Samuel E. Lofland

Material characteristics of perovskite manganese oxide thin films for bolometric applications

We are optimizing thin films of perovskite manganese oxides for bolometric applications. We have studied the relevant material characteristics of several members of this family namely, La0.7Ba0.3MnO3, La0.7Sr0.3MnO3, La0.7Ca0.3MnO3, and Nd0.7Sr0.3MnO3. Here, we discuss issues related to the choice of material, the influence of deposition parameters, and postdeposition heat treatments on the relevant characteristics such as the resistivity-peak temperature (Tp) and the temperature coefficient of resistance (TCR). For a given material, a higher peak temperature implies a larger temperature coefficient of resistance. In contrast, on comparing different material systems, the TCR tends to decrease as Tp increases.

Stress-induced effects in epitaxial (La0.7Sr0.3)MnO3 films

Abstract The transport and magnetic properties of epitaxial (La0.7Sr0.3)MnO3 (LSMO) films deposited on LaAlO3 (Sample L, under a compressive stress) and SrTiO3 (Sample S, under a tensile stress) are compared. The magnetic force microscope image of Sample L shows a ‘maze-like’ pattern indicating a sizable out-of-plane magnetization, while the magnetic image of Sample S shows a ‘feather-like’ pattern indicative of in-plane magnetization. The hysteresis loop and ferromagnetic resonance measurements indicate that the lattice mismatch between the film and the substrate is the origin of the differences in the magnetic anisotropy of the two films. Sample L has a smaller ‘peak’ resistivity and a larger saturation magnetization at room temperature than Sample S. Our results can be understood by the internal ‘pressure’ consequent upon lattice mismatch and agree qualitatively with the recent hydrostatic pressure studies on the doped manganites.

Nanorods of manganese oxalate: a single source precursor to different manganese oxide nanoparticles (MnO, Mn2O3, Mn3O4)

Nanorods of anhydrous manganese oxalate were prepared by the reverse-micellar method using CTAB as the surfactant. Manganese oxalate precursor was used to synthesize single phase nanoparticles of various manganese oxides such as MnO, Mn2O3 and Mn3O4 under specific reaction conditions. Both MnO (28 nm) and α-Mn2O3 (50 nm) are stabilized as cubic phase. α-Mn2O3 shows a weak antiferromagnetic transition (TN = 80 K), while the spinel Mn3O4 (100 nm) particles show a ferrimagnetic transition at 43 K.

Continuous carbide-derived carbon films with high volumetric capacitance

Monolithic porous carbon film has a great potential for integrated supercapacitors due to no polymer binder, reduced macropore volume, and good adhesion between current collector and active material. It is demonstrated that continuous carbide-derived carbon (CDC) films can be synthesized on various substrates by dry etching. CDC films show high volumetric capacitance of ∼180 F cm−3 in 1.5 M TEABF4/acetonitrile electrolyte.

Bulk synthesis and high-temperature ferromagnetism of (In1−xFex)2O3−σ with Cu co-doping

The synthesis and magnetic properties of (In1−xFex)2O3−σ bulk ceramics with Cu co-doping are reported. Magnetic Fe ions are found to have high thermodynamic solubility (up to 20%) in the In2O3 host compound. The lattice constant decreases almost linearly as Fe doping concentration increases indicating the incorporation of Fe ions into the host lattice. The samples with high Fe concentration annealed under Ar reduced atmosphere were found to be ferromagnetic, and the Curie temperature is around 750K. The extensive structural and magnetic studies rule out the possibility that the observed magnetism is derived from magnetic impurity phases.

Growth of colossal magnetoresistance thin films on silicon

We are able to grow high quality La0.67Sr0.33MnO3(LSMO) colossal magnetoresistive (CMR) thin films on Y‐stabilized zirconia (YSZ) buffered (100) Si substrates using a Bi4Ti3O12 texturing and lattice matching layer. The CMR films have very high structural perfection and show excellent transport and ferromagnetic properties, including the almost full saturation magnetization values and narrow ferromagnetic resonance peaks (15 Oe at 290 K). The lattice matching template/buffer layer approach is suitable for the high quality CMR films on Si. A close correlation between the magnetic hysteresis loop and the field dependence of MR is observed at lower temperatures.

