K. M. Reddy

Selective Toxicity of Zinc Oxide Nanoparticles to Prokaryotic and Eukaryotic Systems

We report on the toxicity of ZnO nanoparticles (NPs) to gram-negative and gram-positive bacterial systems, Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), and primary human immune cells. ZnO NP (~13 nm) showed complete inhibition of E. coli growth at concentrations 3.4 mM, whereas growth of S. aureus was completely inhibited for 1 mM. Parallel experiments using flow cytometry based assays clearly demonstrated that growth inhibitory properties of ZnO NP were accompanied by a corresponding loss of cell viability. Identical ZnO NP had minimal effects on primary human T cell viability at concentrations toxic to both gram-negative and gram-positive bacteria. Collectively, these experiments demonstrate selectivity in the toxic nature of ZnO NP to different bacterial systems and human T lymphocytes. Developing selective toxicity to biological systems and controlling it by NP design could lead to biomedical and antibacterial applications.

Magnetic Gas Sensing Using a Dilute Magnetic Semiconductor

The authors report on a magnetic gas sensing methodology to detect hydrogen using the ferromagnetic properties of a nanoscale dilute magnetic semiconductor Sn0.95Fe0.05O2. This work demonstrates the systematic variation of saturation magnetization, coercivity, and remanence of Sn0.95Fe0.05O2 with the hydrogen gas flow rate, thus providing clear experimental evidence of the concept of magnetic gas sensing (using the magnetic property of a material as a gas sensing parameter). Based on the results of using hydrogen as an example for reducing gases, it is believed that any reducing gas capable of changing the oxygen stoichiometry of Sn0.95Fe0.05O2 can be detected using this method. Furthermore, this method presents an alternative gas sensing technology without the use of the electrical contacts.

Structure–magnetic property relationship in transition metal (M=V,Cr,Mn,Fe,Co,Ni) doped SnO2 nanoparticles

This work reports the results of an extensive search for ferromagnetism in SnO2 doped with a wide range of transition metal cations (M=V, Cr, Mn, Fe, Co, and Ni). By varying the dopant concentration in the 0–12% range, signatures of ferromagnetic behavior in varying degrees were observed with most dopants. The room temperature magnetic moments per dopant ion were low in all the systems and Co (0.13μB∕ion), Fe (0.014μB∕ion), and Cr (0.06μB∕ion) showed relatively the strongest ferromagnetic behavior. In these systems, the observed ferromagnetism initially increased reaching a maximum in the 1–12% range and then gradually weakened and eventually disappeared at higher concentration. The limiting dopant concentration xL at which ferromagnetic behavior reaches a maximum varies with dopant type and has a strong relation to structural changes revealed from detailed x-ray diffraction (XRD) analysis. The XRD data indicated that the lattice volume for every Sn1−xMxO2 system decreased with increasing x in the 0⩽xL ra...

Development and Processing Temperature Dependence of Ferromagnetism in Zn0.98Co0.02O

We report the development of room-temperature ferromagnetism (FM), with coercivity Hc=2000Oe and saturation magnetization Ms∼0.01emu∕g, in chemically synthesized powders of Zn0.98Co0.02O processed at 150 °C, and paramagnetism with antiferromagnetic interactions between the Co2+ spins (S=3∕2) in samples processed at higher temperatures 200⩽TP⩽900°C. X-ray diffraction data show a decrease in the lattice parameters a and c with TP, indicating a progressive incorporation of 0.58A sized tetrahedral Co2+ at the substitutional sites of 0.60 A sized Zn2+. Diffuse reflectance spectra show three well defined absorption edges at 660, 615, and 568 nm due to the d‐d crystal field transitions A24(F)→E2(G),A24(F)→T14(P), and A24(F)→T12(G) of high spin (S=3∕2)Co2+ in a tetrahedral crystal field, whose intensities increase with processing temperature. X-ray photoelectron spectroscopy shows that the doped Co2+ ions in the 150 °C processed samples are located mostly on the surface of the particles and they disperse into the...

Magnetic resonance studies of Co2+ ions in nanoparticles of SnO2 processed at different temperatures

Cobalt doping (⩽1%) produces ferromagnetism at room temperature in semiconducting SnO2, presumably due to oxygen vacancies and/or changes in carrier concentration. Electron paramagnetic resonance (EPR) is a sensitive technique to investigate the Co ionic states and their local environments and/or interactions. This paper reports EPR studies of Co2+ ions doped in chemically synthesized nanoparticles of SnO2 carried out at 5K. EPR spectra were recorded from 600°C prepared SnO2 with Co concentrations of 0.5%, 1%, 3%, 5%, 8%, and 12% and from 1% Co-doped SnO2 prepared at temperatures of 150, 250, 350, 450, 600, and 830°C. Each EPR spectrum in samples with cobalt doping can be simulated as an overlap of spectra due to two broad ferromagnetic resonance lines and those due to interstitially and substitutionally incorporated Co2+ ions with effective spin S=1∕2 characterized by their particular g and A tensors. It is concluded that the Co2+ ions occupy substitutional as well as interstitial sites of SnO2 and that ...

