Field-cooled Magnetization Research Articles

The magnetic behaviour of Dy2Ir2O7 – Beyond the mean field approximation

The magnetic properties of pyrochlore iridate Dy2Ir2O7 prepared by CsCl flux method were investigated by means of heat capacity and magnetization measurements. The low temperature behaviour was confirmed to be consistent with previously published results, whereas the high temperature data and their analysis are presented for the first time. The bifurcation of zero field-cooled and field-cooled magnetization curves coincides well with the onset of the anomaly in specific heat pointing to an ordering of the iridium sublattice. We confront the previously determined Dy3+ CF scheme with our high-temperature magnetization and specific heat data. The difference between calculations and experimental data, represented by 20 K specific heat anomaly, suggests the behaviour beyond the mean field approximation. The Dy2Ir2O7 differs from the rest of the heavy rare-earth pyrochlore iridates, where the mean field approximation constitutes an adequate description. The deeper understanding of the Dy2Ir2O7 magnetic behaviour requires, among others, also the phonon spectra studies.

4f-3d interaction dominated field tailoring spin switching in rare earth doped Dy0.5Er0.5FeO3 single crystal

A large single crystal Dy0.5Er0.5FeO3 of 5.2 mm in diameter and 55 mm in length is prepared using the optical floating zone furnace system. The good quality of the single crystal without impurities is confirmed by powder and Laue X-ray diffraction experiments. Magnetic measurements are conducted using 1~2 millimeter cubic sample of Dy0.5Er0.5FeO3 single crystal, which experiences spin switching in two measurement modes of zero-field-cooling (ZFC) and field-cooling (FC) magnetization vs temperature (M-T) curves under magnetic fields less than 48 Oe. The co-occurrence of spin switching (i.e. magnetization drop in FC mode and magnetization jump in ZFC mode) has not been reported in the parent materials of DyFeO3 and ErFeO3 single crystals. More interestingly this kind of spin switching can be easily tailored by using low fields in a large temperature range from below 50 K to nearly 400 K. Spin reorientation transition occurs near 35.9-48 K, where the spin configuration of the iron sublattice changes from Γ4 and Γ2. The magnetic interaction between (Dy, Er)-4f and Fe-3d electrons in the two different sublattices of Dy0.5Er0.5FeO3 single crystal which governs the spin reorientation transition and spin switching under different temperature and magnetic field are discussed.

Structural, chemical and low-temperature magnetic properties of lead-Free 0.6NiFe2O4-0.4Na0.5Bi0.5TiO3 magnetoelectric composite

We report on structural, chemical and low-temperature magnetic properties of lead-free 0.6NiFe2O4-0.4Na0.5Bi0.5TiO3 composite. NiFe2O4 (NFO) and Na0.5Bi0.5TiO3 (NBTO) are seen to crystallize in inverse spinel and perovskite structure, respectively. 0.6NiFe2 O4-0.4Na0.5Bi0.5TiO3 composite exhibits both NFO and NBTO phases in appropriate composition. Zero field-cooled (ZFC) and field-cooled (FC) magnetization measurements carried out from 15 to 300 K shows a large bifurcation at room temperature. ZFC and FC magnetization measurement exhibit a hump at Tm \(\sim\)259.5 K, indicates the possible existence of competing magnetic interactions in 0.6NiFe2 O4-0.4Na0.5Bi0.5TiO3 composite. Saturation magnetization, remnant magnetization and coercivity values are observed to increase with decreasing the temperature. Temperature-dependent saturation magnetization is fit to the Bloch’s law. Magnetocrystalline anisotropy (K1) value at various temperatures are estimated and is seen to increase from 0.23 × 104 erg/cc (at 300 K) to 0.34 × 104 erg/cc (at 15 K).

