Field-cooled Processes Research Articles

Magnetic Field Enhancement in Critical Current and Possible Triplet Superconductivity in LSMO/YBCO/LSMO Heterostructures

The large superconducting critical current (Ic) in superconductors is an important property for applications, but unfortunately, it is easily suppressed in the presence of a magnetic field. The stronger the magnetic field the lower the Ic is the common response of superconductors. In this paper, we discovered a phenomenon that the Ic can be enhanced by a field-cooled process for the La0.67Sr0.33MnO3 (LSMO)/YBa2Cu3O7−δ (YBCO)/La0.67Sr0.33MnO3 (LSMO) trilayer system, in which the high-Tc-superconductor/oxide-ferromagnetic interface plays a crucial role. Three major factors based on a singlet superconducting picture, such as the difference in the work functions, the orbital hybridization, and the magnetic coupling between the induced Cu and Mn moments at the interface, decide the charge transfer and the capability of carrying maximum current. These factors can be preset by a field-cooling process with various field-cooled fields at a temperature higher than the superconducting transition temperature, Tc. While the system is field-cooled below Tc, we found that the competition between these factors and the responses of the interface may enhance or suppress the properties of the superconducting layer and finally gives rise to a maximum Ic at a specific field-cooled field. Moreover, we also observed that the Ic increases with the increase of the applied magnetic field for the sample with a thinner YBCO layer, which cannot be explained by these mechanisms based on singlet superconductivity and strongly implies the possible existence of triplet superconducting pairs at the interface and inside a ferromagnetic (FM) layer.

On the Paramagnetic-Like Susceptibility Peaks at Zero Magnetic Field in hbox{WSe}_{2-x}hbox{Te}_{x} Single Crystals

A weakly temperature-dependent paramagnetic-like susceptibility peak at zero magnetic field is observed in hbox{WSe}_{2-x}hbox{Te}_{x} with only marginal amount of ferromagnetic impurities. The ferromagnetic hysteresis loop and the magnetic moment splitting between zero-field-cooled and field-cooled processes indicate ferromagnetism in the samples. The paramagnetic-like susceptibility peak height is proportional to the remanent magnetic moment of hysteresis loops. High-resolution transmission electron microscope image supports that the observed ferromagnetic feature originates from lattice distortion. These results imply that the weakly temperature-dependent paramagnetic-like susceptibility peak originates from weak lattice distortion and/or superparamagnetism.

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.

Discovery of a new nontoxic cuprate superconducting system Ga-Ba-Ca-Cu-O

Superconductivity is observed in a new nontoxic cuprate system Ga-Ba-Ca-Cu-O, with Tc = 82K for GaBa2Ca5Cu6O14+{\delta} (Ga-1256) and Tc = 116K probably for GaBa2Ca3Cu4O10+{\delta} (Ga-1234) or GaBa2Ca2Cu3O8+{\delta} (Ga-1223), respectively. All compounds are fabricated by solid state reaction method under high pressure and high temperature. Samples are characterized by resistivity, magnetization and X-ray diffraction (XRD) measurements. The temperature dependence of magnetization measured in both zero-field-cooled and field-cooled processes on one sample (S1) shows two superconducting transitions at about 82K and 113K. The estimated shielding fraction for the phase with Tc of 82K is about 67%, while the fraction for another phase with Tc of 113K is quite small. The XRD Rietveld refinement for S1 indicates two main phases existing in the sample, Ga-1256 with fraction of about 58% and non-superconducting Ca0.85CuO2 with fraction of about 42%, respectively. Thus, we can conclude the superconducting phase with transition temperature of 82K is due to Ga-1256. The resistivity measurement also confirms the superconductivity for S1, and the resistivity reaches zero at about 82K. The temperature dependence of magnetization for another sample (S2) shows much higher superconducting shielding fraction for the phase with Tc of 116K, which may be a promising prospective for the synthesis of Ga-1234 or Ga-1223 phase.

Magnetization of potassium-doped p -terphenyl and p -quaterphenyl by high-pressure synthesis

By using high pressure synthesis method, we have fabricated the potassium doped para-terphenyl. The temperature dependence of magnetization measured in both zero-field-cooled and field-cooled processes shows step like transitions at about 125 K. This confirms earlier report about the possible superconductivity like transition in the same system. However, the magnetization hysteresis loop exhibits a weak ferromagnetic background. After removing this ferromagnetic background, a Meissner effect like magnetic shielding can be found. A simple estimate on the diamagnetization of this step tells that the diamagnetic volume is only about 0.0427% at low temperatures, if we assume the penetration depth is much smaller than the size of possible superconducting grains. This magnetization transition does not shift with magnetic field but is suppressed and becomes almost invisible above 1.0 T. The resistivity measurements are failed because of an extremely large resistance. By using the same method, we also fabricated the potassium doped para-quaterphenyl. A similar step like transition at about 125 K was also observed by magnetization measurement. Since there is an unknown positive background and the diamagnetic volume is too small, it is insufficient to conclude that this step is derived from superconductivity although it looks like.

