Adolph L. Micheli

Magnetocaloric properties of doped lanthanum manganite films

LaMnO3 films doped with Ca, Ba, or Sr have been fabricated using the metalorganic decomposition technique. These films exhibit paramagnetic‐to‐ferromagnetic phase transitions at 250, 300, and 350 K, respectively. By measuring the film magnetization as a function of field and temperature we have determined the entropy change associated with these transitions. The large magnetization of these materials results in a total entropy change a factor of five less than that of gadolinium, the prototypical high‐temperature magnetocaloric material. Improvements in film morphology and composition may provide a further increase in the magnetization and total entropy change in these materials.

Slater model applied to polarization graded ferroelectrics

Ferroelectric thin films of BaxSr1−xTiO3 with compositional gradients normal to the growth surface have been formed by the successive deposition and annealing of films having step-variable Ba to Sr ratios. By suitably tailoring the magnitude and sense of the gradient in Ba to Sr ratio, directional potentials can be built into the structures yielding a new, but controllable, hysteresis phenomenon. Slater’s empirical model for ferroelectric materials has been extended to also describe thin films with polarization gradients normal to the growth surface, i.e., graded ferroelectric devices. This model accounts for several aspects of these structures, including: the broadness of the permittivity plots with temperature, the formation of a spontaneous potential upon oscillatory field excitation, offsets in the hysteresis graphs along the displacement axis with directions which are gradient dependent, and the electric field dependence of that offset.

Applicability of effective medium theory to ferroelectric/ferrimagnetic composites with composition and frequency‐dependent complex permittivities and permeabilities

High‐frequency (1 MHz–1 GHz) transmission line measurements were used to determine the composition and frequency‐dependent complex permittivities and complex permeabilities of ferroelectric/ferrimagnetic (barium titanate and a magnesium‐copper‐zinc ferrite) composites. The effective medium rules of Maxwell–Garnett give both lower and upper bounds for the effective permittivities and permeabilities and yield accurate estimates of the bulk electric and magnetic properties at low volume fill fraction of either component provided the proper host matrix is chosen. Bruggeman theory yielded the best predictive values for the permittivity and permeability over the entire composition range. In all cases these complex quantities were shown to be constrained by Bergman–Milton bounds.

Giant effective pyroelectric coefficients from graded ferroelectric devices

Effective pyroelectric coefficients as large as 5 μC/cm2 °C, with peak responsivities at approximately 50 °C, were obtained from compositionally graded barium strontium titanate ferroelectric thin film devices formed on silicon using unbalanced magnetron sputter deposition. These effective pyroelectric coefficients are nearly two orders of magnitude larger than those observed from conventional pyroelectric thin film ferroelectric detectors.

Formation of thin‐film high Tc superconductors by metalorganic deposition

Metalorganic deposition (MOD) is a nonvacuum method of thin‐film deposition which allows easy alteration of chemical components and is compatible with thin‐film processing. We report the preparation of thin‐film superconductors by MOD. Rutherford backscattering spectrometry was used to determine film compositions and thicknesses. Films, approximately 500 nm thick, of YBa2Cu4Oz (z undetermined) were deposited on 〈100〉 single‐crystal SrTiO3. A superconducting onset temperature of 90 K was measured with 37 K the zero resistance temperature. Scanning electron microscopy revealed grain sizes approximately 250 nm in diameter.

TEMPERATURE DEPENDENCE OF CONVENTIONAL AND EFFECTIVE PYROELECTRIC COEFFICIENTS FOR COMPOSITIONALLY GRADED BAXSR1-XTIO3 FILMS

Ferroelectric thin films (∼1.2 μm) of BaxSr1−xTiO3 with gradients in composition normal to the growth surface were formed on platinum substrates by metalorganic decomposition. Effective (pseudo) pyroelectric coefficients as large as 0.06 μC/cm2 K have been obtained from these active ferroelectric devices under the application of an ac field (charge pumping). In contrast, a value of only −0.003 μC/cm2 K has been measured for the conventional pyroelectric coefficient.

Ferroelectric thin films with polarization gradients normal to the growth surface

Thin film ferroelectrics with polarization gradients normal to the growth surface readily form when gradients in temperature, strain, or composition are coupled to the polarization vector in ferroelectric materials. This letter describes the formation of thin films of potassium tantalum niobate with graded polarizations obtained by grading the tantalum to niobium ratio of the ferroelectric phase. Unlike a simple structure consisting of laminated layers of ferroelectric material, the polarization gradient which forms breaks the natural symmetry of the ferroelectric material at any given plane, resulting in a self‐poling of the structure subsequent to excitation by an oscillatory electric field. Once poled, the devices reveal a measurable potential across the structure which varies with temperature.

Preparation of barium strontium titanate thin film capacitors on silicon by metallorganic decomposition

Barium strontium titanate films were deposited on 100‐mm‐diam silicon wafers by metallorganic decomposition. Highly uniform films ranging up to 250 nm in thickness were obtained. Scanning electron beam micrographs revealed dense, crack‐free, low porosity films having grain sizes in the submicron range. Dissipations (tan δ) of less than 0.01 and capacitance densities up to 204 nF/cm2 were measured for 190‐nm‐thick films. These films had dc leakage currents of 0.2 μA/cm2 or less for bias voltages up to 10 V. They also exhibited relatively small temperature coefficients of capacitance. A patterning process was developed that permitted feature resolution down to 5 μm.

Rapid thermal annealing of high Tc superconducting thin films formed by metalorganic deposition

Thin‐film superconductors of Y‐Ba‐Cu and Yb‐Ba‐Cu have been formed by the nonvacuum method of metalorganic deposition (MOD). The films produced in this manner were homogeneous and free of voids and cracks over large dimensions. A two‐step rapid thermal annealing of the MOD films, in oxygen, at 850 °C for 60 s followed by a second annealing at 920 °C for 30 s enhanced grain growth in the films and reduced the effects of substrate interaction. Preferred epitaxial grain growth, in the high Tc films, with the c axis both perpendicular and parallel to the substrate surface, occurred on 〈100〉 SrTiO3. Both the Y‐Ba‐Cu and Yb‐Ba‐Cu films showed superconducting onset temperatures above 90 K and zero resistance at 86 K.

Use of ion beams to decompose metalorganics into patterned thin‐film superconductors

Ion implantation of 400 keV O+2 ions, at a dose of 5×1014 ions/cm2, was used to selectively decompose a high Tc superconductor‐forming metalorganic (MO). Implantation rendered the metal carboxylates insoluble in their solvent, xylene, permitting patterning of the MO prior to pyrolysis and annealing. Fine line superconductors were formed having 90 K onset temperatures and zero resistance at 68 K.