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Papers by Venkata Mudinepalli

Journal of Materials Science: Materials in Electronics, 2007

... Reddy Á M. Venkata Ramana Á E. Rajagopal Á KV Sivakumar (&) Ceramic Composite Materials L... more ... Reddy Á M. Venkata Ramana Á E. Rajagopal Á KV Sivakumar (&) Ceramic Composite Materials Laboratory, Department of Physics, Sri Krishnadevaraya University, SV Puram, Anantapur 515 003, India e-mail: sivani.sivakumar@gmail.com VRK Murthy Microwave Laboratory ...

In our previous work, we synthesized Pb(Zr0.52Ti0.48)O3 (PZT) ceramics by conventional and microw... more In our previous work, we synthesized Pb(Zr0.52Ti0.48)O3 (PZT) ceramics by conventional and microwave sintering methods and studied their structural and electrical properties. We observed that the microwave sintered PZT ceramics show higher densification, fine and uniform grain size, higher dielectric constant, remnant polarization (Pr), and spontaneous polarization (Ps) in comparison to conventional sintered ones. In the present work, we studied the microstructure, phase formation, dielectric, ferroelectric and piezoelectric properties of the PZT ceramics synthesized by using Spark plasma Sintering (SPS) method. The SPS sintering temperature is lower (300 to 400 ◦C) than both conventional and microwave sintering methods. The dielectric constant at room temperature and at transition temperature shows higher values compared to the microwave and conventional sintering methods. This SPS sintering technique is currently attracting growing attention among productions engineers and materia...

Sodium Alginate doped with Copper oxide composite polymer films was developed through Solution ca... more Sodium Alginate doped with Copper oxide composite polymer films was developed through Solution casting method. The structural properties on these polymer nanocomposite thin films were characterized by X-ray diffraction. XRD shows all the individual constituents of these composites and the CuO peaks increases as CuO increased in the composites. Thermogravimetric analysis (TGA) technique was used for thermal stability of these composites. Temperature dependence of dielectric properties of these SA/CuO nanocopmosite polymer thin films we measured in the frequency range from 100 Hz to 5 MHz. Dielectric properties of these composites strongly depends on the percentage of CuO and structure of these matrix. These type of composites are attracted much attention in battery applications.

Journal of Alloys and Compounds, 2021

The combination of yttrium iron garnet (Y3Fe2(FeO4)3, YIG) and ferromagnetic materials or antifer... more The combination of yttrium iron garnet (Y3Fe2(FeO4)3, YIG) and ferromagnetic materials or antiferromagnetic materials has attracted much attention because of its potential in applications such as spin pumping. In this study, a high-temperature and oxygen-poor environment was used to deposit a CoOx top layer on YIG for the observation of magnetic modulation. The temperature- and saturation-field-dependent bias shift of the YIG hysteresis loop was observed. The complex modulation of the magnetic behavior originated from the magnetic coupling of the antiferromagnetic CoO and an additional interfacial coupling of the ferromagnetic CoOx. Depth-profiling X-ray photoemission spectroscopy revealed the nonuniform stoichiometric distribution in the 20-nm CoOx layer, which confirmed the origin of the various magnetic coupling effects on the bottom YIG layer. The comparative ferromagnetic resonance measurements also demonstrated the considerable magnetic coupling effect on the dynamic magnetism...

Ceramics

In our previous work, we synthesized Pb(Zr0.52Ti0.48)O3 (PZT) ceramics by conventional and microw... more In our previous work, we synthesized Pb(Zr0.52Ti0.48)O3 (PZT) ceramics by conventional and microwave sintering methods and studied their structural and electrical properties. We observed that the microwave sintered PZT ceramics show higher densification, fine and uniform grain size, higher dielectric constant, remnant polarization (Pr), and spontaneous polarization (Ps) in comparison to conventional sintered ones. In the present work, we studied the microstructure, phase formation, dielectric, ferroelectric and piezoelectric properties of the PZT ceramics synthesized by using Spark plasma Sintering (SPS) method. The SPS sintering temperature is lower (300 to 400 °C) than both conventional and microwave sintering methods. The dielectric constant at room temperature and at transition temperature shows higher values compared to the microwave and conventional sintering methods. This SPS sintering technique is currently attracting growing attention among productions engineers and materia...

