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Grace Yong

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Papers by Grace Yong

Large photoinduced conductivity reduction in thin films of metallic ferromagnetic manganites

This paper reports on a study of photoinduced resistivity changes in thin films of ferromagnetic ... more This paper reports on a study of photoinduced resistivity changes in thin films of ferromagnetic metallic (FMM) manganites. We have observed a significant increase of resistance in La 0.7 Ba 0.3 MnO 3 thin film under continuous wave argon ion laser illumination presumably associated with photoinduced demagnetization. Strong dependence of resistance on thermal/illumination history is consistent with the coexistence of two phases: ferromagnetic metallic phase and photoinduced less conductive phase. V

$Research paper thumbnail of The effects of annealing on the infrared and optical properties of La $ _ {0.67} $ Sr $ _ {0.33} $ MnO $ _ {3} $ films$

The effects of annealing on the infrared and optical properties of La $ _ {0.67} $ Sr $ _ {0.33} $ MnO $ _ {3} $ films

La 0.67 Sr 0.33 MnO 3 (LSMO) films grown by pulsed laser deposition on lanthanum aluminate substr... more La 0.67 Sr 0.33 MnO 3 (LSMO) films grown by pulsed laser deposition on lanthanum aluminate substrates undergo a phase transition from ferromagnetic metallic state to paramagnetic insulating state at T c of about 350 K. This second-order phase transition proceeds via a phase coexistence regime over an extended temperature range. Annealing affects the strain and oxygen content in films thereby causing significant changes to the magnetic properties, electronic structure, lattice distortion, and possibly the nanoscale properties of coexisting phases. We use ellipsometry and Fourier-transform infrared spectroscopy to investigate the effects of annealing on LSMO films over a broad spectral range from ultraviolet to far infrared. We deduce the Jahn-Teller energy splitting and the Hund's coupling energy from our data on annealed and unannealed films.

Observation of nonbolometric responses to microwave irradiation on YBaCuO films

Journal of Applied Physics, 1991

Nonbolometric response to 8.3-GHz microwave radiation in the resistive transition region of elect... more Nonbolometric response to 8.3-GHz microwave radiation in the resistive transition region of electron-beam-deposited YBaCuO thin films is distinguished from the bolometric response by their different applied microwave power dependence with the nonbolometric effect dominating the voltage response below the boiling temperature of liquid nitrogen. In the presence of a dc magnetic field, the microwave-induced dc voltage in unbiased films shows an oscillatory behavior as a function of microwave power as well as a polarity reversal depending upon the direction of the dc magnetic field. This behavior is consistent with the electromagnetic response arising from the Josephson vortex motion in the junction regions within these granular films.

Thickness Dependence of Electrical and Structural Properties of Tensile Strained Calcium Manganese Oxide Thin Films

Bulletin of the American Physical Society, Mar 15, 2016

investigated the properties of CaMnO 3−δ thin films epitaxially grown by pulsed laser deposition ... more investigated the properties of CaMnO 3−δ thin films epitaxially grown by pulsed laser deposition on lattice mismatched substrates, (100)LaAlO 3 and (100)SrTiO 3 , leading to a tensile strain of ∼4% and 1.5% respectively. For our films these substrates, thickness dependence of the properties is characteristically different from what has been previously observed in thin films of hole-doped manganites. We observe that the resistivity decreases significantly as the film thickness decreases. The decrease in resistivity is more pronounced in the films on (100)SrTiO 3 with the larger lattice mismatch, the resistivity of the thinnest films being about 3 orders of magnitude lower than the of bulk CaMnO 3. Thickness dependence of the lattice constants also show deviations from the behavior expected from strain relaxation. These results suggest a coupling between tensile strain and oxygen deficiency consistent with predictions from models based on density functional theory calculations. Our results are relevant for potential catalytic applications of CaMnO 3−δ thin films.

$Research paper thumbnail of High Resolution X-ray Diffraction Analysis of Residual Stresses in Epitaxial Thin Films of Rare Earth Manganite Thin Films on Silicon$

High Resolution X-ray Diffraction Analysis of Residual Stresses in Epitaxial Thin Films of Rare Earth Manganite Thin Films on Silicon

Bulletin of the American Physical Society, Mar 4, 2014

device functionalities, offers challenges on account of several factors such as reactivity of oxi... more device functionalities, offers challenges on account of several factors such as reactivity of oxide materials with Si at the high temperatures needed for epitaxial growth, the presence/formation of the amorphous native oxide layer, and the thermal expansion mismatch of Si with that of the oxides which leads to the accumulation of stresses during thermal cycling of the film. In order to achieve epitaxial growth circumventing these problems, lattice engineering schemes employing heterostructures of appropriate chemical buffers and structural-templates must be adopted. The film growth parameters and thermal cycling kinetics play a key role in determining the residual stresses in such heterostructures. It is important to monitor the stresses as a function of growth parameters and optimize the parameters to minimize the stresses. We will present our studies of the residual stresses in epitaxial thin films of hole doped rare earth manganite thin films grown on Si using high resolution four-circle x-ray diffraction.

