Zero-field-cooled Processes Research Articles

Observation of spin glass behavior in spinel compound CoGa2O4

Recently, it has been reported that doped Ga ferrite has potential application value in spintronics devices. Therefore, experimental study of the magnetic properties and potential physical mechanisms is essential for the realization of related spin-tronics devices. In this work, the synthesis process, crystal structure, and physical properties of spinel compound CoGa2O4 have been investigated. The competition between antiferromagnetism (AFM) and ferromagnetism (FM) is considered to be the crucial elements for resulting in spin glass (SG) behavior due to magnetic frustration. The observed SG behavior is determined by the temperature dependence of magnetization M(T) curves under the ZFC (zero-field-cooled) and FCC (field-cooled) processes, where the intense irreversibility divergence is formed. Moreover, the corresponding fitting parameters (the freezing temperature T0 = 9.32 K, the flipping time τ0 = 4.49 × 10–10 s, and the dynamical exponent zν = 4.46) strongly indicate the existence of the SG behavior. Meanwhile, as another specific characteristic for SG, in our present work, frequency (f) and magnetic field (H) have a strong influence on the peaks of AC susceptibility. From where, with the increase of f and H, the freezing temperature follows a corresponding peak shift. All the above phenomena and relevant analyses of magnetic frustration behavior confirm the typical SG behavior in CoGa2O4 system.

ZFC process in 2+1 and 3+1 multi-band superconductor

In the present work, we will study the effect of dimensionality (i.e 2 + 1 and 3 + 1) has on the different types of coupling in a two-band superconducting system, specifically for the Josephson, bi-quadratic and weak type couplings. This study will be oriented to a ZFC (Zero-Field-Cooling) process and we will see the effect that the variation of temperature for three special cases of the external field (fixed, H < H1, H = H1 and H > H1) has on the order parameter ψ, phase ψ, magnetization M and free energy G(T) in these superconductors of multibands and a comparison between the behavior of these electronic and thermodynamic variables. This analysis will be carried out using the solution of time-dependent two bands Ginzburg-Landau equations (2B-TDGL).

Mode-I fracture analysis of micro-scale high-temperature superconductors via the double cantilever beam model and gradient elasticity theory

In this paper, fracture behavior of a micro-scale double cantilever beam (DCB) made of superconducting materials is investigated based on the strain gradient theory. Both zero-field cooling (ZFC) and field cooling (FC) magnetization processes are considered. The closed-form solutions of the energy release rates and stress intensity factors are obtained. For ZFC process, superconducting materials are easy to damage during the process of reducing magnetic field rather than increasing magnetic field. For FC process, applied magnetic field will impede superconductors to damage. Moreover, the normalized energy release rate predicted by classical beam theory is larger than that predicted by strain gradient theory. As the characteristic length increases, the normalized energy release rate decreases. The present model may be useful for designing experiments to test the fracture toughness of micro-scale high-temperature superconductors.

Demagnetization of an ultrathin Co/NiO bilayer with creation of submicrometer domains controlled by temperature-induced changes of magnetic anisotropy

We have investigated the magnetization in a ferromagnetic/antiferromagnetic Co/NiO bilayer from room temperature (RT) to 200 °C. Using polar magneto-optical Kerr effect (PMOKE) based magnetometry, we observe the temperature-induced spin reorientation transition of the Co layer at 150 °C (from out-of-plane to in-plane state). Zero field cooling (ZFC) down to RT leads to the appearance of a micrometer size domain structure and demagnetization of the sample. The remanence magnetization after the ZFC process strongly depends on the temperature from which the cooling process starts (onset temperature). The domain structure and its evolution under the external out-of-plane magnetic field were studied with magnetic force and PMOKE microscopies.

