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Search Publications by: William Rippard (Fed)

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Displaying 26 - 50 of 54

Two modes behavior of vortex oscillations in spin-transfer nanocontacts subject to in-plane magnetic fields

June 25, 2010
Author(s)
Michaela Kuepferling, Claudio Serpico, Matthew Pufall, William Rippard, Thomas Mitchell (Mitch) Wallis, Atif A. Imtiaz, Pavel Kabos
The field dependence of vortex oscillations in a spin-transfer metallic nanocontact, subject to in-plane, spatially uniform, external fields, is studied by measuring the power spectral density of the voltage across the device. The measured spectra as a

MgO Tunnel Barriers in Magnetic Nanostructures and Devices

April 28, 2010
Author(s)
Ranko R. Heindl, Matthew Pufall, William Rippard
In recent years, there has been a great deal of success in the development of MgO based tunnel barriers for use in magnetic devices, such as nanoscale magnetic random access memory elements (MRAM) and magnetic field sensors. Much of the focus has been on

Spin-transfer dynamics in spin valves with out-of-plane magnetized CoNi free layers

January 29, 2010
Author(s)
William H. Rippard, Alina M. Deac, Matthew R. Pufall, Justin M. Shaw, Mark W. Keller, Stephen E. Russek, Gerrit E. Bauer, Claudio Serpico
We have measured spin-transfer-induced dynamics in magnetic nanocontact devices having a perpendicularly magnetized Co/Ni free layer and an in-plane magnetized CoFe fixed layer. The frequencies and powers of the excitations agree well with the predictions

Temperature dependence of spin-torque-driven self-oscillators

October 15, 2009
Author(s)
William H. Rippard, Matthew R. Pufall, Thomas Cecil, Thomas J. Silva, Stephen E. Russek, M. L. Schneider
We have measured the temperature dependence of spin-torque-driven self-oscillations in point-contact nanooscillators for both in-plane and out-of-plane applied fields. We find that the linewidth for both field geometries is qualitatively similar. In the

Time domain measurement of phase noise in a spin torque oscillator

May 13, 2009
Author(s)
Mark W. Keller, Anthony B. Kos, Thomas J. Silva, William H. Rippard, Matthew R. Pufall
We measure phase noise from the zero crossings of the voltage vs. time waveform of a spin torque nanocontact oscillating in a vortex mode. The power spectrum of the phase noise varies with Fourier frequency f as 1/f 2, consistent with frequency

Spin Transfer Precessional Dynamics in Co 60 Fe 20 B 20 Nanocontacts

March 14, 2008
Author(s)
William H. Rippard, Matthew R. Pufall, Michael L. Schneider, Kevin Garello, Stephen E. Russek
We report on the precessional dynamics in spin transfer oscillators having Co 60Fe 20B 20 free layers as a function of annealing time at 225 °C. Repeated annealing reduces the critical current I c by roughly a factor of 3 and increases the tunability of

Developments in nano-oscillators based upon spin-transfer point-contact devices

December 31, 2007
Author(s)
Thomas J. Silva, William H. Rippard
We review the current status of research on microwave nano-oscillators that utilize spin transfer devices with point-contact geometry, with an emphasis on the open questions that still prevent our full understanding of device properties. In particular, we

Microwave generation in magnetic multilayers and nanostructures

September 1, 2007
Author(s)
William Rippard, Matthew Pufall
We present an overview of the coherent magnetic precessional dynamics induced by the spin transfer effect in magnetic nanostructures. The precessional frequencies of the magnetic oscillators can be tuned over a range of several gigahertz by varying the dc

Low-field current-hysteretic oscillations in spin-transfer nanocontacts

April 19, 2007
Author(s)
Matthew Pufall, William Rippard, Michael Schneider, Stephen E. Russek
We have measured spin-transfer-driven, large-amplitude, current-hysteretic, low frequency (less than or equal to} 500 MHz), narrowband oscillations in nanocontacts made to spin valve structures. The oscillations occur in zero field, persist up to 5 mT for

Thermal effects on the critical current of spin torque switching in spin valve nanopillars

February 28, 2007
Author(s)
Michael Schneider, Matthew Pufall, William Rippard, Stephen E. Russek, Jordan A. Katine
In spin valve nanopillars, temperature affects the spin torque reversal of the free magnetic layer. The authors compare values of zero temperature critical switching current I c0 extrapolated from room temperature pulsed current switching measurements to

