Nuclear Magnetic Resonance Line Shift Research Articles

5f -electron magnetism in single crystal UN probed by N14 NMR

Spin susceptibility and low-frequency dynamics of uranium $5f$ electrons have been investigated by nuclear magnetic resonance (NMR) on the $^{14}\mathrm{N}$ nuclei in paramagnetic and magnetically ordered phases for single crystalline and polycrystalline samples of uranium mononitride (UN). NMR spectra, shifts of the $^{14}\mathrm{N}$ NMR lines, and the spin-lattice relaxation times ${T}_{1}$ have been obtained in the temperature range $T=10--760\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in magnetic field $B=92.8\phantom{\rule{0.16em}{0ex}}\mathrm{kOe}$. It is shown that in the UN paramagnetic phase, temperature dependence of the $^{14}\mathrm{N}$ NMR line shift is proportional to the spin susceptibility of the uranium $5f$ electrons. Joint analysis of NMR and magnetic susceptibility data allows us to determine temperature dependence of spin fluctuation energy ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{nmr}}(T)$ of the uranium $5f$ electrons and to demonstrate that its temperature variation is close to $\mathrm{\ensuremath{\Gamma}}(T)\ensuremath{\propto}{T}^{0.5}$ dependence which is characteristic of the concentrated Kondo systems above the coherent state formation temperature. In the magnetically ordered UN phase the $^{14}\mathrm{N}$ NMR spectra consist of several lines that can be explained in terms of the model of type $I$ antiferromagnetic order corresponding to $1k$ structure in the presence of magnetic domains.

Spin dynamics in the alternating chain system Li3Cu2SbO6 with defects probed by nuclear magnetic resonance

We report the results of the detailed measurements of $^{7}\mathrm{Li}$ nuclear magnetic resonance (NMR) spectra, NMR line shift, spin-lattice relaxation rate, bulk magnetization, magnetic susceptibility, and heat capacity on the spin-$\frac{1}{2}$ Heisenberg honeycomb compound ${\mathrm{Li}}_{3}{\mathrm{Cu}}_{2}{\mathrm{SbO}}_{6}$. This system consists of weakly coupled alternating Cu chains and, owing to the site inversion, some of the magnetic Cu ions are substituted by nonmagnetic Li ions. As a result, the effective spin model for ${\mathrm{Li}}_{3}{\mathrm{Cu}}_{2}{\mathrm{SbO}}_{6}$ is a set of Cu alternating chain fragments of different length (the ``chain-segments spin model''). Due to this unique structure, various kinds of peculiar magnetic features arise and the combination of local (NMR) and global experimental techniques is especially effective in their study. In order to describe the experimental results theoretically, we perform full diagonalization calculations for the chain-segments spin model taking explicit account of all of the possible dimerized chain fragments of different length. From the fitting of measured susceptibility, we estimate the exchange interactions as ${J}_{\mathrm{FM}}=\ensuremath{-}244\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${J}_{\mathrm{AF}}=146\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and find that about 19% of magnetic Cu ions are replaced by nonmagnetic Li ions. Using those values, we can even quantitatively reproduce the observed NMR line shift and the specific heat as well as field- and temperature-dependent magnetization over a wide temperature range. Although small deviations are seen at very low temperature, they can be corrected by considering the effect of small interchain interactions.