Influence of Non-superconducting Inclusions on Magnetostriction of Bulk Superconductors with Viscous Flux Flow Under Zero-Field Cooling Process

How do non-superconducting inclusions influence the magnetostriction of bulk superconductors with viscous flux flow? This problem has still not been addressed in theoretical models. In the paper, a model is developed focusing on superconducting composites under zero-field cooling process, with the aim to investigate the effects of inclusion on the macroscale magnetostriction of composites, including the elastic properties, such as elastic modulus, volume fraction. Moreover, in order to evaluate the accuracy of the model, the theoretical values in the paper are compared with the results published in the literature, and the results show that the prediction of the model is quantitatively consistent with the data without viscous flux flow. It may be useful to researchers interested in the practical application of the superconductors.

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.

The structure and magnetic properties in Dy3+ doped SmCrO3 ceramics

Abstract The Dy3+ doped SmCrO3 polycrystalline ceramics are prepared by solid state method. The structure and magnetic properties are investigated. All samples show orthorhombic structure with space group Pnma. Three magnetic transitions are detected in Dy3+ doped SmCrO3 samples, which arise from the Cr3+-Cr3+ interactions and the spin reorientation of Cr3+, respectively. Both field cooled (FC) and zero field cooled (ZFC) exchange bias (EB) effects are observed in the prepared Sm1-xDyxCrO3 (x = 0 − 0.5) samples below the spin reorientation temperature (TSR), and the EB field (HEB) increases dramatically below TSR. With the increase of the doping level, the HEB is depressed. Three anomalous variations of magnetic entropy change (ΔSM) derived from the isothermal magnetization are observed, which are consistent with the magnetic transitions. Compared with the ΔSM after ZFC processes, the anomalous variations of ΔSM at ~25 K almost disappear after FC processes due to the enhanced unidirectional anisotropy, and no obvious influence is observed for the other two anomalous variations after FC processes.

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.

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.

Spin-glass and novel magnetic behavior in the spinel-type [formula omitted

A dual non-magnetic substitution system on A- and B-sites in the spinel structure has been studied. The mother compound is a ferromagnet CuCr 2 S 4 with the Curie temperature T c ≃ 380 K . A system of Cu 1 ‐ x Ag x CrSnS 4 , which is the same notation as ( Cu 1 ‐ x Ag x ) ( Cr 0.50 Sn 0.50 ) 2 S 4 , has been prepared over the entire range of 0.00 ≤ x ≤ 1.00 although the Cr-Sn sublattice is unchanged in the fixed composition of 0.50 on B-sites. All these compounds exhibit the spin-glass phase with the freezing temperature T g approximately at 16 K in 100 Oe. Since only Cr ions have the magnetic moment on the B-sites, the substitution of Ag for Cu on the A-sites does not influence strongly the spin-glass freezing behavior over the whole composition range. Nevertheless, the magnetization of Cu 1 ‐ x Ag x CrSnS 4 with x = 0.50 and 0.55 cause a broad upturn hump over 30–130 K where the spin-glass phase is broken. Strong magnetic field dependence of this hump anomaly has been observed with an irreversibility between zero-field-cooled (ZFC) and field-cooled (FC) magnetizations even though above T g . The hump is suppressed in higher fields and collapsed down at approximately 1.0 kOe with a tiny trace quantity of the anomaly where the difference between the ZFC and FC processes disappears. The specimen with x = 0.45 shows a small hump anomaly in low field < 20 Oe which corresponds to a precursor of the huge anomaly for x = 0.50 . The hump anomaly could be attributed to a formation of the cluster-glass. The spin-clusters are embedded in the matrix of spin-glass elements in high degree of disorder without long-range order. All the spins eventually are frozen below T g . The strange magnetic freezing originates from the delicate dual substitutions. The mechanism of the anomaly is far from a complete picture and remains enigmatic.

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.