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.

Magnetic study of M-type doped barium hexaferrite nanocrystalline particles

Co-Ti and Ru-Ti substituted barium ferrite nanocrystalline particles BaFe12−2xCoxTixO19 with (0≤x≤1) and BaFe12−2xRuxTixO19 with (0≤x≤0.6) were prepared by ball milling method, and their magnetic properties and their temperature dependencies were studied. The zero-field-cooled (ZFC) and field-cooled (FC) processes were recorded at low magnetic fields and the ZFC curves displayed a broad peak at a temperature TM. In all samples under investigation, a clear irreversibility between the ZFC and FC curves was observed below room temperature, and this irreversibility disappeared above room temperature. These results were discussed within the framework of random particle assembly model and associated with the magnetic domain wall motion. The resistivity data showed some kind of a transition from insulator to perfect insulator around TM. At 2 K, the saturation magnetization slightly decreased and the coercivity dropped dramatically with increasing the Co-Ti concentration x. With Ru-Ti substitution, the saturation magnetization showed small variations, while the coercivity decreased monotonically, recording a reduction of about 73% at x = 0.6. These results were discussed in light of the single ion anisotropy model and the cationic distributions based on previously reported neutron diffraction data for the CoTi substituted system, and the results of our Mössbauer spectroscopy data for the RuTi substituted system.

Design and Experiment of a New Maglev Design Using Zero-Field-Cooled YBCO Superconductors

The existing high-temperature superconducting magnetic levitation (Maglev) vehicles are based on field-cooled (FC) superconductors mainly to guarantee vehicle lateral stability. Usually, this solution presents two drawbacks: levitation and guidance forces depend on how the FC process has been featured, and flux-flow phenomenon, which induces significant power losses in superconductors during the vehicle's displacement, can put them out of superconductivity state. To minimize these problems, a zero-field-cooled (ZFC) Maglev vehicle was designed and successfully developed in our laboratory. Measurements of its levitation and guidance forces and also its vertical and damping properties were carried out. These results, when compared with those ones from an FC vehicle, show that the ZFC vehicle has higher levitation forces and similar guidance force values. However, guidance forces do not depend on cooling height value as what happens in an FC vehicle, despite that the ZFC vehicle also presents some power losses. The experimental results indicated that power losses from vertical and lateral vehicle oscillations are lower in a ZFC one. However, this effect will have to be compensated by using some technique to reduce ZFC vehicle oscillations in the future.

Some aspects of antiferromagnetic ordering in LiMnP0.85V0.15O4: Neutron diffraction and DC-magnetization studies

V-substituted LiMnPO4 has been synthesized by the solid state reaction route. Combined Rietveld refinement of neutron and X-ray data revealed that all vanadium ions are located in the same positions as the phosphorus ions. The magnetic structure of LiMnP0.85V0.15O4 was found to be the same as that described for undoped antiferromagnetic LiMnPO4 (TN=34.5K). DC-magnetization measurements were carried out to study the peculiarities of magnetic ordering in LiMnP0.85V0.15O4. An irreversible behavior of DC magnetization was found at magnetic fields less than ∼2kOe. It was demonstrated that an increase in the magnitude of the applied field leads to a reduction of the discrepancy between zero-field-cooled and field-cooled processes. These effects were explained by the movement of domain walls and by transition of the system to a monodomain state. The anomaly in the magnetization vs field dependence attributed presumably to the spin-flop transition was observed at ∼40kOe. The existence of magnetic inhomogeneity in the paramagnetic phase of LiMnP0.85V0.15O4 was proved by the analysis of the χ⁎T product. It was concluded that the observed changes in χ⁎T are indicative of a competition between ferromagnetic and antiferromagnetic correlations at temperatures slightly exceeding TN.