Effect of Post-deposition Thermal Treatment on the Structural and Electrical Properties of Oxygen Deficient Perovskite Metal Oxide Thin Films

Bulletin of the American Physical Society, Mar 2, 2020

Ca 3 Co 4 O 9 films obtained by PLD are dense and smooth. • As-grown films are amorphous, resisti... more Ca 3 Co 4 O 9 films obtained by PLD are dense and smooth. • As-grown films are amorphous, resistive and show low thermoelectric power factor. • Post deposition annealing leads to crystalline and thermoelectric efficient films. • A power factor of 2 × 10 −4 W m −1 K −2 is reached at room temperature.

Structural and Electrical Properties of Electron-doped CaMnO3 Thin Films

Bulletin of the American Physical Society, Oct 4, 2014

metal oxides are a class of materials that are predicted to play as big a role in future electron... more metal oxides are a class of materials that are predicted to play as big a role in future electronic technologies as silicon does in today's semiconductor based electronic technologies. Research in thin films of manganites in the past has largely been focused on the hole-doped compositions that exhibit the phenomenon of colossal magnetoresistance. We are currently investigating the properties of thin films of electron-doped calcium manganese oxide. We use the technique of pulsed laser deposition to grow these thin films. The films are grown epitaxially on LaAlO 3 substrates, whose lattice parameters are larger than that of CaMnO 3 , thus causing the films to be under tensile stress. By decreasing the film thickness we can increase the tensile strain. We have studied structural and electrical properties of CaMnO 3 films under tensile strain, by means of X-ray diffraction and temperature dependent resistivity measurements. Our results suggest that tensile strain causes CaMnO 3 to be more susceptible to the formation of oxygen vacancies, thus reducing electrical resistivity.

Tensile Strain Effects on the Magneto-transport in Calcium Manganese Oxide Thin Films: Comparison with its Hole-doped Counterpart

Bulletin of the American Physical Society, Mar 15, 2016

properties of the epitaxial thin films of doped rare earth manganites are known to be influenced ... more properties of the epitaxial thin films of doped rare earth manganites are known to be influenced by the effect of bi-axial strain induced by lattice mismatch with the substrate. In hole-doped manganites, the effect of both compressive and tensile strain is qualitatively consistent with the expected changes in unit cell symmetry from cubic to tetragonal, leading to Jahn-Teller strain fields that affect the energy levels of Mn3+ energy levels. Recent work in our laboratory on CaMnO 3 thin films has pointed out that tetragonal distortions introduced by tensile lattice mismatch strain may also have the effect of modulating the oxygen content of the films in agreement with theoretical models that propose such coupling between strain and oxygen content. Our research focuses on comparing the magneto-transport properties of hole-doped manganite LaCaMnO 3 thin films with that of its electron doped counter parts, in an effort to delineate the effects of oxygen stoichiometry changes on magneto-transport from the effects of Jahn-Teller type strain.

Observation of nonbolometric responses to microwave irradiation on YBaCuO films

Journal of Applied Physics, May 15, 1991

Nonbolometric response to 8.3-GHz microwave radiation in the resistive transition region of elect... more Nonbolometric response to 8.3-GHz microwave radiation in the resistive transition region of electron-beam-deposited YBaCuO thin films is distinguished from the bolometric response by their different applied microwave power dependence with the nonbolometric effect dominating the voltage response below the boiling temperature of liquid nitrogen. In the presence of a dc magnetic field, the microwave-induced dc voltage in unbiased films shows an oscillatory behavior as a function of microwave power as well as a polarity reversal depending upon the direction of the dc magnetic field. This behavior is consistent with the electromagnetic response arising from the Josephson vortex motion in the junction regions within these granular films.

