Electron Magnetic Resonance Spectra Research Articles

Dielectric and magnetic properties of sol-gel-derived lead iron niobate ceramics

In this work, we report the synthesis of sol-gel-derived lead iron niobate [Pb1.10(Fe0.5Nb0.5)O3] (PFN) powders and sintered ceramics. The PFN powders were calcined at (Ta), 973, 1073, 1173, 1273, and 1373 K for 3 h in air. As envisaged from x-ray-diffraction analyses, PFN powder calcined at 1173 K was crystallized into pure monoclinic perovskite phase whereas powders calcined at all other temperatures had varied amounts of retained pyrochlore (Pb3Nb4O13) phase coexisted with dominant monoclinic perovskite phase. The PFN pellet (prepared using the phase pure powder calcined at 1173 K) sintered at 1373 K for 4 h in air also had a minute quantity of retained pyrochlore phase coexisting with desired perovskite phase. From the temperature dependence of measured capacitance and loss tangent at different frequencies, the ferroelectric to paraelectric phase-transition temperature of PFN ceramics was observed at Tc≈370K. The diffused nature of this transition and high dielectric constant of PFN is related to the cation disorder at the B site of A(BI+3BII+5)O3 lattice. For PFN powders, calcined at different temperatures, the temperature dependence of the magnetic susceptibility (χ) was measured in a temperature range of 2–360 K, whereas the magnetic hysteresis loops and electron magnetic resonance (EMR) spectra were measured at room temperature. Room-temperature ferromagnetism is observed in all the calcined powder samples and it was found that the magnetization increases with the increase in calcination temperature (Ta). The symmetric EMR line shape with g≈2.01 observed in all calcined samples was identified to be due to Fe3+ ions. It is suggested that the observed weak ferromagnetism, which increases with an increase in Ta, may be due to canting of the Fe3+ spins. These observations suggest PFN to be a very attractive single-phase ferroelectric/ferromagnetic material for room-temperature applications.

Photogeneration of Distant Radical Pairs in Aqueous Pyruvic Acid Glasses

The lambda > 300 nm photolysis of h4- or d4-pyruvic acid aqueous glasses at 77 K yields identical electron magnetic resonance (EMR) spectra arising from distant (r greater or similar 0.5 nm) triplet radical pairs. Spectra comprise: (1) well-resolved quartets, X, at g approximately ge, that closely match the powder spectra of spin pairs interacting across r approximately 1.0 nm with D approximately 3.0 mT, E approximately 0 mT zero field splittings (ZFS), and (2) broad signals, Y, centered at g approximately 2.07 that display marked g-anisotropy and g-strain, exclude D greater or similar 20.0 mT values (i.e., r less or similar 0.5 spin nm separations), and track the temperature dependence of related g approximately 4 features. These results imply that the n-pi excitation of pyruvic acid, PA, induces long-range electron transfer from the promoted carbonyl chromophore into neighboring carbonyl acceptors, rather than homolysis into contact radical pairs or concerted decarboxylation into a carbene. Since PA is associated into hydrogen-bonded dimers prior to vitrification, X signals arise from radical pairs ensuing intradimer electron transfer to a locked acceptor, while Y signals involve carbonyl groups attached to randomly arranged, disjoint monomers. The ultrafast decarboxylation of primary radical ion pairs, 3[PA+* PA-*], accounts for the release of CO2 under cryogenic conditions, the lack of thermal hysteresis displayed by magnetic signals between 10 and 160 K, and averted charge retrotransfer. All EMR signals disappear irreversibly above the onset of ice diffusivity at approximately 190 K.

EMR studies of La 1−x Ca xMnO 3 ( [formula omitted]; 0.9) manganites with canted antiferromagnetic ground states

X-band electron magnetic resonance (EMR) spectra were recorded at the temperature range of 115⩽ T⩽600 K on powdered samples of single crystalline La 0.9Ca 0.1MnO 3 and polycrystalline La 0.1Ca 0.9MnO 3 manganites, having a mixed magnetic structure in the ground state, which comprises canted antiferromagnetic matrix and nanometer-scale ferromagnetic clusters. A model of the magneto-impurity states for excess holes and electrons, which emerge in the considered compounds due to A-site cation substitution, was used for modeling their paramagnetic susceptibility and spin dynamics measured by means of EMR.

