Successive phase transitions of [(PyO)(H/D)][AuCl4] (PyO=C5H5NO)

³⁵Cl NQR as well as heat capacity measurements of [(PyO)H][AuCl₄] and its deuterated analog [(PyO)D][AuCl₄] revealed successive phase transitions at 70.5 and 62.5 K, and at 71 and 63 K, respectively. The NQR frequency varied continuously through the upper transition point while discontinuously through the lower transition point. In the intermediate-temperature phase a remarkable decrease in the signal intensity was observed. These NQR observations as well as the feature of the heat capacity anomaly in which a broad peak is succeeded by a sharp peak with decreasing temperature suggest a possibility of normal-incommensurate-commensurate phase sequence.     

Directional behavior of nuclear quadrupole resonance in YBa2Cu3O6

Powder samples of YBa₂Cu₃O₆ were magnetically aligned and the anisotropies in the systems were studied by means of Cu(1) nuclear quadrupole resonance (NQR) in the absence of external magnetic fields. Our room temperature measurements of the NQR lineshapes and the spin–lattice and spin–spin relaxation times as a function of the aligning magnetic field indicate that full microscopic alignment can be achieved by using a magnetic field of about 4.7 T, for which doublet line patterns arising from a hyperfine splitting were observed.     

Emission of nuclear quadrupole resonance from polycrystalline hexamethylenetetramine

The angular dependence of the nuclear quadrupole resonance (NQR) signal intensity emitted from polycrystalline hexamethylenetetramine has been analytically investigated for all directions for non-contact detection of chemicals by nuclear quadrupole resonance. The field pattern of the NQR signal from a column sample was measured. The emitted patterns were the same as that from a united single magnetic dipole, which fitted well to the estimation based on quadrupole principle axis system. This result is helpful to design an antenna for NQR remote detection.     

Zeeman shift--a tool for assignment of 14N NQR lines of nonequivalent 14N atoms in powder samples

The use of Zeeman perturbed 14N nuclear quadrupole resonance (NQR) to determine the ν+ and ν-14N lines in polycrystalline samples with several nonequivalent nitrogen atoms was investigated. The 14N NQR line shift due to a weak external Zeeman magnetic field was calculated, assuming isotropic distribution of EFG tensor directions. We calculated the broad line distribution of the ν+ and ν- line shifts and experimentally confirmed the calculated Zeeman field dependence of singularities (NQR peaks) in cyclotrimethylenetrinitramine (RDX) and aminotetrazole monohydrate (ATMH). The calculated and measured frequency shifts agreed well. The proposed measurement method enabled determination of which 14N NQR lines in ATMH belong to ν+ and which to ν- transitions.     

Magnetic properties of PrCu2 at high pressure

We report a study of the low-temperature high-pressure phase diagram of the intermetallic compound PrCu₂, by means of molecular-field calculations and ^63,65Cu nuclear-quadrupole-resonance (NQR) measurements under pressure. The pressure-induced magnetically-ordered phase can be accounted for by considering the influence of the crystal electric field on the 4f electron orbitals of the Pr³⁺ ions and by introducing a pressure-dependent exchange interaction between the corresponding local magnetic moments. Our experimental data suggest that the order in the induced antiferromagnetic phase is incommensurate. The role of magnetic fluctuations both at high and low pressures is also discussed.     

Numerical simulation of NQR/NMR: Applications in quantum computing

A numerical simulation program able to simulate nuclear quadrupole resonance (NQR) as well as nuclear magnetic resonance (NMR) experiments is presented, written using the Mathematica package, aiming especially applications in quantum computing. The program makes use of the interaction picture to compute the effect of the relevant nuclear spin interactions, without any assumption about the relative size of each interaction. This makes the program flexible and versatile, being useful in a wide range of experimental situations, going from NQR (at zero or under small applied magnetic field) to high-field NMR experiments. Some conditions specifically required for quantum computing applications are implemented in the program, such as the possibility of use of elliptically polarized radiofrequency and the inclusion of first- and second-order terms in the average Hamiltonian expansion. A number of examples dealing with simple NQR and quadrupole-perturbed NMR experiments are presented, along with the proposal of experiments to create quantum pseudopure states and logic gates using NQR. The program and the various application examples are freely available through the link http://www.profanderson.net/files/nmr_nqr.php.     

Measurement of the (14)N nuclear quadrupole resonance frequencies by the solid effect

(1)H-(14)N nuclear quadrupole double resonance using magnetic field cycling between high and low magnetic field and solid effect in the low magnetic field is analyzed in details. The transition probabilities per unit time for the solid-effect transitions are calculated. The double resonance spectra are calculated in the limiting cases of fast and slow nitrogen spin-lattice relaxation. The double resonance spectra are measured in histamine and quinolinic acid. The experimental spectra are analyzed and the (14)N NQR frequencies are determined.     