Resonant Ultrasound Spectroscopy of KTa 1-x Nb x O 3 and K 1-x Li x TaO 3 Ferroelectric Relaxor Crystals

Bulletin of the American Physical Society, Mar 16, 2021

ABSTRACT The influence of the development of a ferroelectric state on the elastic properties of K... more ABSTRACT The influence of the development of a ferroelectric state on the elastic properties of KTa1-xNbxO3 relaxor crystals is explored. The high sensitivity of the elastic stiffness tensor to the polar distortions and their reorientational dynamics is individual for each particular component: c11 and c44 are primarily influenced by the reorientational motion of the distortions between the nearest neighboring (111) directions; the c12 , however, depends primarily on the reorientations between the (111) directions neighboring along the cubic face diagonal. Consequently, the temperature behavior of c12 demonstrates a different dependence on the Nb concentration than that of c11 and c44 . In the 1.2% Nb crystal all three elastic coefficients clearly show a softening with the appearance of the dynamic polar distortions while, in the 16% Nb crystal this effect is strong for c11 and c44 , but changes in c12 occur only in the vicinity of the phase transition. Nevertheless, close to the transition the elastic properties become more isotropic for all three crystals. The curves of slowness and Young&#39;s modulus within the main crystallographic planes are presented and the linear compressibility modulus is estimated. The values of the Debye temperatures with an error of ±2K are estimated to be approximately 594, 589, and 592 for the crystals containing 1.2, 8, and 16% of Nb, respectively.

Pretransitional Diffuse Neutron Scattering in Ferroelectric KTa 1-x Nb x O 3

Bulletin of the American Physical Society, Mar 3, 2020

Electrical Transport and Magnetoresistance Properties of Tensile-Strained CaMnO3 Thin Films

Bulletin of the American Physical Society, Mar 13, 2017

will present our studies of the electrical transport and magnetoresistance properties of tensile ... more will present our studies of the electrical transport and magnetoresistance properties of tensile strained CaMnO3 thin films. We observe that the resistivity decreases significantly as the film thickness decreases which is opposite to what is observed in thin films of hole doped manganites. The decrease in resistivity is more pronounced in the films on (100) SrTiO3, with resistivity of the thinnest films being about 3 orders of magnitude lower than that of bulk CaMnO3. Structural changes accompanying resistivity changes cannot be fully explained as due to tensile strain, and indicate the presence of oxygen vacancies. These results also suggest a coupling between tensile strain and oxygen deficiency, consistent with predictions from models based on density functional theory calculations. We observe a change in resistance under the application of moderate magnetic field. Experiments are underway to understand the origin of the magnetoresistance and its possible relation to the tensile strain effects.

$Research paper thumbnail of Possible Mechanisms in Atomic Force Microscope-Induced Nano-Oxidation Lithography (negative AFM tip case) in La$_{0.67}$Ba$_{0.33}$MnO$_{3-\delta }$ Thin Films on SrTiO$_{3}$(001)$

Possible Mechanisms in Atomic Force Microscope-Induced Nano-Oxidation Lithography (negative AFM tip case) in La$_{0.67}$Ba$_{0.33}$MnO$_{3-\delta }$ Thin Films on SrTiO$_{3}$(001)

Bulletin of the American Physical Society, Mar 3, 2015

this paper, we present possible microscopic mechanisms for La 0.67 Ba 0.33 MnO 3−δ films that hav... more this paper, we present possible microscopic mechanisms for La 0.67 Ba 0.33 MnO 3−δ films that have been nano-oxidized by an AFM tip that is negatively biased with respect the sample. Further analysis of comparative EDS elemental profile for an unmodified film versus AFM (negative tip) modified films yield fresh insights. We can qualitatively explain many of the observations with electrochemical half reactions, electrochemical migration and electromigration.

Synthesis and characterization of Nb2AlC thin films

Thin Solid Films, 2009

We report on the synthesis and characterization of epitaxial c-axis oriented Nb 2 AlC thin films ... more We report on the synthesis and characterization of epitaxial c-axis oriented Nb 2 AlC thin films deposited on c-axis sapphire (Al 2 O 3) substrates by magnetron sputtering. Selected area electron diffraction reveal that independent of substrate temperature or film stoichiometry, there is the growth of a secondary phase not found in bulk, Nb 5 Al 3 C x with a-and c-axis lattice constants of 7.746 Å and 5.246 Å, respectively. Scanning electron micrographs reveal large surface features, many with hexagonal shape and faceted texture. Atomic force microscopy topographical measurements indicate a surface roughness of approximately 15% of the total film thickness. Electrical transport measurements show typical metal-like conduction with a room temperature resistivity of ≈0.9 μΩ-m and a residual resistivity ratio of 2.5. A superconducting transition was found at ≈440 mK.