A variable temperature Fe3+ electron paramagnetic resonance study of Sn1−xFexO2 (0.00⩽x⩽0.05)

X-band (∼9.5GHz) electron paramagnetic resonance (EPR) studies of Fe3+ ions in Sn1−xFexO2 powders with 0.00⩽x⩽0.05 at various temperatures (5–300K) are reported. These samples are interesting to investigate as Fe doping (⩽5%) produces ferromagnetism in SnO2 [A. Punnooose et al., Phys. Rev. B 72, 054402 (2005)], making it a promising ferromagnetic semiconductor at room temperature. The EPR spectrum at 5K can be simulated reasonably well as the overlap of spectra due to seven magnetically inequivalent Fe3+ ions: four low-spin (S=1∕2) and three high-spin (S=5∕2) ions, characterized by different spin-Hamiltonian parameters, overlapped by three broad ferromagnetic resonance spectra. The three high-spin ions, situated substitutionally in the interior of nanodomains, are characterized by smaller zero-field splitting (ZFS) parameters D and E, so that all their energy levels are populated at 5K. On the other hand, the four low-spin ions are situated interstitially at the surfaces of nanodomains. They are character...

Effect of interparticle interaction on the magnetic relaxation in NiO nanorods

Temperature variation (5 K–300 K) of the AC magnetic susceptibilities (χ′ and χ″) at frequency f=0.1, 1, 50, 100, 500, 1000, 2000, 5000, and 10, 000 Hz are reported in 5 nm diameter nanorods of NiO, with and without oleic acid (OA) coating. Using the peak in χ′ as the blocking temperature TB, it is observed that TB increases with increasing f. The data for the two samples fit the Vogel-Fulcher law: f=f0exp[−Ea∕k(TB−T0)], with f0=9.2×1011Hz, Ea∕k=1085 K, and T0=165 K (0 K) for the uncoated (coated) particles. This shows that T0 provides a good measure of the effects of interparticle interactions on magnetic relaxation and that these interactions are essentially eliminated with the OA coating.

High-temperature magnetic-field-induced activation of room-temperature ferromagnetism in Ce1−xNixO2

We report room-temperature ferromagnetism in nickel doped ceria (Ce1−xNixO2) powders prepared using the sol-gel process. Magnetometry studies on as-prepared samples reveal a weak ferromagnetic (FM) behavior in the range 0 0.04, the FM magnetization steadily decreases for increased dopant concentration. The weak FM behavior of the samples improved dramatically after undergoing high-temperature (⩽500°C) activation in the presence of an external magnetic field (H⩽1T). The magnitude of the improvement, which was very reproducible, was strongest for 10% Ni doped CeO2 with saturation magnetization Ms increasing by 600 times. Curie temperatures of the activated samples varied in the 597–646K range depending on the Ni concentration and preparation conditions. X-ray diffraction studies did not reveal any noticeable structural changes as a result of the activation process.

Ferromagnetic Resonance Properties and Anisotropy of Ni-Mn-Ga Thin Films of Different Thicknesses Deposited on Si Substrate

Ni-Mn-Ga films of different thicknesses were deposited onto Si(100) substrates by magnetron sputtering and annealed at 1073 K for 1 h in high vacuum. X-ray diffraction analysis showed the formation of 220 fiber texture perpendicular to the film plane. Magnetic properties of thin films were investigated at room temperature using ferromagnetic resonance (FMR) technique. The dependencies of both the FMR absorption maximum position and resonance linewidth on the direction of the external magnetic field with respect to the film normal were studied. The data analysis showed that the direction of magnetocrystalline anisotropy easy axis in the films makes 45° angle with the film normal. The modeling allowed evaluation of the uniaxial anisotropy constant, which is found to increase with thickness of Ni-Mn-Ga films. Uniaxial anisotropy constants were found to be ∼2.8×105 erg/cm3 for 0.1 and 0.5 μm film thickness, ∼4.2×105 erg/cm3 for 1 μm film, and ∼5.1×105 erg/cm3 for 3 μm film.

Role of dopant incorporation on the magnetic properties of Ce1-xNixO2 nanoparticles: An electron paramagnetic resonance study

Nickel doping has been found to produce weak room-temperature ferromagnetism (FM) in CeO2. The saturation magnetization (Ms) of the chemically synthesized Ce1−xNixO2 samples showed a maximum for x=0.04, above which the magnetization decreased gradually. For Ce1−xNixO2 samples with x⩾0.04, an activation process involving slow annealing of the sample to 500°C increased the Ms by more than two orders of magnitude. However, no such activation effect was observed in samples with x 0.04, and (ii) the dramatic increase in Ms in the activated Ce1−xNixO2 samples with x⩾0.04 and the absence of this behavior in samples with x<0.04. Detailed analysis by simulation of the EPR data on several as-prepared Ce1−xNixO2 samples with 0.01⩽x⩽0.10 at 5 and 300K indicates the presence of several paramagnetic species: (i) two magnetically inequivalen...