Cross-polarisation method for improvement of 14N NQR signal detectability

This is a study of the cross-polarization effects in the case of 14N quadrupolar spin-system with a long spin-lattice relaxation time. Two important benefits of the cross-polarization technique were demonstrated for PETN: (i) a polarization transfer resulting in increased NQR single shot signal response and (ii) a dynamic reduction in recovery time of the NQR system allowing scan repetition on a much shorter timescale. It was proved that this technique can reduce the optimal waiting time between pulse sequences up to 60 times through a significant reduction of the relaxation time of the quadrupolar spin-system. All experiments were carried out at room temperature using spin-locking multi-pulse sequences and small external magnetic field.     

Effects of powder flowability on the alignment degree and magnetic properties for NdFeB sintermagnets

The magnetic powders for sintered NdFeB magnets have been prepared by using the strip casting (SC), hydrogen decrepitation (HD) and jet milling (JM) techniques. The effects of powder flowability and addition of a lubricant on the alignment degree and the hard magnetic properties of sintered magnets have been studied. The results show that the main factor affecting powder flowability is the aggregation of magnetic particles for powders in a loose state, but it is the friction between the powder particles for powders that are in a compact state. The addition of a lubricant with suitable dose can slightly prevent the congregating of powders, obviously decrease the friction between the powder particles, improve the powder flowability, and increase the alignment degree, remanence and energy product density of sintered magnets. Mixing a suitable dose of lubricant and adopting rubber isostatic pressing (RIP) with a pulse magnetic field, we have succeeded in producing the sintered NdFeB magnet with high hard magnetic properties of B_r=14.57 KG, _jH_c=14.43 KOe, (BH)_max=51.3 MGOe.     

Spin 3/2 Zeeman perturbed NQR in the presence of slow sample rotation

Theoretical and experimental results are presented for the case of Zeeman perturbed nuclear quadrupole resonance (NQR) using spin-3/2 nuclei with a small Zeeman interaction, gammaB0, while the sample is very slowly rotated. It is found that the decay envelope for a simple two-pulse echo measurement can be strongly affected even though the sample may rotate only a few degrees or less during the course of the measurement. To lowest order the decay envelope can be described using a one dimensional function of the product of gammaB0, the rotation rate, and the square of the pulse spacing. Aside from an indirect and weak dependence on the quadrupole asymmetry parameter, eta, the result is independent of the NQR frequency. Identical results are expected for a stationary sample in a small rotating magnetic field. The effect seen here may be used to advantage to measure rotational motion, for example of particles in fluids, or may be an additional complication for some Zeeman perturbed NQR measurements, including some NQR detection and imaging methods.     

Superconductivity and magnetic fluctuations in Cd(2))Re(2)O(7) via Cd nuclear magnetic resonance and re nuclear quadrupole resonance

We report Cd nuclear magnetic resonance (NMR) and Re nuclear quadrupole resonance (NQR) studies on Cd(2)Re(2)O(7), the first superconductor among pyrochlore oxides (T(c) approximately 1 K). The Re NQR spectrum at zero magnetic field below 100 K rules out any magnetic or charge order. The spin-lattice relaxation rate below T(c) exhibits a pronounced coherence peak and follows the weak-coupling BCS theory with nearly isotropic energy gap. The results of Cd NMR point to a moderate ferromagnetic enhancement at high temperatures followed by a rapid decrease of the density of states below the structural transition temperature of 200 K.     

Detection of ammonium nitrate inside vehicles by nuclear quadrupole resonance

The development of a system for the detection of ammonium nitrate (AN) in vehicles by nuclear quadrupole resonance (NQR) is described. The results from studies of the penetration of radiofrequency (RF) magnetic fields inside certain metal enclosures, including full-scale vehicles, were critical in the design of a novel high-Q resonant probe. The probe was shaped not only for optimal penetration of RF magnetic fields into vehicles, but also for optimal rejection of RF interference and ease of shielding. A full-scale technical demonstrator was designed, built and successfully demonstrated, using novel pulse sequences to generate and detect NQR signals from AN concealed within the boot (trunk) of a car and in the loading bay of a (metal-sided) van. Among the key technical advances that made possible the effective operation of this system was the development of pulse sequences that generate detectable NQR responses for RF magnetic fields that are both very weak and very inhomogeneous.     

Circularly Polarized RF Magnetic Fields for Spin-1 NQR

The low sensitivity of nuclear quadrupole resonance (NQR) of powders is due, in part, to the inability to efficiently excite and detect nuclei at all crystal orientations. Here we describe the use of circularly polarized RF magnetic fields for excitation followed by detection of the resultant circular RF magnetization in I=1 NQR to increase the fraction of nuclei excited and detected. We show that the technique can greatly improve the effective RF field homogeneity and increase the largest signal amplitude by a factor of 1.72. In favorable cases, the resulting circularly polarized NQR signal can be separated from linearly polarized magnetoacoustic and piezoelectric ringing artifacts that occur in some NQR materials detection applications.     

