrenes observed in the 2-MTHF glass. Our new approach to the line broadening analysis invokes both exact analytical solutions for the resonance fields of canonical peaks and a magnetic-parameter gradient method. We have derived the exact analytical expressions for FS canonical peaks for high-spin states, for the first time. A microscopic origin of the line broadening observed for high-spin nitrenes generated by photolysis in rigid glasses is proposed on the basis of quantum chemical calculations of the D-tensor.     

Electron spin resonance study of a CuIr2S4 single crystal

A spinel sulphide CuIr₂S₄ single crystal, which exhibits an orbitally induced Peierls phase transition at ～230?K, is investigated by electron spin resonance (ESR) spectroscopy. The phase transition is clearly manifested on the ESR spectra. It is suggested that the ESR signals are produced by a few non-dimerized Ir⁴⁺ ions. Moreover, an extra ESR spectrum appears at low temperature in addition to the paramagnetic ESR signals of Ir⁴⁺ ions, which is suggested to be caused by the Jahn–Teller effect of the non-dimerized Ir⁴⁺ ions. From the ESR results, it is found that the Jahn–Teller splitting energy ΔE _JT is much smaller than the spin-dimerization gap.     

An investigation of the spin glass behaviour in iron antimonate by iron-57 and antimony-121 Mössbauer spectroscopy

Iron antimonate, which contains a superlattice composed of an ordered array of cations in the rutile-type structure, has been shown to undergo a spin glass transition atca. 20K which is induced by antisite atomic ordering. The iron-57 Mössbauer spectra recorded at 298 and 77K provide information on the relaxation times of short range magnetically ordered clusters identified by magnetic susceptibility measurements. The spectrum of 4K is tentatively interpreted in terms of the existence, at temperatures below the spin glass transition temperature, of a hyperfine magnetic field distribution at Fe³⁺ ions with a mean value of 487 kOe and which contains a supertransferred contribution from the nearest neighbour Fe³⁺ ions. The¹²¹Sb Mössbauer spectra are characteristic of Sb⁵⁺ and the increasing linewidths at lower temperatures are consistent with the presence of a supertransferred hyperfine magnetic field at the Sb⁵⁺ species ofca. 16^kOe.     

Nuclear inelastic scattering study of a dinuclear iron(II) complex showing a direct spin transition

The results of the nuclear inelastic scattering (NIS)/nuclear resonance vibrational spectroscopy (NRVS) for the powder spectra of dimeric [Fe ₂L₅(NCS) ₄] (L = N-salicylidene-4-amino-1,2,4-triazole) complex are presented. This system is spin crossover (SCO) material tagged with a fluorophore that can sense or “feel” the SCO signal ripping through the molecular network and thereby providing an opportunity to register the SCO transition. The spectra have been measured for the low-spin and high-spin phases of the complex. The high-spin isomer reveals one broad band above 200 cm ^?1, while the low-spin one displays two intense bands in the range from 390 to 430 cm ^?1, accompanied by a number of weaker bands below this area and one at ca. 490 cm ^?1. A normal coordinate analysis based on density functional calculations yields the assignment of the spin marker bands to particular molecular modes. In addition the vibrational contribution to the spin transition has been estimated     

Hyperfine structure of ESR spectra as a probe for heisenberg exchange couplings in nitroxide triradicals serving as building blocks for molecule-based ferrimagnets

A study on electron spin resonance (ESR) spectroscopic determination of exchange interactions in organic oligoradicals is given. When the intramolecular exchange couplingJ between, unpaired electron spins in nitroxide-based oligoradicals falls within the order of 10 Oe (1 mK or 10^?3 cm^?1 forg=2), which is on the same order as the hyperfine couplingA of magnetic nuclei such as nitrogen atoms of nitroxide radicals, the magnitude ofJ can be determined from the hyperfine splitting pattern of ESR spectra in solutions. This range of the exchange couplingJ is not detectable in conventional magnetic susceptibility measurements. We demonstrate an application of hyperfine ESR spectroscopy as a probe for the exchange coupling to a series of organic oligoradicals, which the authors have recently developed as building blocks for molecule-based magnetic materials.     

ESR of Mn2+ in La2O3: Position and intensity of forbidden hyperfine lines

Position and intensity of (ΔM=1, Δm=1) forbidden hyperfine lines arising in the central transition of the Mn²⁺:La₂O₃ ESR spectrum are analyzed by means of the existing theories. This permitted us to determine the quadrupole coupling constant and the nuclear Zeeman term.     

