Transport critical-current and voltage-current characteristics of Bi(2223) silver-clamped thick films

The transport critical current properties of Bi(2223) silver-clamped thick films are studied by the measurement of its dependence on magnetic field and temperature close to T _c. It is found that the transport critical current follows a power law J _c(1–T/T _c)^3/2 for the sample with J _c>2.0×10⁴ A/cm² (77K, zero field) and that J _c(H) is basically reversible for increasing and decreasing magnetic field. After the transport current exceeds the critical current, the voltage-current (V-I) characteristics show a flux-creep-like behaviour until they smoothly join the flux-flow state. From the measurement of V-I curves, the pinning property of the sample may be estimated using the flux-creep mode. The flux-flow resistance is found to have a nonlinear magnetic field dependence.     

Commensurability effects in a Josephson tunnel junction in the field of an array of magnetic particles

Commensurability effects have been theoretically studied in a hybrid system consisting of a Josephson junction located in a nonuniform field induced by an array of magnetic particles. A periodic phase-difference distribution in the junction that is caused by the formation of a regular lattice of Abrikosov vortices generated by the magnetic field of the particles in superconducting electrodes is calculated. The dependence of the critical current through the junction I _c on the applied magnetic field H is shown to differ strongly from the conventional Fraunhofer diffraction pattern because of the periodic modulation of the Josephson phase difference created by the vortices. More specifically, the I _c(H) pattern contains additional resonance peaks, whose positions and heights depend on the parameters and magnetic state of the particles in the array. These specific features of the I _c(H) dependence are observed when the period of the Josephson current modulation by the field of the magnetic particles and the characteristic scale of the change in the phase difference by the applied magnetic field are commensurable. The conditions that determine the positions of the commensurability peaks are obtained, and they are found to agree well with experimental results.     

Irreversibility line and flux pinning properties in iron-based high-Tc superconductor SmFeAsO0.85

The irreversibility line and flux pinning properties of high-T_c superconductor SmFeAsO_0.85 were studied using DC magnetization data. Polycrystalline SmFeAsO_0.85 was prepared in a high pressure synthesis apparatus under the pressure of 6 GPa. The results of DC susceptibility showed the superconducting transition at about 55 K. A critical current density J_c(B) was calculated using Bean’s critical state model. At low temperatures (20 K), J_c(B) showed a relatively high value with weak dependence on an applied magnetic field. At higher temperatures, a stronger dependence of the magnetic field was observed, which resulted from decrease in a critical current density probably due to the flux creep effect. The irreversibility line (IL) agreed well with the flux creep theory of Matsushita et al. A comparison of normalized pinning force density with the theoretical models showed that the irreversible behavior in SmFeAsO_0.85 is dominated mainly by normal point pinning (δT_c) and surface pinning mechanisms.     

Mössbauer study on surface magnetic properties in magnetic fluids

The presence of m=0 lines of the Mössbauer spectra of small Fe₃O₄ particles coated with an organic surfactant in an applied field of 7 T at 5 K shows a non-collinear magnetic structure in the surface layer of these particles. From the temperature dependence of the hyperfine field, the anisotropy constantK was calculated.Projects supported by the Science Fund of the Chinese Academy of SciencesNow working at the Shanghai Institute of Education     

Rare Earth systems in a magnetic field: Coupling of elastic and magnetic properties

We study the mutual influence of magnetic and elastic properties of a Rare Earth system as an external magnetic field is applied. It is demonstrated that a magnetic phase transition should show up in the temperature dependence of certain sound velocities. Furthermore the magnetic field should induce a hybridization between the magnetic excitons and phonons. Although the theory is valid for any crystal field level scheme we use for an explicit example a singlet ₁ groundstate with a ₄ triplet first excited state.     

