R. K. Kotnala

High temperature carrier controlled ferromagnetism in alkali doped ZnO nanorods

Recent efforts in developing spintronic and magneto-optoelectric material for applications have relied on the use of magnetic semiconductors doped with transition metals and have met with limited success. Using a fresh synthesis approach using alkali ions we demonstrate that alkali doped zinc oxide can provide high temperature magnetic semiconductors. We report studies on nanocrystalline powder and pellets of p-type ZnO:Li and ZnO:Na that exhibit ferromagnetism up to 554 K. The ferromagnetic behavior was confirmed from magnetic hysteresis, ferromagnetic resonance, magnetic force microscopy, and explained by a model where substitutional Li+/Na+ in cation site induce local magnetic moments on oxygen atoms. Optimum dopant concentrations enable ferromagnetic exchange interaction leading to high Curie temperature.

Photoluminescence and time-resolved spectroscopy in multiferroic BiFeO3: Effects of electric fields and sample aging

We report photoluminescence and time-resolved spectroscopy in bismuth ferrite excited with a 325u2009nm source. The direct-bandgap recombinations near 2.55u2009eV and indirect-bandgap transitions near 2.67u2009eV are presented as functions of applied in-plane electric field with recombination time in the microsecond regime. The applied field moves some conduction electrons away from the Brillouin zone center, increasing significantly the intensity of indirect-gap recombination. An aging phenomenon is manifest in specimens stored for more than twelve months under ambient conditions. Effect of external magnetic field on the surface phase transition is negligible up to Hu2009=u20090.5u2009T.

Room temperature magnetoelectric coupling enhancement in Mg-substituted polycrystalline GdFeO3

The chirality-dependent magnetoelectric properties of Neel-type domain walls in iron garnet films is observed. The electrically driven magnetic domain wall motion changes the direction to the opposite with the reversal of electric polarity of the probe and with the chirality switching of the domain wall from clockwise to counterclockwise. This proves that the origin of the electric field induced micromagnetic structure transformation is inhomogeneous magnetoelectric interaction.

Investigation of structural, dielectric, and magnetic properties of hard and soft mixed ferrite composites

Barium ferrite (hard ferrite) and manganese nickel zinc ferrite (soft ferrite) were successfully synthesized by citrate gel combustion technique. They were used to form the composites by mixing them properly in required compositions (x)BaFe12O19‐(1−x)Mn0.2Ni0.4Zn0.4Fe2O4 (0u2009≤u2009xu2009≤u20091). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized to investigate the different structural and morphological parameters of pure and mixed ferrite composites. XRD and SEM results confirmed the coexistence of both phases in the composite material. Moreover, it has been observed that the composites were constituted by nanosized particles. Structure of pure soft ferrite was found to be cubic and that of pure hard ferrite was hexagonal. Dielectric constant (e′ and e″) and dielectric loss (tan δ) were analyzed as a function of frequency and composition and the behaviour is explained on the basis of Maxwell-Wagner model. It was observed that the dielectric loss decreases with the increase of hard ferrite co...

Defect induced photoluminescence and ferromagnetic properties of bio-compatible SWCNT/Ni hybrid bundles.

Designing of bio-compatible nanomagnets with multiple functionalities receives immense scientific attention due to their potential applications in bio-labeling, medical diagnosis and treatment. Here we report the synthesis of Nickel (Ni) incorporated single-walled carbon nanotube (SWCNT) hybrid and bio-compatible bundles having interesting magnetic and photoluminescence (PL) properties. The SWCNT exhibits a high-crystallinity and it has an average diameter of ∼1.7 nm. Ni particles of 10-20 nm were incorporated within the SWCNT bundles. These hybrid bundles exhibit PL and it is attributed to the presence of delocalized π electrons and their recombination at the defective sites of SWCNT. Magnetic characterization revealed that the SWCNT/Ni hybrid bundle possesses a high (50 Oe) coercivity compared to bulk Ni and a long range ferromagnetic ordering at room temperature. MTT-assay has been conducted to study the cytotoxicity of these hybrid nanostructures.

