Temperature Dependence Of Hyperfine Interaction Research Articles

Temperature Dependence of Hyperfine Interaction for 15N Nitroxide in a Glassy Matrix at 10–210 K

Principal 15 N hyperfine interaction (hfi) values in 15 N-substituted nitr- oxide spin probe 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl dissolved in nematic liquid crystal 4-pentyl-4 0 -cyanobiphenyl (5CB) were measured in a wide temperature range of 10-210 K, for 5CB frozen to a glassy state. X-band continuous- wave electron paramagnetic resonance (CW EPR) and pulse X- and Q-band 15 N electron-nuclear double resonance (ENDOR) techniques were employed. To avoid microwave saturation at low temperatures in CW EPR studies, a holmium complex Ho(Dbm)3Bpy (where Dbm is dibenzoylmethane and Bpy is 2,2 0 -bipyridine) was added. X- and Q-band 15 N-ENDOR data have shown that the nitroxide hfi tensor is axially symmetric. The combination of data from all techniques allowed us to obtain the temperature dependence of isotropic and anisotropic parts of the nitroxide hfi tensor. Above *100 K, a linear dependence of the anisotropic hfi value was observed, whereas below 30 K it was found to be nearly temperature independent. Such a behavior can be interpreted using the model of restricted orientational motions (librations) of a spin probe in a glassy matrix, with quantum effects occurring at low temperature (''freezing'' of the librations). The energy quantum for the libration motion estimated from the temperature dependence of hfi of the spin probe is 84 cm -1 . Low-frequency Raman spectra of 5CB were also obtained, which provided the mean vibrational frequency of 76 cm -1 for glassy 5CB.

Molecular Environment and Temperature Dependence of Hyperfine Interactions in Sugar Crystal Radicals from First Principles

The effect of the molecular environment and the temperature dependence of hyperfine parameters in first principles calculations in alpha-d-glucose and beta-d-fructose crystal radicals have been investigated. More specifically, we show how static (0 K) cluster in vacuo hyperfine calculations, commonly used today, deviate from more advanced molecular dynamics calculations at the experimental temperature using periodic boundary conditions. From the latter approach, more useful information can be extracted, allowing us to ascertain the validity of proposed molecular models.

EPR identification of two types of carbon vacancies in4H−SiC

The EI5 and EI6 centers are typical intrinsic defects in radiation-damaged and semi-insulating $4H\ensuremath{-}\mathrm{SiC}.$ So far, their origins have been assigned to positively charged carbon vacancies ${(V}_{\mathrm{C}}^{+})$ and silicon antisites $({\mathrm{Si}}_{\mathrm{C}}^{+}),$ respectively. However, our complete set of ${}^{29}\mathrm{Si}$ hyperfine (HF) data clearly reveals that both the centers should originate from ${V}_{\mathrm{C}}^{+}$ but their locations are different, i.e., quasicubic sites for EI5 and hexagonal sites for EI6, as recently predicted by the first-principle calculation [M. Bockstedte et al., Phys. Rev. B 67, 193102 (2003)]. The two types of ${V}_{\mathrm{C}}^{+}$ centers showed remarkable differences in their atomic structures as well as in the temperature dependence of HF interactions, which are closely related to the nature of the two sites.

Effect of Fe-exchange-field penetration on the residual amorphous phase in nanocrystalline Fe92Zr8

The temperature dependence of hyperfine interactions has been studied by means of Mössbauer spectroscopy for rapidly quenched amorphous Fe92Zr8 in as-quenched and partially crystallized states. An analysis of the temperature dependence of the mean hyperfine field based on the molecular-field approximation showed that the Curie temperature of the residual intergranular amorphous phase (TCam) is 330±5 K, well above the maximum Curie temperature in amorphous Fe–Zr binary alloys (TC*≈280 K). The difference of TCam−TC* (ΔTCam≈50 K) cannot be attributed to the possible magnetovolume effect in the sample. The difference of ΔTCam could be explained by an enhancement of the molecular field by ∼50 T at 0 K, induced by the Fe exchange field penetrating from the bcc-Fe nanocrystallites.

