Field-cooled Case Research Articles

Magnetic levitation and guidance performance of Y–Ba–Cu–O and Gd–Ba–Cu–O bulk superconductors under low ambient pressure

The high-temperature superconducting Maglev-evacuated tube transport (HTS Maglev-ETT) system has significant potential for practical applications. In order to employ HTS bulks in an ETT system, it is necessary to study the magnetic levitation and guidance performance of bulk superconductors under low ambient pressure conditions that are representative of those inside the tube. As a result, we have investigated the Maglev performance of Y–Ba–Cu–O (YBCO) and Gd–Ba–Cu–O (GdBCO) single grain, bulk superconductors under different ambient pressures of 100 kPa, 60 kPa and 20 kPa using a bespoke pressure-controlling platform, in which the superconductors were positioned above a Halbach permanent magnet guideway. The levitation forces under conditions of field-cooling (FC) and zero-field-cooling (ZFC) have been measured and analyzed. The guidance force for the FC case and the levitation force relaxation for the ZFC case were also measured. It was found from the experimental results that the Maglev performance of both types of bulk superconductor improves significantly by reducing the ambient pressure in their immediate working environment. The levitation forces of YBCO and GdBCO bulk superconductors were observed to increase by 11.6% and 4.4% in the case of FC, and by 20.3% and 13.7% in the case of ZFC, respectively, as the ambient pressure changed from 100 kPa to 20 kPa. An improvement of the guidance force and an inhibition of the levitation force relaxation were also observed for these materials. The supercooled liquid nitrogen cryogen in the low-pressure environment further reduces the temperature, which, in turn, increases the critical current density Jc of the bulk superconductors and improves their levitation performance. These results provide further evidence of the significant advantages of combining HTS Maglev with evacuated tube technologies as a key enabling feature of the HTS Maglev-ETT system. The data further indicate that YBCO bulk superconductors tend to be more sensitive to ambient pressure and temperature variation than GdBCO bulk superconductors. This difference essentially reflects the dependence of Maglev performance of HTS bulk superconductors on Jc when operating in a low-pressure environment.

Levitation Force of Bulk YBaCuO and GdBaCuO Under a Low-Pressure Environment

The high-temperature superconducting (HTS) bulk is the core component of HTS maglev systems. For the potential application of evacuated tube transportation, it is necessary to verify the loading capacity of the bulk under a low-pressure environment. Based on a home-made pressure-reducing platform, we investigated the levitation force of two kinds of typical bulks, that is, the same sized YBaCuO and GdBaCuO, above a Halbach permanent magnet guideway under different pressure conditions. The levitation force in the cases of zero-field-cooling (ZFC) and field-cooling (FC) were measured and analyzed. Experimental results show that the reduced air pressure can significantly improve the levitation force of the two kinds of bulks. The levitation force of YBaCuO and GdBaCuO has increased by 11.6% and 4.4% in the FC case, 20.3% and 13.7% in the ZFC case under 20 kPa compared with the atmospheric pressure (100 kPa), respectively. This universal phenomenon was explained by the increasing critical current density Jc of HTS bulks due to cooler liquid nitrogen under the low-pressure condition. It is interesting to find that the YBaCuO bulk was more sensitive to the pressure variation compared with the GdBaCuO bulk. This difference reflects the improvement extent of levitation force of HTS bulks with different Jc performance working in a low-pressure environment.

Unusual ZFC and FC magnetic behavior in thin Co multi-layered structure

The observation of unusual magnetic phenomena in a Ni -based magnetic memory device ([4] O. Ben-Dor et al., 2013) encouraged us to conduct a systematic research on Co based multi-layered structure which contains a α-helix L polyalanine (AHPA-L) organic compound. The constant Co thickness is 7nm and AHPA-L was also replaced by non-chiral 1-Decanethiol organic molecules. Both organic compounds were chemisorbed on gold by a thiol group. The dc magnetic field (H) was applied parallel and perpendicular to the surface layers. The perpendicular direction is the easy magnetization axis and along this orientation only, the zero-field-cooled (ZFC) plots exhibit a pronounced peak around 55–58K. This peak is suppressed in the second ZFC and field-cooled (FC) runs performed shortly after the virgin ZFC one. Thus, around the peak position ZFC>FC a phenomenon seldom observed. This peak reappears after measuring the same material six months later. This behavior appears in layers with the non-chiral 1-Decanethiol and it is very similar to that obtained in sulfur doped amorphous carbon. The peak origin and the peculiar ZFC>FC case are qualitatively explained.