Modification in Structure, Phase Transition, and Magnetic Property of Metallic Gallium Driven by Atom–Molecule Interactions

The present work supports a novel paradigm in which the surface structure and stacking behavior of metallic gallium (Ga) were significantly influenced by the preparation process in the presence of organic small molecules (ethanol, acetone, dichloromethane, and diethyl ether). The extent of the effect strongly depends on the polarity of the molecules. Especially, a series of new atom-molecule aggregates consisting of metallic Ga and macrocyclic hosts (cyclodextrins, CDs) were prepared and characterized by various techniques. A comprehensive comparative analysis between free metallic Ga and the Ga samples obtained provides important and at present rare information on the modification in structure, phase transition, and magnetic property of Ga driven by atom-molecule interactions. First, there is a notable difference in microstructure and electronic structure between the different types of Ga samples. Second, differential scanning calorimetry analysis gives us a complete picture (such as the occurrence of a series of metastable phases of Ga in the presence of CDs) that has allowed us to consider that Ga atoms were protected by the shielding effect provided by the cavities of CDs. Third, the metallic Ga distributed in the aggregates exhibits very interesting magnetic property compared to free metallic Ga, such as the uniform zero-field-cooled and field-cooled magnetization processes, the enhanced responses in magnetization to temperature and applied field, and the fundamental change in shape of magnetic hysteresis loops. These significant changes in structural transformation and physical property of Ga provide a novel insight into the understanding of atom-molecule interactions between metallic atoms and organic molecules.

Relaxation Phenomena of a Magnetic Nanoparticle Assembly with Randomly Oriented Anisotropy

The effects of a randomly oriented anisotropy on relaxation phenomena including the memory effect of a noninteracting magnetic nanoparticle assembly, are numerically studied with a localized partition function and a master equation, leading to the following results. During the zero-field-cooled (ZFC) process, the energy barrier histogram changes with temperature, while during the field-cooled (FC) process it remains stable. In the relaxation process after ZFC initialization, the effective energy barrier distribution, which is derived from the Tln (t/τ0) (T temperature, t time, and τ0 characteristic time constant) scaling curve, only reflects the low-energy region of the energy barrier histogram. The memory effect with temporary cooling during time evolution occurs in the studied assembly even without volume distribution and particle interaction involved.

Numerical simulation of field-cooled and zero field-cooled processes for assembly of superparamagnetic nanoparticles with uniaxial anisotropy

The results of the numerical simulation of field-cooled and zero field-cooled (ZFC) experiments in a dilute assembly of superparamagnetic nanoparticles with uniaxial anisotropy are presented. The numerical simulation uses a solution of the kinetic rate equations for population numbers of the potential wells. The particle relaxation times are rigorously obtained from the corresponding Fokker–Planck equation. For an assembly of particles with a single diameter a monotonic decrease in a blocking temperature as a function of the applied magnetic field is found, the blocking temperatures of aligned and randomly oriented assemblies being close. For an assembly with lognormal volume distribution the location of the maximum at the ZFC assembly magnetization differs considerably from the blocking temperature of particles with the average diameter. Equating of both quantities may lead to a considerable overestimation of the particle effective anisotropy constant. The effective blocking temperature of the assembly with lognormal volume distribution may experience nonmonotonic magnetic field dependence, as the particles of large diameters begin to contribute to the assembly magnetization with the increase in the applied field. A prominent dependence of the ZFC assembly magnetization maximum on the width of the lognormal distribution is also revealed.

Size effect on the magnetic properties of antiferromagneticLa0.2Ca0.8MnO3nanoparticles

Magnetic properties of electron-doped ${\text{La}}_{0.2}{\text{Ca}}_{0.8}{\text{MnO}}_{3}$ manganite nanoparticles with average particle size ranging from 15 to 37 nm, prepared by the glycine-nitrate method, have been investigated in temperature range 5--300 K and in magnetic fields up to 90 kOe. A monotonous enhancement of weak ferromagnetism linked to the reduction in the particle size was observed for all nanoparticles. Magnetic hysteresis loops also indicate size-dependent exchange bias effect displayed by horizontal and vertical shifts in field-cooled processes. The magnetization data reveal two ferromagnetic components: first one appears at $T\ensuremath{\sim}200\text{ }\text{K}$ and may be attributed to surface magnetization and second one appears as a result of spin canting of antiferromagnetic core or is developed at some interfaces inside nanoparticles. Time evolution of magnetization recorded in magnetic fields after the field cooling to low temperatures exhibits a very noisy behavior that may be caused by formation of collective state of nanoparticles with no clear tendency to reach equilibrium state. Magnetic properties of the nanoparticle samples are compared with those of the bulk ${\text{La}}_{0.2}{\text{Ca}}_{0.8}{\text{MnO}}_{3}$.

