Zero-field NMR Spectra Research Articles

NMR study of magnetic structure and hyperfine interactions in the binary helimagnet FeP

We report a detailed study of the ground state helical magnetic structure in monophosphide FeP by means of ${}^{31}$P NMR spectroscopy. We show that the zero-field NMR spectrum of the polycrystalline sample provides strong evidence of an anisotropic distribution of local magnetic fields at the P site with substantially lower anharmonicity than that found at the Fe site by M\"{o}ssbauer spectroscopy. From field-sweep ${}^{31}$P NMR spectra we conclude that a continuous spin-reorientation transition occurs in an external magnetic field range of 4 - 7 T, which is also confirmed by specific-heat measurements. We observe two pairs of magnetically inequivalent phosphorus positions resulting in a pronounced four-peak structure of the single crystal ${}^{31}$P NMR spectra characteristic of an incommensurate helimagnetic ground state. We revealed a spatial redistribution of local fields at the P sites caused by Fe spin-reorientation transition in high fields and developed an effective approach to account for it. We demonstrate that all observed ${}^{31}$P spectra can be treated within a model of an isotropic helix of Fe magnetic moments in the ($ab$)-plane with a phase shift of 36$^{\circ}$ and 176$^{\circ}$ between Fe1-Fe3 (Fe2-Fe4) and Fe1-Fe2 (Fe3-Fe4) sites, respectively, in accordance with the neutron scattering data.

153Eu and69,71Ga Zero-Field NMR Study of Antiferromagnetic State in EuGa4

We report $^{153}$Eu and $^{69,71}$Ga NMR under a zero magnetic field on the antiferromagnetic state of EuGa$_{4}$ with $T_{\rm{N}}\approx 16$ K. We have successfully observed a $^{153}$Eu zero-field NMR signal with well-resolved nuclear quadrupole splitting in the antiferromagnetic state of EuGa$_{4}$. $^{69,71}$Ga zero-field NMR spectra were also observed below $T_{\rm{N}}$. The internal field and nuclear quadrupole frequency are estimated from a simulation of the spectra by the exact diagonalization of the nuclear spin Hamiltonian matrix. The asymmetrically split zero-field NMR spectra were explained by considering a configuration of the magnetic moments of Eu$^{2+}$ lying in the basal $ab$-plane. The temperature dependence of the internal field, which is proportional to the sublattice magnetization, can be explained by the Brillouin function with $J=S=7/2$

Multi-quantum echoes in GdAl 2 zero-field high-resolution NMR

In this paper we present a series of high-resolution zero-field NMR spectra of the polycrystalline intermetallic compound GdAl 2. The spectra were obtained with the sample at 4.2 K in the ordered magnetic state and in the absence of an external static magnetic field. Using a sequence composed of two RF pulses, we obtained up to five multi-quantum echoes for the 27Al nuclei, which were used to construct the zero-field NMR spectra. The spectra obtained from the FID observed after the second pulse and the even echoes exhibited higher resolution than the odd ones. In order to explain such behavior, we propose a model in which there are two regions inside the sample with different inhomogeneous spectral-line broadenings. Moreover, with the enhanced resolution from the FID signal, we were able to determine quadrupolar couplings with great precision directly from the respective spectra. These results were compared with those obtained from the quadrupolar oscillations of the echo signals, and showed good agreement. Similar data were also obtained from 155Gd and 157Gd nuclei.

A zero-field Cu-NMR study on antiferromagnetic ordered state in four-layered high-[formula omitted] superconductors [formula omitted

We report on superconducting and magnetic characteristics in four-layered high-Tc superconductors Ba2Ca3Cu4O8(FyO1-y)2 with apical fluorine through zero-field Cu-NMR measurements. The zero-field NMR spectra shows that the internal magnetic field, induced by antiferromagnetic (AFM) ordered moments, decreases with increasing the local carrier density (Nh). The AFM phase expands up to Nh∼0.15, which is the magnetic critical point where the AFM order collapses. Moreover, for y=1.0, the AFM order is observed at all CuO2 layers, although superconducting transition takes place at Tc=55K. This result suggests that the AFM order uniformly coexists with superconductivity in a single CuO2 plane in an underdoped region.