Fracture problem for an external circumferential crack in a functionally graded superconducting cylinder subjected to a parallel magnetic field

In this study, the multiple isoparametric finite element method (MIFEM) is used to investigate external circumferential crack problem of a functionally graded superconducting cylinder subjected to electromagnetic forces. The superconducting cylinder is composed by Bi2223/Ag composite with material parameters varying. A crack reference region is defined to reflect the effects of crack on flux and current densities, and the magnetically impermeable crack surface condition and the generalized Irie–Yamafuji critical state model outside the crack region are adopted. The distributions of magnetic flux density in the superconducting cylinder are obtained analytically for both the zero-field cooling (ZFC) and the field cooling (FC) activation processes. Based on the MIFEM, the stress intensity factors (SIFs) at crack fronts in the process of field ascent and/or descent are then numerically calculated. It is interesting to note from numerical results that for the present crack model in the ZFC activation process, the crack is easily propagate and grow with the applied field increases, and that in the field descent process of either the ZFC case or FC case, the crack generally does not propagate. In addition, in the field ascent process of the ZFC case, the SIFs depend on not only the crack depths and model parameters but also the applied field. The present study should be helpful to the design and application of high-temperature superconductors with external edge cracks.

An inner annular crack in a superconducting cylinder and its crack front properties under applied magnetic field

An inner annular crack in an infinitely long superconducting cylinder is modeled, and its fracture properties under electromagnetic forces are investigated. First, the flux density distributions in the superconducting cylinder of the present crack model are obtained analytically for both zero-field cooling (ZFC) and field cooling (FC) activation processes, where the magnetically impermeable crack surface condition and the Bean model outside the crack influence region are adopted. Then the stress intensity factors (SIFs) at both inner and outer crack fronts are calculated by the finite element method (FEM). Some important phenomena are observed. Among others, in the process of field ascent, the state corresponding to the applied maximal magnetic field is the most critical. In the process of field descent, for the FC case, the crack always propagates and grows from the inner crack front, and it is easier to propagate when the crack is close to the cylinder's center. For the ZFC case, although the crack generally propagates from the inner crack front as well, whether the crack would propagate or not depends on the crack position, crack size and the applied field. The crack is generally easier to propagate for the FC process than for the ZFC process. The findings from this study are very useful for detailed analysis and design of superconducting materials.

Fracture analysis for a penny-shaped crack problem of a superconducting cylinder in a parallel magnetic field

In this work, the penny-shaped crack problem is investigated for an infinite long superconducting cylinder under electromagnetic forces. The distributions of magnetic flux density in the superconducting cylinder are obtained analytically for both the zero-field cooling (ZFC) and the field cooling (FC) activation processes, where the magnetically impermeable crack surface condition and the Bean model outside the crack region are adopted. Based on the finite element method (FEM), the stress intensity factor (SIF) and energy release rate (ERR) at the crack tips in the process of field descent are further numerically calculated. Numerical results obtained show that according to the maximal energy release rate criterion, the FC process is generally easier to enhance crack initiation and propagation than the ZFC activation process. On the other hand, for the FC activation process, the larger the maximal applied magnetic field, more likely the crack propagates. Additionally, crack size has important and slightly different effects on the crack extension forces for the ZFC and FC cases. Thus, all of the activation processes, the applied field and the diameter of the penny-shaped crack have significant effects on the intensity analysis and design of superconducting materials.

Flux-pinning-induced stresses in a hollow superconducting cylinder with flux creep and viscosity properties

The magnetoelastic problem for a superconducting cylinder with a concentric hole placed in a magnetic field is investigated, where the flux creep and viscous flux flow have been considered. The stress distributions are derived and numerical calculated for the descending field in both the zero-field cooling (ZFC) and field cooling (FC) processes. The effects of applied magnetic field, flux creep and viscous flux flow on the maximal radial and hoop stresses are discussed in detail, and some novel phenomena are found. Among others, for the FC case, the maximal hoop tensile stress always occurs at the hole edge, whist for the ZFC case, the maximal stresses including both hoop and radial stresses either occur in the vicinity of the hole or occur at the position of flux frontier in the remagnetization process. For the descending field, in general, both the flux creep and viscosity parameters have important effects on the maximal radial and hoop stresses. All these phenomena are perhaps of vital importance for the application of superconductors.

