Spin-lattice Relaxation Behavior Research Articles

Solid-state NMR of vulcanized natural rubber / butadiene rubber blends: Local organization and cross-linking heterogeneities

Elastomer blends, among which natural rubber (NR) and butadiene rubber (BR), are involved in many components of the automotive/tire industry. A comprehensive understanding of their mechanical behavior requires, among other features, a detailed description of the cross-link density in these mixtures. In the case of vulcanized immiscible blends, the distribution of the cross-link density within each of the NR- and BR-rich domains is key information, but difficult to determine using the conventional approaches used for one-component cross-linked elastomers. In this study, the vulcanization within NR/BR blends is investigated using a robust 1H double-quantum (DQ) MAS recoupling experiment, BaBa-xy16. Two kinds of cross-linked NR/BR blends were considered with two different microstructures for the BR component. The bulk organization of the resulting blends was first probed by analyzing the 1H spin-lattice relaxation behavior. In a second step, BaBa-xy16 was used to investigate, in a selective way, the cross-link heterogeneities within NR/BR blends. In particular, for immiscible NR/BR mixtures, the distribution of the cross-link density between both phases was compared and the observed differences were discussed.

Temperature-Dependent Spin-Lattice Relaxation of the Nitrogen-Vacancy Spin Triplet in Diamond.

Spin-lattice relaxation within the nitrogen-vacancy (NV) center's electronic ground-state spin triplet limits its coherence times, and thereby impacts its performance in quantum applications. We report measurements of the relaxation rates on the NV center's |m_{s}=0⟩↔|m_{s}=±1⟩ and |m_{s}=-1⟩↔|m_{s}=+1⟩ transitions as a function of temperature from 9 to 474K in high-purity samples. We show that the temperature dependencies of the rates are reproduced by an abinitio theory of Raman scattering due to second-order spin-phonon interactions, and we discuss the applicability of the theory to other spin systems. Using a novel analytical model based on these results, we suggest that the high-temperature behavior of NV spin-lattice relaxation is dominated by interactions with two groups of quasilocalized phonons centered at 68.2(17) and 167(12)meV.

On the microscopic origins of relaxation processes in aqueous peptide solutions undergoing a glass transition.

We combine broadband dielectric spectroscopy (BDS) with 1H and 2H nuclear magnetic resonance (NMR) to study molecular dynamics in mixtures of ε-polylysine with H2O or D2O. In BDS, four relaxation processes can be attributed to molecular dynamics. While the fastest process P1 obeys the Arrhenius law, the slowest process P4 shows prominent non-Arrhenius behavior typical of structural α relaxation. For the intermediate processes P2 and P3, the temperature dependence changes at the glass transition temperature Tg. The 1H and 2H NMR results yield insights into the molecular origins of these relaxation phenomena. In these NMR analyses, we exploit, in addition to the isotope selectivity of the method, the possibility to distinguish between various types of motion based on their respective line-shape effects and the capability to single out specific molecular moieties based on different spin-lattice relaxation behaviors. In this way, we reveal that process P1 results from the rotation of side and end groups of the peptide, while process P2 is caused by a reorientation of essentially all water molecules, which are quasi-isotropic and survive well below Tg. As for the peptide backbone dynamics, we find evidence that rotational motion of polar groups is involved in process P3 and that nonpolar regions show a dynamical process, which is located between P3 and P4. Thus, the NMR analyses do not yield evidence for coexisting fast peptide-decoupled and slow peptide-coupled water species, which contribute to BDS processes P2 and P3, respectively, but minor bimodality of water motion may remain undetected. Finally, it is demonstrated that the proton/deuteron exchange needs to be considered when interpreting experimental results for molecular dynamics in aqueous peptide solutions.