Zero-field nuclear magnetic resonance in high field by modulated rf sequences

The authors propose a novel approach to design and evaluate sequences for zero-field NMR spectra in high field (ZFHF) by using amplitude and phase modulated rf sequences. ZFHF provide sharp peaks for the dipolar interaction between two nuclear spins even if the orientation of the molecules is distributed. The internuclear distance r can be directly obtained from the peak position which is proportional to r-3. Numerous ZFHF sequences are obtained. A sequence is selected from them by the systematic evaluation of the sequences. The new ZFHF sequence is less affected by chemical shift anisotropy (CSA) than the previous sequences; the sequence can be used for systems with large CSA such as a dipolar coupled 13C-pair system under realistically high field. 13C ZFHF spectra of 13C2 diammonium succinate and 13C2 diammonium oxalate were observed under the 9.4 T field.

Nuclear magnetic resonance study of magnetic properties of [formula omitted

We have carried out Mn 55 and Er 167 NMR measurements on ErMn 6 Sn 6 at 4.2 K. The zero-field NMR spectrum of Mn observed in the frequency range 190–280 MHz is very complex due to anisotropic hyperfine interaction. It consists of the single line around 245.5 MHz and the five equidistant lines in the range 250–270 MHz. For the Er 167 NMR, the resonance frequency of the central line is 955 MHz, and the frequency separation between the adjacent lines split by the electric quadrupole interaction is ∼ 129 MHz . The hyperfine field analysis shows that the magnetic moments are 8.9 μ B for Er and 2.2 μ B for Mn at 4.2 K in the ferrimagnetically ordered state.

Microscopic magnetic properties of (V 1− xTi x) 2O 3 near the phase boundary of the metal–insulator transition

Magnetic susceptibility ( χ) and 51V NMR have been measured in (V 1− x Ti x ) 2O 3 near the phase boundary of the metal–insulator transition. It is established that the transition from antiferromagnetic insulating (AFI) to antiferromagnetic metallic phases near x c≈0.05 is not quantum critical, but is discontinuous with a jump of the transition temperature. In the AFI phase at 4.2 K, we observed the satellite in the zero-field 51V NMR spectrum around 181 MHz in addition to the ‘host’ resonance around 203 MHz. The satellite is also observable in the paramagnetic metallic phase of the x=0.055 sample. We associated the satellite with the V sites near Ti, which are in the V 3+-like oxidation state, but has different temperature dependence of the NMR shift from that of the host V site. The host d-spin susceptibility for x=0.055 decreases below ∼60 K, but remains finite in the low-temperature limit.

195Pt NMR study of antiferromagnetic U(Pt 1− xPd x) 3

We have carried out 195Pt NMR measurement in heavy fermion pseudo-binaries U(Pt 1− x Pd x ) 3 (0< x<0.10) in order to investigate the evolution of magnetic order. At low Pd contents ( x=0 and 0.01), no internal field was observed at Pt site even if small-moment magnetic order was observed by neutron diffraction. At high Pd contents (0.02< x<0.08), however, a large internal field of about 30 kOe was observed associated with the occurrence of large-moment magnetic order. The zero-field NMR spectrum was very broad, which indicated existence of a broad distribution in the size of the ordered moments. With increasing Pd concentration, the average size of U moments increased gradually while its distribution narrowed.

NMR of 31P in CrRuP

NMR properties of 31P in CrRuP at low temperatures are reported. The compound CrRuP, which is a member of a series of compounds MRuP (M=3d transition metal), was expected to be in a nonmagnetic state at low temperature such as a Kondo insulator from the susceptibility data. However, field-swept and zero-field NMR spectra show that it is an antiferromagnet at low temperature. The hyperfine field at the 31P site at 4.2 K is determined to be 9.6 kOe with a half-width of 2.3 kOe. A simulation using the broad zero-field data and a simple antiferromagnetic spin structure explains the field-swept powder spectrum only qualitatively, suggesting a complex antiferromagnetic spin structure in this material.