Spin- 12 chain compound Ba2Cu2Te2P2O13 : Magnetization, specific heat, and local-probe NMR

We present synthesis and characterization of ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{P}}_{2}{\mathrm{O}}_{13}$ (BCTPO) via x-ray diffraction, magnetic susceptibility, heat capacity, and $^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) measurements. BCTPO crystallizes in a monoclinic structure with ${\mathrm{Cu}}^{2+}$ ions forming coupled chains. In magnetization $M(T)$ measurements, a broad maximum is observed around 53 K. Upon the application of magnetic fields, a sharp anomaly evolves around 4 K in the heat capacity ${C}_{P}(T)$ data, indicative of long-range order, while it is absent in zero field. The $^{31}\mathrm{P}$ NMR line shift at about 93.954 kOe tracks the spin susceptibility, whereas the line broadening evidences the onset of magnetic order at about 4 K or lower. The inferred isotropic and axial components of the hyperfine coupling constant are ${A}_{hf}^{\mathrm{iso}}\ensuremath{\simeq}$ 3635 $\text{Oe}/{\ensuremath{\mu}}_{B}$ and ${A}_{hf}^{ax}\ensuremath{\simeq}$ 479 $\text{Oe}/{\ensuremath{\mu}}_{B}$, respectively. The $^{31}\mathrm{P}$ NMR spin-lattice relaxation rate $1/{T}_{1}$ shows a pronounced divergence for $T\ensuremath{\longrightarrow}0$. We assign this behavior to critical fluctuations on the verge of order.

Singlet ground state in the alternating spin- 12 chain compound NaVOAsO4

We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline NaVOAsO$_4$ by means of x-ray diffraction, magnetization, electron spin resonance (ESR), and $^{75}$As nuclear magnetic resonance (NMR) measurements as well as density-functional band structure calculations. Temperature-dependent magnetic susceptibility, ESR intensity, and NMR line shift could be described well using an alternating spin-$1/2$ chain model with the exchange coupling $J/k_{\rm B}\simeq 52$ K and an alternation parameter $\alpha \simeq 0.65$. From the high-field magnetic isotherm measured at $T=1.5 $ K, the critical field of the gap closing is found to be $ H_{\rm c}\simeq 16$ T, which corresponds to the zero-field spin gap of $\Delta_0/k_{\rm B}\simeq 21.4$ K. Both NMR shift and spin-lattice relaxation rate show an activated behavior at low temperatures, further confirming the singlet ground state. The spin chains do not coincide with the structural chains, whereas the couplings between the spin chains are frustrated. Because of a relatively small spin gap, NaVOAsO$_4$ is a promising compound for further experimental studies under high magnetic fields.

Measurement of electron spin-lattice relaxation times in radical doped butanol samples at 1 K using the NEDOR method

The electron spin-lattice relaxation time (T1e) of TEMPO- and trityl-doped butanol samples at 2.5T and temperatures between 0.95K and 2.17K was studied by pulsed nuclear magnetic resonance (NMR) using the nuclear-electron double resonance (NEDOR) method. This method is based on the idea to measure the NMR lineshift produced by the local field of paramagnetic impurities, whose polarization can be manipulated. This is of technical advantage as measurements can be performed under conditions typically used for the dynamic nuclear polarization (DNP) process – in our case 2.5T and temperatures around 1K – where a direct measurement on the electronic spins would be far more complicated to perform.As T1e is a crucial parameter determining the overall efficiency of DNP, the effect of the radical type, its spin concentration, the temperature and the oxygen content on T1e has been investigated.For radical concentrations as used in DNP (several 1019spins/cm3) the relaxation rate (T1e−1) has shown a linear dependence on the paramagnetic electron concentration for both radicals investigated. Experiments with perdeuterated and ordinary butanol have given no indication for any influence of the host materials isotopes. The measured temperature dependence has shown an exponential characteristic. It is further observed that the oxygen content in the butanol samples has a considerable effect on the electron relaxation time and thus influences the nuclear relaxation time and polarization rate during the DNP.The experiments also show a variation in the NMR linewidth, leading to comparable time constants as determined by the lineshift. NEDOR measurements were also performed on irradiated, crystal grains of 6LiD. These samples exhibited a linewidth behavior similar to that of the cylindrically shaped butanol samples.