Analysis of the defect clusters in congruent lithium tantalate

A wide range of technological applications of lithium tantalate (LT) is closely related to the defect chemistry. In literature, several intrinsic defect models have been proposed. Here, using a combinational approach based on DFT and solid-state NMR, we demonstrate that distribution of electric field gradients (EFGs) can be employed as a fingerprint of a specific defect configuration. Analyzing the distribution of $^{7}\mathrm{Li}$ EFGs, the FT-IR and electron spin resonance (ESR) spectra, and the $^{7}\mathrm{Li}$ spin-lattice relaxation behavior, we have found that the congruent LT samples provided by two manufacturers show rather different defect concentrations and distributions although both were grown by the Czochralski method. After thermal treatment hydrogen out-diffusion and homogeneous distribution of other defects have been observed by ESR, NMR, and FT-IR. The defect structure in one of two congruent LT crystals after annealing has been identified and proved by defect formation energy considerations, whereas the more complex defect configuration, including the presence of extrinsic defects, has been suggested for the other LT sample. The approach of searching the EFG fingerprints from DFT calculations in NMR spectra can be applied for identifying the defect clusters in other complex oxides.

Xenon-Water Interaction in Bacterial Suspensions as Studied by NMR

Xenon is a perspective gas for creation of oxygen free environment for different applications of biomaterials. To use xenon in suspensions and products properly it is necessary to know the molecular mechanisms of its interactions with water and cells. This work reports the study of bacterial suspensions of Escherichia coli in the presence of xenon using nuclear magnetic resonance (NMR). The work studied how the spin-lattice relaxation times of water protons in suspension change under xenon conditions. Xenon is able to form clathrate hydrates with water molecules at a temperature above the melting point of ice. The work studied NMR relaxation times which reflect the rotation freedom of water molecules in suspension. Lower relaxation times indicate reduced rotational freedom of water. Single exponential behavior of spin-lattice relaxation of protons in the suspensions of microorganisms has been registered. A recovery of longitudinal magnetization in cell suspensions with xenon clathrates has been characterized by two peaks in T1-distribution. Fast relaxing T1-component was related to the intracellular water and depended on the amount of xenon clathrates. The obtained results elucidate how the NMR method can monitor the process of clathrate formation and how the xenon atoms and hydrates interact with cells.

Thermal Activation of a Pure Montmorillonite Clay and Its Reactivity in Cementitious Systems

Clay minerals are potential candidates as raw materials for new supplementary cementitious materials (SCMs) that can partly replace Portland cement and thereby significantly reduce CO2 emissions associated with cement production. We present the characterization of the complex, disordered structure of a pure montmorillonite clay heated at various temperatures (110–1100 °C), by solid-state 27Al and 29Si MAS NMR methods. The SiO4 tetrahedra and AlO6 octahedral sites become progressively more distorted, exhibit a significant degree of disorder upon dehydroxylation (600–800 °C), and do not lead to the formation of any metastable phase. At high temperatures (1000–1100 °C), the layer structure of the clay breaks down, forming stable crystalline phases. The chemical reactivity, measured as the degree of dissolution/precipitation in an alkaline solution, is found to be proportional to the degree of disorder/dehydroxylation. The maximum reactivity as a function of the heating temperature is achieved at 800 °C prior to the formation of inert, condensed Q4-type phases in the material. At maximum reactivity the calcium silicate hydrate (C-S-H) phase contains silicate chains with the highest aluminum incorporation, leading to blended cements containing a C-S-H phase with longer chain lengths. Most importantly, by exploiting the differential spin–lattice relaxation behavior of the 29Si spins, evidence of multiple sites and components in both the naturally occurring and heated montmorillonite is being reported for the first time.

Bonding characters of Al-containing bulk metallic glasses studied by 27Al NMR

We report very small 27Al metallic shifts in a series of Cu–Zr–Al bulk metallic glasses. This observation and theKorringa type of spin–lattice relaxation behavior suggest that s-character wavefunctionsweakly participate in bonding and opens the possibility of enhanced covalency (pdhybridization) with increasing Al concentration, in good agreement with elastic constantsand hardness measurements. Moreover, ab initio calculations show that this bondingcharacter originates from the strong Al 3p band and Zr 4d band hybridizationsince their atomic energy levels are closer to each other while the Al 3s band islocalized far below the Fermi level. This study might provide a chemical view forunderstanding flow and fracture mechanisms of these bulk glass-forming alloys.

