Arif Mumtaz

Smelting in the age of nano: iron aerogels

Smelting of interpenetrating networks of resorcinol-formaldehyde (RF) and iron oxide (FeOx) aerogels yields porous ferromagnetic and superparamagnetic materials in monolithic form with compositions closely resembling that of pig iron.

Electrical transport and optical studies of ferromagnetic cobalt doped ZnO nanoparticles exhibiting a metal?insulator transition

The observed correlation of oxygen vacancies and room temperature ferromagnetic ordering in Co doped ZnO1−δ nanoparticles reported earlier (Naeem et al 2006 Nanotechnology 17 2675–80) has been further explored by transport and optical measurements. In these particles room temperature ferromagnetic ordering had been observed to occur only after annealing in forming gas. In the current work the optical properties have been studied by diffuse-reflection spectroscopy in the ultraviolet–visible (UV–vis) region and the band gap of the Co doped compositions has been found to decrease with Co addition. Reflection minima are observed at the energies characteristic of Co2+ d–d (tethrahedral symmetry) crystal field transitions, further establishing the presence of Co in substitutional sites. Electrical transport measurements on pelletized samples of the nanoparticles show that the effect of a forming gas is to strongly decrease the resistivity with increasing Co concentration. For the air annealed and non-ferromagnetic samples the variation in the resistivity as a function of Co content are opposite to those observed in the particles prepared in forming gas. The ferromagnetic samples exhibit an apparent change from insulator to metal with increasing temperatures for T>380 K and this change becomes more pronounced with increasing Co content. The magnetic and resistive behaviors are correlated by considering the model by Calderon and Sarma (2007 Ann. Phys. at press) where the ferromagnetism changes from being mediated by polarons in the low temperature insulating region to being mediated by the carriers released from the weakly bound states in the higher temperature metallic region.

Magnetic response of core-shell cobalt ferrite nanoparticles at low temperature

Cobalt ferrite nanoparticles (size: 26±4nm) have been synthesized by coprecipitation route. The coercivity of nanoparticles follows a simple model of thermal activation of particle moments over the anisotropy barrier in the temperature range of 30–300K in accordance with Kneller’s law; however, at low temperatures (<30K), the coercivity shows some deviation from this law. The saturation magnetization follows the modified Bloch’s law in the temperature range of 10–300K. Exchange bias (Hex) studies of the samples show that Hex increases with decreasing temperature of the sample. A strong increase in the Hex values is found below 30K for the low applied field (±20kOe), while a smaller increase is found for the high applied field (±90kOe). The slow increase in the exchange bias at high applied field has been attributed to the high field effects on the surface (shell) spins. These shell spins align along the field direction that weakens the core-shell interface interactions leading to the reduction in the exch...

Effect of Crystallographic Texture on Magnetic Characteristics of Cobalt Nanowires

Cobalt nanowires with controlled diameters have been synthesized using electrochemical deposition in etched ion-track polycarbonate membranes. Structural characterization of these nanowires with diameter 70, 90, 120 nm and length 30 μm was performed by scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray diffraction techniques. The as-prepared wires show uniform diameter along the whole length and X-ray diffraction analysis reveals that [002] texture of these wires become more pronounced as diameter is reduced. Magnetic characterization of the nanowires shows a clear difference of squareness and coercivity between parallel and perpendicular orientations of the wires with respect to the applied field direction. In case of parallel applied field, the coercivity has been found to be decreasing with increasing diameter of the wires while in perpendicular case; the coercivity observes lower values for larger diameter. The results are explained by taking into account the magnetocrystalline and shape anisotropies with respect to the applied field and domain transformation mechanism when single domain limit is surpassed.

A new (Ba, Ca) (Ti, Zr)O3 based multiferroic composite with large magnetoelectric effect

The lead-free ferroelectric 0.5Ba(Zr0.2Ti0.8)O3 − 0.5(Ba0.7Ca0.3)TiO3 (BCZT) is a promising component for multifunctional multiferroics due to its excellent room temperature piezoelectric properties. Having a composition close to the polymorphic phase boundary between the orthorhombic and tetragonal phases, it deserves a case study for analysis of its potential for modern electronics applications. To obtain magnetoelectric coupling, the piezoelectric phase needs to be combined with a suitable magnetostrictive phase. In the current article, we report on the synthesis, dielectric, magnetic, and magnetoelectric characterization of a new magnetoelectric multiferroic composite consisting of BCZT as a piezoelectric phase and CoFe2O4 (CFO) as the magnetostrictive phase. We found that this material is multiferroic at room temperature and manifests a magnetoelectric effect larger than that of BaTiO3 −CoFe2O4 bulk composites with similar content of the ferrite phase.