Journal of Magnetism and Magnetic Materials

Abstract Electric field control of magnetism has many potential applications in magnetic memory s... more Abstract Electric field control of magnetism has many potential applications in magnetic memory storage, sensors and spintronics. Epitaxial heterostructure involving ferroelectric (FE), ferromagnetic (FM) materials is one of the possible routes towards the realization of devices exploiting sizable magnetoelectric effects. In this study, we prepared multiferroic composite ferromagnet/ferrite/ferroelectric thin films on SrTiO3(1 1 1) substrate by electron beam evaporation and pulsed laser deposition. This system represents a prototypical example of interface between body-centered-cubic Fe, spinel NCZF (Ni0.25Cu0.05Zn0.7Fe1.96O3.96) and perovskite PZT (PbZr0.52Ti0.48O3), respectively. Magnetoelectric effects measured as a function of bias voltage on these heterostructures have been investigated via magneto-optical Kerr effect (MOKE). The magnetic coercivity ( H c ) considerably decreased as a voltage was applied. The H c values exhibited the similar variation whether the applied voltage was positive or negative and the reversibility was demonstrated. The strain coupling-mediated magnetoelectric effect and local heating effect are supposed to be the possible origins of H c variation.

Metallurgical and Materials Transactions A

Ba0.8Pb0.2TiO3 nanocrystalline ferroelectric ceramics were prepared by high-energy ball mill foll... more Ba0.8Pb0.2TiO3 nanocrystalline ferroelectric ceramics were prepared by high-energy ball mill followed by spark plasma sintering and conventional sintering techniques. Sintering behavior, microstructure, dielectric properties, and ferroelectric properties were investigated by XRD, SEM, dielectric spectrometer, and ferroelectric P-E loop tracer. It was found that the densification process was greatly enhanced during SPS. The sintering temperature was 673 K (400 °C) lower and the microstructure was much finer than that of conventionally sintered ceramics, and dense compacts with a higher density of over 99 pct were obtained. The average grain size in the conventionally sintered samples is up to about 200 nm, while that in the spark plasma sintered prepared sample is only about 100 nm. The room temperature and transition temperature dielectric constant of spark plasma-sintered samples is higher than that of the conventionally sintered sample. There is a noticeable difference in processing time between conventional sintering and spark plasma sintering, and the latter offers potential advantage in time saving also.

Applied Surface Science

In this study, the self-assembly of surface nanoclusters on 10-20-nm-thick Co 50 Pd 50 (Co-Pd) al... more In this study, the self-assembly of surface nanoclusters on 10-20-nm-thick Co 50 Pd 50 (Co-Pd) alloy thin films deposited on the Al 2 O 3 (0001) substrate was systematically investigated. When the Co-Pd alloy films were stored in an ambient environment, small nanodots gradually gathered to form large nanoclusters. Approximately 30 days after growth, a nanocluster array formed with an average lateral size of 100 ± 20 nm and average height of 10 ± 3 nm. After 100 days, the average lateral size and average height had increased to 140 ± 20 and 25 ± 5 nm, respectively. Cross-sectional investigation through transmission electron microscopy coupled with energy dispersive spectroscopy showed that the nanoclusters were mostly composed of Co oxide. A uniform Pd-rich underlayer had ben maintained underneath the self-assembled Cooxide nanoclusters. With the formation of a Co-oxide nanocluster array, the magnetic easy axis of the Co-Pd film gradually altered its direction from the pristine perpendicular to in-plane direction. The hydrogenation-induced spin-reorientation transition was also suppressed with the evolution of the surface Co-oxide nanoclusters.

Journal of Applied Physics

In this study, the application of bias voltage to 4–8 nm Co/275 nm ZnO heterostructures changed t... more In this study, the application of bias voltage to 4–8 nm Co/275 nm ZnO heterostructures changed the magnetic behavior reversibly or irreversibly, depending on the different voltage-induced mechanisms. The magnetic coercivity (Hc) monotonically decreased 20% when the small voltages of 0–8 V were applied. The Hc reduction was symmetric with the voltage polarity, and the reversibility was demonstrated by cyclically switching the bias voltage between 0 and 7 V. While a large voltage up to 40 V was applied to the Co/ZnO junction, the current heating effect became considerable and the Co-oxide was formed, as confirmed by depth-profiling X-ray photoemission spectroscopy analysis. The presence of Co-oxide in the Co films induced the irreversible reduction of the Kerr signal and Hc at room temperature. The considerable Hc enhancement at 130 K also indicates the exchange bias coupling effect from the antiferromagnetic Co-oxide.