Magnetic phase transition in YCo3B2 studied by magnetic resonance

Magnetic-susceptibility measurements, nuclear magnetic resonance (NMR) of B11, and electron magnetic-resonance (EMR) spectra of YCo3B2 were measured as a function of the temperature. The temperature dependence of the magnetic susceptibility showed a lambda-shaped anomaly at 150K. Whereas upon cooling, a shift in the resonance position of the NMR and EMR spectra is observed at 150K, marking the emergence of an internal magnetic field. The quadrupolar splitting of the NMR lines showed a marked discontinuity at 150K. The (T1T)−1∕2 values were found to be independent of temperature for T>175K, and to increase sharply upon cooling for T<150K. The intensity of the EMR line which is independent of temperature for T>160K is also found to increase markly upon cooling below 150K. All these effects are discussed and it is proposed that YCo3B2 undergoes a magnetic ordering phase transition at 150K.

Electron magnetic resonance of iron oxide nanoparticles mineralized in protein cages

Magnetic and structural properties determined by electron magnetic resonance (EMR) spectroscopy are reported for maghemite (γ-Fe2O3) nanoparticles formed through template-constrained mineralization within three protein cages with nominal diameters of 5, 8, and 24 nm. EMR spectra, obtained at 4.0, 9.2, 34.6, 94.9, and 130.0 GHz, and at ambient temperature, show dramatic frequency dependent effects in the line shapes, line-widths, and resonance-field shifts. Simulations of the spectra are used to obtain moment distribution parameters, which are consistent with size limitations imposed by the protein cages, but which reflect significant departures from bulk magnetization properties.

Interparticle interaction effects in the magnetic properties of NiO nanorods

Magnetic properties of nanorods of NiO (diameter ≈5nm and average aspect ratio ≈12), with and without oleic acid coating, are reported. Temperature variations (5 to 300 K) of the magnetization and electron magnetic resonance (EMR) spectra were measured to determine the respective blocking temperatures Tb(m) and Tb(EMR). The following differences are noted: (1) Tb(m) is reduced from 230 K (uncoated) to 85 K (coated) for H=25Oe(2.5mT); (2) Decrease of Tb(m) with H is weaker and the ratio Tb(EMR)∕Tb(m) is smaller for the uncoated particles. These differences are due to stronger interparticle interaction present in the uncoated particles.

High-frequency electron magnetic resonance and magnetic studies of ferrihydrite nanoparticles and evidence of a phase transition

Temperature variations (300–4 K) of the electron magnetic resonance (EMR) spectra of ferrihydrite (FeOOH· nH 2O) nanoparticles of size ≃5 nm are reported at the frequencies of ν = 190.62 and 285.93 GHz. The EMR line at 300 K, occurring near g ≃ 2 , broadens and shifts to lower fields with decreasing T for T > T s = 30 K . For T < T s , the trend reverses in that the line narrows and shifts to higher fields. In magnetic studies, the coercivity H c is maximum at T s (approaching zero at the blocking temperature T B ≃ 65 K ) with the appearance of exchange bias H E for T < T s . The magnetic viscosity is also maximum near T s , approaching zero above T B . These observations lead to the suggestion that T s represents a phase transition to a new magnetically ordered state, with spin-glass-like ordering of the uncompensated Fe 3+ spins.

Electron magnetic resonance (EMR) study of CaMn1−Ru O3 perovskites: an inhomogeneous ground state induced by Ru-doping

Abstract X-band EMR measurements of polycrystalline CaMn1−xRuxO3 (x=0÷0.40) samples were done at 120⩽T⩽400 K aiming to study the effect of Ru-induced magnetic ordering. High-temperature EMR spectra of pristine CaMnO3 show a Lorentzian-like line with Δ H pp ∼160 mT and g=1.995±0.005, whose intensity diminishes, zeroing at Neel temperature T N =120 K . EMR evidences that Ru-doping modifies both paramagnetic and antiferromagnetic states, and creates an inhomogeneous phase separated ferro- and antiferromagnetic ground states at x>0.06.

