227 research outputs found
Thermoelectric Modeling of the Non-Ohmic Differential Conductance in a Tunnel Junction containing a Pinhole
To test the quality of a tunnel junction, one sometimes fits the
bias-dependent differential conductance to a theoretical model, such as
Simmons's formula. Recent experimental work by {\AA}kerman and collaborators,
however, has demonstrated that a good fit does not necessarily imply a good
junction. Modeling the electrical and thermal properties of a tunnel junction
containing a pinhole, we extract an effective barrier height and effective
barrier width even when as much as 88% of the current flows through the pinhole
short rather than tunneling. A good fit of differential conductance to a
tunneling form therefore cannot rule out pinhole defects in normal-metal or
magnetic tunnel junctions.Comment: Revtex, 5 figure
Non-linear frequency and amplitude modulation of a nano-contact spin torque oscillator
We study the current controlled modulation of a nano-contact spin torque
oscillator. Three principally different cases of frequency non-linearity
( being zero, positive, and negative) are investigated.
Standard non-linear frequency modulation theory is able to accurately describe
the frequency shifts during modulation. However, the power of the modulated
sidebands only agrees with calculations based on a recent theory of combined
non-linear frequency and amplitude modulation.Comment: 4 pages, 4 figure
Bias dependence of perpendicular spin torque and of free and fixed layer eigenmodes in MgO-based nanopillars
We have measured the bias voltage and field dependence of eigenmode
frequencies in a magnetic tunnel junction with MgO barrier. We show that both
free layer (FL) and reference layer (RL) modes are excited, and that a
cross-over between these modes is observed by varying external field and bias
voltage. The bias voltage dependence of the FL and RL modes are shown to be
dramatically different. The bias dependence of the FL modes is linear in bias
voltage, whereas that of the RL mode is strongly quadratic. Using modeling and
micromagnetic simulations, we show that the linear bias dependence of FL
frequencies is primarily due to a linear dependence of the perpendicular spin
torque on bias voltage, whereas the quadratic dependence of the RL on bias
voltage is dominated by the reduction of exchange bias due to Joule heating,
and is not attributable to a quadratic dependence of the perpendicular spin
torque on bias voltage
Temperature dependence of linewidth in nano-contact based spin torque oscillators: effect of multiple oscillatory modes
We discuss the effect of mode transitions on the current (I) and temperature
(T) dependent linewidth (\Delta f) in nanocontact based spin torque oscillators
(STOs). At constant I, \Delta f exhibits an anomalous temperature dependence
near the mode transitions; \Delta f may either increase or decrease with T
depending on the position w.r.t. the mode transition. We show that the behavior
of \Delta f as a function of I can be fitted by the single mode analytical
theory of STOs, even though there are two modes present near the mode
transition, if the nonlinear amplification is determined directly from the
experiment. Using a recently developed theory of two coupled modes, we show
that the linewidth near mode transition can be described by an "effective"
single-oscillator theory with an enhanced nonlinear amplification that carries
additional temperature dependence, which thus qualitatively explain the
experimental results.Comment: 8 page
Super-harmonic injection locking of nano-contact spin-torque vortex oscillators
Super-harmonic injection locking of single nano-contact (NC) spin-torque
vortex oscillators (STVOs) subject to a small microwave current has been
explored. Frequency locking was observed up to the fourth harmonic of the STVO
fundamental frequency in microwave magneto-electronic measurements. The
large frequency tunability of the STVO with respect to allowed the
device to be locked to multiple sub-harmonics of the microwave frequency
, or to the same sub-harmonic over a wide range of by tuning
the DC current. In general, analysis of the locking range, linewidth, and
amplitude showed that the locking efficiency decreased as the harmonic number
increased, as expected for harmonic synchronization of a non-linear oscillator.
