Field-cooling Process Research Articles

Effect of annealing temperature on structural and magnetic properties of Co2TiO4 ceramics prepared by sol-gel method

In this paper, polycrystalline Co2TiO4 ceramics have been synthesized using a sol-gel process followed by annealing at different temperatures. The lattice size and the average grain size of the samples increases with rise in annealing temperature. The temperature-dependent inverse paramagnetic susceptibilities recorded under zero-field-cooling condition have been fitted according to the Néel's expression for ferrimagnets. Subsequently, the molecular field constants and the corresponding exchange constants have been calculated. The fitting result shows that the magnetic interaction in the system becomes weaker as the annealing temperature rises. In addition, negative magnetization is observed during field-cooling process. The higher annealing temperature is beneficial to the growth of tetrahedral sublattice, leading to a decrease on compensation temperature. Furthermore, magnetization hysteresis loops for all the samples demonstrate the crucial role of grain size on the magnetic properties.

Trapped Field and Levitation Performance of a YBCO Superconductor Magnetized in Different External Magnetic Fields

In order to find a relative preferable method to improve the trapped magnetic field (Btrap) of a YBCO superconductor, the influences of field-cooling height (FCH), magnetization intensity, and magnetization process on the trapped fields of the high temperature superconductor (HTS) magnetized in the magnetic fields of the permanent magnet guideway (PMG) and the superconducting magnet (SM) were both studied. In addition, the levitation forces of the bulk magnetized in the two different magnetic fields were also investigated in this paper. The experimental results showed that the maximum trapped field ratio (δtrap) of the bulk magnetized above the PMG attained the largest value, when the initial FCH was 30 mm. The remagnetization process was a preferable method to increase the Btrap, in the case that the FCH was higher than 30 mm. When the exciting current (Ie) of the SM was smaller than 15 A, the remagnetization process was an effective method to improve the Btrap. The remagnetization performance of the bulk attained the saturation, under the condition that the Ie was larger than 15 A. In the aspect of the levitation performance, it can be found that the decreasing amplitude of the force with the Btrap became larger, when the levitation gap was lower. The force acted on the bulk increased with increasing the magnetic fields of the SM. Moreover, the results also indicated that the initial Btrap was larger; the maximum levitation force (Flmax) became smaller. The magnetic hysteresis of the bulk was found to decrease with increasing the Btrap.

Morphology-induced spin frustration in granular BiFeO3 thin films: Origin of the magnetic vertical shift

Pronounced room temperature vertical shifts in the magnetic hysteresis loops of granular, highly polycrystalline and ferromagnetic-like BiFeO3 thin films are observed upon field-cooling from a temperature above the Néel temperature of bulk BiFeO3. This is ascribed to the interplay between the preferential alignment, established by the field-cooling process, of the net magnetic moment, which arises from uncompensated antiferromagnetic spins, and the pinning of a fraction of these spins at the particle boundaries. Conversely, field-cooling of an epitaxially grown BiFeO3 film results in no vertical shift, confirming the effective role played by the particle boundaries (i.e., morphology) of the granular-like BiFeO3 films in the process of spin frustration.

Fracture analysis of a central crack in a long cylindrical superconductor with exponential model

The fracture behavior of a long cylindrical superconductor is investigated by modeling a central crack that is induced by electromagnetic force. Based on the exponential model, the stress intensity factors (SIFs) with the dimensionless parameter p and the length of the crack a/R for the zero-field cooling (ZFC) and field-cooling (FC) processes are numerically simulated using the finite element method (FEM) and assuming a persistent current flow. As the applied field [Formula: see text] decreases, the dependence of p and a/R on the SIFs in the ZFC process is exactly opposite to that observed in the FC process. Numerical results indicate that the exponential model exhibits different characteristics for the trend of the SIFs from the results obtained using the Bean and Kim models. This implies that the crack length and the trapped field have significant effects on the fracture behavior of bulk superconductors. The obtained results are useful for understanding the critical-state model of high-temperature superconductors in crack problem.

Magnetic relaxation dynamics in thermally arrested Cr-modified Mn2Sb

The magnetic phase transitions and their thermal transformation arrest (TTA) behavior of Cr-substituted Mn2Sb were investigated. Mn1.92Cr0.08Sb showed the first-order magnetic transition between antiferromagnetic (AFM) and ferrimagnetic (FRI) state. The TTA phenomenon was observed in field-cooling process under magnetic field of μ0H ≥ 4 T. The relaxation process from the arrested FRI to AFM state was explained by stretched exponential function. As decreasing temperature, the magnetic relaxation changed from Arrhenius-type to non-Arrhenius ones. The characteristic relaxation time was evaluated to be the order of 105 ∼ 106 s, which indicated indeed arresting of the magnetic transition. During the process, the decrease in the magnetization due to the relaxation changed into an increase with the thermal fluctuation of ∼1%. These relaxation behaviors are explained by the density and the frequency of the reversal of triple-layered magnetic moments.