Electron magnetic resonance inLa1−xCaxMnO3(x=0.18,0.20,0.22): Crossing through the boundary between ferromagnetic insulating and metallic ground states

Electron magnetic resonance (EMR) measurements were employed for studying the magnetic state of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ crystalline samples with $x=0.18,$ 0.20, and 0.22. It is shown that EMR spectra of ${\mathrm{La}}_{0.82}{\mathrm{Ca}}_{0.18}{\mathrm{MnO}}_{3}$ and ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3}$ become drastically different from those of ${\mathrm{La}}_{0.78}{\mathrm{Ca}}_{0.22}{\mathrm{MnO}}_{3}$ below the Curie points of these compounds. Such spectra of ${\mathrm{La}}_{0.82}{\mathrm{Ca}}_{0.18}{\mathrm{MnO}}_{3}$ and ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3}$ below ${T}_{C}$ are constituted of low-intensity doublet and relatively intense singlet signals attributed to ferromagnetic insulating and ferromagnetic metallic phases, respectively. The spectra of ${\mathrm{La}}_{0.78}{\mathrm{Ca}}_{0.22}{\mathrm{MnO}}_{3}$ have a shape of a broad asymmetric singlet signal similar to that observed in homogeneous ferromagnetic metallic manganites. The EMR data obtained on ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ were compared with the recent results on magnetic, conductive, and nuclear magnetic resonance properties of these compounds and discussed in the context of transition from electronically separated phase to a homogeneous state with increasing Ca content.

Electron magnetic resonance studies of magnetic inhomogeneities in crystalline and nanosized powders of La1−xSrxMnO3

Electron magnetic resonance (EMR) spectra were measured on different La1−xSrxMnO3 manganites at the interval 100⩽T⩽420 K for studying an influence of the synthesis conditions on the magnetic homogeneity of the sample. The micron-sized crushed crystals La0.833Sr0.126MnO3 and La0.7Sr0.3MnO3 grown by fused-salt electrolysis and by floating zone melting with the Curie temperatures TC=320 K and 350 K, respectively, as well as nanosized sonochemically sintered powders La0.7Sr0.3MnO3 (annealed, TC=340 K) were studied. The results obtained show that samples originated from bulk crystals are less homogeneous than the nanocrystalline one. Except for higher homogeneity, the nanosized sample demonstrates also a weaker magnetic anisotropy. As a result, strongly pronounced magnetic phase separation (revealed as coexisting ferromagnetic and paramagnetic signals in EMR spectra) is observed in crushed crystals.

A tracer study of a radical produced by UV irradiation on BaTi 4O 9 photocatalyst surface

A tracer study was performed to assign a radical produced with UV irradiation on photocatalytically active BaTi 4O 9 for water decomposition. BaTi 4O 9 was reduced in H 2 and oxidized in 16 O 2 , 17 O 2 or 18 O 2 at different temperatures. The behavior of oxygen exchange was investigated by thermal gravimetric analysis and Raman spectroscopy. Electron magnetic resonance spectra showed that new signals with g ⊥=2.038 and g ⊥=2.026 due to hyperfine splitting of 17 O − with A ⊥=19.1 G appeared when oxidized in 17 O 2 , in addition to normal signals with g ⊥=2.018 and g ‖=2.004. The radical is assigned to O − produced from surface lattice oxygen of BaTi 4O 9.

