In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

Superconductivity in iron compounds

Large-scale superconducting electric devices for power industry depend critically on wires with high critical current densities at temperatures where cryogenic losses are tolerable This restricts choice to two high-temperature cuprate superconductors, (Bi,Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Ox, and possibly to MgB2, recently discovered to superconduct at 39 K Crystal structure and material anisotropy place fundamental restrictions on their properties, especially in polycrystalline form So far, power applications have followed a largely empirical, twin-track approach of conductor development and construction of prototype devices The feasibility of superconducting power cables, magnetic energy-storage devices, transformers, fault current limiters and motors, largely using (Bi,Pb)2Sr2Ca2Cu3Ox conductor, is proven Widespread applications now depend significantly on cost-effective resolution of fundamental materials and fabrication issues, which control the production of low-cost, high-performance conductors of these remarkable compounds

High-Tc superconducting materials for electric power applications.

There are numerous potential applications for superconducting tapes based on YBa(2)Cu(3)O(7-x) (YBCO) films coated onto metallic substrates. A long-established goal of more than 15 years has been to understand the magnetic-flux pinning mechanisms that allow films to maintain high current densities out to high magnetic fields. In fact, films carry one to two orders of magnitude higher current densities than any other form of the material. For this reason, the idea of further improving pinning has received little attention. Now that commercialization of YBCO-tape conductors is much closer, an important goal for both better performance and lower fabrication costs is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route that yields a 1.5-5-fold improvement in the in-magnetic-field current densities of conductors that are already of high quality.

Strongly enhanced current densities in superconducting coated conductors of YBa2Cu3O7-x + BaZrO3

In the high-{kappa} cubic superconductor V{sub 3}Si , phase transitions of the flux lattice structure occur as a function of applied field and temperature. With the field parallel to the fourfold [001] axis, the flux lattice transforms from triangular to square symmetry at approximately 1thinspthinspT. With the field parallel to the twofold [110] axis, the lattice, which is a nearly perfect hexagonal array at the lowest fields, distorts as the field is increased; the rate of increase in this distortion changes abruptly at 1.3thinspthinspT. As T{sub c} is approached, the system tends towards a more isotropic hexagonal array of the flux lines. These transitions are largely but not completely in agreement with a recent theory of the effects on the flux line arrangements of nonlocal electrodynamics in the London limit. {copyright} {ital 1999} {ital The American Physical Society }

Flux Lattice Symmetry in V{sub 3}Si : Nonlocal Effects in a High-{kappa} Superconductor

A study has been perfomed on the superconducting critical current density Jc flowing across low-angle grain boundaries in epitaxial thin films of YBa2Cu3O7−δ. The materials studied were dual grain boundary rings deposited on SrTiO3 and containing 2°, 3°, 5°, and 7° tilt boundaries. The current density in self-field was determined by magnetometric methods at temperatures from 5 K to Tc. We conclude that at the higher temperatures of coated conductor applications, there is limited potential for improving Jc by reducing the grain boundary angle below ∼3°.

Critical current density of YBa2Cu3O7−δ low-angle grain boundaries in self-field

An article with an improved buffer layer architecture includes a substrate having a textured metal surface, and an electrically conductive lanthanum metal oxide epitaxial buffer layer on the surface of the substrate. The article can also include an epitaxial superconducting layer deposited on the epitaxial buffer layer. An epitaxial capping layer can be placed between the epitaxial buffer layer and the superconducting layer. A method for preparing an epitaxial article includes providing a substrate with a metal surface and depositing on the metal surface a lanthanum metal oxide epitaxial buffer layer. The method can further include depositing a superconducting layer on the epitaxial buffer layer, and depositing an epitaxial capping layer between the epitaxial buffer layer and the superconducting layer.

Method of depositing an electrically conductive oxide buffer layer on a textured substrate and articles formed therefrom

The continuous deposition of multi-layer oxide films on moving long-length metallic substrates using pulsed laser deposition (PLD) is reported. Epitaxial YBCO/CeO/sub 2//yttria-stabilized zirconia (YSZ)/CeO/sub 2/ layers were grown on 1 cm/spl times/10 cm rolled-Ni tape substrates. The Ni tape was heated radiatively and translated while oxide layers were continuously deposited. The thickness and composition along the moving direction were uniform to within /spl plusmn/7% and /spl plusmn/5%, respectively. The biaxial texture of the 10 cm Ni tape was transferred through the buffer layers to the topmost YBCO during the continuous deposition. The in-plane and out-of-plane texturing of YBCO and buffer layers were uniform along the scanning direction with variations less than /spl plusmn/10% and /spl plusmn/5%, respectively. The average transport zero field J/sub c/ (77 K) for numerous 10 cm samples was about 200,000 A/cm/sup 2/, with the highest J/sub c/ and I/sub c/ values at 77 K, 0 T of 270,000 A/cm/sup 2/ and 16.7 A respectively. This study demonstrates the feasibility of continuous multilayer, multi-component oxide film deposition on rolled-textured metal with uniform thickness, composition, and crystallographic orientation using PLD.

Long length fabrication of YBCO on rolling assisted biaxially textured substrates (RABiTS) using pulsed laser deposition

Flux flow was studied over an entire temperature range down to T{approx}2% of T{sub c} by using intense pulsed current densities to overcome flux-vortex pinning. The resistivity at high vortex velocities is proportional to B and roughly follows {rho}{approx}{rho}{sub n}B/H{sub c2} , with a prefactor of order unity. Contrary to some speculation, {rho}{sub n} saturates to a finite residual value as T{yields}0 , indicating a metallic ({rho}{yields}finite ) rather than insulating ({rho}{yields}{infinity}) normal state, and the vortex dissipation continues to be conventional as T{yields}0 . (c) 2000 The American Physical Society.

David K. Christen

Papers

Flux Lattice Symmetry in V{sub 3}Si : Nonlocal Effects in a High-{kappa} Superconductor

Critical current density of YBa2Cu3O7−δ low-angle grain boundaries in self-field

Method of depositing an electrically conductive oxide buffer layer on a textured substrate and articles formed therefrom

Long length fabrication of YBCO on rolling assisted biaxially textured substrates (RABiTS) using pulsed laser deposition

Metallic Normal State of Y 1 Ba 2 Cu 3 O 7-δ