Low-energy magnetoelectric control of domain states in exchange-coupled heterostructures

The electric manipulation of antiferromagnets has become an area of great interest recently for zero-stray-field spintronic devices, and for their rich spin dynamics. Generally, the application of antiferromagnetic media for information memories and storage requires a heterostructure with a ferromagnetic layer for readout through the exchange-bias field. In magnetoelectric and multiferroic antiferromagnets, the exchange coupling exerts an additional impediment (energy barrier) to magnetization reversal by the applied magnetoelectric energy. We proposed and verified a method to overcome this barrier. We controlled the energy required for switching the magnetic domains in magnetoelectric \cro films by compensating the exchange-coupling energy from the ferromagnetic layer with the Zeeman energy of a small volumetric spontaneous magnetization found for the sputtered \cro films. Based on a simplified phenomenological model of the field-cooling process, the magnetic and electric fields required for switching could be tuned. As an example, the switching of antiferromagnetic domains around a zero-threshold electric field was demonstrated at a magnetic field of 2.6 kOe.

Temperature-induced spin reorientation and magnetization jump of rare-earth orthoferrite Ho0.5Pr0.5FeO3 single crystal

We report temperature-induced spin reorientation and magnetization jump effects in the rare earth (RE) orthoferrite Ho0.5Pr0.5FeO3 single crystal. The single crystal of about 6 mm in diameter and 50 mm in length was successfully grown by optical floating zone method. Both X-ray diffraction and Laue photograph confirmed the homogeneity and high quality of the crystal. Magnetic properties of Ho0.5Pr0.5FeO3 single crystal are studied over a wide temperature range from 2 to 300 K. Spin reorientation transition from Γ2 to Γ4 phase is observed in the temperature range of 75–90 K. At lower temperature, the Ho0.5Pr0.5FeO3 shows an abrupt jump of magnetization along the a-axis, which occurs only in the field-cooling process, and is sensitive to external applied magnetic field. By analyzing the jump temperature and magnitude of the magnetization, we conclude that it is caused by the spin reversal of the rare earth ions. The isothermal magnetization versus field hysteresis loop measurements along a axis explain the spin configuration variation from 3K to 60 K.

High Magnetic Field Generated by Bulk MgB2 Prepared by Spark Plasma Sintering

From the applications' point of view, the advantage of low density given by MgB2 material must be taken into consideration, and the generation of strong magnetic flux densities using MgB2 should be investigated. In this paper, we have studied the magnetic properties of samples processed by a fast spark plasma sintering machine that is able to produce dense and high-quality MgB2 samples. Experiments were carried out both on small pieces of the samples and on large-sized samples performing trapped field measurements in a field-cooling process. For temperatures between 10 and 30 K, the results show a strong dependence of the magnetic behavior of the large-sized samples to the applied magnetic field sweep rate, whereas nothing particular appeared on the small samples. As the magnetic flux density produced by MgB2 bulks is directly linked to the potential of the applications, we report on the field produced at the surface of one single MgB2 sample of 30-mm diameter and the field produced inside a stack of two MgB2 samples of 20-mm diameter. A generation of magnetic flux density (magnetic polarization $\mu_{0}M$ ) up to 4.80 T at 10 K and 3.92 T at 20 K inside the stack of the two MgB2 samples was observed under a negative supporting field of −1.47 and −1.95 T, respectively. According to these values to their very low density and to their ease of manufacturing, MgB2 bulks are promising materials for the applications of superconductors.

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.

Magnetic characteristics of permanent magnet guideways at low temperature and its effect on the levitation force of bulk YBaCuO superconductors

It is important to understand the influence of environmental temperature variation on high temperature superconducting (HTS) maglev performance in practice. As the on-board bulk high temperature superconductors (HTSCs) are kept in liquid nitrogen cryostats at all times, the actual temperature effects relevant to maglev performance are on the permanent magnet guideway (PMG), and these have been examined by exploring the PMG magnetic characteristics and the related interaction force with the on-board bulk HTSCs above it in this paper. The experimental temperature range was set from room temperature to 77 K. The experimental results show that the typical room temperature (RT) vertical magnetic flux density (defined as BPMG), 565 mT, of the PMG segment first increases as the temperature decreases to 138 K, before it reaches a peak of 634 mT within the temperature range from 138 K to 120 K, with a rapid growth rate to 12.2% over the RT value. And then BPMG decreases quickly to a stable value of 576 mT at 77 K. A final 1.9% enhancement ratio was obtained compared with the initial BPMG at room temperature. Furthermore, the levitation force exhibits a similar trend to the BPMG whatever the field cooling process and the zero field cooling process was conducted when the PMG temperature is reduced. Growth of 21.8% and 13.4% for the levitation force occurs for the field-cooling process and the zero-field-cooling process, respectively. The results provide basic data for environmental temperature effects on HTS maglev systems, which implies that the low temperature environment can have benefits towards enhancement of their levitation performance.