A study of exchange bias in BiFeO3 core/NiFe2O4 shell nanoparticles

We have carried out magnetization measurements on BiFeO3 core/NiFe2O4 shell nanoparticles, and searched for the exchange bias phenomenon in this system. The core-shell nature of these nanoparticles has been established from the transmission electron microscopy images. The neutron diffraction study establishes that the core is G-type antiferromagnetic, while the shell is ferrimagnetic in nature. The search for an exchange bias phenomenon in the core-shell system shows a shift of the field-cooled (FC) hysteresis loops, at 5 K, along the magnetic field axis. The present investigation shows an unusual shift of the zero field-cooled (ZFC) hysteresis loop along the magnetic field axis as well. An enhancement of the remanent magnetization along with a decrease in the coercivity is also observed in the FC case, as compared to the corresponding values in the ZFC case, which is not found commonly in any conventional exchange-biased system. All these features indicate the presence of an interface exchange coupling between core and shell of the studied nanoparticles.

Improved Maglev Performance of Bulk High-Temperature Superconductors with a Re-magnetization Process After Zero-Field Cooling

Zero-field cooling (ZFC) and field cooling (FC) are the two most popular activation ways of the bulk high-temperature superconductors (HTSCs). The former can bring a big levitation force but a poor stability, while the latter can bring a good stability but a reduced levitation force due to the trapped flux. Under this rule, it is very difficult to improve the levitation force (load capability) and guidance force (stability) at the same time with the given bulk HTSCs and applied field in practice. In the paper, based on the re-magnetization ability of bulk HTSCs, the maglev performance of bulk HTSCs with a re-magnetization process after ZFC was experimentally investigated above a permanent magnetic guideway (PMG). The bulk HTSCs were firstly cooled down at a far distance above the PMG, but before moving to the working height, an additional process was introduced to descend the bulks to a lower height to magnetize again by the PMG field. Experimental results show that, at certain re-magnetization height above PMG, the levitation force and guidance force could be improved simultaneously compared with the results of normal FC cases, which is different from the present performance improvement with the sacrifice of one important force. This result presents a possible working way for the levitation applications of bulk HTSCs by employing a re-magnetization process after ZFC, and is also useful to optimize the performance of high-temperature superconducting Maglev vehicle systems.

Experimental validation of field cooling simulations for linear superconducting magneticbearings

For practical stability of a superconducting magnetic bearing the refrigeration process mustoccur with the superconductor in the presence of the magnetic field (a field cooling (FC)process). This paper presents an experimental validation of a method for simulating thissystem in the FC case. Measured and simulated results for a vertical force between ahigh temperature superconductor and a permanent magnet rail are compared.The main purpose of this work is to consolidate a simulation tool that can helpin future projects on superconducting magnetic bearings for MagLev vehicles.

Magnetization reversal in circularly exchange-biased ferromagnetic disks

We investigate the reversal behavior of circularly exchange-biased micron-sized bilayer disks of Permalloy (Py)/IrMn and CoFe/IrMn. A circular exchange bias is induced by imprinting the vortex configuration of the ferromagnetic layer into the IrMn when the disks are cooled in zero external field through the blocking temperature of IrMn. The resulting circular exchange bias has a profound effect on the reversal behavior of the ferromagnetic magnetization. In Py/IrMn disks the reversal takes place via vortex motion only, and the behavior is controlled by the exchange bias; it is reversible over a range of small fields and the vortex maintains a single chirality throughout reversal, determined by the chirality of the exchange bias. In CoFe/IrMn disks the non-negligible magnetocrystalline anisotropy causes a reversal via both vortices and domain walls resulting in a finite coercivity, and the behavior is controlled by microstructure. We verify that circular exchange bias does not give rise to a hysteresis loop shift. It lowers coercivity with respect to the field-cooled case, and in Py/IrMn disks it even causes completely reversible magnetic behavior. In both Py/IrMn and CoFe/IrMn disks, circular exchange bias removes the randomness (i.e., stochastic processes due to thermal activation) inherent in single-layer ferromagnetic disks and causes the magnetic behavior to be reproducible over time.