Multiferroic Ni0.6Zn0.4Fe2O4-BaTiO3 nanostructures: Magnetoelectric coupling, dielectric, and fluorescence

Multiferroic nanostructures of Ni0.6Zn0.4Fe2O4-BaTiO3 (NZF/BT) have been prepared by two synthesis routes, i.e., chemical combustion (CNZF/BT) and hydrothermal (HNZF/BT). The synthesis of CNZF/BT results in nanoparticles of average size 4u2009nm at 500u2009°C annealing. However, the synthesis of HNZF/BT with hydrolysis temperature 180u2009°C/48 h shows nanowires of diameter 3u2009nm and length >150u2009nm. A growth mechanism in the fabrication of nanoparticles and wires is given. X-ray diffraction is used to identify the crystalline phase. The transmission electron microscopy shows the dimensions of NZF/BT nanostructures. The ferromagnetism, ferroelectricity, and magnetoelectric coupling show more enhancements in HNZF/BT nanowires than CNZF/BT nanoparticles. The observed polarization depends upon shape of nanostructures, tetragonal phase, and epitaxial strain. The tension induced by the surface curvature of nanowire counteracts the near-surface depolarizing effect and meanwhile leads to unusual enhancement of polarization. T...

Magnetodielectric coupling in epitaxial Nd2CoMnO6 thin films with double perovskite structure

Double perovskite Nd2CoMnO6 thin films have been grown epitaxially on SrTiO3 substrates using pulsed laser deposition and their structural, magnetic, and dielectric properties were investigated. Temperature dependent dielectric (e) constant is measured in the frequency range between 1u2009kHz and 1u2009MHz under applied magnetic fields up to 0.5u2009kOe. The dielectric constant exhibits an anomaly near to the Curie temperature which is independent of magnetic field and can be corroborated with a loop opening temperature in zero-field-cooled and field-cooled magnetization measurements. While, a linear relationship between magnetodielectric constant, δeMD, and M2(magnetization) in the paramagnetic to ferromagnetic regime proves magnetodielectric coupling between ferroelectric and ferromagnetic orders in this single-phase perovskite system.

Influence on ferromagnetic resonance signal of perpendicular magnetic anisotropic Co/Pt bilayer thin film due to microwave induced spin-Hall effect

Microwave induced spin-Hall effect has been investigated in sputtered Co/Pt bilayer thin film for its application in the field of spintronics. Measurements were carried out in the frequency range from 0.1 to 10.0u2009GHz at 10 mW power level with a sweeping magnetic field up to 0.2u2009T. The maximum dc voltage measured was 5.78u2009μV at 0.1u2009GHz in perpendicular applied magnetic field on the bilayer film. The direction of magnetic field, frequency, and power level influence the ferromagnetic resonance signal. Measurements confirm lower effective spin pumping due to a weak perpendicular anisotropy in the film.

Structural, magnetic and magnetoelectric properties of In-Co codoped BiFeO3 nanoparticles

Bi1−xInxFe1−yCoyO3 (0≤x≤0.1, 0≤y≤0.05) nanoparticles have been successively synthesized by low temperature ethylene glycol based solution combustion method. The XRD patterns reveal the single phase rhombohedral destorted perovskite structure for pure BiFeO3 (BFO) sample. The co-doping of In and Co at A-B-sites of BFO results in structural distortation, improves the particles surface morphology and reduces the average particle size from 52nm for BFO to around 12nm for BIFCO. All samples show ferromagnetic behaviour at room temperature which further improves upon codoping with maximum saturation magnetization of 6.30emu/gm. The codoping results in enhancement of magnetoelectric (ME) coefficient (α∼4.92mV/cmOe) at room temperature.

2.8 Magnetic Materials

Growing world population and rapid economic growth are making additional demands on global energy requirement. Carbon dioxide emission has peaked at 32.2 Gt/year and is expected to be at 43 Gt/year by 2016. Such huge emission of carbon dioxide has adverse effect on global climate change. Greenhouse gas emissions of 30% is being generated by the electricity sector followed by 26% from the transportation sector. A huge share of magnetic materials are being used in automotives, transformers, and the power sector, which demands special attention to a new class of magnetic materials. There are two possibilities to reduce deadly air pollution: one is to make energy efficient machines and the other is to explore new green energy sources to provide secure energy supplies. Energy generation focus has also shifted more toward alternative natural resources, such as solar energy, wind energy, bio energy, ocean energy, etc. Use of renewable alternative sources can save money, protect the environment, and ensure clean energy for future generations. Solar energy would yield a never-ending energy supply but the difficulty associated with it is to convert solar energy in an efficient and cost-effective way. The improvement in efficiency of wind turbines demands an increase in height and size that would result in mechanical stresses on the gearbox, blades, and tower.