Semiconducting ferromagnetic spinels

The family of spinel compounds of which CdCr2Se4is the prototype has been found to be simultaneously ferromagnetic and semiconducting. Numerous experiments reveal, moreover, that the electronic states in these materials are strongly coupled to the degree of magnetic order. The interaction between the electronic transport and the magnetic state offers the exciting possibility of constructing electronic devices having magnetic control. This paper presents a review of literature on the magnetic, crystallographic, electrical transport, and optical properties of the chromium chalcogenide spinels as they relate to device applications. The prospects of realizing novel devices utilizing these properties are examined.     

Physical Chemistry of the Magnetic Semiconductor CdCr2Se4

The magnetic semiconductor CdCr₂Se₄ is the best known chalcogenide spinel. This article reviews the available information about this compound: phase diagrams, homogeneity region, vapor pressure, crystal growth, doping of crystals, and defect chemistry. A large body of data in this field have been obtained by the authors.     

New A2+Mo4+O3 oxides with defect spinel structure

New A²⁺Mo⁴⁺O₃ oxides for A = Mn, Co and Zn crystallizing in a defect spinel structure have been prepared by hydrogen-reduction of the corresponding AMoO₄ oxides. X-ray powder diffraction intensity analysis of the zinc compound indicates that the cation distribution is (Zn)_t[Zn_1/3Mo_4/3□_1/3]_oO₄. The defect spinels are metastable decomposing to a mixture of A₂Mo₃O₈ and AO at high temperatures. Electrical and magnetic properties of the spinel phases are reported.     

Dilute Doping,Defects, and Ferromagnetism in Metal Oxide Systems

Over the past decade intensive research efforts have been carried out by researchers around the globe on exploring the effects of dilute doping of magnetic impurities on the physical properties of functional non‐magnetic metal oxides such as TiO₂ and ZnO. This effort is aimed at inducing spin functionality (magnetism, spin polarization) and thereby novel magneto‐transport and magneto‐optic effects in such oxides. After an early excitement and in spite of some very promising results reported in the literature, this field of diluted magnetic semiconducting oxides (DMSO) has continued to be dogged by concerns regarding uniformity of dopant incorporation, the possibilities of secondary ferromagnetic phases, and contamination issues. The rather sensitive dependence of magnetism of the DMSO systems on growth methods and conditions has led to interesting questions regarding the specific role played by defects in the attendant phenomena. Indeed, it has also led to the rapid re‐emergence of the field of defect ferromagnetism. Many theoretical studies have contributed to the analysis of diverse experimental observations in this field and in some cases to the predictions of new systems and scenarios. In this review an attempt is made to capture the scope and spirit of this effort highlighting the successes, concerns, and questions.     

Properties of Mg(Fe0.8Ga0.2)2O4 + δ ceramics and films

The magnetic and semiconducting properties of homogeneous ceramic samples of the diluted magnetic semiconductor Mg(Fe_0.8Ga_0.2)₂O_{4 + δ} have been studied as functions of temperature and magnetic field. The results suggest that spinel phases with this combination of cations are potential candidates for spintronic applications. Using ion-beam sputtering of a ceramic Mg(Fe_0.8Ga_0.2)₂O_{4 + δ} target, we obtained chemically homogeneous films similar in properties to the source material.     

Electrocatalytic Mechanism of Defect in Spinels for Water and Organics Oxidation

Spinels display promising electrocatalytic ability for oxygen evolution reaction (OER) and organics oxidation reaction because of flexible structure, tunable component, and multifold valence. Unfortunately, limited exposure of active sites, poor electronic conductivity, and low intrinsic ability make the electrocatalytic performance of spinels unsatisfactory. Defect engineering is an effective method to enhance the intrinsic ability of electrocatalysts. Herein, the recent advances in defect spinels for OER and organics electrooxidation are reviewed. The defect types that exist in spinels are first introduced. Then the catalytic mechanism and dynamic evolution of defect spinels during the electrochemical process are summarized in detail. Finally, the challenges of defect spinel electrocatalysts are brought up. This review aims to deepen the understanding about the role and evolution of defects in spinel for electrochemical water/organics oxidation and provide a significant reference for the design of efficient defect spinel electrocatalysts.     