High-field study of the magnetization in Ti doped ErCo2 compound

The magnetization of powdered MgCu2 structure-type intermetallic ErCo1.95Ti0.05 was studied in the temperature range of 4.2–123K in magnetic fields up to 23T. A field-cooled magnetization jump at certain temperatures (TC) corresponds to a first-order magnetic phase transition from a paramagnetic to a ferrimagnetic state indicating cobalt moment formation. We observe that TC approach asymptotically T=50K with the cooling field Hc. The magnetization above TC stays constant (temperature independent), but does not saturate even in the field of 23T. The moment per unit formula was found to be approximately 7.25μB for this field. This behavior may be attributed to domain structures with the enhanced magnetocrystalline anisotropy field with titanium impurities. Magnetization versus magnetic field curves has also been recorded for both zero-field-cooled and field-cooled cases at various temperatures below and above TC. These curves below TC show typical ferrimagnetic phase; in the intermediate temperature range of 34–50K metamagnetism was observed and the sample becomes pure paramagnetic above 50K. In addition, the resistivity measurement was performed in the temperature range of 4.2–273K. A jumplike drop in the resistivity was observed at TC. In comparison with that of ErCo2, this transition temperature shifts to higher temperature as much as 2K, consistent with the slight increase of the lattice parameter. This drop in resistivity becomes much deeper presumably due to the internal magnetoresistance within the large domains. Analysis of the resistivity data in terms of spin fluctuations including s-d electron scattering and electron-phonon interaction reveals that T2 behavior becomes dominant in the temperature range of 34–50K where the magnetization data exhibit metamagnetism. Therefore, we assert based on both magnetization and resistivity results that the spin-fluctuation effects are driving force for the cobalt moment formations.

Magnetic phase transition in Ti-doped ErCo2 alloys

Abstract The influence of the non-magnetic Ti substitution for the Co atoms on the electronic structure and the magnetic state of the ErCo2 compound has been investigated by means of electrical resistivity and magnetization measurements. Arc-melted Ti-doped ErCo2 samples, ErCo2−xTix ( x = 0 , 0.04, 0.05, 0.07, 0.10, and 0.20) are crystalized with the same cubic Laves phase C15 (space group F d 3 ¯ m ) at room temperature. The crystal parameter a increases almost linearly with increasing Ti content up to x = 0.20 . In connection with the first-order magnetic phase transition, a sharp drop in the resistivity and a rapid increase in the magnetization are observed in all samples at a transition temperature, T c . The transition temperature slightly decreases at first and then increases gradually with higher concentrations of Ti contents. The overall resistivity of these compounds in the temperature range of 4–300 K increases considerably, but decreases rapidly with decreasing temperature as the Ti concentration is increased. Resistivity data have been analyzed using the existing model in the literature. In addition, magnetization versus temperature curves for both zero-field cooled (ZFC) and field-cooled case (FC) in the measuring field of 50 Oe were recorded. First-order like jump in magnetization and splitting of ZFC and FC curves (irreversibility effects set in) at T c are observed for all samples. The most striking features are: FC magnetization below T c reduces gradually but significantly and coercivity inferred from the hysteresis cycles at T = 4.2 K increases with Ti concentration.

