Shape of the magnetic resonance line in a thin film on the surface of an anisotropic superconductor

The shape of the EPR line in a thin (=λ/2, where λ is the London penetration depth of the magnetic field in the superconductor) paramagnetic film deposited on the surface of an anisotropic superconductor is calculated in an oblique magnetic field with allowance for the inhomogeneity of the local magnetic field of the Abrikosov vortex lattice. It is shown that, as the tilt angle of the external magnetic field is varied, the shape of the EPR line changes noticeably. This fact can give additional information about the superconductor parameters (the symmetry type of the vortex lattice and the anisotropy parameter of the superconductor). Fiz. Tverd. Tela (St. Petersburg) 41, 386–388 (March 1999)     

Experimental observation of the spin screening effect in superconductor/ferromagnet thin film heterostructures

When studying the nuclear magnetic resonance (NMR) of ⁵¹V nuclei in superconductor/ferromagnet thin film heterostructures, anomalous behavior of the NMR signal was observed. This effect was manifested as a systematic distortion of the high-field wing of the resonance line. We consider this to be the first experimental evidence for penetration of ferromagnetism into a superconductor.     

Effect of stabilization processes on NMR signals from 180° domain boundaries in cubic ferrite-garnet crystals

Numerical methods were used to study the effect of stabilization processes on the line shape of NMR signals from 180° domain walls in cubic ferrite-garnet crystals, taking into account the structural change of the boundaries as they shift quasistatically in a magnetic field. The dependences of the amplitude of absorption maxima on the induced anisotropy constants and the amplitude of the external magnetic field are analyzed. Fiz. Tverd. Tela (St. Petersburg) 40, 694–698 (April 1998)     

Drift of scanning cycles of microwave absorption in high-T c superconductors

A change in the position and shape of the hysteresis line of magnetically modulated microwave absorption accompanying repeated magnetization reversal of a high-T _c superconductor was observed. The shift of the signal increases with the magnetic field, and its modulation amplitudes and scanning times vanish at T _c, obeying the basic laws of the temperature dependence of the line in single crystals and ceramics. Its appearance is correlated with the attainment of the critical state of modulated screening currents of weak-link loops separated on the surface of the sample and can be interpreted as their interaction on the basis of a generalized surface barrier model. The observed anisotropy indicates a Néel interaction mechanism. Fiz. Tverd. Tela (St. Petersburg) 41, 567–571 (April 1999)     

NMR and Mössbauer study of multiferroic BiFeO<Subscript>3</Subscript>

Pulse nuclear magnetic resonance (NMR) was applied in studying the effect of ⁵⁷Fe isotope content in multiferroic BiFeO₃ on the shape of NMR spectra at 4.2 K. Strong dependences of the NMR line shape on the isotope content and transverse relaxation time were found. Consideration of these effects on NMR line shape shows that there is an undisturbed (with no anharmonicity effect) space spin-modulated structure of the cycloid type in BiFeO₃. The Mössbauer effect was also used to investigate the perovskite BiFeO₃ at 650, 295, and 87 K. Experimental spectra allowed us to obtain the distribution of hyperfine fields, which was found to be consistent with studies of the NMR line shape. The local electronic and magnetic states of the iron ion were measured.     

Distribution of the local magnetic field of the vortex lattice near the surface of an anisotropic superconductor in oblique external magnetic fields

The distribution of the magnetic field in the unit cell of the Abrikosov vortex lattice near the surface of a uniaxial, anisotropic, type-II superconductor in an oblique external magnetic field is determined on the basis of the London model for the cases in which the symmetry axis is perpendicular and parallel to the boundary of the superconductor. The distribution of the local magnetic field is obtained as a function of the distance from the surface of the superconductor and the inclination angle of the external field. It is shown for an YBaCuO high-T _c superconductor that the investigation of the distribution function of the local magnetic field as a function of the angle of the external magnetic field relative to the symmetry axis and to the surface of the superconductor can yield important information about the anisotropic properties of the superconductor. Fiz. Tverd. Tela (St. Petersburg) 39, 1935–1939 (November 1997)     