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.

Grain-size effects on the charge ordering and exchange bias inPr0.5Ca0.5MnO3: The role of spin configuration

The grain-size effects on the charge ordering and exchange bias in CE-type antiferromagnetic charge ordered manganite compound ${\text{Pr}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_{3}$ are explored by magnetometry. Reducing the size suppresses antiferromagnetism and charge ordering that does not occur any more when the size is smaller than 40 nm. At the same time ferromagnetic clusters appear and gradually become larger; their fraction increases to about 19.7% when the particle size is reduced to 20 nm. The magnetic hysteresis loops of the nanosized samples exhibit both horizontal and vertical shifts in the field-cooled process. The exchange bias field ${H}_{\text{eb}}$ and the coercivity ${H}_{\text{C}}$ do not depend monotonously on size but show a maximum at about 85 nm, which indicates that in antiferromagnetic charge-ordered manganites ${H}_{\text{eb}}$ can be effectively tuned by changing the grain size. These notable phenomena can be understood when the evolution of the spin configuration is considered that results from the surface-phase separation and surface effect.

Size-dependent exchange bias inLa0.25Ca0.75MnO3nanoparticles

Exchange bias phenomena are observed in ${\mathrm{La}}_{0.25}{\mathrm{Ca}}_{0.75}\mathrm{Mn}{\mathrm{O}}_{3}$ nanoparticles with average sizes ranging from $40\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1000\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. It is found that the magnetic hysteresis loops display horizontal and vertical shifts in field-cooled processes. The variations of the exchange bias field $({H}_{\mathrm{EB}})$ and the coercivity $({H}_{c})$ with particle size follow nonmonotonic dependencies and show maxima for particles with diameter around $80\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at $T=5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which can be mainly ascribed to the changes in uncompensated surface spins with nanoparticle size. The peak position for ${H}_{c}$ shifts to larger particle size at higher temperature while that for ${H}_{\mathrm{EB}}$ is unconspicuous. The linear relationship between ${H}_{\mathrm{EB}}$ and vertical magnetization shift $({M}_{\mathrm{EB}})$ further indicates that the characteristics of uncompensated spins play an important role in the variations of ${H}_{\mathrm{EB}}$ for the manganite.

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.

Comparison of the structural and magnetic properties of the two ternary silicides Nd 6Ni 1.67Si 3 and Nd 6Co 1.67Si 3

Single phases of Nd 6Ni 1.67Si 3 and Nd 6Co 1.67Si 3 were prepared by arc melting of a stoichiometric mixture of the elements followed by annealing at 1073 K. The structure of Nd 6Co 1.67Si 3 was determined from single-crystal X-ray data with the space group P6 3/ m and the lattice parameters a = 11.950(4) Å and c = 4.232(2) Å. Its structure derives from the structure of the aristotype compound Ce 6Ni 1.67Si 3. A strong disorder of cobalt atoms in the chains of face-shared octahedra of neodymium is observed. Magnetization measurements reveal the existence of two ferro(ferri)magnetic transitions at 84(1) K and 38(1) K for Nd 6Ni 1.67Si 3. For Nd 6Co 1.67Si 3 it has been previously shown that also two ferro(ferri)magnetic transitions occur at 84 K and 35 K. The comparison between the magnetic properties of Nd 6Ni 1.67Si 3 and Nd 6Co 1.67Si 3 shows a significant difference of the behaviour of the magnetization M versus temperature and field μ 0 H below 84 K. The curves M = f( T) obtained by field-cooled (FC) process and under μ 0 H = 0.1 T show an increase of M below 35 K for Nd 6Co 1.67Si 3 and on the contrary, a decrease of M below 38 K for Nd 6Ni 1.67Si 3. This difference suggests the occurrence at low temperature of different magnetic structures for these two ternary silicides.

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.

Unconventional relaxation in antiferromagneticCoRh2O4nanoparticles

Magnetic relaxation in antiferromagnetic CoRh$_2$O$_4$ nanoparticles is investigated at 2 K by cooling the sample from a temperature (70 K) well above the antiferromagnetic ordering temperature at 27 K, following zero field cooled (ZFC) and field cooled (FC) process. In ZFC process, the sample at 2 K is subsequently followed by magnetic field on and off sequences, whereas in FC process the cooling field is made off during measurement of remanent magnetization as a function of time. The experiments suggest an unconventional relaxation behaviour in the system, as an effect of increasing surface exchange anisotropy with decreasing the size of antiferromagnetic nanoparticles.