Laser-induced shifts and splittings of hyperfine structure lines in electron spin resonance spectra of 35Cl atoms

Laser-induced shifts and splittings of the hyperfine structure (hfs) lines in the electron spin resonance (ESR) spectra of ³⁵Cl atoms have been studied using both dressed-atom theory and semiclassical dispersion theory. In resonant cases, the calculated results by the dressed-atom theory of the ac Stark effect indicate that laser-induced hfs ESR shifts of ³⁵Cl atoms in the ground state 3p^{5 2}P⁰ _3/2 can be more than 1 MHz W^?1 cm², and a single ESR hfs line is split into four lines under the action of a laser beam with intensity 10 W cm^?2. In non-resonant cases, the two theories give similar results for the shifts. The laser-induced broadening of the ESR hfs lines is also calculated and is less than the shifts. The laser-induced shifts in ESR can be observable within the resolution of ESR experiments.     

Free radicals in irradiated wheat flour detected by electron spin resonance

By electron spin resonance (ESR) spectroscopy, we revealed free radicals in wheat flour before and after γ-ray inrradiation and their thermal behavior during heat treatment. The ESR spectrum of wheat flour before irradiation consists of a sextet centered atg=2.0 and a singlet signal at the sameg-value position. The first one is attributable to a signal with hyperfine (hf) interactions of Mn²⁺ ion (hf constant, 7.4 mT). The second signal originates from the carbon-centered radical. Upon γ-ray irradiation, however, a new signal with two triplet lines at the low- and high-field ends was detected on the Mn²⁺ sextet lines. We analyzed the triplet ESR lines as due to powder spectra (rhombicg-tensor symmetry) with nitrogen (¹⁴N) hf interactions. This indicates that a new organic radical was induced in the conjugated protein portion of wheat flour by γ-ray irradiation. The intensity of the organic free radical atg=2.0 detected in irradiated wheat flour increased monotonically under thermal treatment. The analysis of the time-dependent evolution process on the basis of the theory of transient phenomena as well as the nonlinear least-squares numerical method provided a unique time constant for the radical evolution in wheat flour during thermal treatment.     

Variation Angulaire,Transitions Interdites Dans Le Spectre De R.P.E. Des Ions Mn2+ Dans L'alumine α

The spin Hamiltonian with trigonal symmetry for Mn²⁺ in Al₂O₃ has been derived. The line positions have been calculated using perturbation theory up to third order. Three groups of forbidden transitions ΔM = ± 1 Δm = ± 1 have been investigated. Q′ and γ have been deduced from for forbidden hyperfine doublets. The two evaluations of the spin Hamiltonian parameters from allowed (Δm = 0) and forbidden lines (Δm = 1) are discussed.     

Free radicals in irradiated pepper: An electron spin resonance study

With electron spin resonance (ESR) spectroscopy, we revealed various free radicals in commercially available pepper in Japan before and after γ-irradiation. The representative ESR spectrum of the pepper is composed of a sextet centered atg=2.0, a singlet at the sameg-value and a singlet atg=4.0. The first signal is attributable to a signal with hyperfine interactions of the Mn²⁺ ion (hyperfine constant, 7.4 mT). The second signal is due to an organic free radical apparently induced by a sterilization process. The third signal may originate from the Fe³⁺ ion in the nonheme proteins. The progressive saturation behavior at various microwave power levels indicated quite different relaxation behaviors of those radicals. Namely, the peak intensity of the organic free radical component decreases in a monotonic fashion, whereas the Mn²⁺ and Fe³⁺ ESR signals substantially remain constant. This evidences the presence of three independent radicals in the pepper before irradiation. Upon γ-irradiation, a new pair of signals appeared in the pepper. The progressive saturation behavior of the pair peaks after the irradiation showed a quite different behavior as compared with the free radical centered atg=2.0. For the measure of irradiation effects, we propose a universal index for the ESR analysis of irradiated dry foods.     

Novel electron spin resonance spectra of a tetrathiafulvalene salt of copper chloride, (TTF) 2CuCl2

An ESR study has been carried out on electrically conducting (TTF)₂CuCl₂ crystals. The peak-to-peak width W of the ESR spectrum exhibited an unusual angular dependence with respect to the angle θ between the TTF-stack direction and the external field: the W showed a sharp minimum (4.9 G at 300 K) at θ = 0°, a maximum (7.5 G) at 60° (corresponding to 3cos²θ ∼ 1), and another minimum (6.9 G) at 90°. This angular depence has been explained by assuming an anisotropic motional narrowing that may arise from a spin correlation in one-dimensional electroconductive lattices. The temperature dependence of the W showed no anomaly around the temperature of metal-semiconductor transition: the spin relaxation is substantially unchanged by the phase transition.     

Variable temperature EPR study of Fe2+ spin transition in Cu2+ doped [Fe(trz)(Htrz)2](BF4)

The temperature dependence of the EPR spectrum of Cu²⁺ in the range 293–393 K exhibits a low-spin (S=0) to high-spin (S=2) transition of the Fe²⁺ ions, with hysteresis (T _c↑=363 K,T _c↓=343 K). At 103 K, the principal values of theg and hyperfine tensors of Cu²⁺ ions are revealed by hyperfine structure.     