Hysteresis losses of fine filamentary superconductors including field dependent critical current density

Using the force versus displacement curve for the flux lines, the hysteresis losses of thin superconducting slabs in transverse magnetic field were calculated. Especially, Kim's model for the field dependence of the critical current densityJ _c 1/(B _m+B _o) was used (B _m- the applied field,B _o = const.). The results are compared with the corresponding model usingJ _c = const., as well as with the critical state model without the force versus displacement curve. It is shown that the losses per volume are always decreasing with decreasing slab thickness. In the case of large background fieldB ₀ and small amplitude of the ripple fieldb ₀, the corrections are small with respect to the caseJ _c= const. Without background field, the hysteresis losses are somewhat larger than for field independentJ _c. Therefore, in accordance with our previous paper, the strong decrease of hysteresis losses with decreasing filament diameter in lower magnetic fields cannot be explained by the reversible motion of flux lines in the superconductors. It is suggested that this strong decrease is connected with the reversible motion of flux not connected with flux lines in superconductors, the diameter of which is comparable with the London penetration depth .     

U impurities in Au

The properties of U impurities in Au have been studied by magnetic susceptibility, electrical resistivity, and electron spectroscopic measurements. The results indicate that U forms a well-defined local magnetic moment of 3.6 _B , giving rise to a Kondo resistance anomaly at low temperature. This is in agreement with spectroscopic data, which show localized impurity states well removed from the Fermi level. Comparison to calculated atomic multiplets shows the need for inclusion of crystal or ligand field effects for the multiplets. The intra-5f Coulomb correlation energy is found to be 2.6 eV.     

Model temperature dependence of the imaginary ac susceptibility in simple and double superconducting structures

Closed expressions are derived for the temperature dependent imaginary part of the ac susceptibility of simple geometrical forms of superconductors by means of the relation between susceptibility and losses at ac field excitation. The magnetic field dependence of the critical current densityj _c is assumed to follow Bean's or Kim's model. The temperature dependence ofj _c is assumed to be linearly decreasing with a moderate nonlinear tail near the critical temperatureT _c . The Meissner as well as surface barrier shielding effect are also involved into consideration. Simple two component structures are entirely analysed regarding the variety of their relative critical current densities, critical temperatures and dimensions. Using the proposed procedure, it is very simple to derive similar as shown expressions for arbitrary parameters and to use them for getting insight into the inner structure of the investigate superconductors.     

Critical currents of type ii superconductors in non-transversal magnetic fields

The force on flux lines near narrow defects elongated in the direction of the transport current is calculated. This force, together with the maximum interaction forcek _m on the central flux line element, gives the volume pinning forceF _p on the flux line lattice. The dependence ofk _m (and of the critical current densityj _c ) on the angle between the magnetic field and the transport current is calculated for different defect dimensions (with respect to the coherence length of the superconductor) and compared with the experimental results. We obtainj _c sin^–n , where 1n3 in dependence on the defect dimensions and on the angle interval. The possibility of testing theF _p k _m ² dependence of the statistical theory of pinning is proposed.     

Two successive magnetic transitions in Y2Cu2O5 studied by μSR

Zero field muon spin rotation and magnetic susceptibility experiments on investigation of magnetic properties of cuprateY ₂Cu₂O₅ have been performed in the temperature range 4.2–30 K. Transverse fieldSR-experiments have been also carried out in order to obtain accurate information about transition temperature and to study the influence of the external magnetic field. Our data show that two magnetic phase transitions occur inY ₂Cu₂O₅ with lowering temperature. Upper Neel temperatureT _N=13 K is consistent with previous experimental data. We obtained the temperature dependence of the local magnetic field on the muonB (T) in the antiferromagnetic phase.B (T) reveals a peculiarity (some change of the slope) near the temperatureT _N=7.5 K, which can be interpreted as an additional magnetic phase transition caused by a change in magnetic ordering of the copper subsystem. Applying a small external magnetic field 50 Oe leads to smearing of the peculiarity inB (T) dependence.     