Mn local moments prevent superconductivity in iron pnictides Ba(Fe1−xMnx)2As2

75As nuclear magnetic resonance (NMR) experiments were performed on Ba(Fe1−xMnx)2As2 (xMn = 2.5%, 5% and 12%) single crystals. The Fe layer magnetic susceptibility far from Mn atoms is probed by the 75As NMR line shift and is found similar to that of BaFe2As2, implying that Mn does not induce charge doping. A satellite line associated with the Mn nearest neighbours (n.n.) of 75As displays a Curie-Weiss shift which demonstrates that Mn carries a local magnetic moment. This is confirmed by the main line broadening typical of a RKKY-like Mn-induced staggered spin polarization. The Mn moment is due to the localization of the additional Mn hole. These findings explain why Mn does not induce superconductivity in the pnictides contrary to other dopants such as Co, Ni, Ru or K.

Field Dependence of the Ground State in the Exotic SuperconductorCeCoIn5: A Nuclear Magnetic Resonance Investigation

We report 115In nuclear magnetic resonance (NMR) measurements in CeCoIn5 at low temperature (T approximately 70 mK) as a function of the magnetic field (H0) from 2 to 13.5 T applied perpendicular to the c axis. A NMR line shift reveals that below 10 T the spin susceptibility increases as sqrt[H0]. We associate this with an increase of the density of states due to the Zeeman and Doppler-shifted quasiparticles extended outside the vortex cores in a d-wave superconductor. Above 10 T a new superconducting state is stabilized, possibly the modulated phase predicted by Fulde, Ferrell, Larkin, and Ovchinnikov. This phase is clearly identified by a strong and linear increase of the NMR shift with the field, before a jump at the first order transition to the normal state.

27Al nuclear magnetic resonance studies of the Y3-xLuxAl5O12 mixed garnets

The character of the substitutional order, the chemical shift, andthe nuclear quadrupole coupling in the mixed yttrium-lutetiumaluminium garnets Y3-xLuxAl5O12 (0⩽x⩽3)were studied for the first time using 27Al magic angle spinning(MAS) and multiple quantum (MQ) MAS nuclear magnetic resonance(NMR). The relatively narrow distribution of the isotropic NMR lineshifts, revealed by MQ MAS NMR, in crystals with compositionsx = 0.75 and x = 1 compared to mixed garnets of other compositionsshowed that these crystals are partially ordered solid solutions.The aluminium quadrupole coupling constants and isotropic chemicalshifts were evaluated for all garnets under study.

NMR line shift of gallium in GaAs crystals in the temperature range 160–360 K

The shift in the nuclear magnetic resonance (NMR) line of 71Ga isotopes in GaAs crystals is investigated in the temperature range 160–360 K. It is observed that the position of the resonance line is temperature-independent. The value of the shift relative to an isolated nucleus is 440 and 420 ppm for the three samples investigated. The roles of various contributions to the NMR line shift are discussed.

31P nuclear magnetic resonance Knight shift and linewidth in Ni3P and Cu3P: a magic-angle spinning study

Accurate measurements of small 31P Knight shift effects in crystalline Ni3P and Cu3P alloys have been performed by using a magic-angle spinning (MAS) nuclear magnetic resonance (NMR) technique. Both the average Knight shift and the width of its distribution have been found to be different when comparing a normal (equilibrium) and a melt-quenched Ni3P alloy and this is attributed to possible differences in defect concentration and grain size. The 31P NMR line shift of Cu3P is much smaller (K=0.012%) than that of Ni3P (K=0.179%) but spin-lattice relaxation time measurements indicated a Korringa-type relaxation mechanism for both alloys, i.e. Cu3P has also a metallic character.

Conformational changes of ribonuclease A as seen by nuclear magnetic resonance line shifts

An approach to a quantitative experimental description of enzyme denaturations is considered. A parameter, Ω, derived from the first moment of the aromatic region of ribonuclease A nuclear magnetic resonance spectra, is used to follow thermal and lithium bromide denaturations. It is shown that Ω does not depend on widths of individual resonance lines; consequently, it can be defined from even poorly resolved protein spectra. The parameter Ω permits one to follow protein denaturations as shown by a well-defined number of aromatic residues. An interpretation of Ω is given assuming that a protein denaturation, like a homopolypeptide denaturation, is a continuous process within each macromolecule.