Journal of Alloys and Compounds

Because of the Pd-catalyzed hydrogen dissociation and absorption, magnetic Pdalloys provide a mod... more Because of the Pd-catalyzed hydrogen dissociation and absorption, magnetic Pdalloys provide a model system for the investigation of the critical hydrogenation effect on magnetism. In this study, Co 50 Pd 50 (CoPd) alloy thin films were fabricated by e-beam-heated co-evaporation on Al 2 O 3 (0001) substrates. These films exhibited a thickness-dependent spin reorientation transition (SRT) from perpendicular direction to in-plane direction with increase of thickness. For 10-30 nm-thick CoPd alloy films with perpendicular magnetic anisotropy (PMA), hydrogenation triggered a SRT to an in-plane anisotropy. The reversibility of SRT was demonstrated by cyclicly changing the hydrogen gas pressure. Furthermore, hydrogenation-induced SRT randomized the magnetic domain orientation. In comparison with a bare CoPd film , a stronger PMA and a less pronounced hydrogenation-induced SRT were observed in a Pd-capped CoPd film. These observations suggest that the hydrogen content in CoPd alloy films can drastically and reversibly modify PMA, inferring the possible hydrogenationinduced charge transfer and modulation of electronic structure in CoPd. References:

Journal of Alloys and Compounds

Abstract In this study, Co x Pd 1− x alloy thin films and microstructures were grown through an e... more Abstract In this study, Co x Pd 1− x alloy thin films and microstructures were grown through an e-beam heated co-evaporation method on Al 2 O 3 (0001) substrates. The Fourier transformation analysis of high-resolution transmission electron microscopy images revealed a structural transition from a partial crystalline structure in the bottom layer region to a well-ordered crystalline structure in the top layer region. Increasing the Pd content in the Co x Pd 1− x thin films shifted the magnetic anisotropy toward the perpendicular direction. Correspondingly, magnetic force microscopy revealed submicrometer-scale magnetic domain structures for Co x Pd 1− x thin films, where x = 37%–50%. By combining oblique deposition and a grid mask, this study fabricated a 1-dimensional (1-D) microstructured Co 50 Pd 50 thin film that exhibited uniaxial magnetic anisotropy (UMA). Hydrogenation exerted a more sensitive effect on the magnetic properties of this structured thin film than it did on those of a continuous thin film. After hydrogen absorption, the magnetic coercivity was enhanced by approximately 50% and squareness by more than 100%. The reversibility of the hydrogenation effect on the UMA was also demonstrated. These observations suggest that the 1-D microstructured Co Pd film is sensitive and suitable for applications related to sensing hydrogen gas.

Advanced Materials Letters, 2015

ABSTRACT This work focuses on the high temperature dielectric and mechanical spectroscopic proper... more ABSTRACT This work focuses on the high temperature dielectric and mechanical spectroscopic properties of lead free relaxor Sodium Bismuth Titanate (NBT) ceramics, fabricated by conventional ceramic double sintering method. Systematic measurements of dielectric and mechanical properties have been performed as a function of temperature. A sequence of phase transitions has been studied by both dielectric and anelastic measurements. Three internal friction peaks were observed near 350, 200 and 120 °C. The 350 °C-peak corresponds to a transition associated with the tetragonal (P4bm) to rhombohedral (R3c) phase, and the 200 °C-peak is related to the ferroelectric to antiferroelectric phase transition. The 120 °C-peak could be ascribed to the interaction between the domain walls and the diffusion of oxygen vacancies in the domains

Journal of Alloys and Compounds, 2016

Abstract The effects of reversible hydrogen absorption on the magnetism of Co–Pd alloy thin films... more Abstract The effects of reversible hydrogen absorption on the magnetism of Co–Pd alloy thin films were investigated. Evident enhancement of magnetic coercivity Hc and squareness, measured by magneto optical Kerr effect (MOKE), was observed in the magnetic hysteresis loops of films with Pd content of 61%, 76%, and 86%, whereas no observable changes were noted in the MOKE hysteresis loops of Co-rich alloy films. For Co39 Pd61, perpendicular magnetic anisotropy increased as film thickness increased from 10 to 20 nm. Hydrogenation induced three times enhancement of perpendicular Hc for 20 nm Co39 Pd61, whereas the Hc change in 10 nm Co39 Pd61 was within 10%. A Pd-rich alloy composition and appropriate film thickness for perpendicular magnetization are both crucial for hydrogen absorption-induced effect on magnetic properties, i.e. the enhancement of Hc and squareness as well as the reduction of Kerr rotation angle.