Solid-state 13C NMR analyses of the structures of crystallized and quenched poly(lactide)s: Effects of crystallinity, water absorption, hydrolytic degradation, and tacticity

The effects of crystallinity, water absorption, hydrolytic degradation, and tacticity on the solid structure and chain mobility of poly(lactide)s were investigated by solid-state 13C NMR spectroscopy. The following results were obtained from the line shapes of the carbonyl and methine carbons in 13C NMR spectra and their spin-lattice relaxation behavior. The crystallized poly( l-lactide) (PLLA) specimens in the dried, hydrated, and hydrolyzed states had two components, rigid and mobile components which can be, respectively, assigned to the crystalline and non-crystalline components. Upon water absorption, the chain mobility in the non-crystalline component of PLLA-C remained unvaried, reflecting a very small effect of the incorporated water molecules at room temperature. In contrast, the elevated chain mobility in the crystalline component and unclear splitting of carbonyl carbon strongly suggest that the water molecules are incorporated in the crystalline lattice. Upon removal of the non-crystalline components by hydrolytic degradation of crystallized PLLA, the chain mobility was slightly elevated in both crystalline and non-crystalline components by the lowered crystalline thickness and shortened non-crystalline chains. The non-crystalline specimens, PLLA (PLLA-Q) and poly( dl-lactide) (PDLLA), could be analyzed to contain two components, rigid and soft components, with the similar conformation but different restricted states of chains which cause high and low chain mobility. The insignificant difference in the spectral shapes and T 1C values between PLLA-Q and PDLLA strongly suggests that the effects of difference in the chain regularity and interaction on the spectral shapes and T 1C values are very low.

Investigation of Trans Content in Amorphous Phase of stretched Polyethylene Terephthalate by Solid-state High resolution 13C NMR Spectroscopy

Crystalline, constrained noncrystalline, and amorphous phase structures for polyethylene terephthalate (PET) stretched at 75, 80, and 90°C were investigated by solid-state high resolution 13 C NMR. Saturation recovery pulse sequence is a very powerful tool for investigating not only the molecular structure but also the molecular mobility. Spin-lattice relaxation behavior of trans methylene carbon was decomposed by three components, which are assignable to crystalline, constrained noncrystalline, and amorphous phases. The weight content of each component can be estimated by the relaxation behavior. The stretching process induced the transformation from gauche to trans conformation. The trans content with the longest T 1c corresponds to the crystallinity, which was in good agreement with DSC measurement. The trans content in the amorphous phase was also estimated by the ratio between the gauche content assignable to the amorphous phase and the amorphous component of the trans conformer decomposed by relaxation measurement. The trans content of unstretched PET was as low as 21%, whereas that for five-times stretched PET was about 60%.

Study of the Adsorption of Organic Molecules on Transition Metal Exchanged Zeolites via Solid State NMR. Part 2: Adsorption of Organic Molecules on Zeolite NaX, CaX, and CaCoX