Seed‐Induced Growth of Flower‐Like Au–Ni–ZnO Metal–Semiconductor Hybrid Nanocrystals for Photocatalytic Applications

The combination of metal and semiconductor components in nanoscale to form a hybrid nanocrystal provides an important approach for achieving advanced functional materials with special optical, magnetic and photocatalytic functionalities. Here, a facile solution method is reported for the synthesis of Au–Ni–ZnO metal–semiconductor hybrid nanocrystals with a flower‐like morphology and multifunctional properties. This synthetic strategy uses noble and magnetic metal Au@Ni nanocrystal seeds formed in situ to induce the heteroepitaxial growth of semiconducting ZnO nanopyramids onto the surface of metal cores. Evidence of epitaxial growth of ZnO{0001} facets on Ni {111} facets is observed on the heterojunction, even though there is a large lattice mismatch between the semiconducting and magnetic components. Adjustment of the amount of Au and Ni precursors can control the size and composition of the metal core, and consequently modify the surface plasmon resonance (SPR) and magnetic properties. Room‐temperature superparamagnetic properties can be achieved by tuning the size of Ni core. The as‐prepared Au–Ni–ZnO nanocrystals are strongly photocatalytic and can be separated and re‐cycled by virtue of their magnetic properties. The simultaneous combination of plasmonic, semiconducting and magnetic components within a single hybrid nanocrystal furnishes it multifunctionalities that may find wide potential applications.     

Morphology and structure of magnetic spheres based on hematite or spinel and glass

Magnetic spheres of various origins, isolated from fly ashes of the main Russian thermal coals, have been studied by electron microscopy, optical microscopy, X-ray diffraction, and Mössbauer spectroscopy. The magnetic spheres differ in morphology and have the form of multilevel core-shell nanocomposites, with a hematite or spinel Fe-rich core and a Fe-containing silicate glass shell. We have derived structural formulas of the spinels, which are consistent with all our experimental data. The most promising application fields of such magnetic spheres, after proper selection and modification, are catalysis and composites.     

Novel synthesis of high surface area MgAl2O4 spinel as catalyst support

A modified sol–gel route, by combining gelation and coprecipitation processes, was developed for the synthesis of high surface area MgAl₂O₄ spinel precursors. The obtained precursors were then calcined in flowing air at temperatures ranging from 500 to 900 °C. The formation of new phases upon calcinations was investigated using X-ray diffraction, thermal gravimetric analysis, and Fourier transform infrared spectroscopy (FTIR). Single-phase spinel powder with uniform pore size distribution was formed at temperatures as low as 600 °C. It was found that the thermal stability of the as-synthesized spinels is higher than that reported by other preparation methods. After calcinations at 800 and 950 °C for 8 h, the specific surface area reaches a level of 182 and 136 m²·g⁻¹, respectively. And the degree of crystallinity is higher than other preparation methods as illustrated by samples calcined at 800 °C. The amount of PVA added significantly affects the surface area of the samples. With increasing the ratio of M/PVA, the surface area of the resulting spinels increased accordingly.     

Probing exotic magnetic phases in ferrites with neutrons

The decades of the sixties and early seventies saw the neutron diffraction technique playing a major role in the elucidation of magnetic structures in a variety of ferrites in our laboratory. In 1979, Villain in his seminal paper on insulating spin glasses argued that the spinel structure affords topological frustration which can give rise to a variety of perturbed magnetic ordering, depending upon magnetic dilution in the ferrite. The neutron is a unique probe to explore the nature of spatial correlations of magnetic moments in such systems. This paper describes the exciting work carried out at Trombay recently in mixed ferrites which have led to the discovery of exotic magnetic phases like the uniaxial random ferrimagnetic phase and the canted random ferrimagnetic phase involving the coexistence of long-range magnetic order and disorder.     