Temperature- and magnetic field dependence of the Hall effect in the heavy fermion system CeCu6

The temperature dependence of the Hall coefficient and the magnetic field dependence of the Hall resistivity of CeCu₆ have been determined in the temperature range 80 mK<T<10 K and in magnetic fields up to 10T. The Hall coefficientR _H shows a very strong temperature dependence with two extrema and a change of sign, and the Hall resistivity _xy has a strong field dependence with up to two changes of sign. The observed behavior can partially be explained by the field- and temperature dependence of the skew scattering contribution to the Hall coefficient.     

Comparison of probe and microwave measurements of electron density in plasmatic cylinder

The paper deals with a comparison of the measurement of the electron density of a plasmatic cylinder in a weak magnetic field (radius of cyclotron rotation of ionsr _Hi is larger than radius of probea) parallel to the axis of the plasma by means of a cylindrical Langmuir probe and a microwave interferometer in the X band. A Langmuir probe is used to measure the electron density distribution across the cross-section of the plasmatic cylinder, i.e. in the plane perpendicular to the axis of the cylinder. The measured density distribution is used to determine the mean electron density over the cylinder cross-section. This mean electron density is compared with the density found with a microwave interferometer. In evaluating the measurements the concentration of the microwave energy through the antennas of the interferometer in the cross-section of the plasmatic cylinder is considered. The difference of the mean electron density found by the microwave method from the mean density found by the probe method was in the least favourable case 30%. In conclusion, the paper gives a comparison of the probe and microwave measurements of the plasma density in a stronger magnetic field, whenr _Hi a.The authors thank J. Teichmann, C. Sc., R. Klíma, C.Sc. and other members of the department for discussions of the results, V. Kopecký for aid in the probe measurements and the director of the institute, Ing. J. Váa, for his interest in the experiments.     

Magnetic properties on holmium iron garnet in high magnetic field. Optical absorption spectra in the infra-red region

On single crystals of holmium iron garnet (HoIG), magnetic properties have been studied in magnetic field up to 150kOe applied parallel to the main crystallographic directions in the 4.2–300K temperature range. Above 130 K, the magnetization is isotropic and linear magnetic field dependent as previously found in polycrystals and predicted by Néel's ferrimagnetic theory. Nevertheless the paramagnetic Curie temperature is much higher than the polycrystal value. Below 130K, due to the onset of the umbrella structure, the ferrite magnetization presents a non linear field variation with [111] as easy direction. The field evolutions of the anisotropy constants (K ₁ andK ₂) were calculated. Optical absorption measurements of both 5₈5I ₆ and 5I ₈5I ₇ transitions are reported and compared to the results of the literature in terms of inequivalent magnetic sites.     

A study of the mixed state of the YBa2Cu3O6.9 superconducting ceramics by neutron depolarization

The mixed state of the YBaCu₂O_6.9 superconducting ceramics is investigated by neutron depolarization (ND). The magnetic flux penetration into superconductor's granules can be described in terms of Bean's critical state model. The field and temperature dependence of the average angle of the vortex line deflection from the direction of the applied magnetic field are determined. In particular, this angle is zero atT/T _c 0.5 andH/H _c20.015, which is at the sameT/T _c nearH/H _c20.017 for the metalloceramic SnMo₆S₈ superconductor. The latter is also studied in the present work for comparison with the YBa₂Cu₃O_6.9 sample. TheseH/H _c2 are far below the corresponding values ofH/H _c2 for Nb, Nb–Ta. The creep of the flux line misalignments from the applied field direction is examined.     