Literature [Denayer et al. Microporous Mesoporous Mater. 2007, 103, 1 and Denayer et al. Microporous Mesoporous Mater. 2007, 103, 11] shows that zeolite NaX exchanged with Ca(2+) and Co(2+) ions is able to remove cyclopentadiene (CPD) impurities from a 1-octene feed with high selectivity. In the present work, the adsorption of dicyclopentadiene (DCPD), CPD, 1-octene, and n-octane on zeolite X, exchanged with Ca(2+) and/or Co(2+) ions, has been investigated via (1)H magic-angle spinning (MAS) NMR spectroscopy. The liquid adsorbate was dosed under inert atmosphere in an MAS rotor filled with dry adsorbent, at a pore filling degree of 70%. Next, the evolution in time was recorded of the (1)H MAS NMR spectrum and the (1)H spin-lattice and spin-spin relaxation times of the adsorbed components. For the various adsorbate-adsorbent systems, a plot is made of the signal intensity versus the square root of the contact time. It is found that, over the considered time interval, Fickian diffusion takes place. On the basis of the change in time of the spin-lattice relaxation time, a transport diffusion coefficient ranging between 1 and 2 x 10(-15) m(2) x s(-1) is calculated. Moreover, there appear to be two sorption regimes, with different diffusivities. A comparison is made between the (1)H spin-lattice relaxation behavior of DCPD, 1-octene, and n-octane, indicating that 1-octene and n-octane are located closer to the paramagnetic ions than DCPD. The average distance between the adsorbate molecules and the paramagnetic ions is derived from relaxometric data. By analyzing the chemical shifts of the resonance lines, it is found that the pi-interaction of CPD and 1-octene is stronger than that of DCPD.

Phase-transition mechanisms of [N(C 2H 5) 4] 2BCl 4 and [N(CH 3) 4] 2BCl 4 (B= 63Cu and 67Zn) single crystals studied by proton NMR

The proton line widths and spin-lattice relaxation rates of [N(C 2H 5) 4] 2BCl 4 (hereafter denoted [TEA] 2BCl 4, where B= 63Cu or 67Zn) and [N(CH 3) 4] 2BCl 4 (hereafter denoted [TMA] 2BCl 4) single crystals were investigated in the temperature range 150–430 K. Our experimental results show that the relaxation rates of [TEA] 2CuCl 4 and [TMA] 2CuCl 4 differ from those of [TEA] 2ZnCl 4 and [TMA] 2ZnCl 4. In the case of [TEA] 2ZnCl 4 and [TMA] 2ZnCl 4, the maxima in the spin-lattice relaxation rates are attributed to the molecular motions of the ethyl and methyl groups in the [TEA] + and [TMA] + ions, respectively. The NMR line widths indicate that the ethyl groups in the [TEA] 2BCl 4 compounds have one more degree of freedom than the methyl groups in the [TMA] 2BCl 4 compounds. The differences between the 1H spin-lattice relaxation rates of these compounds are probably due to differences between the electronic structures of the metal ions, in particular between the structures of the d electrons, which screen the nuclear charge from the motion of the outer electrons. Further, the 1H spin-lattice relaxation behaviors of the Cu-based compounds are driven by fluctuations of the dipolar magnetic field of the Cu 2+ ion, and are dominated by Cu spin fluctuations.

New Approach for Characterization of Gelatin Biopolymer Films Using Proton Behavior Determined by Low Field 1H NMR Spectrometry

The behavior of protons in biopolymer films (BFs) formed with gelatin, water, and glycerol was investigated at various relative humidities (RHs) and concentrations of glycerol using a low field 1H NMR spectrometer. At a RH of approximately 0%, the distributed spin-spin relaxation times (T2) of protons in BFs showed two components: a rapidly relaxing proton with the shortest T2 derived from protons in the rigid backbone of the gelatin polymer such as CH1-, CH2-, and CH3-, and a slowly relaxing component with longer T2 from protons of the functional groups in amino acid residues in gelatin such as -OH, -COOH, and -NH3. These two components are referred to as nonexchangeable (T2N) and exchangeable protons (T2E), respectively, indicating the different mobility of the protons. The T2E increased as RH increased indicating the increase in relative mobility of protons due to the larger free volume in the BF matrix. Above a RH of 33%, the slowest relaxing component was found in all BFs and referred to as hydration-water protons (T2W) with the highest relative mobility of all protons in the films. It suggests that the free volume in BFs can be formed above a RH of 33% in the absence of glycerol. The behaviors of T2N, T2E, and T2W reveal the formation of free volume in the BF matrix associated with the presence of plasticizers (water and glycerol). The T2 behavior in BFs is consistent with the behavior of spin-lattice relaxation (T1). Our result is the first attempt to characterize using low field 1H NMR technology how all protons in a film matrix behave and to develop correlations between proton mobility and free volume in protein-based BFs plasticized with water and glycerol.