Correlation between magnetic homogeneity, oxygen content, and electrical and magnetic properties of perovskite manganite thin films

Perovskite manganese oxide materials known for the phenomenon of colossal magnetoresistance often exhibit anomalously large 1/f noise and large, temperature-dependent ferromagnetic resonance (FMR) linewidths. We show that in epitaxial films, these anomalies are very sensitive to oxygen partial pressure during film growth and to postdeposition thermal processing in oxygen, suggesting that oxygen stoichiometry plays a key role. We find that the temperature coefficient of resistance (TCR) at the metal–insulator transition increases and the FMR linewidth decreases as we increase the oxygen partial pressure during growth. Postdeposition heat treatment in oxygen leads to further increase in TCR and decrease in FMR linewidth, accompanied by a dramatic reduction in 1/f noise magnitudes.

Temperature and field dependence of microwave losses in manganite powders

We report systematic measurements of the temperature and field dependence of microwave losses in micron-size powders (∼3 μm) of the manganites Nd0.7Sr0.3MnO3, La0.7Ca0.3MnO3, La0.8Ba0.2MnO3, and La1.4Ca1.6Mn2O7. We fully confirm previous findings of a large increase in zero-field absorption, in La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3, accompanying the onset of ferromagnetism and a concomitant colossal low-field magnetoimpedance (up to 80% at 600 Oe at room temperature). An oversimplified set of computations renders plausible support for interpreting these observations as arising from absorption in the spin system. Possible applications of these novel phenomena are also described.

Development of a microemulsion-based process for synthesis of cobalt (Co) and cobalt oxide (Co3O4) nanoparticles from submicrometer rods of cobalt oxalate.

Rod-shaped nanostructures of cobalt oxalate dihydrate were synthesized at room temperature by the microemulsion (reverse micellar) route. These rods are highly uniform in length and can be modified with temperature (from approximately 6.5 microm at 50 degrees C to approximately 2.5 microm at 150 degrees C) while keeping the diameter nearly constant (200-250 nm). Thermal decomposition of these rods in a controlled atmosphere (air and H(2)) leads to nanoparticles of Co(3)O(4) and Co, respectively, while in a helium atmosphere a mixture of Co and CoO nanoparticles is obtained. Co(3)O(4) nanoparticles (approximately 35 nm) were slightly agglomerated, while Co nanoparticles were monodispersed and highly uniform (approximately 25 nm). The oxalate rods and Co(3)O(4) nanoparticles show an antiferromagnetic ordering at 54 and 35 K, respectively.

Microemulsion-mediated synthesis of cobalt (pure fcc and hexagonal phases) and cobalt-nickel alloy nanoparticles

By choosing appropriate microemulsion systems, hexagonal cobalt (Co) and cobalt-nickel (1:1) alloy nanoparticles have been obtained with cetyltrimethylammonium bromide as a cationic surfactant at 500 degrees C. This method thus stabilizes the hcp cobalt even at sizes (<10 nm) at which normally fcc cobalt is predicted to be stable. On annealing the hcp cobalt nanoparticles in H(2) at 700 degrees C we could transform them to fcc cobalt nanoparticles. Microscopy studies show the formation of spherical nanoparticles of hexagonal and cubic forms of cobalt and Co-Ni (1:1) alloy nanoparticles with the average size of 4, 8 and 20 nm, respectively. Electrochemical studies show that the catalytic property towards oxygen evolution is dependent on the applied voltage. At low voltage (less than 0.65 V) the Co (hexagonal) nanoparticles are superior to the alloy (Co-Ni) nanoparticles while above this voltage the alloy nanoparticles are more efficient catalysts. The nanoparticles of cobalt (hcp and fcc) and alloy (Co-Ni) nanoparticles show ferromagnetism. The saturation magnetization of Co-Ni nanoparticles is reduced compared to the bulk possibly due to surface oxidation.