Colossal Magneto Resistive (CMR) Bolometric Sensors as a Total Energy Monitor for the Linac Coherent Light Source (LCLS) Free Electron Laser (FEL)

Bulletin of the American Physical Society, Mar 8, 2007

We are developing a CMR based bolometric x-ray detector as a total energy monitor for the LCLS FE... more We are developing a CMR based bolometric x-ray detector as a total energy monitor for the LCLS FEL to be built at the Stanford Linear Accelerator Center. The FEL will produce ∼200 femtosecond pulses in the energy range 0.8 to 8 keV with 10 12 photons per pulse. The bolometer is designed to measure the total energy of each laser pulse with repeatability below 1% and an absolute accuracy below 10%. The detector is fabricated using epitaxial thin films of Nd 0.67 Sr 0.33 MnO 3 grown on Si by Pulsed Laser Deposition. An epitaxial buffer layer of SrTiO 3 is used as a chemical barrier and an additional template layer of Bi 4 Ti 3 O 12 is employed to facilitate lattice match with the Nd 0.67 Sr 0.33 MnO 3 layer. We have been able to obtain good quality epitaxial thin films by this approach. Prototype detectors have been fabricated photolithographically, and are operated in a pulse tube refrigerator at temperatures between 100 to 150 K. Initial tests with a thermal heater pulse show that the film properties are sufficient for detector functionality. We will discuss the details of material optimization, characteristics of the sensor material such as the temperature coefficient of resistance and 1/f noise, and projections of the detector response under photon illumination. G. Yong Towson U.

Origin of the non-thermal photoresponse in thin films of two-phase manganites

Bulletin of the American Physical Society, Mar 13, 2006

UNIVERSITY TEAM-Our studies of light-induced resistance changes (photoresponse) in thin films the... more UNIVERSITY TEAM-Our studies of light-induced resistance changes (photoresponse) in thin films the colossal magnetoresistive manganite material (La,Pr) 0.67 Ca 0.33 MnO 3 experiments have revealed a non-thermal component of the light-induced resistance change .This non-thermal component is also observed in thin films of oxygen deficient La 0.67 Ca 0.33 MnO 3. The common feature of both these material systems is the coexistence of metallic and insulating phases. Our results indicate that this component may be associated with the light-induced resistance decrease in the insulating regions through an electronic mechanism. Previous studies have shown insulator-metal transitions induced by magnetic fields as well as electric fields in these materials. We will present our studies of the correlation of the observed non-thermal photoresponse with magnetoresistance as well as current-voltage characteristics.

Colossal Magnetoresistive Manganite Based Fast Bolometric X-ray Sensors for Total Energy Measurements of Free Electron Lasers

Sensor Letters, 2008

Bolometric detectors based on epitaxial thin films of rare earth perovskite manganites have been ... more Bolometric detectors based on epitaxial thin films of rare earth perovskite manganites have been proposed as total energy monitors for X-ray pulses at the Linac Coherent Light Source free electron laser. We demonstrate such a detector scheme based on epitaxial thin films of the perovskite manganese oxide material Nd{sub 0.67}Sr{sub x0.33}MnO, grown by pulsed laser deposition on buffered silicon substrates.

Ultrasound Pulse-Echo Setup for Studying Elastic Properties of the Electric Field Induced Phase Transitions in Ferroelectric Materials

Bulletin of the American Physical Society, 2018

Structural and Electrical Properties of Thin Films of Electron-doped Mixed-Valent Rare Earth Manganites

Bulletin of the American Physical Society, 2014

in thin films of mixed valent rare earth manganese oxides has largely been focused on hole-doped ... more in thin films of mixed valent rare earth manganese oxides has largely been focused on hole-doped manganites that exhibit colossal magnetoresistance. Hole doped manganites are derived from trivalent rare earth manganese oxides, where the hole doping (introduction of Mn 4+ ions to replace the Mn 3+ ions) is the result of substitution of the trivalent rare earth site (such as La 3+) by a divalent alkaline earth element (such as Ca 2+). In contrast, electron doped manganites can be obtained by introducing Mn 3+ ions to replace Mn 4+ ions in an alkaline earth manganese oxide. We are currently investigating the properties of electron-doped manganites which are derived from CaMnO 3. We use Pulsed Laser Deposition to grow these epitaxial thin films. One way to introduce electron carriers in the film is by creating an oxygen poor environment during the deposition, causing the film composition to be of the form Ca 2+ Mn 4+ 1−2x Mn 3+ 2x O 2 − 3−x. Another method is by substitution of the Ca 2+ site by rare earth elements of valency 3+ or higher (such as Ce 4+ or Ho 4+) to introduce electron carriers. We will report our study of the structural, electrical, and magneto-transport properties of electron doped manganite thin films, focusing on the sensitivity of these properties to growth parameters.

Design of a bolometer for total-energy measurement of the linear coherent light source pulsed X-ray laser

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2006

We are developing a cryogenic bolometer to measure the total energy of the linear coherent light ... more We are developing a cryogenic bolometer to measure the total energy of the linear coherent light source (LCLS) free electron X-ray laser to be built at the Stanford Linear Accelerator Center. The laser will produce ultrabright X-ray pulses in the energy range between 0.8 and 8keV with ∼1012 photons per ∼200fs pulse at a repeat interval of 8ms, and will