The short range magnetic ordering in RbNiF 3 above the Curie point according to NMR data

The temperature dependence of the nuclear magnetic resonance line shift (Δ H) has been investigated in single crystal RbNiF 3. From these data conclusions about the existence of two oppositely directed magnetic sublattices and short range magnetic ordering have been drawn.

SOLVENT SHIFT OF THE CHLOROFORM NUCLEAR MAGNETIC RESONANCE LINE

The chloroform and dioxane proton magnetic resonance line positions have been measured relative to an internal tetramethylsilane reference signal as a function of concentration in binary chloroform −1,4-dioxane mixtures at 28 °C. Corrections have been applied to eliminate the effects of the chloroform self-association and of the non-specific interactions of the chloroform dipole. Analysis of the remaining nuclear magnetic resonance line shift in terms of an equilibrium between free chloroform and 1:1 and 2:1 chloroform-dioxane complexes gives the position of the signal from chloroform in the 1:1 complex at −1.20 p.p.m. and in the 2:1 complex at −0.85 p.p.m. relative to the position of the signal from chloroform at infinite dilution in cyclohexane. Data on other binary chloroform systems have been collected from the literature and have been similarly corrected. The results support the hypothesis of a monotonic relation between the proton magnetic resonance line shift and the energy of the hydrogen bonds which are believed to cause the formation of complexes in these systems. The accuracy of the data is not sufficient to give conclusive evidence regarding the linearity of this relation or the structure of the 2:1 chloroform–dioxane complex.

Nuclear Magnetic Resonance Line Shifts of Fluorine in AgF and Ag2F1,2

Nuclear Magnetic Resonance Line Shifts of Fluorine in AgF and Ag₂F^1,2

Nuclear resonance in solid and liquid metals: A comparison of electronic structures

The electronic structures of ten solid and liquid metals are investigated experimentally and theoretically, and the results of recent nuclear resonance measurements are correlated with already published data concerning magnetic susceptibility, electrical conductivity and atomic arrangement. It is concluded that the electronic structure of each of the metals investigated, with the notable exceptions of Bi and Ga, does not change appreciably at the melting point. Moreover, in Ga and Bi there is a correlation between a change in the electronic structure and structural rearrangements of neighboring atoms at short range; the other metals considered show neither sort of change. A principal conclusion is that a liquid metal possesses a band structure which is very like that of its solid, provided the short-range structure is not altered during melting. Included are hitherto unpublished values for the nuclear magnetic resonance line shifts in liquid Al, Sn, In, and Bi, and the nuclear spin relaxation times in solid and liquid Ga. The nmr shifts in solid and liquid Al are equal within experimental error. The same is true for Sn. This is in agreement with published results on the alkali metals and Hg, where the corresponding changes are known to be small, suggesting that the electronic structures of the respective solids and liquids are the same. The large quadrupole coupling in Ga, In, and Bi prevents observation of the nmr in the solid state. However, the value of the nmr shift in liquid In is consistent with values of the hyperfine coupling constant and the electronic specific heat at low temperatures, a situation which is true for most of the other metals. Ga and Bi are the exceptions for which the nmr shift in liquids is much larger than the hyperfine and low-temperature data would lead one to expect. This is taken as evidence for a reduced density of states at the Fermi surface in solid Ga and Bi. Confirmation is provided by the fact that the nuclear spin relaxation time of Ga increases sharply when the metal freezes. The measurement of relaxation in solid Ga is significant because the nuclear quadrupole resonance is directly observed and the relaxation process is of the same type, so that a change in its magnitude may also be related to a density of states.