The role of metal nanoparticles and nanonetworks in alloy degradation

Oxide scale, which is essential to protect structural alloys from high-temperature degradation such as oxidation, carburization and metal dusting, is usually considered to consist simply of oxide phases. Here, we report on a nanobeam X-ray and magnetic force microscopy investigation that reveals that the oxide scale actually consists of a mixture of oxide materials and metal nanoparticles. The metal nanoparticles self-assemble into nanonetworks, forming continuous channels for carbon transport through the oxide scales. To avoid the formation of these metallic particles in the oxide scale, alloys must develop a scale without spinel phase. We have designed a novel alloy that has been tested in a high-carbon-activity environment. Our results show that the incubation time for carbon transport through the oxide scale of the new alloy is more than an order of magnitude longer compared with commercial alloys with similar chromium content.     

On the formation of solid solution in Co3−xMnxO4 system

The formation of solid solution in the Co_3−xMn_xO₄ system in atmospheres of oxygen, air and argon was examined at a constant temperature of 1000 °C. In oxygen, a small amount of the NaCl-type compound was found to co-exist with the cubic spinel in the composition rangex≤0.1. A single phase of the cubic spinel was found in the range 0.1 to 1.3 and the tetragonal spinel above 1.9. In the rangex=1.3 to 1.9 where the cubic and tetragonal spinels co-exist they both have very broadened diffraction line profiles. In air, the identified phases and the changes in their lattice constants with composition were very similar to those in oxygen, except that the NaCl-type compound and the cubic spinel co-existed over a larger range. In argon, the cubic spinel was not observed over any of the composition range and the NaCI-type compound and the tetragonal spinel co-existed in the wide range of 1.1 to 2.3. The experimental results are discussed with regard to the cation distribution in the spinel and also to the relative stability of Co³⁺- and Mn³⁺-ions under the low partial pressure of oxygen.     

CuCr<Subscript>2</Subscript>S<Subscript>4</Subscript>-Based Quaternary Cation-Substituted Magnetic Phases

CuCr₂S₄-based solid solutions containing solute atoms on the Cu or Cr site were studied with the aim of changing the band structure of the spinel to semiconducting, while maintaining a high magnetic ordering temperature. Cu_{1 ? x}M_xCr₂S₄ and CuCr_{2 ? x}M_xS₄ (M = Al, Ga, In, Fe, Co, As) quaternary phases were synthesized, and CuCr₂S₄, Cu_0.3Co_0.7Cr₂S₄, and Cu_0.5Fe_0.5Cr₂S₄ single crystals were prepared using high-temperature solution growth and chemical vapor transport. The magnetic materials were characterized by a variety of techniques.     

Electronic and Magnetic Properties of SnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Spinel Ferrites

In this work, the electronic and magnetic properties of SnFe₂O₄ spinel ferrite with various cation distributions (mixed, normal and inverse spinel phases) were studied. The calculations were performed by Korringa–Kohn–Rostoker (KKR)-coherent potential approximation (CPA) method with generalized gradient (GGA) approximation for the exchange and correlation functional. Our spin-polarized calculations give a half-metallic character for SnFe₂O₄ inverse spinel phase and near half-metallic character for SnFe₂O₄ mixed spinel phase and metal character for SnFe₂O₄ normal spinel phase. These results, which contribute to understanding the effect of the cation distribution in SnFe₂O₄ ferrite with spinel structure in the existence of gap states occupied by majority and minority spin electrons, the spin magnetic moments, the magnetization and the half-metallic behaviour.     

Improvement of Thermal Characteristics of Refractory Castable by Addition of Gel-Route Spinel Nanoparticles

Selected physical and thermal properties of conventional spinel-alumina castables were compared with those of castables produced using spinel fines prepared from a prepared gel. Limited numbers of hydroxyl groups created around the spinel precursor helped to improve thermal shock resistance. Micrographic examination confirmed that retained nanodimensional spinels firmly connected the hibonite and corundum grains in the castable, developing multiple interfaces after densification. Castable-containing spinel with excess alumina powder was found to have the best combination of bulk density, apparent porosity, and hot modulus of rupture. The reactive magnesia fine used to prepare in situ spinel-bonded castable was not found to give satisfactory results, owing to progressive disintegration of brucite-type compounds and abnormally grown spinels. Thermal characteristics of castables were assessed by differential thermal analysis (DTA), measurement of porosity, and percent linear change (PLC). Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS) analysis of the gel-derived spinel and castable confirmed their differences, particularly with commercial spinel having comparable chemistry but lacking nanocrystalline structure.     