Temperature Changes and Magnetic Properties of Melt-textured Superconducting Bulk Magnets in the Magnetizing Processes using Quasi-static Magnetic Fields

The temperature changes and magnetic-field trapping behaviors of melt-textured, single-domain Sm-Ba-Cu-O bulk superconductors have been precisely investigated for magnetizing processes by means of quasi-static magnetic fields such as field cooling (FC) and zero field cooling (ZFC). The experiments were conducted in the temperature range of 40-80 K by simultaneously measuring both the temperature and magnetic flux density. According to the balance of heat generation and drainage, the temperature evolution profiles show distinctive behaviors of the invading and exhausting magnetic fluxes into and out of the HTS bulk magnet. The average temperature change during the FC process reached the maximum value of 4.63 K when the sweep rate was 11.3 mT/s and the initial temperature was 45.6 K. In the ZFC process, both the temperature and magnetic flux density kept increasing gradually even after the external magnetic field stopped growing at 5 T in the ascending field process. The highest temperature rise due to the flux motion reached 4.6 K even when the sample was magnetized with at sweeping rate of 11.3 mT/s. As the behavior of temperature rise was different between the ascending and descending field processes, it is suggested that the magnetic fluxes in the processes invade and diffuse under different heating characteristics.

Spin-glass state in hexagonal Li 2.5Co 0.5N

Polycrystalline lithium nitridocobaltate Li 2.5Co 0.5N was prepared by solid-state reaction. Both field-cooled (FC) and zero-field-cooled (ZFC) magnetization were measured, in which irreversibility between the ZFC and FC processes was observed and the freezing temperature ( T f) shifted to low temperature with increasing of the applied field. This indicates lack of long range magnetic order in Li 2.5Co 0.5N .We studied the effect of frequency on the ac susceptibility. It was found out that there was a peak and the peak position shifting with the frequency. A criterion parameter δ, Δ T'/[ T'Δlog v], is calculated to be 0.04(1) based the experimental data. The observed magnetic behavior indicates a spin-glass ground state in Li 2.5Co 0.5N.

Ferroelectric-field-induced spin-pinning effect in Pb(Zr0.5Ti0.5)O3∕La0.9Sr0.1MnO3 bilayers

The magnetic properties of Pb(Zr0.5Ti0.5)O3∕La0.9Sr0.1MnO3 bilayers epitaxially grown on Nb-doped SrTiO3 (001) substrate were studied. Bilayers with a fixed Pb(Zr0.5Ti0.5)O3 (PZT) layer thickness of 200nm and different La1−xSrxMnO3 layer thicknesses varying from 10to40nm show a divergence between field-cooled (FC) and zero-field-cooled (ZFC) temperature-dependent magnetization measurements. Moreover, the polarization state of the PZT ferroelectric layer induces a spin-pinning effect causing a variation of the M∕M(10K) ratio in ZFC measurement and the divergent temperature (Td) between FC and ZFC processes. The magnetic hysteresis loops measured in FC and ZFC processes confirm the spin-pinning effect induced from the ferroelectric polarization field, leading to an alternation of the hysteresis loop characteristics for samples polarized with different signs of voltage.

Neutron diffraction study of field-cooling effects on the relaxor ferroelectricPb[(Zn1/3Nb2/3)0.92Ti0.08]O3

High-temperature (T) and high-electric-field (E) effects on Pb[(Zn_{1/3} Nb_{2/3})_{0.92} Ti_{0.08}]O_3 (PZN-8%PT) were studied comprehensively by neutron diffraction in the ranges 300 <= T <= 550 K and 0 <= E <= 15 kV/cm. We have focused on how phase transitions depend on preceding thermal and electrical sequences. In the field cooling process (FC, E parallel [001] >= 0.5 kV/cm), a successive cubic (C) --> tetragonal (T) --> monoclinic (M_C) transition was observed. In the zero field cooling process (ZFC), however, we have found that the system does not transform to the rhombohedral (R) phase as widely believed, but to a new, unidentified phase, which we call X. X gives a Bragg peak profile similar to that expected for R, but the c-axis is always slightly shorter than the a-axis. As for field effects on the X phase, we found an irreversible X --> M_C transition via another monoclinic phase (M_A) as expected from a previous report [Noheda et al. Phys. Rev. Lett. 86, 3891 (2001)]. At a higher electric field, we confirmed a c-axis jump associated with the field-induced M_C --> T transition, which was observed by strain and x-ray diffraction measurements.