Simulation Methods for Looping Transitions

Looping transitions occur in field-swept electron magnetic resonance spectra near avoided crossings and involve a single pair of energy levels that are in resonance at two magnetic field strengths, before and after the avoided crossing. When the distance between the two resonances approaches a linewidth, the usual simulation of the spectra, which results from a linear approximation of the dependence of the transition frequency on magnetic field, breaks down. A cubic approximation to the transition frequency, which can be obtained from the two resonance fields and the field-derivatives of the transition frequencies, along with linear (or better) interpolation of the transition-probability factor, restores accurate simulation. The difference is crucial for accurate line shapes at fixed angles, as in an oriented single crystal, but the difference turns out to be a smaller change in relative intensity for a powder spectrum. Spin-32Cr3+in ruby and spin-52Fe3+in transferrin oxalate are treated as examples.

Zero-field electron magnetic resonance spectra of copper carboxylates

The design of a new zero-field electron magnetic resonance spectrometer including cryogenic capabilities and a new tuning mechanism for sweeping over approximately two octaves is described. The tuning mechanism is based on a one-loop, two-gap resonator mounted on quartz rods in such a manner that rotating the rods changes the width of the gap and the resonant frequency of the resonator. The spectrometer was used to measure the zero-field emr spectra of several copper carboxylate dimers at 90 K and 70 K. The parallel hyperfine structure was well resolved in all of the spectra and this allowed accurate determination of D, E, and A‖. A⊥ was found to be poorly defined in general and least square refinements attached large errors to this term in the spin Hamiltonian. The zero-field terms in the spin Hamiltonian at 90 K were determined to be D=9.828 GHz, E=83 MHz, A‖=217 MHz for [Cu(CH3CO2)2(CO(NH2)2)]2⋅2H2O; D=9.985 GHz, E=43 MHz, A‖=221 MHz for [Cu(C2H5CO2)2(H2O)]2; D=10.107 GHz, E=49 MHz, A‖=218 MHz for [Cu(C2H5CO2)2(CO(NH2)2)]2; D=9.979 GHz, E=192 MHz, A‖=223 MHz for [Cu(C6H5CO2)2(C6H5CO2H)]2. A model which assumes A⊥=0 is discussed in detail as it was found that all of the features observed in the spectra could be interpreted using simple explicit expressions derived from the model.

Studies of fusion processes of natural glasses by electron magnetic resonance spectroscopy

Electron magnetic resonance (EMR) spectra of three silicate glasses, tektite, fulgurite, and Libyan Desert glass formed by natural processes have been measured. These measurements were made to determine the nature of the natural fusion processes. One of the major trace elements, iron, was detected in the EMR spectra of samples of the three glasses. In the Libyan Desert glass, the iron was primarily Fe 3+ and randomly dispersed. In the tektite, approximately 1% of the iron was Fe 3+, the remaining 99% was Fe 2+, and both were randomly dispersed. In the fulgurite, the iron was in a metallic state and concentrated in spherical particles whose size ranged from multidomain to single domain. The results indicate that the Libyan Desert glass, in contrast to fulgurites and tektites, was fused under oxidizing conditions.

Localized Spins inp-Phenylene Diamine-Bromanil Complexes

Abstract In a number of studies of the electron magnetic resonance spectra of donoracceptor charge transfer complexes,1–3 the temperature dependence of the absorption approximately follows the Curie Law, x ∼ 1/T at temperatures near that of liquid nitrogen. It is generally assumed that in this temperature region most of the absorption is of extrinsic origin. Indeed, Pott and Kommandeur4 showed that such effects could be produced in their samples by varying the method of preparation. The magnitudes of the absorption are outside the limits expected for impurities. Hughes and Soos5 have proposed that the low temperature absorption in p-phenylene diamine-chloranil is due to molecular chains of only finite length with an odd number of radicals. The purpose of this note is to report that in some samples of p-phenylene diamine-bromanil the number of spins appears to be constant over a wide range of temperature, from 77°K to 300°K and that the spins appear to be localized on bromanil radical ions.

