Search Results - (Author, Cooperation:R. Ramesh)

2018-03-27

American Physical Society (APS)

1098-0121

1095-3795

Physics

Magnetism

2018-01-04

Nature Publishing Group (NPG)

2041-1723

Biology

Chemistry and Pharmacology

Natural Sciences in General

Physics

2014-12-19

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2016-01-28

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2016-03-05

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2011-11-19

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1432-1114

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Abstract The freejet characteristics of radially lobed nozzles were studied experimentally. Schlieren photographs of the freejets revealed that the lobes of the nozzles gave rise to alternate expansion-compression regions. For two of the lobed nozzles the potential core was reduced by a factor of three relative to the conical nozzle tested. The jet spread and the shear layer thickness of the lobed nozzles were considerably higher than those for the conical nozzle. The jet width and the shear layer thickness were higher in the major plane; however, the shear layer growth rate was higher in the minor plane of the lobed nozzles. Jet axial thrust measurements revealed a thrust loss of 14% for two of the lobed nozzles tested.

Electronic Resource

2018-11-24

American Association for the Advancement of Science (AAAS)

2375-2548

Natural Sciences in General

1089-7550

AIP Digital Archive

Physics

Incorporation of magnetic, metallic quantum wells in semiconductors has added new dimensions to epitaxial heterostructures. Magnetotransport experiments explore the interplay of bandstructure, quantum confinement, dimensionality and magnetism. Rare earth monoarsenides, like ErAs, have been grown as buried layers in GaAs. The material is a semimetallic antiferromagnet that exhibits spin-disorder scattering at the Néel point. By diluting the ErAs with ScAs, lattice matching to GaAs can be achieved and these materials exhibit Shubnikov–de Haas (SdH) oscillations. The exchange interaction between the conduction band and the 4f spin deduced from SdH agrees with that derived from spin-disorder scattering. Surprisingly, down to 2 monolayers, quantum confinement does not convert the semimetal into a semiconductor. This is not understood. But, the Neel point can be seen to drop by a factor of two by following the spin-disorder scattering. In the few monolayer regime the magnetotransport can change its character and is more aptly described by bound magnetic polarons. More recently, ferromagnetic, metastable, τ-MnAl layers have also been grown on GaAs. Thin layers of τ-MnAl grow in such a way that the tetragonal axis is perpendicular to the interface. The extraordinary Hall effect and longitudinal magnetotransport confirm that the moment has the desired property of being oriented perpendicular to the interface.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