Applied Surface Science, 2016

Abstract The bilayers of Pd and Fe with different thickness and relative positions were grown on ... more Abstract The bilayers of Pd and Fe with different thickness and relative positions were grown on Si(1 1 1)-7 × 7 surface at room temperature. For the investigation of the thermal annealing induced inter-diffusion and the corresponding magnetic behavior, Auger electron spectroscopy (AES) measurement was carried out after various annealing processes, including the variation of annealing duration and temperature. With the annealing temperature of 300–500 K, the Pd/Fe bilayers were stable. Slight Si segregated into the thin film at around 700 K. Above 700 K, more serious Si segregation occurred and most of the Pd/Fe bilayer was mixed with Si, forming a silicide layer. 700–800 K annealing also induced change of Pd/Fe AES ratio, indicating the inter-diffusion between Pd and Fe layers. To overcome the unavoidable silicide formation induced magnetic dead layer, a relative thick Fe film of 20 ML capped with 1.5 ML Pd was chosen for the investigation of magnetism. The magnetic coercivity (H c ) increased by 2–3 times with the annealing temperature up to 740 K. Obvious hydrogenation effect was observed in 710 K-annealed sample; the in-plane H c increased by more than 10% when the hydrogen pressure was above 200 mbar. After further annealing at 740–800 K, the hydrogenation effect on H c became nearly unobservable. The annealing induced Pd-rich magnetic interface is supposed to dominate the hydrogenation effect on magnetism.

Applied Surface Science, 2015

Abstract The surface morphology, magnetism and chemical state of Fe coverage on the surface of mo... more Abstract The surface morphology, magnetism and chemical state of Fe coverage on the surface of molybdenum disulfide (MoS2) were investigated using scanning tunneling microscopy, magneto-optical Kerr effect, and depth-profiling X-ray photoemission spectroscopy (XPS). Fe deposition on the MoS2 substrate resulted in a nanoparticle array with the particle size ranged few nanometers (∼3±1 nm). For low-coverage Fe deposition

Journal of Electronic Materials, 2015

A combination of mechanical alloying and spark plasma sintering (SPS) was used to process Ba0.9Sr... more A combination of mechanical alloying and spark plasma sintering (SPS) was used to process Ba0.9Sr0.1TiO3 (abbreviated as BST) with a high-density and homogeneous microstructure, at a temperature 300°C to 400°C lower than that used in conventional sintering. The SPS technique was employed to prepare dense Ba0.9Sr0.1TiO3 nano-ceramics at different temperatures ranging from 800°C to 1000°C within a very short time. The SPS samples were characterized by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, electrical, ferroelectric and piezoelectric property measurements. The dielectric constant both at room temperature and Curie temperature increased with increasing SPS temperature. The coercive field increased continuously with rising SPS temperature, but the spontaneous polarization increased gradually up to 900°C and then decreased considerably with further increases in SPS temperature. In addition, the prepared nano-ceramic exhibited excellent piezoelectric properties.

Journal of the European Ceramic Society, 2015

ABSTRACT BiFeO3 ferrite ceramics were synthesized by spark plasma sintering (SPS) and conventiona... more ABSTRACT BiFeO3 ferrite ceramics were synthesized by spark plasma sintering (SPS) and conventional solid-state synthesis in conjunction with high-energy ball milling. They were evaluated by different characterization techniques such as X-ray diffraction, scanning electron microscopy, and dielectric, ferroelectric and magnetic measurements. The 750 °C-SPS prepared ceramic samples possessed a single perovskite structure and they were compared with the conventionally synthesized ones. The relative density and average grain size were approximately 95% and <200 nm for the SPS-prepared samples, while they were about 89% and >300 nm for the conventionally prepared ones. In comparison with the conventionally synthesized ceramics, the SPS-prepared ones exhibited improved dielectric, ferroelectric and magnetic properties. These should be associated with high-density, fine-grained and uniform microstructure created by SPS in conjunction with high-energy ball milling, which could be a promising technique for fabricating high-quality BiFeO3 multiferroic ceramics.