Paramagnetic resonance spectra of Ti3+, Fe3+, and Mn2+in lunar plagioclases

At least three paramagnetic and one ferromagnetic components have been resolved in the electron magnetic resonance spectra of plagioclase fractions of two lunar samples, 14053-47 and 14321-166. Measurements were made at 9 and 35 GHz and at 346° 300°, and 130°K. One of the paramagnetic components is attributed to Fe3+ in crystal sites of low symmetries for which the axial terms D are 0.4 ≤ D ≤ 0.6 cm−1 and the ratio of rhombic E to axial terms is 0.24 30 and 3, respectively. The absolute concentration η(Ti3+) in 14053-47 and 14321–166 is ∼3 × 1017 and ∼6 × 1017 g−1, respectively. The concentrations of Fe3+ and Ti3+ are inversely correlated. The ferromagnetic resonance is attributed to single-domain iron particles on the basis of comparison with many lunar samples. A plagioclase fraction from Stillwater specimens was measured for purposes of comparison. The spectrum of Fe3+ in this material was quite similar to the two lunar plagioclases in shape, geff values, and relative proportions in various crystal sites. The concentration was approximately five times greater than that in the sample from 14053-47. A spectral component attributable to Ti3+ was not observed, nor was a ferromagnetic component detected. A Mn2+ spectrum was observed.

A precise g-factor standard for esr measurements in a wide temperature range

The characteristics and use of charred dextrose as a standard for determination of the g-factor in electron magnetic resonance spectra ire described. An arrangement for measuring the temperature dependance of the g factor is given and experimental precautions are discussed.

Paramagnetic resonance of Mo 5+ in tetragonal GeO 2

Electron magnetic resonance spectra of Mo 5+ in tetragonal GeO 2 has parameters: g(001) = 1.857, g(110) = 1.910, g( 110) = 1.951 , A(001) = 2.4×10 −3 cm −1, A(110) = 1.8×10 −3 cm −1, A( 110) = 5.9×10 −3 cm −1 and occupies Ge sites. These values give λ = 0.08 and <1/ r 3>/ N = 3.10 a.u. Excess oxygen is assumed to furnish charge compensation.

Ferromagnetic and paramagnetic resonance spectra of lunar material: Apollo 12

Electron magnetic resonance spectra of specimens of two crystalline rocks (12021-55 and 12075-19) and of four specimens of fines < 1 mm (12001-16, 12030-16, 12033-50 and 12070-125) have been obtained as a function of spectrometer frequency (9 and 35 GHz), temperature (78 to 300K), heat treatments (to 960°C), and mineral phases (plagioclase, olivine, pyroxferroite, glass, and basaltic fragments). Three paramagnetic ions, Fe3+, Ti3+ and Mn2+, are identified on the basis of spectral characteristics in plagioclase fractions, with concentrations ⩽ 1019 ions g−1. Spectral components of at least two phases with exchange coupling of unpaired spins are resolved in ‘whole’ rock samples of the crystalline rock specimens. These disappear upon heat treatment in air at 250°C and are presumably the result of an oxidation of the phases. It is suggested that these are non-stochiometric iron-rich oxide phases which approach stochiometry with heating in air. Some of the spectral properties of the ‘characteristic’ resonance in fines are shown to be inconsistent with the hypothesis that the resonance is due to spherical iron particles. Another intense spectral component observed in samples of 12033-50 (ΔH ~ 3000 Oe,g ~ 4 at 9 GHz andΔH ~ 3000 Oe,g ~ 2.2 at 35 GHz) is also observed in basaltic appearing fragments selected from 12001-16. It is estimated that the source of this component has concentrations in 12001-16, 12030-16, and 12075-125 that are < 2% the concentration in 12033-50. A similarity of this component to one observed in a crystalline rock sample from 12021-55 heat-treated in air and then left in air for 27 days and to one observed in a sample of fines heat treated at low pressure (< 10−3 mm Hg) to 800°C is the basis for suggesting that it is also due to a ferromagnetic oxide phase. The spectrum of a ferromagnetic metal platlet from 12001-16, in which the resonance and antiresonance were well resolved, was used to calculate the saturation magnetization and gave a value in reasonable agreement with that of iron as expected.

Electron and nuclear magnetic resonance of three chondritic meteorites

Chondritic meteorites electron and nuclear magnetic resonance spectra at various frequencies