Large magnetic entropy change in Nd<Subscript>2/3</Subscript>Sr<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>

In order to search for new materials for the application of magnetic refrigeration, the polycrystalline perovskite compound Nd_2/3Sr_1/3MnO₃ was prepared by a solid-state method. The dependence of the magnetization on the applied field and temperature was measured near the Curie temperature. In terms of Maxwells equation, the temperature dependence of the absolute value of the isothermal magnetic entropy change |S_M| at various applied fields from 1 T to 5 T was determined. The results showed that a large magnetic entropy change was observed in this compound. The maximum magnetic entropy change |S_M^max|can reach 3.25 J/kgK with an applied field of 1 T at the Curie temperature of 257.5 K, which equals that of Gd. At 5 T applied field, it is 7.57 J/kgK. Such good magnetocaloric properties make this compound a promising candidate for the application of magnetic refrigeration in the room-temperature range. PACS 74.25.Ha; 75.30.-m; 75.30.Sg; 75.50.-y; 75.60.-d     

Magnetized cosmological model

The object of this paper is to investigate the behavior of the magnetic field in a cosmological model for perfect fluid distribution. The magnetic field is due to an electric current produced along thex axis. It is assumed that expansion () in the model is proportional to ₁ ¹ , the eigenvalue of the shear tensor _i ^j . The behavior of the model when the magnetic field tends to zero and other physical properties are also discussed.     

4f-electron-phonon coupling and lattice dynamics in the ferromagnet LiTbF4

The polarized Raman spectra of LiTbF₄ have been investigated at several temperaturesT150 K, and the temperature dependence of the selfenergies of optical phonons due to the magnetoelastic interaction have been determined. The sizes of these temperature shifts, of the magnetic phonon splittings and of the level repellings between resonant optical phonons and crystal field states have been calculated using the theory of magnetoelastic interaction by Fulde and Thalmeier without any adjustable parameters, considering the modulation of the crystalline electric field by the optical phonons. The phonon modes of LiTbF₄ have been determined in a rigid-ion approximation. Reasonable to good agreement with the experimental results has been obtained for most of the phonon modes.     

Resonant magnon—optical phonon coupling in metamagnetic FeCl2·2H2O

The resonant coupling between magnons and optical phonons of symmetryB _g was observed in the Raman spectra at different magnetic fields in the antiferro-, ferri- and ferromagnetic phases of FeCl₂·2H₂O at 2K. The line positions of the coupled modes were calculated taking a field dependence of the coupling strength into account. Varying this strength and the frequencies of the uncoupled modes as parameters of this calculation a fit of the experimental data was obtained also at low fields where former interpretations were not satisfying. However, the field dependence of the coupling strength disagrees with a corresponding term of an appropriate microscopic coupling mechanism in which the local spins are disturbed by a modulation of the ctystal field via spin-orbit coupling. In accordance with the former results of the far infrared absorption measurements zero field splitting of the magnonlike mode was found to exist. In the AF-phase two magnonlike modes can be observed, but the Raman scattering cross section of one of these modes tends to zero at zero field corresponding to its vanishing phonon contributions responsible for the scattering cross sections of the coupled modes. The relationship between cross sections of the coupled modes and their phonon contributions was confirmed by analysing the field dependence of the mode intensities in the Raman spectra.A project of the Sonderforschungsbereich 65 Festkörperspektroskopie Darmstadt-Frankfurt, supported by the Deutsche Forschungsgemeinschaft     

Temperature and pressure dependence of magnetic and electric hyperfine interactions in metallic Samarium

The magnetic and electric hyperfine interaction at¹¹¹Cd impurities in Samarium has been investigated by TDPAC measurements. The quadrupole frequency is _Q=20.0(2) MHz at 290 K and has a linear temperature dependence with the same slope (dln_Q/dT)_290K=–7.3(2) 10^–4 K^–1 in the rhombohedral and the hcp phase. The pressure dependence up to 7 kbar is (dln _Q/dT)=+8.7(1.4) 10^–3 kbar^–1. The magnetic hyperfine field of¹¹¹Cd in Sm is H_hf=242(6) kG at 4.2 K. Its temperature dependence confirms the existence of 2 different magnetic phases in Sm. The crystal field parameters B ₂ ⁰ and B ₄ ⁰ have been estimated from a comparison of H_hf(T) with molecular field models. The TDPAC spectra in the magnetic phases suggest that the impurities preferentially occupy the hexagonal Sm sites.