Indian Journal of Physics, 2015

ABSTRACT

Journal of Materials Science: Materials in Electronics, 2007

... Reddy Á M. Venkata Ramana Á E. Rajagopal Á KV Sivakumar (&) Ceramic Composite Materials L... more ... Reddy Á M. Venkata Ramana Á E. Rajagopal Á KV Sivakumar (&) Ceramic Composite Materials Laboratory, Department of Physics, Sri Krishnadevaraya University, SV Puram, Anantapur 515 003, India e-mail: sivani.sivakumar@gmail.com VRK Murthy Microwave Laboratory ...

In our previous work, we synthesized Pb(Zr0.52Ti0.48)O3 (PZT) ceramics by conventional and microw... more In our previous work, we synthesized Pb(Zr0.52Ti0.48)O3 (PZT) ceramics by conventional and microwave sintering methods and studied their structural and electrical properties. We observed that the microwave sintered PZT ceramics show higher densification, fine and uniform grain size, higher dielectric constant, remnant polarization (Pr), and spontaneous polarization (Ps) in comparison to conventional sintered ones. In the present work, we studied the microstructure, phase formation, dielectric, ferroelectric and piezoelectric properties of the PZT ceramics synthesized by using Spark plasma Sintering (SPS) method. The SPS sintering temperature is lower (300 to 400 ◦C) than both conventional and microwave sintering methods. The dielectric constant at room temperature and at transition temperature shows higher values compared to the microwave and conventional sintering methods. This SPS sintering technique is currently attracting growing attention among productions engineers and materia...

Sodium Alginate doped with Copper oxide composite polymer films was developed through Solution ca... more Sodium Alginate doped with Copper oxide composite polymer films was developed through Solution casting method. The structural properties on these polymer nanocomposite thin films were characterized by X-ray diffraction. XRD shows all the individual constituents of these composites and the CuO peaks increases as CuO increased in the composites. Thermogravimetric analysis (TGA) technique was used for thermal stability of these composites. Temperature dependence of dielectric properties of these SA/CuO nanocopmosite polymer thin films we measured in the frequency range from 100 Hz to 5 MHz. Dielectric properties of these composites strongly depends on the percentage of CuO and structure of these matrix. These type of composites are attracted much attention in battery applications.

Journal of Alloys and Compounds, 2021

The combination of yttrium iron garnet (Y3Fe2(FeO4)3, YIG) and ferromagnetic materials or antifer... more The combination of yttrium iron garnet (Y3Fe2(FeO4)3, YIG) and ferromagnetic materials or antiferromagnetic materials has attracted much attention because of its potential in applications such as spin pumping. In this study, a high-temperature and oxygen-poor environment was used to deposit a CoOx top layer on YIG for the observation of magnetic modulation. The temperature- and saturation-field-dependent bias shift of the YIG hysteresis loop was observed. The complex modulation of the magnetic behavior originated from the magnetic coupling of the antiferromagnetic CoO and an additional interfacial coupling of the ferromagnetic CoOx. Depth-profiling X-ray photoemission spectroscopy revealed the nonuniform stoichiometric distribution in the 20-nm CoOx layer, which confirmed the origin of the various magnetic coupling effects on the bottom YIG layer. The comparative ferromagnetic resonance measurements also demonstrated the considerable magnetic coupling effect on the dynamic magnetism...

Ceramics

In our previous work, we synthesized Pb(Zr0.52Ti0.48)O3 (PZT) ceramics by conventional and microw... more In our previous work, we synthesized Pb(Zr0.52Ti0.48)O3 (PZT) ceramics by conventional and microwave sintering methods and studied their structural and electrical properties. We observed that the microwave sintered PZT ceramics show higher densification, fine and uniform grain size, higher dielectric constant, remnant polarization (Pr), and spontaneous polarization (Ps) in comparison to conventional sintered ones. In the present work, we studied the microstructure, phase formation, dielectric, ferroelectric and piezoelectric properties of the PZT ceramics synthesized by using Spark plasma Sintering (SPS) method. The SPS sintering temperature is lower (300 to 400 °C) than both conventional and microwave sintering methods. The dielectric constant at room temperature and at transition temperature shows higher values compared to the microwave and conventional sintering methods. This SPS sintering technique is currently attracting growing attention among productions engineers and materia...