Modeling and simulation on the magnetization in field-cooling and zero-field-cooling processes

By using Monte Carlo simulation and thermally activated flux motion (TAFM) model, we calculate the magnetic field profile and thus the magnetization in the field-cooling (FC) and zero-field-cooling (ZFC) processes. Based on the simple Kim–Anderson U( j) dependence, the major experimental observations can be nicely reproduced. It is found that the calculated field profile B( x) is close to the Bean critical state model in the ZFC process, in sharp contrast to that in the FC process which shows a curved frozen flux pattern. The magnetization in both the ZFC and FC processes is strongly dependent on the pinning potential and the critical current density, but weakly dependent on the temperature-sweeping rate.

Magnetic and resistivity investigation of Dy(Co 1− xSi x) 2 (if0 < x ⩽ 0.2) compounds

The results of experimental investigations of the magnetic and transport (electrical resistivity) properties of Dy(Co 1− x Si x ) 2, where x = 0.0, 0.1 and 0.2 compounds are presented. These systems crystallize in the whole concentration range of x, in the MgCu 2 Laves-phase-type crystal structure. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements were performed on the pseudobinary Dy(Co 1− x Si x ) 2 cubic compounds. The thermomagnetic cycle of ZFC and FC processes exhibits an irreversible behaviour. The composition dependence of the Curie temperature, 3d-magnetic moment and electrical resistivity is discussed.

Nonmetallic spin glass behavior by spin frustration at low temperature in nonstoichiometric BaSn 1− xFe xO 3− y system

The solid solutions of BaSn 1− x Fe x O 3− y ( x=0.00–1.00) have been synthesized and investigated as materials being expected to show the spin glass behavior by spin frustration at low temperature. The Fe ions substituted in place of Sn ions could have the mixed valence states of Fe 3+ and Fe 4+ ions. The formation of Fe 4+ ion began at x=0.50, which has been identified by the potentiometric titration and the Mössbauer resonance spectra. Magnetic susceptibilities after the zero field cooling (ZFC) and field cooling (FC) processes have shown the irreversibility at the compositions of x=0.50, 0.75 and 1.00. This result has indicated the spin frustration between Fe 3+OFe 4+ ferromagnetic coupling and Fe 3+OFe 3+ or Fe 4+OFe 4+ antiferromagnetic coupling on the magnetic domain boundaries. The field dependence of magnetization at 5 K has shown that there is no net magnetization at zero field in the ZFC process; on the other hand, the remnant magnetization exists in the FC process. In the ZFC process, the magnetization can be relaxed to an equilibrium resulting in the zero magnetization. In the FC process, however, the ferromagnetic couplings incompatible with antiferromagnetic couplings are frozen on the boundaries and kept on, which shows the spin glass behavior.

Anomalous peak effect deduced from the temperature dependence of the magnetization in the Bi 2Sr 2CaCu 2O 8+ y system

We obtained a new aspect in the experimental study of the anomalous peak effect: magnetization curves as a function of magnetic field were deduced from the temperature dependence of the magnetization curves. In this method we can utilize the two different flux distributions in the sample corresponding to the two thermal processes: heating the sample in the field after zero-field cooling (ZFC) and cooling it in the field (FC). The ordinary diamagnetic hysteresis curve was obtained from the data observed in the ZFC process, and a new magnetization curve was obtained from the data taken in the FC process. More than two anomalous peaks were observed in the latter process, suggesting the presence of the higher- and sub-harmonics and the fundamental peak. The sample dependence of the anomalous peak field was investigated for the undoped as-grown sample, an oxygen-annealed sample and a Zn-substituted one.