In [formula omitted]-Fe2MnGa Heusler alloy do Fe and Mn sublattices magnetically couple parallel or antiparallel at low temperatures?

Bulk and local magnetic properties of the polycrystalline γ-Fe2MnGa Heusler alloy have been studied using X-ray diffraction, magnetization measurements and zero-field and in-field Mössbauer spectroscopy. X-ray diffraction data indicate stabilization of a L12-type structure and no structural phase transformation in the temperature range 30–300K. While Mn atomic magnetic moments order magnetically well above 300K according to magnetization data, 57Fe Mössbauer spectroscopy suggests that the Fe-sublattice ordering temperature is ca. 220K, when it couples antiparallel to the Mn-sublattice. Large vertical (magnetization-axis) and horizontal (field-axis) magnetization loop shifts are observed in field-cooling process (up to 70kOe); these effects may be associated with magnetic frustrated state that occurs by interaction of the two sublattices.

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.

Low-temperature synthesis of K0.5FeF3 with tunable exchange bias

Fluorides K0.5FeF3 with tetragonal tungsten bronze structure have been fabricated by solid state reaction at low sintering temperature in the range between 150 °C and 400 °C with the assistance of crystal water during the grinding and sintering processes. Unusual magnetic properties have been observed, including positive exchange bias field (HE) with negative vertical magnetization shift (Mshift), and smaller field cooling (FC) magnetization than the zero field cooling one below 53 K. The results are explained by a core-shell structure consisting of antiferromagnetic core and spin glass (SG) shell with antiferromagnetic interfacial coupling between the pinned interface spins and the SG shell spins. The sign of HE and Mshift can be changed by increasing the cooling field in the FC process, which is attributed to the competition between the antiferromagnetic interfacial coupling and the Zeeman energy of magnetization of the SG shell.

Magnetoresistance of nanogranular Ni/NiO controlled by exchange anisotropy

A link between exchange anisotropy and magnetoresistance has been found to occur in a Ni/NiO sample consisting of Ni nanocrystallites (mean size ∼13nm, Ni content ∼33vol%) dispersed in a NiO matrix. This material shows metallic-type electric conduction and isotropic spin-dependent magnetoresistance as well as exchange bias effect. The latter is the outcome of an exchange anisotropy arising from the contact interaction between the Ni phase and the NiO matrix. Combined analysis of magnetization M(H) and magnetoresistance MR(H) loops measured in the 5–250K temperature range after zero-field-cooling (ZFC) and after field-cooling (FC) from 300K reveals that the magnetoresistance is influenced by exchange anisotropy, which is triggered by the FC process and can be modified in strength by varying the temperature. Compared to the ZFC case, the exchange anisotropy produces a horizontal shift of the FC MR(H) loop along with a reduction of the MR response associated to the reorientation of the Ni moments. A strict connection between magnetoresistance and remanent magnetization of FC loops on one side and the exchange field on the other, ruled by exchange anisotropy, is indicated.

Switching current modulations induced by vortices rearrangement in mesoscopic superconducting loops

Periodic modulations in the switching current of niobium mesoscopic thin loops have been observed as a function of a perpendicular magnetic field. These modulations reflect the formation of specific vortices configurations in the arms and in the contacts of the loops. The voltage measured just before the current-driven transition to the normal state takes either zero or nonzero values depending on whether the vortices are efficiently pinned or move under the action of the applied current. This switching voltage varies sharply in the vicinity of the regularly spaced matching fields at which the switching current is minimum. This reflects the rearrangement of the vortices pattern from an unstable configuration into a more stable one. In addition, multimodal switching current distributions obtained at a constant field highlight that the few vortices involved in the field-cooling process freeze into a limited number of configurations. Finally, the spacing between the matching fields allows us to extract the critical field for complete vortex expulsion of the samples. Electrical transport measurements appear to be an efficient tool to determine the superheating field of submicrometer-wide structures, which is not obvious when measured by scanning probe microscopy.

