Charles T. Rogers

Inductive measurement of ultrafast magnetization dynamics in thin-film Permalloy

An inductive technique for the measurement of dynamical magnetic processes in thin-film materials is described. The technique is demonstrated using 50 nm films of Permalloy (Ni81Fe19). Data are presented for impulse- and step-response experiments with the applied field pulse oriented in the plane of the film and transverse to the anisotropy axis. Rotation times as short as 200 ps and free oscillations of the magnetization after excitation are clearly observed. The oscillation frequency increases as the dc bias field parallel to the anisotropy axis increases as predicted by classical gyromagnetic theory. The data are fitted to the Landau–Lifshitz equation, and damping parameters are determined as a function of dc bias field. Damping for both impulse and step excitations exhibits a strong dependence on bias field. Damping for step excitations is characterized by an anomalous transient damping which rapidly increases at low dc bias field. Transformation of the data to the frequency domain reveals a higher or...

a‐axis oriented epitaxial YBa2Cu3O7−x‐PrBa2Cu3O7−y heterostructures

We have grown YBa2Cu3O7−x‐PrBa2Cu3O7−y heterostructures with the perovskite a axis normal to the surface of a variety of (001) oriented substrates using the pulsed laser deposition technique. X‐ray diffraction studies indicate little or no formation of the traditional c‐axis normal orientation usually nucleated on these substrates, while ion channeling, transmission electron microscopy (TEM), and Raman scattering studies reveal a highly ordered crystalline structure similar in quality to that obtained in the best c‐axis oriented films. Superconducting transition temperatures, for current transport in the plane of the films, consistently exceed 80 K, but are probably lower than the optimum 92 K transition expected for YBa2Cu3O7−x due to the presence of growth twin boundaries between grains with the c axis oriented along the two substrate directions.We have grown YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}}-PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}} heterostructures with the perovskite {ital a} axis normal to the surface of a variety of (001) oriented substrates using the pulsed laser deposition technique. X-ray diffraction studies indicate little or no formation of the traditional {ital c}-axis normal orientation usually nucleated on these substrates, while ion channeling, transmission electron microscopy (TEM), and Raman scattering studies reveal a highly ordered crystalline structure similar in quality to that obtained in the best {ital c}-axis oriented films. Superconducting transition temperatures, for current transport in the plane of the films, consistently exceed 80 K, but are probably lower than the optimum 92 K transition expected for YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} due to the presence of growth twin boundaries between grains with the {ital c} axis oriented along the two substrate directions.

Fabrication of heteroepitaxial YBa2Cu3O7−x‐PrBa2Cu3O7−x‐YBa2Cu3O7−x Josephson devices grown by laser deposition

We have fabricated Josephson weak links from i n s i t u laser‐deposited four‐layer structures of YBa2Cu3O7−x /PrBa2Cu3O7−x /YBa2Cu3O7−x Au. These devices show superconductor/normal‐metal/superconductor‐like current‐voltage characteristics with good areal scaling of both the critical currentsI c and resistancesR j , with I c R j ≊3.5 mV. Constant‐voltage current steps observed in response to 84 GHz mm‐wave radiation and modulation of the dc supercurrent in a transverse magnetic field demonstrate that both the ac and dc Josephson effects occur in these devices.

All a‐axis oriented YBa2Cu3O7−y‐PrBa2Cu3O7−z‐YBa2Cu3O7−y Josephson devices operating at 80 K

We have demonstrated the controllable, reproducible fabrication of nonhysteretic Josephson devices with excess‐current weak‐link characteristics at temperatures up to 80 K. The devices are patterned from in situ deposited a‐axis oriented YBa2Cu3O7−y‐PrBa2Cu3O7−z‐YBa2Cu3O7−y trilayers grown on SrTiO3(001) substrates. Control of the critical current density and resistance is achieved by varying the thickness of the PrBa2Cu3O7−z barrier layer. Critical current densities in excess of 104 A/cm2 have been reproducibly measured; good uniformity across the wafer is obtained with device parameters scaling with device area. Strong constant‐voltage current steps are observed under 11.2 GHz microwave radiation at temperatures up to and above 80 K.

Improved low frequency and microwave dielectric response in strontium titanate thin films grown by pulsed laser ablation

We have grown epitaxial strontium titanate films on lanthanum aluminate substrates at a range of oxygen pressures and substrate temperatures. The complex dielectric function was measured as a function of temperature and electric field bias using a microwave ring resonator and a flip-chip technique. The films having the highest dielectric constant were grown with an oxygen pressure of 600 mTorr and showed large grains in the plane of the film. The small-signal dielectric constant of these films could be changed by a factor of 4 by applying an electric field. The films with the highest dielectric constant showed increased losses, but an improved figure of merit for application to tunable circuits.