Low-field magnetic-properties of ErCo 2 intermetallic compound

The magnetic behavior of ErCo 2 at low fields has been investigated over the temperature range extending from 4 to 300 K. Magnetic hysteresis loops, at 4.2 K between the fields of ∓ 1.5 kOe, have been recorded for both zero field-cooled (ZFC) and field-cooled (FC) cases. Strong thermo-magnetic hysteresis effects were observed in the dc temperature dependence of magnetization measurements at temperatures below T = 35 K. A sharp drop occurs in both the ac susceptibility and the resistivity at the same temperature. All these behaviors indicate an apparent first-order magnetic transition. The same experiments were repeated on the same sample, but for two different thermal histories (annealed at 670 ° C for 2 months, then slowly cooled down to room temperature and annealed at 900 ° C for 2 days, then quenched to room temperature). Neither the transition temperature nor the magnetic hysteresis has been affected considerably by these thermal treatments. From magnetization measurements for different magnetic histories such as ZFC and FC cases and also thermal cycling of the remnants, it is concluded that the cobalt moments are created at T c accompanied by a lattice expansion with a small distortion along the 〈 1 1 1 〉 direction (easy axes). Each induced cobalt moment has an orbital component coupled to the 〈 1 1 1 〉 direction. Close to its critical temperature, the slightest applied field, even earth’s field, will have an effect first on the erbium sites whose magnetic moments are oriented along the field direction and then will trigger the cobalt moment formation process in the opposite direction. The existence of the cobalt orbital moment can account consistently for many aspects of low-field M versus H curves below the Curie temperature for both ZFC and FC cases. Furthermore, we have observed that the magnetization, at temperatures below T c , depends on the history. The reduction in the magnetization after each M versus H cycle (the reduction in the magnetization occurs substantially after the first cycle of the applied field and followed by the relaxation to the equilibrium state where the rate of reduction decreases for the following cycles). This behavior may be attributed to the spin melting of some cobalt atoms, initially quenched along the direction of erbium magnetic moments. We also discuss the Dzyaloshinsky–Moriya (DM) anisotropic interactions as one of the possible mechanisms to explain the origin of the large coercivity observed at low fields.

Thermomagnetic hysteresis and electrical transport in amorphous [formula omitted] films

We report resistivity and magnetization measurements on an amorphous Ni 74 Mn 24 Pt 2 thin film in the temperature range of 3–300 K. Two significant features are apparent in both the magnetic susceptibility and electrical resistivity. A low-temperature (low- T ) anomaly is observed at about 40 K, where a cusp appears in the resistivity, while a concomitant step-like increase in zero-field-cooled (ZFC) magnetization ( M ) appears with increasing temperature. The low- T anomaly is attributed to a crossover from a pure re-entrant spin-glass within individual domains to a mixed ferro-spin-glass regime at lower temperatures. By contrast, the high-temperature (high- T ) anomaly, signaled by the appearance of hysteresis below 250 K, corresponds to the freezing of transverse spins in individual domains acting independently. Between the low- T and high- T anomalies a small but discernable magnetic hysteresis is observed for warming vs. cooling in the field-cooled (FC) case. This behavior clearly indicates the presence of domain structure in the sample, while the disappearance of this hysteresis at lower temperatures indicates the complete freezing of the spin orientation of these domains. According to these results, we have divided the magnetic state of this sample into three regions: at temperatures above 250 K, the sample behaves like a soft ferromagnet, exhibiting M vs. H loops with very small hysteresis (less than 5 Oe). As the temperature is lowered into the intermediate region (the range 40–250 K), spins become frozen randomly and progressively within the individual domains. These domains behave independently, rather than as a cooperative behavior of the sample. Weak irreversibility sets in, indicating the onset of transverse spin freezing within the domains. At temperatures below 40 K, the M vs. H loops exhibit larger hysteresis, for both the ZFC and FC cases, as in a pure spin-glass. We have also demonstrated giant noise in the resistivity at temperatures just below 250 K. Such noise can originate from fluctuations of the domains near the film surface because of competing effective bulk and surface anisotropy fields. The large observed amplitude may be explained by means of a large ferromagnetic anisotropy in the resistivity due to the large spin–orbit effect seen in NiMn systems. Finally, the low- T peak in the resistivity has been analyzed using Fisher and Langer's expression based on the Friedel Model proposed for critical transitions in transition metals ( s – d systems). The fitted results are in satisfactory agreement with the predictions of this model.

Levitation force and guidance force of YBaCuO bulk in applied field

Levitation force of YBaCuO bulk below 77 K and trapped flux dependence of levitation force are studied. Experimental results show that levitation forces increase with decreasing of temperature and decrease approximately linearly with increasing of trapped flux. Guidance forces of a single YBaCuO bulk and YBaCuO bulk arrays are investigated in different field-cooling cases. Results show that guidance force exhibits much hysteresis; moreover, the hysteresis increases with decreasing of measurement height. Both trapped flux and applied field have much influence on the guidance force. Smaller lateral displacement may lead to the elastic lateral motion of the YBaCuO bulk arrays.