Cerium influence on the spin reorientation in Er2−xCexFe14B evidenced by Mössbauer and DSC techniques

Polycrystalline Er_2−xCe_xFe₁₄B (x=0.25, 0.5, 1.0) compounds have been studied by ⁵⁷Fe Mössbauer spectroscopy and by differential scanning calorimetry (DSC) in the temperature range 80–470 K. The spin reorientation phenomenon has been studied extensively by narrow step temperature scanning in the vicinity of the spin reorientation temperature. It was found that in the region of transition, each Mössbauer subspectrum splits into two Zeeman sextets, which are characterised by different hyperfine magnetic fields and quadrupole splittings. A consistent way of describing the Mössbauer spectra in the reorientation region, below and above, was proposed. The composition and temperature dependencies of hyperfine interaction parameters and subspectra contributions were derived from experimental spectra.The endothermic DSC peaks were observed for all studied compounds, which correspond to the transition from basal plane to axial easy magnetisation direction on increasing the temperature. The spin reorientation temperatures and the enthalpies of transitions were established from DSC data. The spin arrangement diagram was constructed and the spin reorientation temperatures obtained by the two methods were compared.     

A high resolution study of the transitions 4f 76s 2→4f 76s6p in the Eu I-spectrum

Eight transitions in the Eu I-spectrum connecting the ground state with configuration 4f ⁷ 6s ² with states of the configuration 4f ⁷ 6s6p were studied with high resolution laseratomic-beam spectroscopy. CW dye lasers operating in the wavelength regions 435–470 nm and 560–630 nm were used for this study. New data for the hyperfine structure in¹⁵³Eu were obtained as well as new and more accurate values for the isotope shifts between¹⁵¹Eu and¹⁵³Eu. The existing data for the hyperfine structure in¹⁵¹Eu were reproduced with an exception for the levelz ⁶ P _7/2.     

The use of electron spin resonance spectroscopy for determining the provenance of classical marbles

The possibility of identifying the provenance of classical marbles and solving related questions, such as the joining fragments problem, via electron spin resonance spectroscopy has been reexamined. The method is based on characterization of the Mn²⁺ impurity ubiquitously present in marbles. Six different, although correlated, spectroscopic variables, such as intensity, linewidth, metal hyperfine splitting and others have been measured, together with some petrographic properties, for over 500 samples belonging to 14 different Italian, Greek and Turkish quarrying sites. The work, still in progress, is aimed to establish a marble database including all the historically relevant sites within the Mediterranean basin. The experimental data matrix has been analyzed with the aid of multivariate statistical techniques, primarily linear and quadratic discriminant analysis, and the ability of the method to classify correctly unknown samples has been estimated through standard techniques (resubstitution, jackknife), but also employing control “unknown” samples. The essential result is that, although the all-variables approach may describe the data set very accurately, the predictive power is due to four variables only, which are a suitable combination of spectroscopic and petrographic information. In this way 82.4% of the control samples could be assigned correctly, whereas inclusion of additional variables in the classification rule may result in substantially poorer performance. The conclusion is that ESR spectroscopy, although not providing a complete and general solution for the marble provenance problems, is probably, at the moment, the most developed methodology for identifying marbles. Its results, coupled with artistic historical information, deal correctly with a number of relevant archaeometric problems.     

Nuclear spin induced collapse and revival shape of Rabi oscillations of a single electron spin in diamond

A collapse and revival shape of Rabi oscillations in an electron spin of a single nitrogen-vacancy centre has been observed in diamond at room temperature. Because of hyperfine interaction between the host ¹⁴N nuclear spin and the nitrogen-vacancy centre electron spin, different orientations of the ¹⁴N nuclear spins lead to a triplet splitting of the transition between ground state (m_s =0) and excited state (m_s =1). The manipulation of the single electron spin of nitrogen-vacancy centre is achieved by using a combination of selective microwave excitation and optical pumping at 532 nm. Microwaves can excite three transitions equally to induce three independent nutations and the shape of Rabi oscillations is a combination of the three nutations.     

Die Kernmomente und die relative Isotopenlage des Os187

Using 4 mg of radiogenic Os¹⁸⁷ the hyperfine structure of this isotope has been investigated with a recording Fabry-Perot interferometer. From the measurement of intensities and distances of the hyperfine-structure components in three fine-structure lines the spin and the magnetic moment of the Os¹⁸⁷ nucleus have been determined to beI=1/2 andμ _I =+(0,0652±0,0011) n.m. In three fine-structure lines the isotopic shift has been measured and the relative isotopic shift of Os¹⁸⁷ has been found to be\(\frac{{\Delta v(Os^{192} - Os^{187} )}}{{\Delta v(Os^{192} - Os^{190} )}} = 3,09 \pm 0,03\). The two classifications of the fine-structure line λ 4261 Å have been verified by observing both transitions in Os¹⁸⁷.