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1.
We investigated influence of bidirectional square wave current with long periods and dc current on the evolution of the voltage-time
(V - t) curves in superconducting polycrystalline bulk Y1Ba2Cu3O7-δ (YBCO) material at the temperatures near the critical temperature. In a well-defined range of amplitudes and periods of driving
current, and temperatures, novel type of non-linear dynamic responses was observed by means of the V-t curves. It was seen
that such a non-linear response to bidirectional square wave current sometimes reflects itself as regular sinusoidal-like
voltage oscillations. The sinusoidal-like and non-sinusoidal oscillations were discussed mainly in terms of the dynamic competition
between pinning and depinning and
significant relaxation effects which appear in this competing process. The density fluctuations associated with the current
induced self-magnetic flux (SMF) lines and semi-elastic coupling of SMF lines with the pinning centers were also considered
as possible physical mechanisms in the interpretation of the experimental results. 相似文献
2.
We have studied the field dependence of the sublattice magnetization of ferrimagnetic yttrium iron garnet (YIG) using neutron scattering. In contrast to the macroscopic spontaneous magnetization that shows the normal field dependence of a soft ferromagnet (sudden saturation at the demagnetization field and no hysteresis) in neutron scattering a field induced first order spin flop transition with considerable hysteresis is observed at a critical field of Hc∼580 G (external field). Considering that with neutron scattering the antiferromagnetic component of ∼4/5 of the total moment is detected preferentially while in the macroscopic magnetization samples the ferromagnetic component of ∼1/5 exclusively it becomes clear that ferromagnetic and antiferromagnetic component have a completely independent field (and temperature) dependence. This indicates that the two magnetic structures have to be viewed as two weakly coupled order parameters. In the zero field ground state the moment orientations of the two ordering structures are orthogonal. Only for fields H0>Hc a nearly collinear ferrimagnetic order is established by the field. 相似文献
3.
利用半导体霍尔探头测量法在液氮温度下研究了低场下圆柱状熔融织构YBa2Cu3O7-δ样品的局域磁通蠕动行为.对瞬时加减场样品端面局域磁场的磁弛豫,磁滞回线以及不同扫场速率下局域磁场的变化进行了测量,观察到了对数时间磁弛豫的行为,发现由加场和去场数据所得的激活能U0有明显的不同.磁滞回线测量中,局域点的磁场随外场几乎同步变化,不同扫场速率下局域磁场随外场变化的差别不大.实验结果表明,外场除对样品侧面外,对端面也有明显的穿
关键词:
局域磁通蠕动
激活能
磁弛豫 相似文献
4.
B V B Sarkissian A K Grover G Balakrishnan Ravi Kumar P L Paulose R Vijayaraghavan V Sankaranarayanan C K Subramanian 《Pramana》1992,38(6):641-667
The results of experimental studies on hysteresis in magnetization, thermomagnetic history effects, anomalous variations in
magnetic hysteresis curves and the decay rates of magnetization obtained under different thermomagnetic histories in specimens
of conventional and high temperature superconductors are presented. The Bean’s critical state model is considered adequate
to explain magnetic behaviour in conventional hard superconductors. The similarity in the general features of the results
of different experiments on specimens of the two families of superconductors underscores the efficacy of the said model to
understand some aspects of the macroscopic magnetic response of high temperature superconductors as well. For instance, the
isothermal magnetization hysteresis loop which comprises of magnetization curves along forward (−H
max to +H
max) and reverse (+H
max to −H
max) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There
exist inequality relationship between various field values identified asH
peak,H
I,H
II etc. in isothermal magnetization hysteresis as well as magnetic relaxation data. The entire field span of an isothermal magnetization
hysteresis data set can be considered to comprise of three parts corresponding to (M
rem(H)−M
FC(H)+M
ZFC(H)) being equal to, less than or greater than zero, whereM
rem(H) are the remanent magnetization values obtained on reducing field to zero after having the specimen in different applied
field (H) values. There are, however some situations amongst thermomagnetic history effects in specimens which show incomplete flux
trapping on field cooling, where the critical state model has been found inadequate. 相似文献
5.