Evolution from sinusoidal to collinear A-type antiferromagnetic spin-ordered magnetic phase transition in Tb1−x Pr x MnO3 solid solution

The present study reports on the structural and magnetic phase transitions in Pr-doped polycrystalline Tb0.6Pr0.4MnO3, using high-resolution neutron powder diffraction (NPD) collected at SINQ spallation source, to emphasize the suppression of the sinusoidal magnetic structure of pure TbMnO3 and the evolution to a collinear A-type antiferromagnetic ordering. The phase purity, Jahn–Teller distortion, and one-electron bandwidth for eg orbital of Mn3+ cation have been calculated for polycrystalline Tb0.6Pr0.4MnO3, in comparison to the parent materials TbMnO3 and PrMnO3, through the Rietveld refinement study from x-ray diffraction data at room temperature, which reveals the GdFeO3 type orthorhombic structure of Tb0.6Pr0.4MnO3 having Pnma space group symmetry. The temperature-dependent zero field-cooled and field-cooled dc magnetization study at low temperature down to 5 K reveals a variation in the magnetic phase transition due to the effect of Pr3+ substitution at the Tb3+ site, which gives the signature of the antiferromagnetic nature of the sample, with a weak ferromagnetic component at low temperature-induced by an external magnetic field. The field-dependent magnetization study at low temperatures gives the weak coercivity having the order of 2 kOe, which is expected due to the canted-spin arrangement or ferromagnetic nature of Terbium ordering. The NPD data for Tb0.6Pr0.4MnO3 confirms that the nuclear structure of the synthesized sample maintains its orthorhombic symmetry down to 1.5 K. Also, the magnetic structures have been solved at 50 K, 25 K, and 1.5 K through the NPD study, which shows an A-type antiferromagnetic spin arrangement having the magnetic space group Pn′ma′.

Detection of 1H NMR signal in a trapped magnetic field of a compact tubular MgB2 superconductor bulk

We measured 1H nuclear magnetic resonance (NMR) spectra from silicon rubber in a magnetic field trapped by a compact tubular MgB2 superconductor bulk magnet (60 mm outer diameter, 40 mm inner diameter, and 60 mm height) which was magnetized by field-cooled magnetization from an applied field, , of 0.4778 T at 20 K. The sharpest 1H NMR spectrum of the resonant frequency, , of 20.314 MHz, which corresponded to a field strength of 0.4771 T, was observed at 1 mm below the center of the bore of the MgB2 tube. The homogeneity of the trapped field was within 100 ppm between z −2 and +2 mm along the central z-axis, which was evaluated by the position-dependent . No change in the trapped field was observed for about 25 d, which corresponded to a decay rate of less than 0.1 ppb h−1. The obtained results suggest that the MgB2 bulk magnet is a promising candidate for a magnetic pole in an NMR apparatus.

Effect of doping concentration and heat treatment on the refrigerant capacity of Pr0.63Dy0.37-xSrxMnO3

Based on the evolution of the quantity of Sr in Pr0.63Dy0.37-xSrxMnO3 (x = 0.00 and x = 0.30) systems and the different heat treatments (500, 800 and 1300 °C), various physical changes are illustrated by these samples. X-ray diffraction patterns were carried out revealing that the heat temperature can affect the structure of the systems.Zero-Field-Cooled and Field-Cooled magnetizations revealed the appearance of different magnetic transitions; from the paramagnetic state to the antiferromagnetic one. Once increasing the temperature of heat up to 1300 °C and for x = 0.30, the system presents a ferromagnetic order at Curie temperature that is around 200 K. Magnetic hysteresis loops confirm the transitions presented by the magnetic measurements.The heating temperature affected also the evolution of the magnetic entropy change. All samples show an important increase in the data values of the maximum of the magnetic entropy change while augmenting the applied magnetic field. We note an important result that is the refrigerant capacity of the x = 0.30 sample heated at 1300 °C is 725% compared to that at 500 °C.

A novel approach to determine blocking temperature in bulk materials

Blocking temperature (θB) holds utmost significance in magnetic devices. A novel approach to ascertain the θB for bulk materials has been propounded. The θB has been correlated with the magnetic entropy change (ΔS) during zero-field cooled (ZFC) and field cooled (FC) magnetization. The values of ΔS have been calculated in µB/f.u.K. The θB has been investigated for two distinct cases – (I) with ZFC magnetization reversal and (II) without ZFC magnetisation reversal. Based on the reported experimental approach, a novel definition of θB has been propounded for bulk materials. The combined analysis of ZFC and FC magnetization behaviour provides a better approximation of θB for bulk materials.