The NMR line shape of a gas of hydrogen molecules in nanopores

We consider a closed nanopore filled with hydrogen gas in an external magnetic field. Only hydrogen molecules with spin I = 1 contribute to nuclear magnetic resonance (NMR) spin dynamics. Rapid molecular diffusion leads to partial averaging of dipole-dipole interactions (DDI) of spins I = 1 of different molecules. A method for the calculation of the NMR line shape of a hydrogen gas in nanopores is developed. Significant technical difficulties in calculations of the NMR line shape of the system of spins I = 1 coupled by DDI can be overcome with the introduction of a self-interaction with an averaged DDI coupling constant. The accuracy of this procedure is proportional to , where N is the number of hydrogen molecules in the nanocavity. In this approximation the NMR line is Gaussian. The second and fourth moments of the NMR line shape are calculated both from the known line shape and with the Van Vleck theory. They coincide up to the terms of the order of 1/N. The obtained results are in good agreement with experimental data for thin films with ellipsoidal nanopores. The developed theory allows one to extract information about sizes, shapes and orientations of nanopores in solids.     

Motions of the flux lines in high-T c superconductors from NMR linewidth,relaxation and spin echo decay

The motion of the flux lines (FL) in high temperature superconductors and their relationship with the NMR quantities are reviewed and discussed in the light of recent⁸⁹Y NMR experiments in YBCO-type compounds. In particular measurements involving the⁸⁹Y spin echo attenuation induced both by the thermal excitation of the FL’s and by motions driven by DC current and pulsed magnetic fields are presented, with preliminary results and lines of interpretation. Flux line motion as observed with¹⁹⁹Hg NMR in HgBa₂CuO_4+δ high temperature superconductor is discussed.     

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. Fiz. Tverd. Tela (St. Petersburg) 40, 1418–1419 (August 1998)     

Proton nuclear magnetic resonance in paramagnetic CoCl2.6 H2O

The present paper deals with a study of nuclear magnetic resonance (NMR) of protons of the crystal water of paramagnetic CoCl₂.6 H₂O. The measurements were carried out on powdered samples at room temperature, for values of the external magnetic field ranging from 0.3 to 1.0 T.The NMR signals of protons of the crystal water exhibit asymmetric shape which changes with the applied external magnetic field. We found that the second moment of the resonance line shows a linear dependence on the square of the induction of the externally applied magnetic field.Both the cause of the asymmetry of the NMR line of protons of the crystal water and the dependence of the second moment of the resonance line on the induction of external magnetic field are interpreted in the paper.     

Longitudinal critical current in a ferrite-type-II superconductor structure

The effect of the interaction of Abrikosov vortices with the magnetization on the longitudinal vortical instability in a layered ferromagnet-type-II superconductor structure is analyzed. It is shown that in the vicinity of the orientational phase transition in the magnet, where the transverse magnetic susceptibility is large, the magnitude of the longitudinal critical current in the structure can be almost 1.5 times smaller than in the isolated superconductor. The reason for this is compensation of stray field sources outside the superconductor by “magnetic charges” arising from a jump in the transverse magnetization on the surface of the magnet. A structure is considered in which the thickness of the superconductor significantly exceeds the London penetration depth of the magnetic field and the wavelength of the critical mode. For this reason (in light of the absence of high-quality bulk high-temperature superconductors), to experimentally study the described phenomenon it is necessary to use conventional low-temperature superconductors. Fiz. Tverd. Tela (St. Petersburg) 39, 231–235 (February 1997)     

Influence of an isolated magnetic impurity on an unconventional superconducting state

The effect of the moment of a magnetic impurity on the order parameter of an unconventional superconductor is examined. The coupling of the magnetic moment to the order parameter induces a locally time-reversal symmetry-breaking state which generates a magnetic field distribution in the vicinity of the impurity. The magnetic field can cause precession of the magnetic moment. The case of a spin polarized muon injected into the superconductor is discussed. Zh. éksp. Teor. Fiz. 112, 304–312 (July 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

On the unified nature of the longitudinal and transverse acoustoelectric effects in type-II superconductors

A longitudinal ultrasonic wave should drag the vortex structure of superconductors at an acute angle with respect to the direction of wave propagation. This motion engenders longitudinal and transverse electric fields in the superconductor and, moreover, effects of the “optical rectification” type contribute to the observed longitudinal and transverse electric fields. The effect should be observed in all type-II superconductors above the irreversibility line. Both fields have extrema with respect to temperature and magnetic field. In contrast to the acoustoelectric effect in the normal state, the sign of the induced electric field depends on the external magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 222–227 (10 February 1998)     

