High Irreversibility Field Research Articles

Field-induced spin–flop transitions of interacting nanosizedα-Fe2O3 particles dispersed in a silica glass matrix

Nanoparticles of 10 mol% Fe2O3 doped in silica glass (Fe10) samples prepared by a sol–gel methodfollowed by calcination at various temperatures in the range700–1000 °C are studied by x-ray, transmission electron microscopy and magnetic methods, includingelectron paramagnetic resonance (EPR). X-ray studies reveal the presence of bothγ-Fe2O3 and α-Fe2O3 nanocrystals in varying proportion, the latter being more abundant in samples subject tocalcination at higher temperature. Nanocrystals have mean sizes in the range 10–40 nm andare larger in the samples calcined at higher temperatures. The relatively narrow EPR linehaving its origin in superparamagnetism is observed at room temperature and is transformedto an asymmetrically broad ferromagnetic resonance signal at 77 K. Mössbauer spectra of the700 °C calcined sample at room temperature show two doublet structures due toγ-Fe2O3 and α-Fe2O3 nanoparticlessignifying their superparamagnetic character, whereas those of samples calcined at higher temperatures(≤1000 °C) displaytwo sextets indicating that the nanoparticles are magnetically ordered. At higher calcination temperatures(≥900 °C) the hyperfine lines become asymmetrically broadened and the average hyperfine fielddiminishes in a way typical for interacting magnetic nanoparticles. Zero-field-cooledmagnetization and hysteresis studies of Fe10 samples calcined at higher temperatures(≥800 °C) in the temperature range 5–300 K reveal unusual ferromagnetic behaviour with substantialmagnetization and large coercivities at low temperature (5 K). The presence of a highirreversibility field and shifted hysteresis loop in the Fe10 sample calcined at1000 °C at 5 K has been verified from field-cooled magnetization versus the magnetic field curve. Onlyα-Fe2O3 nanoparticles generatedin the Fe2O3:SiO2 sample calcinedat 1000 °C exhibit a Morintransition like bulk α-Fe2O3 crystals but at a muchlower temperature ∼100 K. Spin–flop like transitions have been observed for the first time at temperatures abovethe Morin temperature, possibly induced by an external dc magnetic field of appropriatemagnitude in conjunction with inter-nanoparticle exchange.

A thermally actuated superconducting flux pump

The concept of a superconducting flux pump is relatively straightforward. A small magnetic field repeatedly applied will lead to a much larger field being trapped within the superconductor. This field is limited by the volume of the superconductor and by its critical current but not by the excitation field. Here we will describe a new technique which facilitates the creation of high magnetic fields and where the magnitude of the trapped field is limited by the superconductor not the magnetising field. The technique is demonstrated using measurements taken using samples of bulk YBCO as YBCO has a very high irreversibility field and has the potential to trap high magnetic fields. The technique could be applied to other superconductors such as BSCCO or MgB 2 and in other forms such as thin or thick films.

Irreversibility Field and c-Axis Correlated Pinning in High-$J_{c}$ SmBCO Films

A high critical current density Jc at 77.3 K and a high irreversibility field for B par c were achieved for low-temperature-growth processed SmBa2Cu3O7 (LTG-Sm123) films. Transport properties of the LTG-Sm123 films with high Jc were measured in high magnetic fields up to 17 T. We found a small enhancement of the irreversibility field due to the c-axis correlated pinning. We also observed the large peak for B par c in a low field region below 1 T and 77.3 K. Those behaviors are originated with the c-axis correlated pinning. The peak at B par c decreases in a low field region but increases again above 3 T with increasing a magnetic field. These Jc behaviors are considerably related with the enhancement of irreversibility field. From experimental results obtained in this study, we proposed new pinned glass states near the irreversibility field.

Enhanced flux pinning in YBa2Cu3O7 layers by the formation of nanosized BaHfO3 precipitates using the chemical deposition method

Both a high critical current density and a cost-competitive, easily upscalable procedure are among the most critical requirements for coated conductors. In this work, a low-cost method based on the trifluoroacetic acid process was used to prepare nanosized BaHfO3 particles inside a YBa2Cu3O7 film matrix. The inclusions have a perovskite structure and grow epitaxially in a cube-on-cube relationship. A drastically improved pinning force density and high irreversibility fields in the YBa2Cu3O7 layers were found and are discussed in respect to the Hf doping level in the precursor solutions, which resulted in an increased nanoinclusion content.