Purity analysis of hydrogen cyanide, cyanogen chloride and phosgene by quantitative 13C NMR spectroscopy

Hydrogen cyanide, cyanogen chloride and phosgene are produced in tremendously large quantities today by the chemical industry. The compounds are also particularly attractive to foreign states and terrorists seeking an inexpensive mass-destruction capability. Along with contemporary warfare agents, therefore, the US Army evaluates protective equipment used by warfighters and domestic emergency responders against the compounds, and requires their certification at > or = 95 carbon atom % before use. We have investigated the (13)C spin-lattice relaxation behavior of the compounds to develop a quantitative NMR method for characterizing chemical lots supplied to the Army. Behavior was assessed at 75 and 126 MHz for temperatures between 5 and 15 degrees C to hold the compounds in their liquid states, dramatically improving detection sensitivity. T(1) values for cyanogen chloride and phosgene were somewhat comparable, ranging between 20 and 31 s. Hydrogen cyanide values were significantly shorter at 10-18 s, most likely because of a (1)H--(13)C dipolar contribution to relaxation not possible for the other compounds. The T(1) measurements were used to derive relaxation delays for collecting the quantitative (13)C data sets. At 126 MHz, only a single data acquisition with a cryogenic probehead gave a signal-to-noise ratio exceeding that necessary for certifying the compounds at > or = 95 carbon atom % and 99% confidence. Data acquired at 75 MHz with a conventional probehead, however, required > or = 5 acquisitions to reach this certifying signal-to-noise ratio for phosgene, and >/= 12 acquisitions were required for the other compounds under these same conditions. In terms of accuracy and execution time, the NMR method rivals typical chromatographic methods.

Heterogeneities in sol–gel-derived paramagnetics-doped forsterites and willemites — Electron microprobe analysis and stretched-exponential29Si MAS NMR spin–lattice relaxation studies

We report the synthesis and analysis of sol–gel-derived samples of forsterite (Mg2SiO4) and willemite (Zn2SiO4), doped with paramagnetic Cu2+, Ni2+, and Co2+, at a range of dopant concentrations. Electron probe microanalysis and backscattered electron imaging show the presence of major micrometre-scale heterogeneities in the distribution of paramagnetic centres. Despite the inhomogeneities, the29Si NMR spin–lattice relaxation behaviour is well-behaved and is consistent with the stretched-exponential expression Mz(t) = Mz(∞){1 – a exp[–(t/T′)n]}. The exponent n is 0.5 within the experimental error in some samples. This value is consistent with relaxation by immobile isolated paramagnetic impurities with negligible29Si spin diffusion from the impurity centres, but careful curve fitting confirms that n is significantly larger than 0.5 in other samples. Relaxation efficiency is highly dependent on the dopant ion and its concentration. Although the purely empirical stretched-exponential function does not provide a unique physical picture, it is noteworthy that it is sufficiently robust to describe spin–lattice relaxation even in highly inhomogeneous systems. Spin–lattice relaxation is a useful probe of paramagnetics-doped solid samples, but NMR does not provide information on homogeneity. Careful sample characterization on the micrometre scale is highly desirable, as a complement to NMR studies.Key words: MAS NMR, spin–lattice relaxation,29Si, forsterite, willemite, stretched-exponential relaxation, sol–gel, minor-component heterogeneity, backscattered electron analysis.

