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

Technological applications of high temperature superconductors (HTS) require high critical current density, Jc, under operation at high magnetic field strengths This requires effective flux pinning by introducing artificial defects through creative processing In this work, we generated correlated disorder for strong vortex pinning in the YBa2Cu3O7- (YBCO) films by replacing the standard LaMnO3 (LMO) cap buffer layers in ion beam assisted deposited MgO templates with LMO:MgO composite films Such films revealed formation of two phase-separated, but at the same time vertically aligned, self-assembled composite nanostructures that extend throughout the entire thickness of the film Measurements of magnetic-field orientation-dependent Jc of YBCO coatings deposited on these nanostructured cap layers showed correlated c-axis pinning and improved in-field Jc performance compared to those of YBCO films deposited on standard LMO buffers The present results demonstrate feasibility of novel and potentially practical approaches in the pursuit of more efficient, economical, and high performance superconducting devices

Flux pinning enhancements in YBa2Cu3O7-8 superconductors through phase separated, self-assembled LaMnO3-MgO nanocomposite films.

Single-crystal substrates allow the epitaxial deposition of high-T c superconductor (HTS) films that exhibit high critical current densities (J c ). The decrease in J c observed for films deposited on polycrystalline crystals is attributed to misorientation between adjacent grains. To understand better this phenomenon, deposition of HTS films on bicrystal substrates is used to model the effects of a single grain boundary. The surfaces of strontium titanate (001) bicrystals with tilt boundary orientations of 7°, 15° and 24° were analyzed by atomic force microscopy (AFM). Annealing conditions were found to be a key factor for obtaining atomically smooth surfaces. The grain boundary morphologies were analyzed from sections of the AFM images. Images of YBa 2 Cu 3 O 7-x (YBCO) films produced on these substrates by pulsed-laser deposition are shown for regions that include the grain boundaries. Different morphologies of YBCO thin films are presented.

Atomic force microscopy study of bicrystal SrTiO3 substrates and YBCO thin films

A number of fundamental physical phenomena unfold in the mixed state of superconductor, when subjected to enormous current and power-dissipation levels. A sufficiently large current can destroy the superconducting state itself--the so-called pair-breaking effect. At intermediate current densities, below the onset of pair-breaking, one expects to see the free viscous flow of flux vortices. In the present work a pulsed-current technique was used to explore this dissipative regime of high-{Tc} superconductors, verifying both free flux flow and the pair-breaking effect, as predicted by traditional theories. This paper concentrates on the dissipation and Hall behavior in the free flux flow state.

https://digital.library.unt.edu/ark:/67531/metadc674515/m2/1/high_res_d/392826.pdf

Transport behavior in superconductors at extreme dissipation levels

The correlation between defect formation and carrier doping in epitaxial films of the infinite layer compound SrCuO{sub 2} has been studied via molecular beam epitaxy controlled layer-by-layer growth experiments, chemically resolved scanning transmission electron microscopy, scanning tunneling microscopy, x-ray diffraction, electrical transport measurements, and post-growth oxidation-reduction annealing. Based on the complementary information provided by these experiments, it is concluded that the carrier doping is dominated by the formation of an electron-doped, Sr and O deficient matrix under mildly oxidizing growth conditions. Hole-doping is induced by extended defects containing excess Sr atoms and may lead to superconductivity after high-temperature oxidation.

/pdf/defect-formation-and-carrier-doping-in-epitaxial-films-of-5fm1f5faph.pdf

Defect formation and carrier doping in epitaxial films of the infinite layer compound

The effects of varying oxygen content on the superconducting transition temperature Tc, resistivity, and crystal structure of HoBa2Cu3O7−x (0 < x < 1) have been investigated. The variation of Tc wltn 8 is very similar to that previously reported by several authors for YBa2Cu3O7−x [1,2]. In particular a plateau in the Tc vs x curve is observed for values of x between 0.3 and 0.4 witfi the value of Tc,(10 % onset measured magnetically) remaining relatively constant at about 55 K. The width of the superconducting transition, as measured both resistively and magnetically, is substantially narrower for x= 0.35 + 0.05 than for slightly larger or smaller values of x. High resolution powder x-ray diffraction analysis was used to measure the lattice constants of both the orthorhombic and tetragonal phases as x was varied. With increasing x the temperature coefficient of the resistivity changed from positive (metallic) for x ≤0.2 to negative (semiconducting) for x ≥, 0.3. Since the general shape of the Tc vs x curve was unaffected by the complete substitution of magnetic Ho for Y, these results suggest an ordering of the oxygen vacancies in the a-b plane and the existence of an ordered compound with a Tc of 55 K.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400545060

David K. Christen

Papers

Flux pinning enhancements in YBa2Cu3O7-8 superconductors through phase separated, self-assembled LaMnO3-MgO nanocomposite films.

Atomic force microscopy study of bicrystal SrTiO3 substrates and YBCO thin films

Transport behavior in superconductors at extreme dissipation levels

Defect formation and carrier doping in epitaxial films of the infinite layer compound

Oxygen Stoichiometry and Superconductivity in Ho1Ba2Cu3O7−x