Electric transport and magnetic properties of LaBaCo2O6−δ single-crystal films grown on SrTiO3 substrates

Deposition atmosphere was found to have significant impacts on the growth of single-crystal LaBaCo2O6−δ (LBCO) films. In 2 × 10−4 Pa, the LBCO film grew basically along the c-axis of LaBaCo2O5.5 and high-quality single-crystal LBCO film was obtained after O2 annealing. In 2 Pa of O2, the dominant domains in the LBCO film became a- and/or b-axis oriented and a phase separation appeared in the O-annealed film. The O-annealed LBCO films exhibited an obvious paramagnetic to ferromagnetic transition at ~200 K and ~1.3 μB/Co of saturated magnetic moment was obtained at 10 K. Ferromagnetic and spin-glass-like states may coexist in the LBCO films, making a deviation of the zero-field-cooled from the field-cooled magnetization at temperatures below an irreversible temperature. The LBCO films exhibited the magnetic properties close to that of nanoscale-ordered LaBaCo2O6, but the electric transport behaviors are close to that of ordered LaBaCo2O6.

Experimental realization of an all-(RE)BaCuO hybrid trapped field magnet lens generating a 9.8 T concentrated magnetic field from a 7 T external field

In this work, we have verified experimentally an all-(RE)BaCuO hybrid trapped field magnet lens (HTFML) using only one cryocooler and a special technique named the ‘loose contact method’. In the experimental setup, only the inner magnetic lens was tightly connected to the cold stage and cooled at all times, and the outer trapped field magnet (TFM) cylinder was loosely connected to the cold stage before the magnetizing process by introducing a gap between the outer TFM and cold stage of the cryocooler. As a result, the superconducting state for zero-field cooled magnetization of the inner magnetic lens and the non-superconducting (normal) state for field-cooled magnetization of the outer TFM cylinder can co-exist at the same time. A maximum concentrated field of B c = 9.8 T was achieved for the magnetizing process with an applied field of B app = 7 T in the present HTFML, consistent with the numerical estimation in our previous conceptual study. These results validate the HTFML concept as a compact and desktop-type magnet device that can provide 10 T-class magnetic field enhancement from the viewpoint of the magnetizing method. However, during magnetization with a higher B app of 10 T, thermal instability of the outer stacked TFM cylinder caused flux jumps to occur, resulting in mechanical fracture of multiple bulks. These results suggest that the further development of a practical cooling method that can realize a stable and controllable cooling process for each part of the HTFML is necessary based on fundamental studies relating to the thermal stability of the large stacked TFM cylinder.

Effect of Ni and Al substitution on the magnetic properties of Y-type hexaferrite Ba0.5Sr1.5Zn0.5Ni1.5Fe11.92Al0.08O22 powders

The effect is reported of substituting the non-magnetic Zn2+ cations with magnetic Ni2+ cations, and of the magnetic Fe3+ cations with non-magnetic Al3+ cations in Ba0.5Sr1.5Zn0.5Ni1.5Fe11.92Al0.08O22 on the resulting magnetic properties. The Y-type hexaferrite powders were synthesized by citric acid sol-gel auto-combustion, followed by appropriate thermal annealing. The saturation magnetization values (Ms ) in a magnetic field of 50 kOe were 36 emu/g and 30 emu/g at 4.2 K and 300 K, respectively. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization vs. temperature (4.2 – 300 K) were measured in dc magnetic fields of 50 Oe, 100 Oe and 500 Oe. The changes resulting from the dissimilar cationic substitutions were identified and discussed.

A conceptual study of a high gradient trapped field magnet (HG-TFM) toward providing a quasi-zero gravity space on Earth

In this work, we propose a new concept of a high gradient trapped field magnet (HG-TFM). The HG-TFM is made from (RE)BaCuO bulk superconductors, in which slit ring bulks (slit-TFMs) are tightly stacked with TFM cylinders (full-TFMs), and state-of-the-art numerical simulations were used to investigate the magnetic and mechanical properties in detail during and after magnetization. A maximum value of the magnetic field gradient product of = 6040 T2 m−1 was obtained after conventional field cooled magnetization (FCM) with an applied field, B app, of 10 T of the HG-TFM with 60 mm in outer diameter and 10 mm in inner diameter. This value may be the highest value ever reported compared to any other magnetic sources. The value increased with decreasing inner diameter of the HG-TFM and with increasing B app during FCM. The electromagnetic stress in the HG-TFM during the FCM process mainly results from the hoop stress along the circumferential direction. The simulations suggested that there is no fracture risk of the bulk components during FCM from 10 T in a proposed realistic configuration of the HG-TFM where both TFM parts are mounted in Al-alloy rings and the whole HG-TFM is encapsulated in a steel capsule. A quasi-zero gravity space can be realized in the HG-TFM with a high value in an open space outside the vacuum chamber. The HG-TFM device can act as a compact and cryogen-free desktop-type magnetic source to provide a large magnetic force and could be useful in a number of life/medical science applications, such as protein crystallization and cell culture.