Flux pinning properties of (Nd,Eu,Gd)Ba2Cu3Oy (NEG-123) superconductor with 211 phase particles

Flux pinning properties were investigated by focusing on the origin ofthe high peak critical current density and the high irreversibility field forNd0.33Eu0.38Gd0.28Ba2Cu3Oy (NEG-123) bulk superconductors with the addition of 211 secondaryphase particles of volume fractions up to 10 mol%. It was found thata negative correlation exists between the peak critical current densityJcp and theirreversibility field Bi for various fractions of added 211 phase. This suggests that the mechanisms that determineJcp and Bi are different. The 211 particles do not contribute to the peak effect neither directly throughthe pinning mechanisms of the condensation energy interaction nor indirectly with the aidof the order–disorder transition of flux lines. Other possible defects are nano-lamellastructures. Although these defects do not directly contribute to the peak effect through thepinning mechanism of the condensation energy interaction, they may contribute to itwith the aid of the order–disorder transition of flux lines. On the other hand, asBi deteriorates with the addition of 211 particles, it is not determined by the flux pinning of 211 particles. Thedecrease in Bi is considered to be caused by the proximity effect of nano-lamella structures.

Critical radius for exchange bias in naturally oxidized Fe nanoparticles

Details of synthesis and structural characterization of highly crystalline iron oxide nanoparticles are presented together with results on the magnetic investigation as a function of the temperature and applied field. Monodisperse Fe nanoparticles were prepared by thermal decomposition of iron pentacarbonyl in the presence of oleic acid. These iron nanoparticles were readily oxidized on exposure to air. The resulting nanocrystals have been identified as inverse spinels, being $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ the dominant phase of the small $5\text{\ensuremath{-}}\mathrm{nm}$ iron oxide nanocrystals, whereas the proportion of the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ component gradually increases on increasing the particle size. The small particles volume resulted in finite-size effects, for instance, the magnetization deviates from the ${T}^{3∕2}$ Bloch's law. At the same time, high field irreversibility and shifted hysteresis loops after field-cooled processes have been detected, and attributed to a low-temperature surface spin-glass layer. Moreover, there is a critical diameter below which the surface spin-glass behavior and exchange bias effect abruptly disappear.

Synthesis of Hg(Re)1223 Tapes by PIT Method Using NiO/Ni Sheath

Fabrication of Hg1-xRexBa2Ca2Cu3Oy [Hg(Re)1223] tapes has been attempted by the PIT(Powder-In-Tube) method using a NiO/Ni sheath. It is confirmed that Hg(Re)1223 tapes with a high Re substitution level, 25% for Hg, showed sharp superconducting transitions at ∼134 K and high irreversibility fields up to ∼9 T at 77 K. In addition, application of magnetic grain alignment was effective for development of c-axis oriented Hg(Re)1223 tapes showing high transport properties.

Magnetic properties of nanosized Sr 2CrReO 6 powders prepared by a combustion synthesis process

Double perovskite, Sr 2CrReO 6, nanoparticles in the size range 10–25 nm have been prepared by a combustion method. Its coercivity ( H c) at 5 and 300 K are determined to be 16.1 and 6.5 kOe, respectively. Using the law of approach to saturation, the obtained saturation magnetization σ s (5 K) and σ s (300 K) are 0.41 and 0.32 μB per formula unit (f.u.), respectively. The values of saturation magnetization are lower than the expected value 1 μB/f.u. considering the antiferromagnetic coupling between Cr 3+ (3d 3, S = 3 / 2) and Re 5+ (5d 2, S = 1). At the same time high field irreversibility in M( H) curves and zero field cooled–field cooled (ZFC–FC) processes has also been found. A field-cooled (FC) hysteresis loop shift decreases with increasing temperature and vanishes near 350 K. It is suggested that the observed reduction of σ s and shifted hysteresis loops are caused by the exchange interaction between the ferromagnetic core of the Sr 2CrReO 6 nanoparticles and its surface layer with disordered spins.

ＮＥＧ１２３およびＹ１２３バルクの強磁場輸送特性

The bulk material comprised of (Nd, Eu, Gd)Ba2Cu3Ox(NEG123) shows a high irreversibility field over 14 T at 77.3 K. In order to study the origin of the high irreversibility field, transport measurements of NEG123 and Y123 bulk samples were performed as functions of temperature, high magnetic field and field angle, and transport properties were compared with each other. Dip structures for the angular dependence of resistivity are observed around B//c for all samples. These results indicate that c-axis correlated disorder exists. The NEG123 bulk sample with a larger dip shows a higher irreversibility temperature. In addition, we found that the differences of the irreversibility temperature between on and 12° off the dip are scaled universally as a function of the depth of the dip for NEG123 and Y123 bulk samples. It is considered that c-axis correlated disorder is important for high irreversibility fields.