E. Mezzetti A. Chiodoni R. Gerbaldo G. Ghigo L. Gozzelino B. Minetti C. Camerlingo C. Giannini 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(3):357-362
This paper reports on the striking correlation between nanosize mosaic domain walls in YBCO films and 1D rows of parallel
Josephson junctions, determining the J
c
vs.B curves. From X-ray data analysis, it results that the average “hidden" domain wall, faceted at a nanometric scale, is almost
mimicking the Josephson Junction (JJ) 1D array. The assumption that the JJs and the domain-wall arrays are coincident, enables
to find out the particular scaling field, making the J
c
vs.B curves independent of temperature. This scaling field can be interpreted in terms of the Josephson nature of the transport
current across these particular patterns in the intermediate temperature range. By means of our model it is also possible
to calculate two asymptotic behaviors of the pinning force as a function of field, for low and high fields, respectively.
These behaviors are punctually repeated by the experimental results in the same asymptotic limit, so that two corresponding
vortex regimes are clearly pointed out. All results can be interpreted by concluding that in the intermediate temperature
range, the strong pinning observed in high quality YBCO films is due to the Josephson Junctions average patterns. These patterns
are the counterpart related to the transport mechanisms of “hidden" structural nano-domains.
Received 16 October 2000 and Received in final form 28 November 2000 相似文献
6.
S. Takács 《Czechoslovak Journal of Physics》1988,38(8):899-909
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 . 相似文献
7.
8.
D. A. Balaev S. I. Popkov K. A. Shaikhutdinov M. I. Petrov D. M. Gokhfeld 《Physics of the Solid State》2014,56(8):1542-1547
The hysteretic dependences of the magnetoresistance of porous (38% of the theoretical density) granular high-temperature superconductor (HTSC) Bi1.8Pb0.3Sr1.9Ca2Cu3O x have been analyzed in the model of the effective intergranular field. This effective field has been defined by the superposition of the external field and the field induced by magnetic moments of superconducting grains. The magnetic flux compression in an intergranular medium, characterized by the effective field, controls the hysteretic behavior of the magnetoresistance. It has been found that the magnetoresistance hysteresis width for the studied porous HTSC depends on the transport current, in contrast to the superconductor of the same composition with high physical density (more than 90% of the theoretical value). For a porous superconductor, a significant current concentration occurs in the region of the grain boundaries, which is caused by features of its microstructure. A current-induced increase in the effective boundary length results in a decrease in the flux compression, a decrease in the effective field in the intergranular medium, and a magnetoresistance hysteresis narrowing with increasing current. 相似文献
9.
D. B. Marshall R. E. DeWames P. E. D. Morgan J. J. Ratto 《Applied Physics A: Materials Science & Processing》1989,48(1):87-91
Two new effects associated with flux penetration and pinning in superconducting YBa2Cu3Ox are demonstrated. One of these is focussing of magnetic field lines by the superconductor and the other is magnetic suspension. The existence of stable equilibrium in suspension is shown to be related directly to hysteresis observed in the force-separation relation for a magnet and the superconductor. We also observe that the levitation height of a magnet increases with the magnet volume, contrary to results in the literature. 相似文献
10.
A. Kniffka T. Trautmann 《Journal of Quantitative Spectroscopy & Radiative Transfer》2011,112(8):1383-1393
A fast method is presented for gaining 3D actinic flux density fields, Fact, in clouds employing the Independent Pixel Approximation (IPA) with a parameterized horizontal photon transport to imitate radiative smoothing effects. For 3D clouds the IPA is an efficient method to simulate radiative transfer, but it suffers from the neglect of horizontal photon fluxes leading to significant errors (up to locally 30% in the present study). Consequently, the resulting actinic flux density fields exhibit an unrealistically rough and rugged structure. In this study, the radiative smoothing is approximated by applying a physically based smoothing algorithm to the calculated IPA actinic flux field. 相似文献
11.
Y. Ding Y. Sun X.D. Wang H.C. Wang Z.X. Shi Z.A. Ren J. Yang W. Lu 《Physica C: Superconductivity and its Applications》2010,470(22):2051-2056
Polycrystalline NdFeAsO0.88F0.12 superconductors prepared by high pressure (HP) and ambient pressure (AP) methods were comparatively studied by magnetization and transport measurements. Upper critical field Hc2, irreversibility field Hirr and the anisotropy parameter Γ were estimated from resistance transition curves. The broadening of transition width was observed, and was ascribed to both Hc2 anisotropy and superconductivity inhomogeneity of samples. Magnetic hysteresis loops (MHLs) in low fields were measured to detect the trace of weak-link behavior. The reclosed hysteresis loops in low fields indicate that there are weak links in both samples. Magnetization critical current density Jcm were derived from MHLs. Sample HP shows higher Jcm than sample AP. Direct transport I–V measurements show that the transport critical current density Jct are very low but persist up to 9 T, suggesting intrinsic strong-link existing in both samples. 相似文献
12.