Role of Surfaces in the Magnetic and Ozone Gas-Sensing Properties of ZnFe2O4 Nanoparticles: Theoretical and Experimental Insights.

The magnetic properties and ozone (O3) gas-sensing activity of zinc ferrite (ZnFe2O4) nanoparticles (NPs) were discussed by the combination of the results acquired by experimental procedures and density functional theory simulations. The ZnFe2O4 NPs were synthesized via the microwave-assisted hydrothermal method by varying the reaction time in order to obtain ZnFe2O4 NPs with different exposed surfaces and evaluate the influence on its properties. Regardless of the reaction time employed in the synthesis, the zero-field-cooled and field-cooled magnetization measurements showed superparamagnetic ZnFe2O4 NPs with an average blocking temperature of 12 K. The (100), (110), (111), and (311) surfaces were computationally modeled, displaying the different undercoordinated surfaces. The good sensing activity of ZnFe2O4 NPs was discussed in relation to the presence of the (110) surface, which exhibited low (-0.69 eV) adsorption enthalpy, promoting reversibility and preventing the saturation of the sensor surface. Finally, the O3 gas-sensing mechanism could be explained based on the conduction changes of the ZnFe2O4 surface and the increase in the height of the electron-depletion layer upon exposure toward the target gas. The results obtained allowed us to propose a mechanism for understanding the relationship between the morphological changes and the magnetic and O3 gas-sensing properties of ZnFe2O4 NPs.

Optimization of structure-property relationships in nickel ferrite nanoparticles annealed at different temperature

In this report, a detail analysis of the impact of annealing temperature on the structural, elastic, morphological, optical, and magnetic behavior of NiFe2O4 nanoparticles prepared by the citrate sol-gel method is presented. Analyzing the XRD patterns by the Rietveld method confirms that all the annealed samples have been crystallized to cubic spinel structure belonging to Fd3−m space group with a single phase. Rietveld analysis demonstrates the change in structural and microstructural parameters and movement of cations from tetrahedral to octahedral sites and vice-versa upon annealing. The quantitative estimation of Ni2+ & Ni3+ and Fe2+ & Fe3+ has been carried out using XPS analysis. Decreases in peak broadening and shift of five Raman active peaks towards higher frequency upon annealing have been analyzed using the phonon confinement model. The variation in elastic parameters with annealing temperature has been assessed by FTIR analysis. The UV analysis reveals the increase of the optical energy band gap and the decrease of Urbach energy with annealing temperature enhancement. A noticeable sharp absorption band at 748 nm in UV spectra is attributed to A32g(3F)→T31g(3F) electronic transition. Room temperature magnetic hysteresis loops exhibit an increase of saturation magnetization upon annealing which is discussed with reference to finite size effects and disorderly surface spins. The estimated value of magnetocrystalline anisotropy constant by Law of Approach to saturation (LAS) theory as well as coercivity value elucidates the annealing effect in changing the magnetic single domain state of the particle to a multidomain state. Analysis of ZFC and FC magnetization curve measured at 100 Oe in the temperature range 400 K–60 K reveals the significant impact of annealing temperature on magnetic anisotropy, inter-particle interaction, and blocking temperature. Exploring the magnetic hysteresis loop measured in the temperature range 60–400 K over field strength of ± 3 T demonstrates the significant role of annealing on magnetic exchange interaction. Temperature dependent behavior of saturation magnetization and coercivity has been analyzed using modified Bloch's law and Kneller's relation. The magnetic heating efficiency examined by the induction heating system reveals that the sample has enough potential for hyperthermia application.

Magnetic and dielectric property control in the multivalent nanoscale perovskite Eu0.5Ba0.5TiO3.