Spinel structure and liquidus temperature relationships in nuclear waste glass

The liquidus temperature of spinel (AB²O⁴) crystals frequently limits the waste loading of vitrified nuclear waste. In other studies, glass structure-spinel liquidus temperature relationships were discovered for the non-spinel forming cations in glass, but no such relationship was identified for the spinel-forming metal-ions. In this paper, coefficients from an empirical first order mixture model describing spinel liquidus temperature were correlated with the octahedral site preference energy (OSPE) of the spinel-forming metal ions. The OSPE was calculated using approximations of glass and spinel structure made by extrapolating from data on similar glasses and spinels. A literature review indicated that Cr(III), Ni(II), and Fe(II) are most likely to be in the octahedral sites in spinel, Mn(II) in the tetrahedral site, and that Fe(III) will be common in both sites. These assignments were used as starting assumptions to calculate the OPSE for each metal ion. A strong correlation between the OSPE and the liquidus temperature coefficients of spinel-forming metal ions was observed. Further correlations determined that the Crystal Field Stabilization Energy was the most important portion of the OSPE for these spinels. These results indicate that the thermodynamic properties of spinel could be predicted based on assumptions of spinel structure, underpinned by fundamental properties, when scaled empirically.     

Application of thermomagnetometry to the study of siderite

Thermogravimetry in the presence of an external magnetic field is used to detect the presence or absence of magnetic intermediates and products during the thermal decomposition of several siderite minerals (FeCO₃). The Curie temperature of the product phase indicates that several of the major impurities are incorporated into the spinel product in nitrogen. In oxygen no spinel intermediate phases are detected.     

Magnetic behaviour and microstructural properties of amorphous and crystallized Y3−x GdxFe5O12 prepared by the amorphous citrate process (x=0, 0.5 and 3)

The systems Y_3–x GdxFe₅O₁₂ withx=0, 0.5 and 3 have been synthesized by the amorphous citrate process. Their structural and magnetic properties have been studied for various heat-treatment schedules. The systems withx=0, 0.5 and 3 are amorphous when heat treated up to 650, 600 and 500° C, respectively, and the results of amorphous, as well as crystallized, samples are discussed. The volume fraction of the garnet phase is found to increase with higher heat-treatment temperature. Besides the garnet phase, two other phases appear to grow, the volume fraction of the spinel phase increasing with increase in gadolinium concentration. Microstructral properties have been further correlated with the magnetic properties.     

Synthesis and physical properties of chalcogenide chromium spinel films

This work is a report of the present authors' experimental results on the synthesis and investigation of thin films of the chalcogenide chromium spinels CdCr₂Se₄, CuCr₂Se₄ and CuCr₂Te₄. Films were made by the method of vacuum condensation from separate sources. As substrates glass, mica, MgO, CaF₂, Y₂O₃ and NaCl were used. With the last of these vacuum condensation results in the formation of a single-crystal film CdCr₂Se₄, the physical characteristics of which are, however, worse than those of polycrystalline films.

The measured temperature dependences of magnetization, ferromagnetic resonance and electron paramagnetic resonance linewidths, electric resistance and magnetic conductivity are given. We have realized and investigated stripe domain structures in CdCr₂Se₄. Photoelectric, photomagnetic and magneto-optical properties are studied. The magnetic and electric characteristics of stoichiometric polycrystalline films are close to those of bulk materials.

The effects of diffuse substitution with indium, gallium (donors) and silver, copper (acceptors) and of non-stoichiometry on the magnetic and electric properties of CdCr₂Se₄ semiconductor films are investigated.