A field-cycling NMR study of nematic 4-pentyl-4′-cyanobiphenyl confined in porous glasses

The proton spin-lattice relaxation time T1, in the nematic liquid crystal 4-pentyl-4′-cyanobiphenyl confined in a glassy porous matrix has been measured in a wide Larmor frequency range of 1 · 102−2 · 107 Hz employing the fast field-cycling NMR technique. A strong influence of the restricted geometry on the character of the T1 dispersion was found. Our investigation clearly demonstrates the importance of the translationally induced molecular reorientations in inhomogeneous director field for the relaxation in the samples with 200 and 80 nm mean pore size. The experimental results are in a good agreement with the theoretical predictions. In the sample with 7 nm pore size the main contribution to the relaxation is ascribed to the slowing down of the molecular motion in the near-surface layer. Zero-field 1H NMR spectra of a microconfined liquid crystal are reported for the first time.

The nuclear hexadecapole interaction of iodine-127 in cadmium iodide measured using zero-field two dimensional nuclear magnetic resonance

Two dimensional nuclear quadrupole correlation spectroscopy has been used to measure simultaneously the two nuclear spin transition frequencies for the iodine resonance of cadmium iodide at zero magnetic field. Because of the layer structure and resultant polymorphism in this substance, conventional zero-field NMR spectra are inhomogeneously broadened by several hundred kHz. However, the 2D spectra obtained by our method are subject only to homogeneous linewidth, which for this compound is of the order of 5 kHz. This greatly increased precision allows more accurate evaluation of the nuclear hexadecapole coupling in cadmium iodide; which we measure as 0.216±0.514 kHz (i.e., indistinguishable from zero). This gives a maximum value of the nuclear hexadecapole moment several times lower than that recently reported for the same nucleus in potassium iodide.

NMR evidence of metal-support interaction in syngas conversion catalyst Co-TiO2

To examine the relation between catalytic and magnetic properties, the zero-field NMR spectra and hysteresis loops of cobalt supported on silica, alumina, magnesia, titania, and ZSM-5 with and without the promoter thoria were investigated. Cobalt was incorporated on the support by simple physical admixture of precipitated cobalt and support, and by aqueous impregnation technique. Our studies indicate that the particle sizes are consistently lower in the presence of thoria. Of all the catalysts examined, the Co/Th/TiO2 catalyst exhibits a high saturation magnetization value—about 20% higher than pure cobalt. In addition, the NMR spectrum of the aqueous impregnation Co/TiO2 catalyst is distinctly different from the rest. All the NMR lines are shifted to a higher frequency by about 4 MHz. These two features—enhancement of the magnetic moment of cobalt atoms and increases in the hyperfine field at the Co nucleus—clearly indicate that there occurs strong metal-support interaction between cobalt and titania support. The higher hydrocarbon yields observed by the earlier investigators with Co/TiO2 catalysts might be related to this phenomenon.

Calculation of zero-field NMR spectra of the ammonium ion undergoing tunneling motion and fast random reorientation

Zero-field NMR spectra of the ammonium ion in the case of tunneling, tunneling and fast reorientation by 180° jumps around theC2 axis, tunneling and fast reorientation by 120° jumps aroundC3 axis are calculated.

Quadrupole interactions at the27Al nuclei of GdAl2as a function of pressure, temperature, holmium substitution and the application of an external field

The 27Al zero-field NMR spectrum of GdAl2 at 4.2 K and 77 K has been shown to contain signals emanating from nuclei in both domains and all regions of domain walls. This interpretation is based on detailed measurements of quadrupole frequencies, and is supported by readings on a holmium-doped GdAl2 sample and measurements in an external magnetic field. The results are incompatible with a significant magnetic contribution to the electric field gradient at the AI sites unless such a contribution has its principal axis along the (111) axes in both domains and domain walls. Pressure measurements, giving a value of +(14+or-2) kbar-1 for dlnPt/dlnP where Pt is the quadrupole parameter and P the pressure, have been used to show that lattice expansion cannot explain the temperature dependence of the electric field gradient. Lattice vibrations, however, may provide an explanation.