Size-dependent properties ofTl2Sestudied by NMR spectroscopy

We report on size-dependent properties of dithallium selenide, ${\mathrm{Tl}}_{2}\mathrm{Se}$. We have carried out a comparative nuclear magnetic resonance (NMR) study of ${\mathrm{Tl}}_{2}\mathrm{Se}$ nanorods and bulk samples, measuring NMR spectra and spin-lattice relaxation rate of $^{203}\mathrm{Tl}$ and $^{205}\mathrm{Tl}$ isotopes. Though bulk ${\mathrm{Tl}}_{2}\mathrm{Se}$ was reported to be a metal, the Korringa-like spin-lattice relaxation behavior is observed only at low temperatures and is transformed to an activation regime above $\ensuremath{\sim}200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This finding is interpreted assuming a two-band model in the semimetallic compound. Our measurements show significant difference in the Knight shift and indirect nuclear exchange coupling for the bulk and nanorod ${\mathrm{Tl}}_{2}\mathrm{Se}$ samples, reflecting noticeable distinction in their electronic structure. At that, ${\mathrm{Tl}}_{2}\mathrm{Se}$ nanorods are semiconductors and exhibit a characteristic activation behavior in the spin-lattice relaxation rate due to the thermal excitation of carriers to the conduction band. The obtained size dependence of the ${\mathrm{Tl}}_{2}\mathrm{Se}$ properties is interpreted in terms of the semimetal-semiconductor transformation due to the quantum confinement.

C13NMR spectroscopy of carbon nanohorns

We report the results of a carbon-13 nuclear magnetic resonance spectroscopic investigation of the structure of carbon nanohorn aggregates (CNHs). The results show that CNHs consist of two components, characterized by different chemical shifts and spin lattice relaxation $({T}_{1})$ behavior. The first component has a chemical shift of $124\phantom{\rule{0.3em}{0ex}}\mathrm{ppm}$ and displays rapid spin-lattice relaxation behavior and is assigned to the nanotubelike horns on the particles' surfaces. The second component has a chemical shift of 116 ppm and much slower spin-lattice relaxation behavior and is assigned to the graphitelike part of the CNH aggregrate. The results of integrated peak area measurements indicate a 1:2 ratio of nanohorns to the graphitelike substrate. The absence of a clear Korringa behavior for the temperature dependence of ${T}_{1}$ and the lack of a Knight shift ruled out any metallic behavior and indicated instead behavior characteristic of semiconductor materials with paramagnetic centers due to structural defects providing an effective relaxation mechanism in the nanohorn domains. We also observed an anomalous change in ${T}_{1}$ near $17\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the nanohorn domains suggesting the development of an antiferromagnetic correlation between localized electron spins.

Magnetization-recovery experiments for static and MAS-NMR of I = 3/2 nuclei

Multifrequency pulsed NMR experiments on quadrupole-perturbed I = 3/2 spins in single crystals are shown to be useful for measuring spin–lattice relaxation parameters even for a mixture of quadrupolar plus magnetic relaxation mechanisms. Such measurements can then be related to other MAS-NMR experiments on powders. This strategy is demonstrated by studies of 71Ga and 69Ga (both I = 3/2) spin–lattice relaxation behavior in a single-crystal (film) sample of gallium nitride, GaN, at various orientations of the axially symmetric nuclear quadrupole coupling tensor. Observation of apparent single-exponential relaxation behavior in I = 3/2 saturation-recovery experiments can be misleading when individual contributing rate processes are neglected in the interpretation. The quadrupolar mechanism (dominant in this study) has both a single-quantum process ( T 1Q1) and a double-quantum process ( T 1Q2), whose time constants are not necessarily equal. Magnetic relaxation (in this study most likely arising from hyperfine couplings to unpaired delocalized electron spins in the conduction band) also contributes to a single-quantum process ( T 1M). A strategy of multifrequency irradiation with observation of satellite and/or central transitions, incorporating different initial conditions for the level populations, provides a means of obtaining these three relaxation time constants from single-crystal 71Ga data alone. The 69Ga results provide a further check of internal consistency, since magnetic and quadrupolar contributions to its relaxation scale in opposite directions compared to 71Ga. For both perpendicular and parallel quadrupole coupling tensor symmetry axis orientations small but significant differences between T 1Q1 and T 1Q2 were measured, whereas for a tensor symmetry axis oriented at the magic-angle (54.74°) the values were essentially equal. Magic-angle spinning introduces a number of complications into the measurement and interpretation of the spin–lattice relaxation. Comparison of 71Ga and 69Ga MAS-NMR saturation-recovery curves with both central and satellite transitions completely saturated by a train of 90° pulses incommensurate with the rotor period provides the simplest means of assessing the contribution from magnetic relaxation, and yields results for the quadrupolar mechanism contribution that are consistent with those obtained from the film sample.