Journal of Magnetism and Magnetic Materials

Abstract Electric field control of magnetism has many potential applications in magnetic memory s... more Abstract Electric field control of magnetism has many potential applications in magnetic memory storage, sensors and spintronics. Epitaxial heterostructure involving ferroelectric (FE), ferromagnetic (FM) materials is one of the possible routes towards the realization of devices exploiting sizable magnetoelectric effects. In this study, we prepared multiferroic composite ferromagnet/ferrite/ferroelectric thin films on SrTiO3(1 1 1) substrate by electron beam evaporation and pulsed laser deposition. This system represents a prototypical example of interface between body-centered-cubic Fe, spinel NCZF (Ni0.25Cu0.05Zn0.7Fe1.96O3.96) and perovskite PZT (PbZr0.52Ti0.48O3), respectively. Magnetoelectric effects measured as a function of bias voltage on these heterostructures have been investigated via magneto-optical Kerr effect (MOKE). The magnetic coercivity ( H c ) considerably decreased as a voltage was applied. The H c values exhibited the similar variation whether the applied voltage was positive or negative and the reversibility was demonstrated. The strain coupling-mediated magnetoelectric effect and local heating effect are supposed to be the possible origins of H c variation.

Metallurgical and Materials Transactions A

Ba0.8Pb0.2TiO3 nanocrystalline ferroelectric ceramics were prepared by high-energy ball mill foll... more Ba0.8Pb0.2TiO3 nanocrystalline ferroelectric ceramics were prepared by high-energy ball mill followed by spark plasma sintering and conventional sintering techniques. Sintering behavior, microstructure, dielectric properties, and ferroelectric properties were investigated by XRD, SEM, dielectric spectrometer, and ferroelectric P-E loop tracer. It was found that the densification process was greatly enhanced during SPS. The sintering temperature was 673 K (400 °C) lower and the microstructure was much finer than that of conventionally sintered ceramics, and dense compacts with a higher density of over 99 pct were obtained. The average grain size in the conventionally sintered samples is up to about 200 nm, while that in the spark plasma sintered prepared sample is only about 100 nm. The room temperature and transition temperature dielectric constant of spark plasma-sintered samples is higher than that of the conventionally sintered sample. There is a noticeable difference in processing time between conventional sintering and spark plasma sintering, and the latter offers potential advantage in time saving also.

Applied Surface Science

In this study, the self-assembly of surface nanoclusters on 10-20-nm-thick Co 50 Pd 50 (Co-Pd) al... more In this study, the self-assembly of surface nanoclusters on 10-20-nm-thick Co 50 Pd 50 (Co-Pd) alloy thin films deposited on the Al 2 O 3 (0001) substrate was systematically investigated. When the Co-Pd alloy films were stored in an ambient environment, small nanodots gradually gathered to form large nanoclusters. Approximately 30 days after growth, a nanocluster array formed with an average lateral size of 100 ± 20 nm and average height of 10 ± 3 nm. After 100 days, the average lateral size and average height had increased to 140 ± 20 and 25 ± 5 nm, respectively. Cross-sectional investigation through transmission electron microscopy coupled with energy dispersive spectroscopy showed that the nanoclusters were mostly composed of Co oxide. A uniform Pd-rich underlayer had ben maintained underneath the self-assembled Cooxide nanoclusters. With the formation of a Co-oxide nanocluster array, the magnetic easy axis of the Co-Pd film gradually altered its direction from the pristine perpendicular to in-plane direction. The hydrogenation-induced spin-reorientation transition was also suppressed with the evolution of the surface Co-oxide nanoclusters.

Journal of Applied Physics

In this study, the application of bias voltage to 4–8 nm Co/275 nm ZnO heterostructures changed t... more In this study, the application of bias voltage to 4–8 nm Co/275 nm ZnO heterostructures changed the magnetic behavior reversibly or irreversibly, depending on the different voltage-induced mechanisms. The magnetic coercivity (Hc) monotonically decreased 20% when the small voltages of 0–8 V were applied. The Hc reduction was symmetric with the voltage polarity, and the reversibility was demonstrated by cyclically switching the bias voltage between 0 and 7 V. While a large voltage up to 40 V was applied to the Co/ZnO junction, the current heating effect became considerable and the Co-oxide was formed, as confirmed by depth-profiling X-ray photoemission spectroscopy analysis. The presence of Co-oxide in the Co films induced the irreversible reduction of the Kerr signal and Hc at room temperature. The considerable Hc enhancement at 130 K also indicates the exchange bias coupling effect from the antiferromagnetic Co-oxide.