Magnetic control of relaxor features in BaZr0.5Ti0.5O3 and CoFe2O4 composite

We report the effect of magnetic field on the dielectric response in a relaxor ferroelectric and ferromagnetic composite (BaZr0.5Ti0.5O3)0.65-(CoFe2O4)0.35. Relaxor characteristics such as dielectric peak temperature and activation energy show a dependence on applied magnetic fields. This is explained in terms of increasing magnetic field induced frustration of the polar nanoregions comprising the relaxor. The results are also consistent with the mean field formalism of dipolar glasses. It is found that the variation of the spin glass order parameter q(T) is consistent with increased frustration and earlier blocking of nanopolar regions with increasing magnetic field.

Non-linear susceptibility studies of La0.85Ca0.15Mn0.95Fe0.05O3

Linear and non-linear dynamic properties of a re-entrant ferromagnetic polycrystalline bulk La0.85Ca0.15Mn0.95Fe0.05O3 sample are studied using AC susceptibility. This sample composition shows a transition from paramagnetic (PM) to ferromagnetic (FM) at around 174 K and to spin glass (SG) at ~100 K. The dynamic behaviour is investigated in the intermediate temperature range (Tg<T<Tc). A prominent non-linear susceptibility is observed in this range, signifying the decomposition of the FM network into clusters by a random magnetic field which is expected to be generated by PM spins in the FM region. The results are supported by a study of the AC frequency and DC field effect on the non-linear susceptibility.

Order parameter and scaling behavior in BaZrxTi1−xO3 (0.3 < x < 0.6) relaxor ferroelectrics

We report the relaxor behavior of the zirconium doped barium titanate BaZrxTi1−xO3 solid solutions and discuss the temperature, frequency, and concentration dependence in terms of correlations among the polar nanoregions. The relaxor behavior is analyzed within the mean field theory by estimating the Edward-Anderson order parameter qEA. Additionally, we find that qEA calculated for the different concentrations obeys a scaling behavior qEA=1−(T/Tm)n, where Tm are the respective dielectric maxima temperatures and n = 2.0 ± 0.1. The frequency dependence of the qEA also shows results consistent with the above mentioned picture.

Magnetic study of Cu1−xMnxO (0≤x≤0.08) nanoparticles

Cu1−xMnxO (0 ≤ x ≤ 0.08) nanoparticles were synthesized by a coprecipitation method. Powder x ray analysis reveals that samples show a pure CuO phase for x ≤ 0.06 and the appearance of a secondary phase CuMn2O4 for x > 0.06. The unit cell volume expands with increasing Mn concentration up to x = 0.06 and remains constant on further increase in Mn concentration. The undoped sample shows the behavior of pure CuO with antiferromagnetic transitions at TN1 = 228 K and TN2 = 214 K. Inclusion of Mn into the Cu sublattice turned antiferromagnetic CuO into a ferromagnet. The origin of ferromagnetism is analyzed on the basis of different possible interactions. It is believed that the indirect coupling between Mn ions mediated by O and Cu ions helps to stabilize the ferromagnetism.

Frustrated metastable behavior of magnetic and transport properties in charge ordered La1−xCaxMnO3+δ manganites

We have studied the effect of metastable, irreversibility induced by repeated thermal cycles on the electric transport and magnetization of polycrystalline samples of La1-xCaxMnO3 (0.48 ≤ x ≤ 0.55) close to charge ordering. With time and thermal cycling (T < 300 K) there is an irreversible transformation of the low-temperature phase from a partially ferromagnetic and metallic to one that is less ferromagnetic and highly resistive for the composition close to charge ordering (x = 0.50 and 0.52). Irrespective of the actual ground state of the compound, the effect of thermal cycling is toward an increase of the amount of the insulating phase. We have observed the magnetic relaxation in the metastable state and also the revival of the metastable state (in a relaxed sample) due to high temperature thermal treatment. We observed changes in the resistivity and magnetization as the revived metastable state is cycled. The time changes in the magnetization are logarithmic in general and activation energies are cons...