High upper critical field and irreversibility field in MgB2 coated-conductor fibers

We report on structural and superconducting properties of round MgB2 coated-conductor fibers deposited by hybrid physical-chemical vapor deposition on SiC fibers. The coating is polycrystalline and composed of elongated crystallites with dimensions less than 1μm in length and 0.2μm in width. The pure MgB2 fiber shows a zero-resistance Tc of 39.3 K. The carbon-alloyed fibers show a high upper critical field of 55 T at 1.5 K and a high irreversibility field of 40 T at 1.5 K. The result demonstrates great potential of MgB2 coated conductors for superconducting magnets.

Mechanism of a High Irreversibility Field for (Nd, Eu, Gd)<tex>$rm Ba_2rm Cu_3rm O_7$</tex>Bulk

In order to study the mechanism of the high irreversibility field, we measured the high field transport properties for bulk NEG123 samples with high irreversibility field. It was found that the Bose glass behavior is observed for both NEG123 bulk samples with 3% and 40% NEG211 additions. However, the 40%NEG211 sample with a low irreversibility field showed a higher effective matching field of the c-axis correlated disorder than the 3%NEG211 sample with a high irreversibility field. It is suggested that the c-axis correlation varies among samples and the strong correlation along the c-axis, which may arise from the alignment of the RE-rich NEG211 clusters, is important for the realization of the high irreversibility field.

High-<tex>$J_c$</tex>Gd-Ba-Cu-O Epitaxial Films Prepared by Pulsed Laser Deposition

For applications of high-temperature cuprate superconductors in magnetic fields at 77 K, a RBa/sub 2/Cu/sub 3/O/sub x/ (R123; R=Nd, Sm, Gd, etc.) film is one of the promising materials due to its high critical temperature T/sub c/ and high irreversibility field B/sub irr/. Especially, a substitution between R and Ba sites that results in a T/sub c/ degradation is not likely to occur in GdBa/sub 2/Cu/sub 3/O/sub x/ (Gd123) films, compared to the cases of Nd and Sm ions. This is an advantage for an application of Gd123 film. We studied the c-axis oriented Gd123 film for obtaining a high critical current density J/sub c/ in a magnetic field at 77 K. The substrate temperature to deposit the epitaxial films on MgO(100) was varied between 700-800/spl deg/C under an oxygen partial pressure of 200 mTorr. T/sub c/ values of the films were within 89.8-92.7 K. Interestingly, the J/sub c/ at 77 K for 5 T (B//c) reached 0.2 MA/cm/sup 2/. The values are three times higher than those of Y123 films, which indicate that Gd123 films have the superior potentiality in magnetic fields at 77 K.

Temperature and Magnetic Field Dependence of Critical Currents in YBCO Coated Conductors With Processing-Induced Variations in Pinning Properties

Several applications of high-temperature super-conducting wire require high currents at intermediate magnetic fields B and over a range of orientations; however, such conditions are at present achievable only at low temperatures (/spl sim/30 K). The goal of this study is to determine the feasibility of higher operating temperatures for these applications by investigating temperature dependent, low- and high-field pinning properties of YBCO coated conductor samples. The YBCO films were grown on RABiTS templates by a PVD ex situ BaF/sub 2/ process. Variations in pinning properties were induced by introducing excess yttrium (Y) in the precursor and controllably increasing the growth rate. The main result is a more uniform dependence of J/sub c/ over all orientations of B, along with high irreversibility field B/sub irr/ and high critical current densities J/sub c/. Results also show that for films with various pinning properties and processed under different conditions the self-field J/sub c/ at 77 K is an effective indicator of performance in the temperatures and fields of interest.

Optimization of Critical Currents in<tex>$rm MgB_2$</tex>Wires and Coils

We present an analysis of the effect of SiC doping and heat treatments, including hot isostatic pressing (HIPing), on the microstructure, magnetization and transport properties of MgB/sub 2/ wires and coils prepared by the powder in tube (PIT) method. The HIPed wires have higher critical current densities (J/sub c/) than the annealed wires, especially at high temperatures and magnetic fields, and higher irreversibility field. The improvements are due to the reduction of porosity and to a high density of structural defects, which are the likely source of vortex pinning. We demonstrate that the high-quality properties mentioned above can be retained in longer wires. At 25 K our champion 6-layer coil, made with 25 meters of wire, was able to generate a field of 1 T at self-field. We were able to achieve even higher J/sub c/'s in another coil by combining the addition of SiC and HIPing. We also observed a larger upper critical field due to the SiC doping, which drives the sample to the dirty limit.