Microwave properties of highly oriented YBa2Cu3O7−x thin films

We have performed intra‐ and extra‐cavity microwave frequency (1–100 GHz) measurements on high quality Y1Ba2Cu3O7−x superconducting thin films on (100) LaAlO3 substrates. The ∼0.3 μm thin films fabricated by the pulsed laser deposition technique exhibit superconducting transition temperatures >90 K, as determined by resistivity and ac susceptibility measurements, and critical current densities of 5×106 A/cm2 at 77 K. Moreover, ion beam channeling minimum yields of ∼3% were measured, indicating the extremely high crystalline quality of films grown on the LaAlO3 substrate. Microwave surface resistance values at 77 K for these films are found to be more than one to two orders of magnitude lower than for copper at 77 K for almost the entire frequency range explored. We postulate that the reason we observe such low surface resistances in these films is the virtual absence of grain and phase boundaries coupled with the high degree of crystallinity. Furthermore, we believe that the residual resistance measured b...

High-Q GaN nanowire resonators and oscillators

We report high mechanical quality factors Q for GaN nanowire cantilevers grown by molecular beam epitaxy. Nanowires with 30–500nm diameters and 5–20μm lengths having resonance frequencies from 400kHzto2.8MHz were measured. Q near room temperature and 10−4Pa ranged from 2700 to above 60 000 with most above 10 000. Positive feedback to a piezoelectric stack caused spontaneous nanowire oscillations with Q exceeding 106. Spontaneous oscillations also occurred with direct e-beam excitation of unintentionally doped nanowires. Doped nanowires showed no oscillations, consistent with oscillation arising via direct actuation of piezoelectric GaN.

Epitaxial Y1Ba2Cu3O7−y/Y1−xPrxBa2Cu3O7−y heterostructures

For a variety of device applications, junction devices in particular, we have demonstrated a heterostructure system of Y{sub 1}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}}/Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}} which maintains epitaxy over the entire Pr composition range {ital x}=0--1. We have grown both trilayer and multiperiod superlattices which show nearly single crystalline helium ion backscattering minimum yields of {lt}6% in the topmost layer. X-ray diffraction measurements indicate {ital c}-axis orientation by a transverse scan across (005) line with a full width at half maximum of 0.6{degree} and 0.4{degree} on MgO and SrTiO{sub 3} substrates, respectively. Scanning Auger electron depth profiles and cross-sectional transmission electron micrographs indicate abrupt Pr/Y interfaces within one unit cell and virtually no disruption of the layered structure at the interface. These results indicate the potential for the growth of excellent heterostructures and superlattices of the high-temperature superconductors.For a variety of device applications, junction devices in particular, we have demonstrated a heterostructure system of Y1Ba2Cu3O7−y/Y1−xPrxBa2Cu3O7−y which maintains epitaxy over the entire Pr composition range x=0–1. We have grown both trilayer and multiperiod superlattices which show nearly single crystalline helium ion backscattering minimum yields of <6% in the topmost layer. X‐ray diffraction measurements indicate c‐axis orientation by a transverse scan across (005) line with a full width at half maximum of 0.6° and 0.4° on MgO and SrTiO3 substrates, respectively. Scanning Auger electron depth profiles and cross‐sectional transmission electron micrographs indicate abrupt Pr/Y interfaces within one unit cell and virtually no disruption of the layered structure at the interface. These results indicate the potential for the growth of excellent heterostructures and superlattices of the high‐temperature superconductors.

Subnanosecond magnetization dynamics measured by the second-harmonic magneto-optic Kerr effect

We have measured the in-plane magnetization dynamics of Ni81Fe19 films using the surface- and interface-sensitive second-harmonic magneto-optic Kerr effect. The dynamical magnetization was measured on patterned Ni81Fe19 stripes as a function of an in-plane magnetic field applied parallel to the anisotropy axis. The excitation sources were 100 ps risetime magnetic field impulses and steps. The minimum magnetization switching times were <300 ps, and precessional free-induction decay was observed. The dynamics for both impulse and step excitation are fitted to the Landau–Lifshitz equation, yielding values for the anisotropy field, gyroscopic splitting factor, and damping. The local surface precessional frequency and anisotropy are different from the average bulk values, demonstrating that this technique possesses the necessary sensitivity to detect variations in localized surface and interface dynamics.

Inclusion Compound Based Approach to Arrays of Artificial Dipolar Molecular Rotors. A Surface Inclusion

We describe an approach to regular triangular arrays of dipolar molecular rotors based on insertion of dipolar rotator carrying shafts as guests into channels of a host, tris(o-phenylenedioxy)cyclotriphosphazene (TPP). The rotor guests can either enter the bulk of the host or stay at or near the surface, if a suitable stopper is installed at the end of the shaft. Differential scanning calorimetry, solid-state NMR, and powder X-ray diffraction were used to examine the insertion of a dipolar rotor synthesized for the purpose, 1-n-hexadecyl-12-(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane, and it was found that it forms a surface inclusion compound. Rotational barriers from 1.2 to 9 kcal/mol were found by dielectric spectroscopy and were attributed to rotors inserted into the surface to different degrees, some rubbing the surface as they turn.