We report nanoscale Eu0.5Ba0.5TiO3, a multiferroic in the bulk and candidate in the search to quantify the electric dipole moment of the electron. Eu0.5Ba0.5TiO3, in the form of nanoparticles and other nanostructures is interesting for nanocomposite integration, biomedical imaging and fundamental research, based upon the prospect of polarizability, f-orbital magnetism and tunable optical/radio luminescence. We developed a [non-hydrolytic]sol-[H2O-activated]gel route, derived from in-house metallic Ba(s)/Eu(s) alkoxide precursors and Ti{(OCH(CH3)2}4. Two distinct nanoscale compounds of Ba:Ti:Eu with the parent perovskite crystal structure were produced, with variable dielectric, magnetic and optical properties, based on altering the oxidizing/reducing conditions. Eu0.5Ba0.5TiO3 prepared under air/O2 atmospheres produced a spherical core-shell nanostructure (30-35 nm), with perovskite Eu0.5Ba0.5TiO3 nanocrystal core-insulating oxide shell layer (∼3 nm), presumed a pre-pyrochlore layer abundant with Eu3+. Fluorescence spectroscopy shows a high intensity 5D0→7F2 transition at 622 nm and strong red fluorescence. The core/shell structure demonstrated excellent capacitive properties: assembly into dielectric thin films gave low conductivity (2133 GΩ mm-1) and an extremely stable, low loss permittivity of εeff∼25 over a wide frequency range (tan δ < 0.01, 100 kHz-2 MHz). Eu0.5Ba0.5TiO3 prepared under H2/argon produced more irregular shaped nanocrystals (20-25) nm, with a thin film permittivity around 4 times greater (εeff 101, tan δ < 0.05, 10 kHz-2 MHz, σ∼59.54 kΩ mm-1). Field-cooled magnetization values of 0.025 emu g-1 for EBTO-Air and 0.84 emu g-1 for EBTO-Argon were observed. X-ray photoelectron spectroscopy analysis reveals a complex interplay of EuII/III/TiIII/IV configurations which contribute to the observed ferroic and fluorescence behavior.

An investigation on cubic and monoclinic phase coexistence in sol-gel derived LaFeO3‐Na0.5Bi0.5TiO3 ceramics

LaFeO3 modified Sodium Bismuth Titanate (NBT) with composition (LaFeO3)x-(Na0.5Bi0.5TiO3)1−x where x = 0, 0.01, 0.03 and 0.05 (abbreviated as LFNBT0, LFNBT1, LFNBT3 and LFNBT5) have been synthesized via sol-gel method. The Rietveld refinement of room temperature XRD data certified the phase purity and the coexistence of a cubic (Pm3̅m) and a monoclinic phase (Cc). An increase in molar % of monoclinic phase takes from 1.66% to 16.53% with increasing LaFeO3 content. The Raman spectra cross verifies the XRD results and SEM micrographs reveal the dense microstructure and well-packed distribution of grains. Almost lossy P-E loops have been observed for all the samples and the ferroelectric behavior reduces with doping. LFNBT1 sample is found to be paramagnetic where as weak ferromagnetic character is observed in LFNBT3 and LFNBT5. The overlapped ZFC and FC magnetization curves in both LFNBT3 and LFNBT5 assured the absence of any kind of magnetic ordering.

Thermally Reduced Soft Magnetic CuFe Nanoparticles for High-Performance Electrical Devices

Developing economically soft magnetic materials for high-performance electrical devices is indispensable. Here, we present the structural and magnetic properties of thermally reduced soft CuFe nanoparticles. The fcc cubic structure of iron-rich Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">37</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">63</sub> and their composition was confirmed by Rietveld refinement. Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">37</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">63</sub> nanoparticles exhibited high saturation magnetization and coercivity of 127 emu/g (142 emu/g) and 4.3 Oe (31 Oe), respectively, at 300 K (5 K). They showed transitions at ~34 and ~249 K due to the Kondo temperature of CuFe and minor fraction of CuFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , respectively. The exchange coupling between Cu and Fe was not significant, as demonstrated by field-cooled magnetization curves at 5 K. The magnetocaloric effect (MCE) in the range of fields and temperatures was estimated whereas the maximum MCE of -8.71 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> J.kg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> was achieved at 222 K. These soft magnetic materials, which exhibited stable high saturation magnetization with less heating effect during magnetization and demagnetization cycles, would be suitable candidates for magnetic applications.