Mn-site hyperfine fields inLaMnO3andNdMnO3measured using perturbed-angular-correlation spectroscopy

Using perturbed-angular-correlation spectroscopy, we have measured the temperature dependence of hyperfine interactions at ${}^{181}\mathrm{Ta}$ nuclei at Mn sites in ceramic samples of nominally stoichiometric ${\mathrm{LaMnO}}_{3}$ and ${\mathrm{NdMnO}}_{3}.$ Magnetic hyperfine fields (MHF's) and electric-field gradients (EFG's) were measured from 10 to 296 K. The perturbation functions measured for both compounds show sharp line shapes, indicating low defect populations in the vicinity of the probe ions. The Mn-site EFG's show a weak dependence on temperature in both compounds, and the associated quadrupole frequencies and EFG asymmetries are very large. At laboratory temperature, ${\ensuremath{\omega}}_{Q}=158.3(2){\mathrm{Mrad}\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and $\ensuremath{\eta}=0.82(1)$ for ${\mathrm{LaMnO}}_{3},$ and ${\ensuremath{\omega}}_{Q}=190.9(3){\mathrm{Mrad}\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and $\ensuremath{\eta}=0.80(2)$ for ${\mathrm{NdMnO}}_{3}.$ Below the respective magnetic ordering temperatures of 138 and 85 K for the La and Nd compounds, the dependence on reduced temperature of the observed supertransferred MHF's follows a power law, with an exponent of 0.41(1) in each case. The analysis of the combined magnetic-dipole and electric-quadrupole interaction yields the angle \ensuremath{\beta} between the principal z axis of the EFG tensor and the direction of the MHF, which is 56.5(5)\ifmmode^\circ\else\textdegree\fi{} at 10 K for ${\mathrm{LaMnO}}_{3}$ and 62.5(5)\ifmmode^\circ\else\textdegree\fi{} at 10 K for ${\mathrm{NdMnO}}_{3}.$ For ${\mathrm{LaMnO}}_{3},$ the angle \ensuremath{\beta} shows negligible temperature dependence, whereas for ${\mathrm{NdMnO}}_{3}$ this angle decreases linearly with increasing temperature.

A 155Gd Mössbauer study of exchange interactions in Gd2Fe14B

The temperature dependence of hyperfine interactions at 155Gd nuclei in Gd2Fe14B has been studied in the temperature range from 4.2 to 120 K. The results indicate that the Fe-Gd exchange interactions at the two Gd sites may be different. The frequency spectrum of Gd vibrations has been derived from the temperature dependence of the absorption intensity.

Temperature dependence of hyperfine interactions in incommensurate systems

One investigates the influence of thermally excited collective modes in incommensurate systems on the temperature dependence of hyperfine interactions. The observed hyperfine fields are reduced by a temperature-dependent Debye-Waller factor and the hyperfine field amplitude can eventually decrease to zero well below the transition temperature.

Pb3+-ions in CaWO4: Temperature dependence of hyperfine interaction

The hyperfine interaction of the (6s)1 electronic configuration of Pb3+ with Pb-207-nuclei (I=1/2) shows a temperature dependent Fermi-contact-term obeying aT4 × Debyeintegral law. This effect is correlated to increasing delocalisation with temperature.

VOLUME DEPENDENCE OF HYPERFINE INTERACTIONS

The influence of changes in atomic volume on the three types of hyperfine interactions (the isomer shift, the quadrupole coupling and the magnetic dipole interaction) is reviewed. The volume dependence of a hyperfine interaction is usually obtained from high-pressure experiments, which are discussed in some detail. Particular emphasis is given to the interpretation of the volume coefficients of the Mossbauer isomer shift in metals and compounds. The volume coefficient of a hyperfine interaction can be used to interpret other phenomena which are accompanied by volume changes. A typical example is the temperature dependence of hyperfine interactions in solids. A selection of measurements of this kind is discussed to give some insight into the problems connected with such investigations.

Temperature dependence of hyperfine interactions in Dy-Gd alloys

The magnetic hyperfine field and quadrupole interaction have been measured in pure dysprosium and three gadolinium-dysprosium alloys using the M\"ossbauer effect on the dysprosium nucleus. The temperature dependences of ${H}_{\mathrm{hf}}$ and ${V}_{\mathrm{zz}}$ were determined from 4 K up to the ordering temperatures and compared to theory. The hyperfine field obeyed a conventional spin-wave analysis with an anisotropy gap of order 20 K. The quadrupole interaction was found to have the same temperature dependence as predicted theoretically for the single-ion anisotropy. No systematic variation in the low temperature values of ${H}_{\mathrm{hf}}$ or ${V}_{\mathrm{zz}}$ was observed with concentration.

Temperature Dependence of Hyperfine Interactions in Near‐Stoichiometric FeS I. Experiment

AbstractMössbauer measurements as a function of temperature are presented for three synthetic 57Fe‐enriched polycrystalline samples of FeS in the composition region near stoichiometry. The independence of the Morin and α‐transitions, previously concluded from neutron diffraction measurements, has been verified. A low‐temperature transition has been found near 200 °K. It appears necessary to postulate the existence of a reversible, crystallographic two‐phase region between the α‐transition and about 200 °K in order to explain the Mössbauer measurements. This hypothesis also seems to provide a logical explanation of several anomalies in FeS.

Interpretation of Temperature Dependence of Hyperfine Interactions in Metal—Amine Solutions

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