Magnetic History Effects Research Articles

Effects of Magnetic History on AC Loss Minimum in BSCCO-2223 Composite Tape

Effects of Magnetic History on AC Loss Minimum in BSCCO-2223 Composite Tape

A comparative study of magnetic behaviors in TbNi2, TbMn2 and TbNi2Mn

All TbNi2, TbMn2, and TbNi2Mn compounds exhibit the cubic Laves phase with AB2-type structure in spite of the fact that the ratio of the Tb to transition-metal components in TbNi2Mn is 1:3. Rietveld refinement indicates that in TbNi2Mn the Mn atoms are distributed on both the A (8a) and B (16d) sites. The values of the lattice constants were measured to be a = 14.348 Å (space group F-43 m), 7.618 Å, and 7.158 Å (space group Fd-3 m) for TbNi2, TbMn2, and TbNi2Mn, respectively. The magnetic transition temperatures TC were found to be TC = 38 K and TC = 148 K for TbNi2 and TbNi2Mn, respectively, while two magnetic phase transitions are detected for TbMn2 at T1 = 20 K and T2 = 49 K. Clear magnetic history effects in a low magnetic field are observed in TbMn2 and TbNi2Mn. The magnetic entropy changes have been obtained.

Effects of magnetic history on the levitation characteristics in a superconducting levitation system

Using an updated high-temperature superconductor maglev measurement system, influences of the motion mode in which both the superconductor and the magnet are placed close to each other on the magnetic forces and their relaxation are measured. It is found that Method 1, in which the magnet coaxially approaches the superconductor which is subsequently displaced laterally, leads to reduce the relaxation both in the levitation force and in the lateral force in comparison to Method 2, where lateral displacement of superconductor occurs before vertical motion of the magnet. The results are relevant for the practical levitation systems.

Angular dependence of the order-disorder transition in proton irradiated single crystalMgB2

We present magnetization results on a proton irradiated $\mathrm{Mg}{\mathrm{B}}_{2}$ single crystal that displays a peak in magnetization for the field applied parallel to the $c$ axis. Magnetic history effects are observed, which are ascribed to the occurrence of a disorder driven phase transition close to an inflection point in the magnetization-field curve. We demonstrate that the angular and temperature dependence of this feature is significantly different to that of the lower and upper critical fields.

Magneto-optical investigation of bi-crystalline thin films of Y-123

We have performed a deliberate study on the transport properties of Y-123 thin films on bi-crystalline substrates containing low angle grain boundaries (LAGBs). By magneto-optical (MO) imaging we are able to map the magnetic field and current distribution of wide current bridges. A quantitative analysis of the distribution of currents over the entire grain boundary (GB) is presented. We discuss the effects of magnetic history on the transport properties of Y-123 thin films with LAGBs. The results are compared with artificially created two-dimensional (2D) current distributions.

Effect of Mn substitution on the volume and magnetic properties of Er2Fe17

The effects of Mn substitution on the magnetic properties of Er2Fe17−xMnx compounds have been investigated by different experimental techniques. An unusual composition dependence of the unit cell volume at room temperature occurs due to a strong magnetovolume effect, which has been confirmed by high temperature linear thermal expansion measurements. From magnetization measurements the composition dependence of the spontaneous magnetization of the 3d-sublattice magnetic moment and of the Curie temperature has been determined. Spin reorientation has been detected for compounds with 3⩽x⩽5.5, as well as remarkable magnetic history effects.

Magnetic-history effects in Hall resistivity of UNiAl

We report on magnetic-history effects in the Hall resistivity (ρH) of UNiAl with respect to temperature and a magnetic field applied along the c-axis. The magnetic-field induced irreversibility of ρH is the strongest at the lowest temperature of measurement (2.5 K), decays with increasing temperature and vanishes around 7 K, where simultaneously the metamagnetic transition (MT) in UNiAl loses its first-order character. We have also observed that exposing UNiAl to fields larger than the critical field for MT (11.4 T at 2.5 K) is not the necessary condition to observe the irreversibility. In particular, the irreversibility in UNiAl can be induced by fields ≥10 T. Using available data we propose a tentative magnetic phase diagram for UNiAl and discuss it in the context of recent neutron-diffraction results.