Journal of Alloys and Compounds

Because of the Pd-catalyzed hydrogen dissociation and absorption, magnetic Pdalloys provide a mod... more Because of the Pd-catalyzed hydrogen dissociation and absorption, magnetic Pdalloys provide a model system for the investigation of the critical hydrogenation effect on magnetism. In this study, Co 50 Pd 50 (CoPd) alloy thin films were fabricated by e-beam-heated co-evaporation on Al 2 O 3 (0001) substrates. These films exhibited a thickness-dependent spin reorientation transition (SRT) from perpendicular direction to in-plane direction with increase of thickness. For 10-30 nm-thick CoPd alloy films with perpendicular magnetic anisotropy (PMA), hydrogenation triggered a SRT to an in-plane anisotropy. The reversibility of SRT was demonstrated by cyclicly changing the hydrogen gas pressure. Furthermore, hydrogenation-induced SRT randomized the magnetic domain orientation. In comparison with a bare CoPd film , a stronger PMA and a less pronounced hydrogenation-induced SRT were observed in a Pd-capped CoPd film. These observations suggest that the hydrogen content in CoPd alloy films can drastically and reversibly modify PMA, inferring the possible hydrogenationinduced charge transfer and modulation of electronic structure in CoPd. References:

Journal of Alloys and Compounds

Abstract In this study, Co x Pd 1− x alloy thin films and microstructures were grown through an e... more Abstract In this study, Co x Pd 1− x alloy thin films and microstructures were grown through an e-beam heated co-evaporation method on Al 2 O 3 (0001) substrates. The Fourier transformation analysis of high-resolution transmission electron microscopy images revealed a structural transition from a partial crystalline structure in the bottom layer region to a well-ordered crystalline structure in the top layer region. Increasing the Pd content in the Co x Pd 1− x thin films shifted the magnetic anisotropy toward the perpendicular direction. Correspondingly, magnetic force microscopy revealed submicrometer-scale magnetic domain structures for Co x Pd 1− x thin films, where x = 37%–50%. By combining oblique deposition and a grid mask, this study fabricated a 1-dimensional (1-D) microstructured Co 50 Pd 50 thin film that exhibited uniaxial magnetic anisotropy (UMA). Hydrogenation exerted a more sensitive effect on the magnetic properties of this structured thin film than it did on those of a continuous thin film. After hydrogen absorption, the magnetic coercivity was enhanced by approximately 50% and squareness by more than 100%. The reversibility of the hydrogenation effect on the UMA was also demonstrated. These observations suggest that the 1-D microstructured Co Pd film is sensitive and suitable for applications related to sensing hydrogen gas.

Advanced Materials Letters, 2015

ABSTRACT This work focuses on the high temperature dielectric and mechanical spectroscopic proper... more ABSTRACT This work focuses on the high temperature dielectric and mechanical spectroscopic properties of lead free relaxor Sodium Bismuth Titanate (NBT) ceramics, fabricated by conventional ceramic double sintering method. Systematic measurements of dielectric and mechanical properties have been performed as a function of temperature. A sequence of phase transitions has been studied by both dielectric and anelastic measurements. Three internal friction peaks were observed near 350, 200 and 120 °C. The 350 °C-peak corresponds to a transition associated with the tetragonal (P4bm) to rhombohedral (R3c) phase, and the 200 °C-peak is related to the ferroelectric to antiferroelectric phase transition. The 120 °C-peak could be ascribed to the interaction between the domain walls and the diffusion of oxygen vacancies in the domains

Journal of Alloys and Compounds, 2016

Abstract The effects of reversible hydrogen absorption on the magnetism of Co–Pd alloy thin films... more Abstract The effects of reversible hydrogen absorption on the magnetism of Co–Pd alloy thin films were investigated. Evident enhancement of magnetic coercivity Hc and squareness, measured by magneto optical Kerr effect (MOKE), was observed in the magnetic hysteresis loops of films with Pd content of 61%, 76%, and 86%, whereas no observable changes were noted in the MOKE hysteresis loops of Co-rich alloy films. For Co39 Pd61, perpendicular magnetic anisotropy increased as film thickness increased from 10 to 20 nm. Hydrogenation induced three times enhancement of perpendicular Hc for 20 nm Co39 Pd61, whereas the Hc change in 10 nm Co39 Pd61 was within 10%. A Pd-rich alloy composition and appropriate film thickness for perpendicular magnetization are both crucial for hydrogen absorption-induced effect on magnetic properties, i.e. the enhancement of Hc and squareness as well as the reduction of Kerr rotation angle.