Effect of frozen spin on the magnetocaloric property of La 0.7Ca 0.3CoO 3 polycrystalline and single crystal samples

Polycrystalline (PC) and single crystalline (SC) samples of La 0.7Ca 0.3CoO 3 (LCCO) with the perovskite structure were synthesized by conventional solid-state reaction and the floating-zone growth method. We conducted isothermal magnetization measurements of the PC and SC samples at temperatures from 2.8 K to 140 K, and evaluated the magnetic entropy change (−Δ S M) under zero field cooling (ZFC) and field cooling (FC) to 2.8 K. An interesting result has been obtained, where the −Δ S M– T curves in the low temperature range show totally different features between the ZFC and FC cooling procedures. The −Δ S M shows a large inverse irreversibility value for the ZFC process, while the −Δ S M also shows a normal positive value, but one that is slightly larger at 2.8 K for the FC process for both samples. We also present the results of a comprehensive investigation of the magnetic properties of the LCCO system. Systematic measurements have been conducted on DC magnetization, AC susceptibility, and exchange-bias. These findings suggest that complex structural phases, including ferromagnetic and spin-glass/cluster-spin-glass (SG/CSG)) states and their transitions, exist in PC samples, while there is a much simpler magnetic phase regime in SC samples. It was also of interest to discover that the CSG induced a magnetic field memory effect and an exchange-bias-like effect.

Dependence of exchange coupling direction on cooling-field strength

We studied the dependence of exchange coupling on cooling-field strength in an exchange-biased spin valve with a synthetic antiferromagnetic layer by experiment and theory. Our theory calculates magnetic anisotropy energies in each magnetic layer composing the spin valve during the field-cooling process, finds the minimum state of total energy, and explains how the magnetizations in the layers interact with one another during field-cooling under various cooling-field strengths. Calculations based on the theory well match results of the experimental measurements. Our observation shows that one has to carefully choose the cooling-field strength optimal for designing exchange-biased spin devices having a synthetic antiferromagnetic layer; otherwise the exchange coupling direction can significantly deviate from the cooling-field direction, which impairs performance.

Application to Magnetic Levitation

Magnetic levitation systems using HTS bulks, such as magnetic bearing devices for flywheel energy storage systems, transporters, linear actuators, magnetic gears and so on, have been developed by many research groups. Most of the HTS magnetic levitation systems are composed of HTS bulks and permanent magnets. "Stable magnetic levitation without any other control system" is a specific characteristic of the HTS bulks. A non-contact spin processor and a magnetic-levitation seismic isolation device are introduced as magnetic levitation systems that are composed of HTS bulks and permanent magnets. Furthermore, an active magnetic levitation system using spherical HTS bulks for inertial nuclear fusion and a three-dimensional HTS actuator are introduced as magnetic levitation systems with HTS bulks and electromagnets. Levitation force, levitation gap and stability are closely related to the size and shape of the HTS bulk, the magnetic field distribution around the HTS bulk, the conditions in the field-cooling process of the HTS bulk and so on. Achieving large levitation force, large levitation gap and highly stable levitation is very important for practical use of the HTS magnetic levitation system. Theoretical analysis using a numerical simulation code for understanding electromagnetic behaviors within the HTS bulk is useful for improving the levitation force, levitation gap and stability. Finally, numerical techniques based on the finite element method are introduced in this paper.

Thermal behavior and field-trapping property of melt-textured superconducting bulk magnets activated by quasi-static magnetic fields

The thermal and field trapping behaviors of melt-processed RE-Ba-Cu-O (RE=Y, Sm) bulk superconductors during their magnetization processes conducted in the zero field cooling (ZFC) were investigated in comparison with the field cooling (FC) operations in the temperature range around 50 K. The temperature changes were precisely measured by five thermocouples attached on the sample surface. The authors discuss the relationship between the heat generation and resultant trapped magnetic field ability by means of the direct temperature measurement during these magnetizing processes. As the applied field is not satisfactorily given to the sample, the trapped field distributions activated in ZFC process show trapezoid shapes, whereas they show a corn shape without any strain when magnetized in FC process. The trapped magnetic field gradually increases even after when the applied magnetic fields reaches the highest value. According to the balance on the heat generation and drain, the temperature evolution shows us an almost stable state. These data suggest that the improvement with respect to the heat propagation of the bulk material must suppress the temperature rises and enhance the resultant flux trapping ability.

Elliptical hole in a bulk superconductor under electromagnetic forces

A simple model is presented for the distribution of flux-pinning-induced stress in asuperconductor around an elliptical hole and the singular stress field near crack tips. Themagnetic behavior is described by the critical state, the original Bean model. It is assumedthat the perturbation brought upon by the elliptical hole on the critical current density isnot significant. Explicit expressions for the stress field in the vicinity of an elliptical holeare derived based on the complex variable method. Furthermore, the stress intensityfactor at the tip of a slender crack is determined. An exact solution is foundduring the decreasing field and field-cooling process. Dependence of the stressfield on the parameters including the applied field, shape of the elliptical holeor superconductor slab is investigated. The results show that the applied fieldand geometry parameter have obvious effects on the distribution of the stress.