Magnetic history effects in the Heusler compound Co 2NbSn

The magnetic properties of the Heusler compound Co 2NbSn have been investigated. Whereas Co 2NbSn is paramagnetic above T c =116 K , it changes into a ferromagnetic state below T c. Based upon the observation of an anomalous thermomagnetic irreversibility below T c we argue that this indicates the presence of narrow Bloch walls, probably caused by large magnetic anisotropy induced by a structural transition at T s =235 k .

Rare earth effects on the magnetic behavior of R 6Fe 11− xAl 3+ x compounds

R 6Fe 11− x Al 3+ x (R=Nd, La) samples were investigated with magnetometry and neutron diffraction. Both compounds are antiferromagnetic and undergo a metamagnetic transition at moderate fields. In contrast to La 6Fe 10Al 4, Nd 6Fe 10Al 4 exhibits a large transition hysteresis and magnetic history effects at T<30 K. These effects are related to a Nd spin canting revealed by neutron diffraction in this temperature range.

Electric-field-driven vortex lattice-glass transition and history effect inRBa2Cu3O7−δ(R=Y,Nd)single crystals

The lattice-glass transition in the solid vortex state is investigated in very clean, naturally twin-free $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ $(R=\mathrm{Y},\mathrm{N}\mathrm{d})$ single crystals. We demonstrate that this transition is driven by the electric field E in addition to magnetic field and disorder. With increasing sweep rate of the magnetic field $dB/dt\ensuremath{\sim}E,$ the transition field shifts below its equilibrium value. This interesting and unexpected observation is related to the injection of disorder into the vortex lattice from large electric fields at the sample surface. The fast annealing of introduced disorder in the ordered phase results in an unusual time-dependent decay of the irreversible magnetic moment. The correlation of this metastable behavior with path-dependent magnetic history effects is further discussed.

Anomalous magnetic behavior in holmium doped La 0.7Sr 0.3MnO 3 film

Low field magnetization measurements are reported in a highly oriented La 0.55Ho 0.15Sr 0.3MnO 3 film on LaAlO 3 substrate in the temperature range of 2–300 K and in fields up to 50 mT. Interesting magnetic history effects are observed at low temperatures which are not seen in high magnetic fields. It appears that the inhomogeneous magnetic state at low temperatures evolves due to competing magnetic interactions leading to the observed irreversibility behavior in these materials.

Low field ac susceptibility studies of gamma irradiated granular YBCO superconductors: Magnetic history effects

We report here the temperature dependence of the out-of-phase susceptibility of 60Co γ-irradiated granular YBa 2Cu 3O 7−δ samples under the zero field cooling (ZFC) and field cooling (FC) configurations. DC field of different magnitudes was superimposed on a fixed 1 Oe ac field of frequency 111.1 Hz. A modified form of the Anderson-Kim model has been invoked to compute the critical current density ( J c) and the pinning force density ( f p). J c has been seen to be a non-linear function of the increasing radiation dosage. Both J c and f p are found to be higher for the FC case than for the ZFC case. Attempts have been made to account for the experimental results in terms of the formation of point defects like vacancies, interstitials and their recombination during irradiation.

Effects of magnetic history on the ac magnetic susceptibility of granular YBa2Cu3O7- delta superconductors in weak fields.

We investigate the ac susceptibility of both field-cooled and zero-field-cooled sintered ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ granular thin slabs and powders in low magnetic fields. The ac susceptibility of the granular thin slabs appears to depend upon the magnetic history for dc fields larger than a certain value, ${\mathit{H}}_{\mathrm{dc}}^{\mathrm{*}}$, with the existence of the stronger diamagnetic signal (the in-phase part of the ac susceptibility) in the field-cooled case. Our results show that the difference in local flux distributions between the field-cooled case and the zero-field-cooled case exists even for a dc field as low as several Oe, a value much lower than the bulk lower critical field (${\mathit{H}}_{\mathit{c}1}$). The upper limit of a homogeneous superconducting area determined from ${\mathit{H}}_{\mathrm{dc}}^{\mathrm{*}}$ appears comparable to the average grain size in our granular thin slabs. The existence of ${\mathit{H}}_{\mathrm{dc}}^{\mathrm{*}}$ is attributed to the superconducting glass behavior of granular ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with disorder in the intergranular Josephson junctions.