Applied Surface Science, 2016

Abstract The bilayers of Pd and Fe with different thickness and relative positions were grown on ... more Abstract The bilayers of Pd and Fe with different thickness and relative positions were grown on Si(1 1 1)-7 × 7 surface at room temperature. For the investigation of the thermal annealing induced inter-diffusion and the corresponding magnetic behavior, Auger electron spectroscopy (AES) measurement was carried out after various annealing processes, including the variation of annealing duration and temperature. With the annealing temperature of 300–500 K, the Pd/Fe bilayers were stable. Slight Si segregated into the thin film at around 700 K. Above 700 K, more serious Si segregation occurred and most of the Pd/Fe bilayer was mixed with Si, forming a silicide layer. 700–800 K annealing also induced change of Pd/Fe AES ratio, indicating the inter-diffusion between Pd and Fe layers. To overcome the unavoidable silicide formation induced magnetic dead layer, a relative thick Fe film of 20 ML capped with 1.5 ML Pd was chosen for the investigation of magnetism. The magnetic coercivity (H c ) increased by 2–3 times with the annealing temperature up to 740 K. Obvious hydrogenation effect was observed in 710 K-annealed sample; the in-plane H c increased by more than 10% when the hydrogen pressure was above 200 mbar. After further annealing at 740–800 K, the hydrogenation effect on H c became nearly unobservable. The annealing induced Pd-rich magnetic interface is supposed to dominate the hydrogenation effect on magnetism.

Applied Surface Science, 2015

Abstract The surface morphology, magnetism and chemical state of Fe coverage on the surface of mo... more Abstract The surface morphology, magnetism and chemical state of Fe coverage on the surface of molybdenum disulfide (MoS2) were investigated using scanning tunneling microscopy, magneto-optical Kerr effect, and depth-profiling X-ray photoemission spectroscopy (XPS). Fe deposition on the MoS2 substrate resulted in a nanoparticle array with the particle size ranged few nanometers (∼3±1 nm). For low-coverage Fe deposition

Journal of Electronic Materials, 2015

A combination of mechanical alloying and spark plasma sintering (SPS) was used to process Ba0.9Sr... more A combination of mechanical alloying and spark plasma sintering (SPS) was used to process Ba0.9Sr0.1TiO3 (abbreviated as BST) with a high-density and homogeneous microstructure, at a temperature 300°C to 400°C lower than that used in conventional sintering. The SPS technique was employed to prepare dense Ba0.9Sr0.1TiO3 nano-ceramics at different temperatures ranging from 800°C to 1000°C within a very short time. The SPS samples were characterized by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, electrical, ferroelectric and piezoelectric property measurements. The dielectric constant both at room temperature and Curie temperature increased with increasing SPS temperature. The coercive field increased continuously with rising SPS temperature, but the spontaneous polarization increased gradually up to 900°C and then decreased considerably with further increases in SPS temperature. In addition, the prepared nano-ceramic exhibited excellent piezoelectric properties.

Journal of the European Ceramic Society, 2015

ABSTRACT BiFeO3 ferrite ceramics were synthesized by spark plasma sintering (SPS) and conventiona... more ABSTRACT BiFeO3 ferrite ceramics were synthesized by spark plasma sintering (SPS) and conventional solid-state synthesis in conjunction with high-energy ball milling. They were evaluated by different characterization techniques such as X-ray diffraction, scanning electron microscopy, and dielectric, ferroelectric and magnetic measurements. The 750 °C-SPS prepared ceramic samples possessed a single perovskite structure and they were compared with the conventionally synthesized ones. The relative density and average grain size were approximately 95% and <200 nm for the SPS-prepared samples, while they were about 89% and >300 nm for the conventionally prepared ones. In comparison with the conventionally synthesized ceramics, the SPS-prepared ones exhibited improved dielectric, ferroelectric and magnetic properties. These should be associated with high-density, fine-grained and uniform microstructure created by SPS in conjunction with high-energy ball milling, which could be a promising technique for fabricating high-quality BiFeO3 multiferroic ceramics.

Indian Journal of Physics, 2015

ABSTRACT