Heteronuclear zero-field NMR of liquid crystals

Heteronuclear spin systems in solids have previously been studied by zero-field NMR (I, 2). Inequivalent nuclear spins (e.g., 13C-lH) behave identically to homo- nuclear spins (e.g., ‘H-‘H) except that they can be independently manipulated in high and zero fields (2, 3). In combination with anisotropic liquid crystal solvents, such spin systems often produce interesting and unexpected features in the zero-field NMR spectra. This paper presents the simplest case of an I-S (I = ‘H, S = 13C) spin pair in nematic and smectic liquid crystalline phases, the latter displaying the effects of small asymmetries. A comparison between the high- and zero-field NMR Hamiltonians illustrates a few of the relevant features of heteronuclear spin systems in zero field. For a 13C-‘H pair, the high-field NMR Hamiltonian in a uniaxial phase may be written as %+ffF = -4, - W& - 9(2I&)(3

Magnetically induced electric quadrupolar interaction of Gd nuclei at cubic lattice sites in ferromagnetically ordered intermetallic compounds

Typical appearance of quadrupolar splitting in zero-field Gd NMR spectra is presented for GdIr 2 and GdAl 2. The splitting varies with temperature roughly as the square of magnetization in GdAl 2. Its analysis may become an additional source of information on electronic structure of intermetallics.

Zero field NMR and NQR with selective pulses and indirect detection

Zero field NMR and NQR spectra are obtained by the application of dc magnetic field pulses to a demagnetized sample. Pulsed dc fields allow for selective excitation of isotopic species and provide a means for coherent manipulation of the spin system in zero field. Using these selective pulses and level crossing techniques, indirect detection of a quadrupolar nucleus may be accomplished via protons without obtaining the proton background signal in the NQR spectrum. Experimental results from a variety of 1H, 2H, and 14N homo- and heteronuclear systems are presented as an illustration of these techniques.

55Mn NMR study of the collective excitations in spin-glasses.

The first direct observation of zero-field NMR spectra of $^{55}\mathrm{Mn}$ in $\mathrm{Cu}\mathrm{Mn}$, $\mathrm{Ag}\mathrm{Mn}$, and $\mathrm{Au}\mathrm{Mn}$ spin-glasses allows a detailed study of the concentration and $T$ dependence of the Mn local magnetization for $T\ensuremath{\lesssim}0.2{T}_{g}$. The quantum reduction of the local spin is found smaller than numerical estimates, while the thermal part is found quadratic in $T$. Such a result agrees with expectations for magnons with a linear dispersion relation and a large stiffness constant, at short times, for the spin system.

Theory of zero-field NMR spectra. Effects of motion

Zero-field NMR spectra of tunnelling methyl group, classically reorienting methyl group and of a pair of spins 1/2 jumping between two perpendicular orientations are calculated. A formula for the second moment of a rigid lattice is derived from a formal analogy between zero-field and high-field spectra. Fast reorientation about a perpendicular axis reduces the intramolecular second moment to 1/4 of its rigid value. The zerofield method seems to be a promising tool for the determination of tunnelling splitting or of jumping rates.

NMR Study of11B in DyRh4B4and Dy(Ir0.7Rh0.3)4B4

Pulsed NMR study of 11 B has been performed in DyRh 4 B 4 and Dy(Ir 0.7 Rh 0.3 ) 4 B 4 at the resonance frequencies of 10–17 MHz and at temperatures of 0.5 K–300 K. From the nuclear spin lattice relaxation time T 1 , we obtain JN ( E F ) at Dy site. The crystal field effect on Dy is also investigated. A probable spin structure of the antiferromagnetic Dy(Ir 0.7 Rh 0.3 ) 4 B 4 is also proposed from the zero-field NMR spectrum of 11 B, whose hyperfine field is generated by the classical dipole interaction with RE element.