A. Volmer S. Mukherji T. Nattermann 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):627-645
In anisotropic or layered superconductors thermal fluctuations as well as impurities induce a van der Waals (vdW) attraction
between flux lines, as has recently been shown by Blatter and Geshkenbein in the thermal case [#!BlatterGeshkenbein!#] and
by Mukherji and Nattermann in the disorder dominated case [#!NattermannMukherji!#]. This attraction together with the entropic
or disorder induced repulsion has interesting consequences for the low field phase diagram. We present two derivations of
the vdW attraction, one of which is based on an intuitive picture, the other one following from a systematic expansion of
the free energy of two interacting flux lines. Both the thermal and the disorder dominated case are considered. In the thermal
case in the absence of disorder, we use scaling arguments as well as a functional renormalization of the vortex-vortex interaction
energy to calculate the effective Gibbs free energy on the scale of the mean flux line distance. We discuss the resulting
low field phase diagram and make quantitative predictions for pure BiSCCO (Bi2Sr2CaCu2O8). In the case with impurities, the Gibbs free energy is calculated on the basis of scaling arguments, allowing for a semi-quantitative
discussion of the low-field, low-temperature phase diagram in the presence of impurities.
Received: 9 February 1998 / Accepted: 17 April 1998 相似文献
13.
A.V. Pan P. Esquinazi 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,17(3):405-410
We have investigated the influence of a driving force on the elastic coupling (Labusch parameter) of the field-cooled state
of the flux line lattice (FLL) in 400 nm thick YBa2Cu3O7 superconducting films. We found that the FLL of a field-cooled state without driving forces is not in an equilibrium state.
Results obtained for magnetic fields applied at and 30 relative to CuO2 planes, show an enhancement of the elastic coupling of the films at driving current densities several orders of magnitude
smaller than the critical one. Our results indicate that the FLL appears to be in a relatively ordered, metastable state after
field cooling without driving forces.
Received 21 December 1999 相似文献
14.
H. Ohoyama Y. Nagamachi T. Kasai 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2006,38(1):231-236
A pure and high intense pulsed supersonic CH (X2Π ) radical beam
source was developed via the C(1D) + H2 reaction. An electrostatic
hexapole field was used to state-select CH radicals. The focusing curves for
the single rotational states of CH were measured for the first time by a
saturated laser-induced fluorescence (SLIF) spectroscopy for the R-branch in
A2Δ3/2 ← X2Π 1/2 transition. The
focusing curves were simulated by the classical trajectory simulation based
on a Stark energy analysis of the rotational energy levels, including
spin-orbit and Λ-doubling coupling effects. In addition,
orientational distribution functions were calculated for the selectable
states. 相似文献
15.
P. Mun F. C. Fonseca R. Muccillo R. F. Jardim 《Physica C: Superconductivity and its Applications》2003,390(4):363-373
A systematic study of the magnetic hysteresis in transport properties of polycrystalline YBa2Cu3O7−δ–Ag compounds has been made based on two kinds of measurements at 77 K and under applied magnetic fields up to 30 mT: critical current density Jc(Ba) and magnetoresistance R(Ba). The R(Ba) curves show a minimum in their decreasing branch occurring at B=Bmin which was found to be both the excitation current Iex and the maximum applied magnetic field Bam dependent. In addition, for a certain value of Bam>5 mT, we have observed that Bmin increases with increasing Iex and reaches a saturation value. The Jc(Ba) curves show a maximum in decreasing applied magnetic fields occurring at B=Bmax. We have also found that Bmax increases with increasing Bam and reaches a saturation value. The minimum in the R(Ba) and the maximum in Jc(Ba) curves were found to be related to the trapped flux within the grains. All the experimental results are discussed within the context of the flux dynamics and transport mechanisms in these high-Tc materials. 相似文献
16.
