Exchange bias and Verwey transition in Fe5C2/Fe3O4 core/shell nanoparticles.

This report presents new findings of exchange bias and related structural and magnetic properties in iron carbide/magnetite (Fe5C2/Fe3O4) core/shell nanoparticles. The exchange bias emerges from an energetic landscape, namely a first-order phase transition-the Verwey transition at 125 K, during which the Fe3O4 shell changes from the cubic to monoclinic structure. The phase transition leads to the exchange bias because it results in abrupt changes in magnetocrystalline anisotropy and exchange coupling. Another unique phenomenon identified in this composite system is enhanced magnetic coercivity due to the uniaxial anisotropy of the monoclinic phase. An analysis of the correlations between the observed phenomena is given based on the temperature dependence of the coercivity, the exchange bias field values, and the Verwey transition temperature.

Magnetic-field-induced self-assembly of FeCo/CoFe2O4 core/shell nanoparticles with tunable collective magnetic properties.

Self-assembly of nanoparticles into ordered patterns is a novel approach to build up new consolidated materials with desired collective physical properties. Herein, nanoparticle assemblies of composition-modulated bimagnetic nanoparticles have been produced via slow evaporation of their colloidal suspension in the absence or presence of magnetic fields. The assemblies obtained in the presence of the magnetic fields exhibit oriented nanoparticle chains in face-centered cubic superlattice structures, compared with the hexagonal closed-packed superlattice obtained without the magnetic field. The oriented structure has an alignment of the easy magnetization axis along the chains. This alignment leads to enhanced intra-superlattice interactions. As a result, the field-induced assembly displays collective magnetic properties with significantly enhanced magnetic anisotropy, remanent magnetization and coercivity. It is also found that the bimagnetic FeCo/CoFe2O4 core/shell nanostructure enhances the intra-particle interaction and thus is beneficial for the growth of oriented assembly of nanoparticles. Furthermore, the collective magnetic behavior is evidenced by the observation of a superferromagnetic-like magnetization relaxation in the ac-susceptibility curves.

Novel Molten Salt Assisted Autocombustion Method for the Synthesis of Aluminum-Doped SrFe12-xAlxO19 Hexaferrite Nanoparticles.

The study presents a novel molten salt assisted autocombustion synthesis of SrFe12-xAlxO19 particles. The extrinsic magnetic properties such as coercivity and the remanence of sintered M-type ferrites are highly dependent on the microstructure viz. morphology and size, of the ferrite particles. The control of the microstructures of ferrite particles is usually achieved via control nucleation and grain growth process. In this study, NaCl salt was used to control the crystal shape and size of Al3+ doped SrFe12-xAlxO19 particles. The presented novel method couples advantage of deriving homogenized particles via auctocombustion first and later sintering in the presence of NaCl salt. Highly dispersed, homogeneous, and hexagonal shaped SrFe12-xAlxO19 ferrite particles were achieved with this method. The particles derived via the molten salt assisted method presented a high coercivity and squareness ratio (>0.5) as compared to that obtained via autocombustion method only.

Influence of Urea on the Synthesis of NiCo2O4 Nanostructure: Morphological and Electrochemical Studies.

The widespread use of miniature electronic devices calls for energy-dense storage strategies. The supercapacitor-based energy storage devices with high areal capacitance are desired energy storage alternative. It is still a challenge to fabricate supercapacitor-based energy devices with consistent performance. The porous metal oxides with large areal capacitance are desired materials for electrode, but there exists a limited understanding of the influence of synthesis parameters on microstructural properties, which largely govern their electrochemical performance. In the present work, hierarchal spinel nickel cobaltite (NiCo2O4) nanostructures were synthesized in the presence of the varying amount of hydrolyzing agent via a simple hydrothermal method coupled with a simple post-annealing process. This work focuses on understanding the influence of hydrolyzing agent in controlling the microstructure and hence ensuing electrochemical properties of the NiCo2O4 based electrode. Based on the urea hydrolyzing content, the as synthesized NiCo2O4 nanostructure varied from the rod, plate to nanoflower. The mesoporous nanostructures, with urea content 1.49 gm, exhibit a sizeable BJH surface area (79.2 m² g-1) and high mesopore volume (0.140 cm³ g-1). Remarkably, the NiCo2O4 nanoflower shows high specific capacitance of 3143.451 F/g at 2 mV/s scan rate, 1264.5 F/g at 1 A/g current density, energy density of 56 Wh/kg and power density of 8,400 W/kg in 3 M KOH electrolyte. The capacitance loss after 5000 cycles is 48% at the current density of 10 A/g, indicating their excellent cycling stability. The impressive electrocatalytic activity is largely ascribed to the high intrinsic electronic conductivity, superior mesoporous nanostructures and rich surface Ni active species of the NiCo2O4 materials, which can largely boost the interfacial electroactive sites and charge transfer rates indicating promising applications as electrodes in future supercapacitors.

Facile Synthesis of Bio-Templated Tubular Co3O4 Microstructure and Its Electrochemical Performance in Aqueous Electrolytes.

Template-assisted facile synthesis of tubular Co3O4 microstructures and its electrochemical performance was studied to understand its use as a potential electrode material for supercapacitors. Tubular porous Co3O4 microstructures were synthesized using cotton fibers as bio-template. The as-obtained templated Co3O4 structure inherits the morphology and microstructure of cotton fiber. The electrochemical performance of the electrode made up of tubular Co3O4 structure was evaluated in 3 M KOH, NaOH, and LiOH aqueous electrolytes. The large-surface-area of tubular Co3O4 microstructure has a noticeable pseudocapacitive performance with a capacitance of 401 F/g at 1 A/g and 828 F/g at 2 mV/s, a Coulombic efficiency averaging ~100%, and excellent cycling stability with capacitance retention of about 80% after 5,000 cycles. Overall, the tubular Co3O4 microstructure displayed superior electrochemical performance in 3 M KOH electrolyte with peak power density reaching 5,500 W/kg and energy density exceeding 22 Wh/kg. The superior performance of tubular Co3O4 microstructure electrode is attributed to its high surface area and adequate pore volume distribution, which allows effective redox reaction and diffusion of hydrated ions. The facile synthesis method can be adapted for preparing various metal oxide microstructures for possible applications in catalysis, electrochemical, sensors, and fuel cells applications.