Pressure effect on CeAl 2 investigated by AC- [formula omitted] and 27Al-NQR measurements

To elucidate microscopically the transformation from the Fermi liquid to non-Fermi liquid states of an antiferromagnetic (AFM) Kondo compound CeAl 2, we have carried out AC- χ and 27Al-NQR measurements at pressures up to P = 2.4 GPa . The AFM transition temperature T N is suppressed from 3.8 ( P = 0 ) to ≃ 2.6 K (2.4 GPa), and the NQR spectrum in the AFM ordered state varies in shape from a complex structure at P = 0 with large linewidth ( Δ H ∼ 400 kHz ) to a Lorentzian-like structure with Δ H ≃ 200 kHz . This is ascribed to the transformation from a mixing-phase of the incommensurate and commensurate orderings at P = 0 to a single phase of commensurate above ∼ 2 GPa . Variations in the spin–lattice relaxation behavior of 27Al with pressure indicate the increase of the itinerant electron character and the density of states at the Fermi level.

Nuclear magnetic moment ofAs69from on-line β-NMR on oriented nuclei

A precise value for the magnetic moment of the $^{69}\mathrm{As}$ $5/{2}^{\ensuremath{-}}$ ground state has been obtained from nuclear magnetic resonance on oriented nuclei (NMR/ON) using the NICOLE $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ dilution refrigerator setup at ISOLDE/CERN. The NMR/ON signal was observed by monitoring the anisotropy of the $^{69}\mathrm{As}$ \ensuremath{\beta} particles. The center frequency $\ensuremath{\nu}[{B}_{\mathrm{ext}}=0.0994(10)T]=169.98(9)$ MHz corresponds to $\ensuremath{\mu}[$$^{69}\mathrm{As}$$]=+1.6229(16){\ensuremath{\mu}}_{N}$. This result differs considerably from the $\ensuremath{\pi}{f}_{5/2}$ single-particle value obtained with g factors for a free proton but is in reasonable agreement with the value obtained with effective g factors and with values from a core polarization calculation and from calculations in the framework of the interacting boson-fermion model. Assuming a single exponential spin-lattice relaxation behavior a relaxation time ${T}_{1}^{'}=10(25)$ s was observed for $^{69}\mathrm{As}$$\underset{\ifmmode\bar\else\textasciimacron\fi{}}{\mathrm{Fe}}$ at a temperature of about 20 mK in a magnetic field $B=0.1$ T.

NMR and Mössbauer study of spin-glass behavior inFeAl2

We have used NMR and M\"ossbauer spectroscopy to investigate the magnetism and spin-glass behavior in $\mathrm{Fe}{\mathrm{Al}}_{2}$. The results show that the observed behavior can be interpreted as local-moment magnetism residing on the Fe sites. An increase in $^{57}\mathrm{Fe}$ M\"ossbauer magnetic hyperfine field was observed at the $35\text{\ensuremath{-}}\mathrm{K}$ spin-glass freezing temperature, with a reduced low-temperature value consistent with unquenched fluctuations in the spin-glass state. However, the $^{27}\mathrm{Al}$ NMR shifts and spin-lattice relaxation behavior are consistent with transfer hyperfine interactions to Fe local moments, with a magnitude of approximately $2.5\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$. This value is consistent with the bulk magnetization, but surprising compared to other Fe aluminides, given the small $\mathrm{Fe}\ensuremath{-}\mathrm{Fe}$ coordination number and associated weakening expected for the magnetic moment.