D. A. Balaev A. A. Dubrovskiy S. I. Popkov K. A. Shaikhutdinov O. N. Mart’yanov M. I. Petrov 《Journal of Experimental and Theoretical Physics》2010,110(4):584-593
The transport properties (R(T) and R(H) dependences at various values of the transport current in magnetic fields up to 65 kOe) and low-temperature heat capacity
in magnetic fields up to 90 kOe of the BaPb0.75Bi0.25O3 superconductor (T
C
≈ 11.3 K) are investigated with the goal of clarifying the mechanisms determining the nonmonotonic behavior and hysteresis
of its magnetoresistance R(H). The type of R(H) hysteretic dependences for BaPb0.75Bi0.25O3 is analogous to that observed in granular high-T
c
superconductors (HTSCs); however, unlike classical HTSC systems, the field width of the magnetoresistance hysteresis loop
for polycrystalline BaPb0.75Bi0.25O3 depends on the transport current. This means that although the mechanisms responsible for the magnetoresistance hysteresis
(the influence of the magnetic flux trapped in superconducting regions on the effective field in Josephson interlayers) are
identical in these objects, the transport current in BaPb0.75Bi0.25O3 may considerably affect the diamagnetic response of the superconductor. A considerable effect of transport current on the
field in which the R(H) dependences have a peak and exhibit hysterestic properties is observed. Such a behavior can be adequately interpreted using
the model of the spatially inhomogeneous superconductor-insulator state proposed by Gorbatsevich et al. [JETP Lett. 52, 95 (1990)]. The nonmonotonic dependence of quantity C/T (C is the heat capacity) on the magnetic field discovered in the present study also agrees with the conclusions based on this
model. 相似文献
17.
D.H. Kim Jun-Ki Chung T.J. Hwang W.N. Kang Kook Chae Chung 《Physica C: Superconductivity and its Applications》2009,469(21):1950-1953
We measured the transport properties of MgB2 films having columnar grain structure with their axis normal to the substrate. When an external magnetic field was applied parallel to the grain axis, an enhanced critical current density has been observed, and this result has been ascribed to flux pinning induced by grain boundaries. The shape of the angular dependence of critical current density and its magnetic field dependence showed a quite similar resemblance to those of YBa2Cu3Ox films containing columnar defects, implying a possible existence of linear defects in MgB2 films of columnar structure. We propose that the amorphous regions at the vertex points of three or more grain boundaries observed in microstructural studies correspond to the linear defects and these linear defects anchor the end points of the flux line dislocations of Frank-Read sources, by which the shear in the flux line lattice is actuated. This assumed mechanism is found to reasonably explain the magnetic field dependence of the flux pinning force density of MgB2 films with columnar grain structure. 相似文献
18.
H. Lengfellner A. Schnellbögl J. Betz K. F. Renk W. Prettl 《International Journal of Infrared and Millimeter Waves》1990,11(5):631-639
A Nernst effect has been observed in a high temperature superconductor for the first time. Irradiating superconducting Tl–Ba–Ca–Cu–O thin films by short pulses of a TEA-CO2 laser, a photovoltaic signal is detected perpendicular to a magnetic field applied parallel to the film surface. The signal is attributed to magnetic flux line depinning and flux line transport driven by the laser induced temperature gradient. The results are described by thermal flux line activation leading to a calculated distribution of pinning energies from 100 K to 4000 K. 相似文献
19.
Mehdi Vaez-zadeh Ali Mohammadi 《Applied Physics A: Materials Science & Processing》2014,115(1):341-345
MnFe2O4 nanoparticles were prepared by a coprecipitation chemical method. The average size of the obtained nanoparticles was about 30 nm. The hysteresis measured at T=300 K clearly shows ferromagnetic order at room temperature while that measured at T=450 K shows superparamagnetic behavior. The difference in the magnetization curves in the field increasing cycle and field decreasing cycle at higher temperatures (450 K or higher) is very unusual. In this case, a hysteresis in magnetization in higher magnetic fields with an opening up of the magnetization curve was observed. In this work, the effect of temperature and time of application of the applied field on the magnetization behavior was studied. 相似文献
20.
D.H. Kim T.J. Hwang Y.J. Cha W.K. Seong W.N. Kang 《Physica C: Superconductivity and its Applications》2009,469(15-20):1059-1062
We studied the flux pinning properties by grain boundaries in MgB2 films prepared by using a hybrid physical chemical vapor deposition method on the c-axis oriented sapphire substrates. All the films we report here had the columnar grains with the growth direction perpendicular to the substrates and the grain sizes in the range of a few hundred nanometers. At very low magnetic fields, no discernable grain-boundary (GB) pinning effect was observed in all measuring temperatures, but above those fields, the effect of GB flux pinning was observed as enhanced critical current densities (Jcs) and reduced resistances when an external magnetic field (B) was aligned parallel to the c-axis. We interpret the B dependence of Jc in the terms of flux line lattice shear inside the columnar grains activated by dislocations of Frank–Read source while the flux lines pinned by GB act as anchors for dislocations. Magnetic field dependence of flux pinning force density for B parallel to the c-axis was reasonably explained by the above model. 相似文献