In situ superconducting YBa2Cu3O7−x films with Tc0 up to 87 K and Jc, 77 K up to 6×104 A/cm2 were prepared on Si substrates with MgAl2O4 and BaTiO3 double-buffer layers. The epitaxial relations between various layers were established by transmission electron microscopy. The MgAl2O4 layer is heavily faulted. The subsequent BaTiO3 layer stops most of the faults, provides a template for the YBa2Cu3O7−x growth, and partially screens off the stress due to different thermal expansion coefficients. The microstructure of the YBa2Cu3O7−x layer is very similar to that of the films deposited directly on SrTiO3, exhibiting a homogeneous heavily faulted single-crystal-like structure free from secondary phases and grain boundaries. The slight degradation of the transport properties is attributed to residual thermal stress.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The effect of post sintering heat treatments on the intrinsic coercivity of Fe-Nd-B magnets is studied. It is shown that the large anisotropy in thermal expansion coefficients in the a and c directions of the tetragonal unit cell as well as the anisotropy in thermal expansion properties between the matrix and the grain boundary phase, will lead to stresses upon quenching from the sintering temperature. The effect of stresses due to water quenching have been incorporated into the model developed in the companion paper (Part I). This quenching stress manifests itself as a stress anisotropy constant, which opposes the magnetocrystalline anisotropy constant. Thus, the effective anisotropy field, near the interfaces, is reduced from its value in an optimally cooled sample, leading to a lower iHc. It is also shown through step quenching experiments that the effect of quenching stresses can be nullified by cooling slowly to below a specific temperature following which quenching does not influence the intrinsic coercivity.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Ultrathin coherent epitaxial films of ferromagnetic τ(Mn,Ni)0.60Al0.40 have been grown by molecular beam epitaxy on GaAs substrates. X-ray scattering and cross-sectional transmission electron microscopy measurements confirm that the c axis of the tetragonal τ unit cell is aligned normal to the (001) GaAs substrate. Measurements of the extraordinary Hall effect (EHE) show that the films are perpendicularly magnetized, exhibiting EHE resistivities saturating in the range of 3.3–7.1 μΩ-cm at room temperature. These values of EHE resistivity correspond to signals as large as +7 and −7 mV for the two magnetic states of the film with a measurement current of 1 mA. Switching between the two magnetic states is found to occur at distinct field values that depend on the previously applied maximum field. These observations suggest that the films are magnetically uniform. As such, τ(Mn,Ni)Al films may be an excellent medium for high-density storage of binary information.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A microstructure based magnetization reversal model is presented for sintered Fe-Nd-B magnets. The model makes use of a position dependent domain wall energy profile to correlate the intrinsic coercivity, iHc (and the nucleation field) to the magnetocrystalline anisotropy field. The inferences of the model are verified through the study of the effect of Dy and Ce additions on the iHc as well as by examining the temperature dependence of iHc and anisotropy field existing in published literature. It is inferred from this model that selective modifications of the interfaces could lead to appreciable improvements in the magnetic properties.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Single-crystal films of τ-MnAl are grown by molecular-beam epitaxy on GaAs substrates. The extraordinary Hall effect shows a large hysteresis loop, and the magnetization is found to have a substantial component perpendicular to the plane of the film.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We have studied the effect of misfit strain on the microstructure and properties of ferroelectric lead zirconate titanate thin films. We have changed the misfit strain by varying the film thickness and studied the thickness effect on the domain formation of epitaxial PbZr0.2Ti0.8O3 (PZT) films grown by pulsed laser deposition on (001) LaAlO3 substrates with La0.5Sr0.5CoO3 (LSCO) electrodes. The nominal thickness of the PZT films was varied from 60 to 400 nm with the LSCO electrode thickness kept constant at 50 nm. X-ray diffraction experiments show that the films relax via the formation of a domains, the fraction of which increase with the ferroelectric film thickness. The c-axis lattice constant of PZT films calculated from the 002 reflection decreases with increasing film thickness and approaches the bulk value of ∼0.413 nm in the films thicker than 300 nm. Cross-sectional transmission electron microscopy images reveal that the a-domain fraction and period increase with increasing film thickness. The relaxation of misfit strain in the film is accompanied by systematic changes in the polarization properties, as well as the switching fields, quantified by the coercive field and the activation field. © 1999 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Voltage-modulated scanning force microscopy (Piezoresponse microscopy) is applied to investigate the domain structure in epitaxial PbZr0.2Ti0.8O3 ferroelectric thin films grown on (001) SrTiO3. By monitoring the vertical and lateral differential signals from the photodetector of the atomic force microscope it is possible to separate out and observe the out-of-plane and in-plane polarization vectors in the thin film individually. The relative orientation of the polarization vectors across a 90° domain wall is observed. Nucleation of new reversed 180° domains at the 90° domain wall is studied and its impact on the rotation of polarization within the a domain is analyzed as a function of reversal time. © 2002 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Electrodes can impact the device performance of ferroelectric capacitors in several ways. The present controlled studies on Pb (Nb, Zr, Ti)O3 with Pt, (La, Sr)CoO3 and SrRuO3 is a clear demonstration of the role of electrodes in impacting the leakage current mechanism of the ferroelectric capacitors and their reliability properties. The oxide electrode capacitors show predominantly nonblocking contact and good fatigue and imprint properties. Pt electrode capacitors show blocking contacts, long term leakage current relaxation, and poor fatigue and imprint properties. The nature of the temperature and voltage dependence of leakage current relaxation in Pt capacitors indicates trapping of charge carriers to be the cause for the observed relaxation. A good correlation between leakage current relaxation and the rate of polarization loss during fatigue and the similarity in their voltage and temperature dependence suggests trapping (of charged carriers/domains, respectively) as common to both phenomena. © 2001 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Epitaxial La0.7Ca0.3MnO3 thin films on the SrTiO3(100) surface have been irradiated with 250 MeV Ag17+ ions at different nominal fluence values in the range of 5×1010–4×1011 ions/cm2, resulting in columnar defects. At low fluences these defects cause changes in material properties that are small and scale linearly with dosage. Above a threshold fluence value ∼3×1011 ions/cm2 dramatic changes are observed, including an order of magnitude increase in the resistivity and 50 K drop in the Curie temperature. Transmission electron microscopy measurements show that the changes are associated with a phase transformation of the undamaged region between the columnar defects. The transformed phase has a diffraction pattern very similar to that seen in charge-ordered La0.5Ca0.5MnO3. We propose that above a critical level of ion damage, strains caused by the presence of the columnar defects induce a charge-ordering phase transition that causes the observed dramatic changes in physical properties. We speculate that a conceptually similar surface-induced charge ordering may be responsible for the "dead layer" observed in very thin strained films, and the dramatic changes in optical properties induced by polishing, and that an impurity-induced charge ordering causes the extreme sensitivity of properties to lattice substitution. © 2000 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Using the phenomenon of ferromagnetic antiresonance (FMAR) in ceramic samples of La0.67Ba0.33MnO3 at 10 GHz, we report a large magneto-impedance MI=Rs(H(parallel))−Rs(H⊥)]/Rs(H(parallel)), where Rs is the microwave surface resistance and H the applied field. The MI reaches 30% at a field of 30 mT near room temperature. The FMAR also lets us measure M(T) by following Rs as a function of T and H. ©1997 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The effect of an electric field on GMR oxide films was studied in a MOSFET configuration where the gate dielectric was a layer of SrTiO3 epitaxially grown on an underlying layer of the manganate which served as the source/drain. The response of the manganate channel was studied for different gate voltages. The following significant features were observed. The peak resistance temperature shifted to lower temperature for both polarities of the field. The resistance change varied quadratically with the field indicating the dominance of strain or polarization effects. In dynamical studies of the system using the gate voltage as an excitation the system showed anomalous slowing down near the peak of the resistivity. These results are understood on the basis of a stress effect on the film due to electro-elastic effects in the SrTiO3 layer, which introduces a tensile stress in the manganate layer upon the application of a gate voltage. The anomalous slowing down of the system near the ferromagnetic phase transition suggests a strong coupling between the spins, transport and structural distortions in the system. © 1996 American Institute of Physics.

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