New Methods for Detection of 14N NQR Frequencies

Nitrogen atoms are present in a number of solid explosives and illicit substances. The nuclear quadrupole resonance (NQR) spectra and spin?Clattice relaxation of the nitrogen atomic nucleus ¹⁴N can be used to characterize these compounds and to distinguish between possible crystal polymorphs. After the characteristic ¹⁴N NQR frequencies and spin?Clattice relaxation rates in a compound are determined, NQR can be used to detect this compounds and, in case of crystal polymorphs, also to determine the method of preparation. The ¹⁴N NQR frequencies and spin?Clattice relaxation rates are measured either by pulse NQR or by nuclear quadrupole double resonance (NQDR) based on magnetic field cycling. Here, we discuss several ¹H?C¹⁴N NQDR techniques which can be used to measure the ¹⁴N NQR frequencies and spin?Clattice relaxation rates under various experimental conditions. Some characteristic applications of these techniques are presented and discussed in details.     

Polarization enhancement technique for nuclear quadrupole resonance detection

We demonstrate a dramatic increase in the signal-to-noise ratio (SNR) of a nuclear quadrupole resonance (NQR) signal by using a polarization enhancement technique. By first applying a static magnetic field to pre-polarize one spin subsystem of a material, and then allowing that net polarization to be transferred to the quadrupole subsystem, we increased the SNR of a sample of ammonium nitrate by one-order of magnitude.     

Determination of the electric field gradient asymmetry from 2D nutation NQR spectra of 75As nuclei in oriented samples of As2Se3 semiconductor

The method of two-dimensional nutation nuclear quadrupole resonance in time domain is used to study the chalcogenide semiconductor As(2)Se(3). In this system, the nuclear quadrupole resonance (NQR) resonance line width is as large as 10 MHz; therefore, the radiofrequency field produced by a pulsed NQR spectrometer can excite only a portion of the nuclear spins. The proposed method relies on polarizing the melted glass specimen in a strong magnetic field so that orientational disorder is partially removed. After hardening the sample is placed in a spectrometer r.f. coil in such a way that that the axes of sample polarization and r.f. coil coincide. We demonstrate the application of this method to determine eta in glassy As(2)Se(3).     

Spin and temperature dependence of the magnetic hyperfine field of Cd in the rare earth-aluminum Lavesphase compounds RAl2

Perturbed angular correlation spectroscopy has been used to investigate the combined magnetic and electric hyperfine interaction of the probe nucleus ¹¹¹Cd in ferromagnetically ordered rare earth (R)-dialuminides RAl₂ as a function of temperature for the rare earth constituents R=Pr, Nd, Sm, Eu, Tb, Dy, Ho and Er. In compounds with two magnetically non-equivalent Al sites (R=Sm, Tb, Ho, Er), the magnetic hyperfine field was found to be strongly anisotropic. This anisotropy is much greater than the anisotropic dipolar fields, suggesting a contribution of the anisotropic 4f-electron density to magnetic hyperfine field at the closed-shell probe nucleus. The spin dependence of the magnetic hyperfine field reflects a decrease of the effective exchange parameter of the indirect coupling with increasing R atomic number. For the compounds with the R constituents R=Pr, Nd, Tb, Dy and Ho the parameters B₄, B₆ of the interaction of the crystal field interaction have been determined from the temperature dependence of the magnetic hyperfine field. The ¹¹¹Cd PAC spectrum of EuAl₂ at 9 K confirms the antiferromagnetic structure of this compound.     

Field sweep rate dynamics in magnetic tunnel junctions

The dynamical behavior of magnetic tunnel junctions (MTJs) was investigated by varying the magnetic field sweep rate from 0.01 mT/s to 10 T/s in a magneto optical Kerr effect set-up. The bias fields of the pinned and free ferromagnetic electrodes were found to drastically decrease above a field sweep rate of 1 T/s. This decrease in the bias fields coincides with a change in the magnetization reversal process from domain wall motion at low-field sweep rates to domain nucleation at high-field sweep rates. The nucleation of inverse domains in the ferromagnetic layer changes the interfacial spin structure of the antiferromagnetic layer and therefore the magnitude of the exchange bias effect. Furthermore, the nucleation of domains induces a discontinuous magnetic charge density at the tunnel barrier interfaces and this reduces the interlayer coupling between the two ferromagnetic electrodes of the MTJ.     

NMR and NQR studies on superconducting Sr2RuO4

We review our nuclear-magnetic resonance (NMR) and nuclear-quadrupole-resonance (NQR) studies in superconducting Sr₂RuO₄, which have been performed in order to investigate the gap structure and the pairing symmetry in the superconducting state and magnetic fluctuations in the normal state. The spin-lattice relaxation rate (1/T₁) of a high-quality sample with shows a sharp decrease without a coherence peak just below T_c, followed by a T³ behavior down to 0.15 K. This result indicates that the superconducting gap in pure Sr₂RuO₄ is a highly anisotropic character with a line-node gap. The Knight shift, which is related to the spin susceptibility, is unchanged in the superconducting state irrespective of the direction of the applied fields and various magnitude of the field. This result strongly suggests that the superconducting pairs are in the spin-triplet state, and the spin direction of the triplet pairs is considered to be changed by small fields of several hundred Oe.