Origins of switching field distributions in perpendicular magnetic nanodot arrays

January 19, 2007
Author(s)
Justin Shaw, William Rippard, Stephen E. Russek, Tim Reith, Charles M. Falco
We studied the reversal properties of perpendicularly magnetized Co/Pd nanodots from 100 to 50 nm in diameter fabricated using electron beam lithography. Polycrystalline Co/Pd multilayers show considerable differences in the switching field distribution

Mutual phase-locking of microwave spin torque nano-oscillators

September 15, 2005
Author(s)
Shehzaad F. Kaka, Matthew Pufall, William Rippard, Thomas J. Silva, Stephen E. Russek, Jordan A. Katine
The spin torque effect that occurs in nanometre-scale magnetic multilayer devices can be used to generate steady- state microwave signals in response to a d.c. electrical current. This establishes a new functionality for magneto-electronic structures that

Injection Locking and Phase Control of Spin Transfer Nano-Oscillators

August 5, 2005
Author(s)
William Rippard, Matthew Pufall, Shehzaad F. Kaka, Thomas J. Silva, Stephen E. Russek, Jordan A. Katine
We have directly measured phase locking of spin transfer oscillators to an injected ac current. The oscillators lock to signals up to several hundred megahertz away from their natural oscillation frequencies, depending on the relative strength of the input

Finite-temperature modeling of nanoscale spin-transfer oscillators

March 31, 2005
Author(s)
Stephen E. Russek, Shehzaad F. Kaka, William Rippard, Matthew Pufall, Thomas J. Silva
Magnetization dynamics induced by spin-polarized currents in magnetic nanodevices have been numerically simulated using a single-domain model proposed by Slonczewski extended to include temperature effects. For currents with a spin polarization

Frequency modulation of spin-transfer oscillators

February 18, 2005
Author(s)
Matthew Pufall, William Rippard, Shehzaad F. Kaka, Thomas J. Silva, Stephen E. Russek
Spin-polarized dc electric current flowing into a magnetic layer can induce precession of the magnetization at a frequency that depends on current. We show that addition of an ac current to this dc bias current results in a frequency modulated (FM)

Spin transfer switching of spin valve nanopillars using nanosecond pulsed currents

October 29, 2004
Author(s)
Shehzaad F. Kaka, Matthew Pufall, William Rippard, Thomas J. Silva, Stephen E. Russek, Jordan A. Katine, Matt Carey
Spin valve nanopillars are reversed via the mechanism of spin momentum transfer using current pulses applied perpendicular to the film plane of the device. The applied pulses were varied in amplitude from 1.8 to 7.8 mA, and varied in duration within the

Large-angle, gigahertz-rate random telegraph switching induced by spin-momentum transfer

July 27, 2004
Author(s)
Matthew Pufall, William Rippard, Shehzaad F. Kaka, Stephen E. Russek, Thomas J. Silva, Jordan A. Katine, Matt Carey
We show that a spin-polarized dc current passing through a small magnetic element induces two-state, random telegraph switching of the magnetization via the spin-momentum transfer effect. The resistances of the states differ by up to 50% of the change due

Superconducting transition edge sensor using dilute AlMn alloys

July 6, 2004
Author(s)
Steven Deiker, William B. Doriese, Gene C. Hilton, Kent D. Irwin, William H. Rippard, Joel N. Ullom, Leila R. Vale, Steve T. Ruggiero, Anthony Williams, B. Young
We have fabricated a bolometer using a transition-edge sensor (TES) made of Al doped with Mn to suppress the superconducting critical temperature !Tc" of Al from #1 K to #100 mK. The resulting detector exhibits low-frequency noise consistent with theory

Direct-Current Induced Dynamics in Co 90 Fe 10 /Ni 80 Fe 20 Point Contacts

January 16, 2004
Author(s)
William Rippard, Matthew Pufall, Shehzaad F. Kaka, Stephen E. Russek, Thomas J. Silva
We have directly measured coherent high-frequency magnetization dynamics in ferromagnetic films induced by a spin-polarized dc current. The precision frequency can be tuned over a range of several gigahertz by varying the applied current. The frequencies
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