Polarization and energy barriers in ferroelectric pyridinium tetrafluoroborate

Measurements of the ¹H and ¹⁹F nuclear magnetic resonance (NMR) second moments were performed for a polycrystalline sample of (PyH)BF₄, whereas the shape of the ²H NMR line was analysed for a polycrystalline sample of (d₅PyH)BF₄. Asymmetry parameter δ has been calculated for four models of pyridinium cation reorientation among inequivalent potential energy minima, using the experimental value of the ¹H NMR second moment as well as the ²H NMR line width. From knowledge of the potential shape and the population of its minima, the temperature dependence of polarization for all the models has been found. From the comparison of the experimentally determined polarization with the calculated polarization, the most appropriate model of the pyridinium cation reorientation has been chosen.     

Alignment of Anisotropic Particles by Magnetic Field as Seen by NMR

We applied ¹³C and ²⁰⁵Tl NMR for studying alignment of particles of graphene and high temperature superconductor (Tl_0.5Pb_0.5)(Ba_0.2Sr_0.8)₂Ca₂Cu₃O_y caused by magnetic field. These compounds have layered structure and reveal anisotropic magnetic susceptibility. We found that the field of 8 T causes minor alignment of powder graphene and somewhat better alignment of fluffy graphene particles. Herewith the effect of alignment is well pronounced in ²⁰⁵Tl spectra of the superconducting particles fixed in epoxy in the field of 8 T. This effect is reflected in the ²⁰⁵Tl line shape measured in a magnetic field of 1.17 T and becomes much more pronounced in measurements made in high magnetic field of 8 T. Spectra simulations allow determining the degree of the particles’ alignment.     

核磁共振磁体超导接头工艺研究进展

高度稳定的磁场对于核磁共振（NMR）波谱仪至关重要.为了保持磁场的稳定性,高质量的超导接头必不可少.它在过去几十年中,受到NMR超导磁体研究人员的广泛关注.本文从五个部分介绍了超导接头技术的研究进展：第一部分简要介绍了NMR超导磁体和超导接头的发展;第二部分概述了低温超导体材料之间超导接头的研究进展;第三部分介绍了高温超导体材料之间的接头;第四部分讨论了有关超导接头电阻的测量技术;最后,提出了对超导接头技术研究的展望.     

<Superscript>33</Superscript>S MAS NMR of a disordered sulfur-doped silicate: Signal enhancement via RAPT,QCPMG and adiabatic pulses

Three different signal enhancement techniques have been applied to ³³S magic-angle spinning nuclear magnetic resonance (MAS NMR) of a disordered silicate containing 1.15 wt% ³³S. Partial saturation of the satellite transitions was achieved using a rotor-assisted population transfer (RAPT) pulse sequence, resulting in a signal enhancement of 1.63, albeit with a slight distortion of the line shape due to selective excitation. Adiabatic inversion of the satellite transitions by various amplitude-and frequency-modulated pulse shapes (such as hyperbolic secant and wideband uniform-rate smooth truncation) was also attempted, resulting in a signal enhancement of up to 1.85, with no apparent line shape distortion. Quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) and RAPT-QCPMG sequences were also used, both of which yielded spikelet spectra that accurately reflected the MAS line shape with a greatly improved signal-to-noise ratio. It is hoped that this study demonstrates that ³³S solid-state MAS NMR is now feasible even on disordered, low-sulfur-content systems.     

Electrical and superconducting properties of BiSrCaCuO ceramics superconductors

Electrical and superconducting properties of BiSrCaCuO superconductor prepared by partial melting method were determined to study its superconducting mechanism. Induced voltage of the BiSrCaCuO superconductor was not so significantly dependent upon on the shape and size of the superconductor. Anti-magnetism was less dependent upon the melting and the annealing temperatures. BiSrCaCuO film prepared at 1158 K shows the maximum magnetic susceptibility in this study. Differential conductance behavior of the superconductor with magnetic flux was not well matched by a conventional flux flow model. It suggests that a superconducting mechanism for type II superconductor should be able to explain the fact that magnetic properties arise from the interaction between the trapped magnetic flux and weak link of the superconducting connectors forms in the superconducting material.     

Field dependence of the ground state in the exotic superconductor CeCoIn5: 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.