Superconductivity at an elevated temperature in a new class of quaternary rare earth-transition metal boro-carbides

Our discovery of superconductivity at an elevated temperature (∼ 12 K) in the system YNiBC has opened up an entirely new area of research in superconductivity of intermetallics. We briefly present the results of our recent studies on magnetic history effects in YNi 2B 2C, the coexistence of superconductivity and magnetism in ErNi 2B 2C, the effect of substituents on superconductivity in YNi 2B 2C, the mixed valence of Ce in CeNi 2B 2C and superconductivity in Th-based borocarbides.

Metamagnetic transitions and giant magnetoresistance in UNiGe

We report on the temperature dependence of the metamagnetic transitions, which occur in UNiGe in magnetic fields applied along the b and c axis. Above the ordering temperature T N = 50 K, a strong magnetic and thermal history-dependendce is found. Furthermore, the development of a small ferromagnetic component (0.02μ B/f.u.) at 4.2 K after releasing the field applied indicates magnetic-history effects also at low temperatures. The presumed irreversibility in the magnetization process at low temperatures is corroborated by irreproducible changes in the magnetoresistance response at 4.2 K. Nevertheless, tentative magnetic phase diagrams have been obtained for both orientations.

Magnetization studies on YNi 2B 2C

We present magnetization studies on the quaternary intermetallic superconductor YNi 2B 2C. We have observed magnetic history effects that establish its response as consistent with that of a conventional hard type-II superconductor, and we also report irreversibility line measurements.

Energy dissipation and flux trapping in a high- Tc superconductor in low AC and DC magnetic fields

The results of measurements of low frequency AC losses in a cylindrical YBa 2Cu 3O 7− x sample subjected to a sinusoidally varying magnetic field, with a coaxially superimposed DC-bias field, are presented. It is found that magnetic history effects are not observed if only the AC field is applied. A quite different behavior is observed if a DC magnetic field is trapped in the sample. In such a case, a significant increase, above one order of magnitude, of the intergranular loss is observed, whereas the dissipation inside the granules is practically unchanged. Moreover, there exists a threshold value of the applied DC magnetizing field, below which changes of the AC losses are not observed. Its value is approximately equal to the first penetration field for the grains.

AC LOSSES IN YBa2Cu3O6+x WITH TRAPPED MAGNETIC FLUX

Results of measurements of low frequency ac losses in a cylindrical YBa 2 Cu 3 O 6+x sample subjected to a sinusoidally varying magnetic field are presented. Magnetic history effects are not observed if only the ac field is cycling. A quite different behavior is observed if dc magnetic field is trapped in the sample. In such a case, a significant increase, above one order of magnitude, of intergranular losses is observed, whereas the dissipation in granules remains practically unchanged. Moreover, there exists a threshold value of the applied dc magnetizing field below which changes of the ac losses connected with the magnetic history of the sample are not observed. Its value is approximately equal to the first penetration field for the grains.

Effect of magnetic history on transport critical currents

We measured field dependent critical of a nionium sample with different magnetic history. The effect of magnetic history on the measured critical density for a fixed applied field is explained by a macroscopic model based on the critical state model.

Spin Glass Magnetic Behavior of Iron/Silica Gel Nanocomposites

ABSTRACTHomogeneous gelled composites of iron and silica containing 5-40 wt.%Fe prepared by low temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst) were heated to 380°C in the presence of hydrogen gas. X-ray diffraction and M6ssbauer effect measurements, and transmission electron microscope (TEM) observations show these materials are comprised of nanometer-sized regions of iron compounds embedded in a silica gel matrix. Magnetic susceptibility data indicate the materials became either superparamagnetic or ferromagnetic at room temperature. On cooling, the magnetization data furthermore show that the hydrogenated materials containing ll-30% Fe become magnetic spin glasses at temperatures less than 30 K. Magnetic history effects are observed in addition to displaced hysteresis loops below their spin freezing temperatures (Tf). For field-cooled materials at 10 K, the displacement of the hysteresis loops along the field axis indicates the presence of a unidirectional anisotropy which decreases with the cooling field. Both superparamagnetic-to-spin glass and ferromagnetic-to-spin glass transitions are observed in these nanocomposites. Tf varies with the Fe content from ∼30 K for the 11%Fe nanocomposite to ∼10 K for a content near 33%Fe.