Field- and temperature-controlled magnetic phase separation in(La0.25Pr0.75)0.7Ca0.3Mn18O3manganite as seen by La NMR and magnetization measurements

We present a detailed La NMR and magnetization study of $({\mathrm{La}}_{0.25}{\mathrm{Pr}}_{0.75}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ manganite, in which an insulator-metal transition can be induced by magnetic field and also by oxygen isotope substitution. NMR results demonstrate directly the presence of magnetic phase separation at a local level. The microscopic magnetic structure of the sample depends on cooling conditions [namely, zero field cooling (ZFC) or field cooling (FC)] and can be continuously controlled through the phase separation range by magnetic field and temperature. The magnetic phase diagrams of the sample have been obtained in a wide ranges of fields and temperatures for both ZFC and FC cases.

Magnetic properties of strip-like Josephson-junction arrays

Zero-field-cooled (ZFC) and field-cooled (FC) magnetic propertiesof strip-like Josephson-junction (JJ) arrays with very strong demagnetizingeffects are calculated from basic laws. Similar to slab-like JJ arrayswithout considering demagnetizing effects, a vortex state evolves to acritical state (CS) with increasing maximum JJ currents in the ZFC case,and a vortex state always remains with a negative low-field susceptibilityin the FC case. However, the strong demagnetizing effects cause qualitativechanges in the CS, where the overall feature of the field and currentprofiles turns out to be similar to that in type-II superconducting strips,but not like the ordinary Bean CS in slab-like JJ arrays, the CS currentprofile is never flat and the critical current is no longer a step functionof the maximum JJ current as in slab-like JJ arrays. The calculated resultsof different types of JJ arrays indicate that although the intergranular CSin granular superconductors may have a common origin, the discoveredparamagnetic Meissner effect in them is still difficult to explain.

Hysteresis Loop Shifts in Magnetic Field Cooled FeOOH Nanoparticles

ABSTRACTMeasurements of the magnetization M as a function of temperature (5K - 300K) and applied magnetic field H (up to 50 kOe) in 30 Å particles of FeOOH are reported. M increases with decreasing T, peaking at TB = 65 K below which the ZFC (zero-field-cooled) and the FC (field-cooled) data separate. Hysteresis loop measured at 10 K for ZFC shows an open loop up to 40 kOe with coercivity = 2 kOe. For the FC case, the loop shifts and the loop-shift increases with the cooling field ItL, approaching saturation above Hc = 20 kOe. From the variation of M vs H above TB, a magnetic moment/particle μp = 300 μB is determined. These results suggest that the FeOOH nanoparticles have an antiferromagnetically ordered core with uncompensated surface spins yielding μp and the surface spins order in a spin-glass-like state below TB, possibly due to interparticle interactions.

Vortex behavior in a rotated YBa 2Cu 3O 7− δ polycrystal

Magnetic measurements on a rotated polycrystalline YBa 2Cu 3O 7− δ (YBCO) disk at 4.2 K yield polar plots of the longitudinal-vs.-transverse components of the vortex flux density ( B) relative to the fixed applied field ( H) for various angles of rotation ( θ) up to 360°. For the sample initially in a field-cooled (FC) state, detailed analyses of these plots reveal that B at any θ consists of a B R component, which rotates rigidly with the sample, plus a B F component, which stays at a constant frictional angle relative to H. At each H, B R and B F undergo large characteristic changes with θ until a rotational steady state is reached for θ above 180°. Strikingly, their numerical sum B R+ B F is constant for θ up to ∼90°, then decreases linearly up to ∼180°, above which it remains constant at its reduced value. This reduction in B R+ B F is attributed to an exiting of vortices from the sample, primarily those associated with the weakly pinned B F component. With increasing H, the relative vortex reduction rises to a broad maximum (of nearly 40%) and then slowly decreases, which reflects a competition between the intervortex repulsion and the confining forces produced by H. For the sample initially in a zero-field-cooled (ZFC) state, B R goes to zero while B F reaches the same steady-state value as in the corresponding FC case but without any vortex exiting from the sample.