Exchange bias and training effect in an amorphous Zn-Fe-O/nanocrystalline GaFeO3 bilayer thin film

In this paper, I report the exchange bias effect in a bilayer thin film of amorphous Zn-Fe-O (ZFO) and nanocrystalline GaFeO3 (GFO). I deposited the amorphous ZFO layer on top of a nanocrystalline GFO thin film using pulsed laser ablation of a Zinc ferrite (ZnFe2O4) target at room temperature. This bilayer film showed a large exchange bias effect (HE ∼ 418 Oe at 2 K). The exchange bias shift decreased exponentially as the temperature increased and disappeared for T > 30 K. Along with the exchange bias shift, the film also showed enhanced magnetization in Field Cooled (FC) measurements as compared to the Zero Field Cooled (ZFC) magnetization. The magnetization decreased during consecutive M-H loops cycling in training effect measurements. This decrease in the magnetization of the film followed the spin configurational relaxation model. The observed exchange bias effect can be attributed to a unidirectional anisotropy originated due to the pinning effect of the interfacial spins of the ZFO layer.

Structure and physical properties of the solid solution Gd2–xNdxPdSi3 (x = 0, 0.75, 1, 2)

The structural and magnetic properties of Gd2−xNdxPdSi3 (x = 0, 0.75, 1, 2) have been studied using X-ray diffraction (XRD), exchange bias and magnetization measurements. The end compounds (x = 0, 2) along with the solid solutions of them are found to crystallize in the hexagonal superstructure having space group P6/mmc. The rare-earth (RE) ions are occupying at two different crystallographically inequivalent sites (2b and 6h), which is similar to all ordered members of the RE2PdSi3 series. The absence of chemical phase separation and formation of true solid solutions are confirmed by the XRD analysis. No signature of exchange bias has been observed signifying the absence of coexistence of magnetic phases. The origin of complex magnetic behaviour in Gd2−xNdxPdSi3 is observed due to the separate magnetic ordering and two available crystallographic sites for magnetic ion. The spin-glass behaviour near ferromagnetic transition in Nd2PdSi3 and anomalous bifurcation in zero-field-cooled and field-cooled magnetization curves in Gd2PdSi3 originates from the proximity and overlap of the transition temperatures of the two RE magnetic ions present in two inequivalent sites, and strongly correlated to the interatomic and intra-atomic distances between RE atoms occupying two crystallographic sites.

Spin-glass magnetism of the non-equiatomic CoCrFeMnNi high-entropy alloy

The CoCrFeMnNi high-entropy alloy (HEA) is a magnetically concentrated crystalline system with all lattice sites magnetic, containing randomness (five different types of spins are randomly positioned on the lattice) and frustration (a consequence of mixed ferromagnetic and antiferromagnetic interactions). The sample material was prepared as a non-equiatomic, fully random solid solution of the five magnetic elements and we have studied experimentally the nature of the magnetic state. Upon cooling, no long-range magnetic ordering takes place, but the spin system undergoes a kinetic freezing transition to a spin glass phase, where below the spin freezing temperature Tf≈ 20 K, ergodicity of the system is broken. The observed broken-ergodicity phenomena include zero-field-cooled – field-cooled magnetization splitting in low magnetic fields, a frequency-dependent cusp in the ac susceptibility, an ultraslow time-decay of the thermoremanent magnetization and the memory effect, where the state of the spin system reached upon isothermal aging at a certain temperature can be retrieved after a reverse temperature cycle. All these phenomena are associated with the out-of-equilibrium dynamics of a nonergodic, frustrated system of coupled spins that approach thermal equilibrium, but can never reach it on a finite experimental time scale, so that we are observing only transient effects of partial equilibration within localized spin domains.

Magnetization Current Simulation of High-Temperature Bulk Superconductors Using the ANSYS Iterative Algorithm Method

The A-V formulation based iterative algorithm method (IAM), developed at Paul Scherrer Institute, shows excellent performance in modeling the critical state magnetization-current in the ReBCO tape stacks after field-cooling magnetization. This article upgrades the function of the IAM, making it realistic to take into account the widely used E-J power law, as well as to model the magnetization-current in bulk high-temperature superconductors under a zero-field-cooling cycle, having both the magnetization and demagnetization stages. The simulation results obtained from the ANSYS-IAM are validated with the well-known H-formulation method for both the critical state model and the flux creep model. The computation times from using the two different numerical methods are compared and discussed. The influence factors, including the iteration steps, the initial superconductor resistivity, the ramping time, and the reservation coefficient, are studied in detail.