Surface effects in $\mathsf{\alpha}$-Fe$\mathsf{_{2}}$O$\mathsf{_{3}}$ nanoparticles

The magnetic properties of 5 nm \(\alpha \)-Fe2O3 nanoparticles have been investigated by magnetization measurements on a sample consisting of homogeneously dispersed, non-interacting, nanoparticles in a polymer matrix. The results indicate that the magnetic properties are mainly determined by surface effects, which manifest themselves in high coercive field, high irreversibility field and shifted hysteresis loop, after field cooling. These effects come from surface anisotropy and exchange anisotropy, due to the coupling between the disordered surface magnetic structure, with multiple spin configurations, and the core antiferromagnetically ordered structure.

Magnetic behavior of nickel ferrite–polyethylene nanocomposites synthesized by mechanical milling process

Magnetic properties of nanocrystalline nickel ferrite–polyethylene nanocomposites synthesized by mechanical milling process has been studied by superconducting quantum interference device (SQUID) magnetometer. Both zero-field-cooled (ZFC) and field-cooled (FC) magnetic measurements were made. The absence of hysteresis, remanence, and coercivity at room temperature suggests the superparamagnetic nature of the nanocomposite. The nickel ferrite–polyethylene nanocomposite exhibits a blocking temperature of ∼20 K and does not attain saturation magnetization even at a high field of 50 kOe. The lack of saturation magnetization at high field occurs in association with high field irreversibility and open loop at 50 kOe. This is attributed to the spin-glass like surface layer on the nanocrystalline nickel ferrites with a ferrimagnetically aligned core.

Bose glass state in bulk(Nd,Eu,Gd)Ba2Cu3Oxwith a high irreversibility field

$(\mathrm{Nd},\mathrm{Eu},\mathrm{Gd}){\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ (NEG123) bulk shows the high irreversibility properties. In order to clarify the origin of high irreversibility fields, the transport properties of NEG123 bulk with the high irreversibility field were investigated against high magnetic fields, temperatures, and field angles. We found that a dip in the angular dependent resistivity appears at $B∕∕c$, originated to the correlated disorder along the $c$ axis. In addition, the Bose glass phase was observed around $B∕∕c$ by the detailed analysis of the glass transition temperature and exponent. Therefore the Bose glass state plays an important role for realization of the high irreversibility field. It is suggested that new effective pinning centers correlated along the $c$ axis are responsible for the high irreversibility field.

Magnetic behavior of nanocrystalline nickel ferrite synthesized by the reverse micelle technique

Nanocrystalline nickel ferrite of crystallite size 5–8 nm, synthesized by the reverse micelle technique were characterized by high-resolution transmission electron microscopy and X-ray diffraction techniques, and the magnetic behavior studied by superconducting quantum interference device. Nanocrystalline nickel ferrite exhibit a blocking temperature of 16 K. They do not attain saturation magnetization even at a high field of 50 kOe. The lack of saturation in high field occurs in association with high field irreversibility and open loop at 50 kOe. The saturation magnetization at 300 K is 25.4 emu/g and at 2 K is 35.5 emu/g, which is significantly lower than that reported for the multidomain bulk nickel ferrite (55 emu/g). This is discussed in terms of a two-component nanoparticle system consisting of a spin glass-like surface layer of a few atomic layers thick and ferrimagnetically aligned core spins.

Magnetization measurement of (Nd, Eu, Gd)Ba 2Cu 3O y having very high irreversibility fields in pulsed high magnetic fields

Magnetization measurement in pulsed high magnetic fields up to 28 T in the range of temperature from 55 to 85 K have been performed for (Nd 0.33Eu 0.38Gd 0.28)Ba 2Cu 3O y (NEG123) having very high irreversibility fields B irr We have evaluated temperature dependence of B irr by taking account of heat generation. Critical current density in pulsed high magnetic fields was somewhat overestimated in comparison with that in fields, but values of B irr nearly coincide. B irr did not strongly depend on the content of Gd 2BaCuO 5 (Gd211) secondary phase between 2 and 10 mol%. The temperature dependence of B irr can be expressed as B irr (T)=80.2−0.878 T (K).

Structure and magnetic properties of polycrystalline Fe3O4 films deposited by reactive sputtering at room temperature

Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature, which is much lower than the reported temperature of above 500 K to deposit high quality Fe3O4 films. Transmission electron microscopy image shows that uniform Fe3O4 grains are well separated by grain boundaries and their size increases with film thickness. The magnetization of the Fe3O4 films measured at 300 K under 50 kOe magnetic field is much lower than the reported value of 471 emu/cc, which can be interpreted as the existence of strong antiferromagnetic coupling within the grain boundaries. The coupling between frozen surface spins and core spins and the switch of surface spin configurations cause hysteresis loop shift below 200 K and high field irreversibility, respectively. The observed decrease of magnetization with film thickness can be explained as the reduction in magnetization caused by the increased volume fraction of surface layer and the grain boundaries. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)