Time-resolved scanning Kerr microscopy (TRSKM) revealed significant differences
in the spatial character of the magnetization dynamics of states locked to the
fundamental and harmonic frequencies, suggesting significant differences in the
core trajectories within the same device. Super-harmonic injection locking of a
NC-STVO may open up possibilities for devices such as nanoscale frequency
dividers, while differences in the core trajectory may allow mutual
synchronisation to be achieved in multi-oscillator networks by tuning the
spatial character of the dynamics within shared magnetic layers.Comment: 21 pages, 8 figure
Direct observation of magnetization dynamics generated by nano-contact spin-torque vortex oscillators
Time-resolved scanning Kerr microscopy has been used to directly image the
magnetization dynamics of nano-contact (NC) spin-torque vortex oscillators
(STVOs) when phase-locked to an injected microwave (RF) current. The Kerr
images reveal free layer magnetization dynamics that extend outside the NC
footprint, where they cannot be detected electrically, but which are crucial to
phase-lock STVOs that share common magnetic layers. For a single NC, dynamics
were observed not only when the STVO frequency was fully locked to that of the
RF current, but also for a partially locked state characterized by periodic
changes in the core trajectory at the RF frequency. For a pair of NCs, images
reveal the spatial character of dynamics that electrical measurements show to
have enhanced amplitude and reduced linewidth. Insight gained from these images
may improve understanding of the conditions required for mutual phase-locking
of multiple STVOs, and hence enhanced microwave power emission.Comment: 10 pages, 3 figure
Propagating spin waves excited by spin-transfer torque: A combined electrical and optical study
Nanocontact spin-torque oscillators are devices in which the generation of propagating spin waves can be sustained by spin transfer torque. In the present paper, we perform combined electrical and optical measurements in a single experimental setup to systematically investigate the excitation of spin waves by a nanocontact spin-torque oscillator and their propagation in a Ni80Fe20 extended layer. By using microfocused Brillouin light scattering we observe an anisotropic emission of spin waves, due to the broken symmetry imposed by the inhomogeneous Oersted field generated by the injected current. In particular, spin waves propagate on the side of the nanocontact where the Oersted field and the in-plane component of the applied magnetic field are antiparallel, while propagation is inhibited on the opposite side. Moreover, propagating spin waves are efficiently excited only in a limited frequency range corresponding to wavevectors inversely proportional to the size of the nanocontact. This frequency range obeys the dispersion relation for exchange-dominated spin waves in the far field, as confirmed by micromagnetic simulations of similar devices. The present results have direct consequences for spin wave based applications, such as synchronization, computation, and magnonics
Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting
This is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this record.Nano-contact spin-torque vortex oscillators (STVOs) are anticipated to find application as nanoscale sources of microwave emission in future technological applications. Presently the output power and phase stability of individual STVOs are not competitive with existing oscillator technologies. Synchronisation of multiple nano-contact STVOs via magnetisation dynamics has been proposed to enhance the microwave emission. The control of device-to-device variations, such as mode splitting of the microwave emission, is essential if multiple STVOs are to be successfully synchronised. In this work a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM) was used to demonstrate how mode splitting in the microwave emission of STVOs was related to the magnetisation dynamics that are generated. The free-running STVO response to a DC current only was used to identify devices and bias magnetic field configurations for which single and multiple modes of microwave emission were observed. Stroboscopic Kerr images were acquired by injecting a small amplitude RF current to phase lock the free-running STVO response. The images showed that the magnetisation dynamics of a multimode device with moderate splitting could be controlled by injecting an RF current so that they exhibit similar spatial character to that of a single mode. Significant splitting was found to result from a complicated equilibrium magnetic state that was observed in Kerr images as irregular spatial characteristics of the magnetisation dynamics. Such dynamics were observed far from the nano-contact and so their presence cannot be detected in electrical measurements. This work demonstrates that TRSKM is a powerful tool for the direct observation of the magnetisation dynamics generated by STVOs that exhibit complicated microwave emission. Characterisation of such dynamics outside the nano-contact perimeter permits a deeper insight into the requirements for optimal phase-locking of multiple STVOs that share common magnetic layers.The authors gratefully acknowledge the financial support of the Engineering and Physical Sciences Research Council under grants EP/I038470/1 and EP/K008501/1, the Royal Society under grant UF080837, the Swedish Research Council (VR), the Swedish Foundation for Strategic Research (SSF), and the Knut and Alice Wallenberg Foundation (KAW). The authors and co-authors declare that there are no conflicts of interes
Hysteresis and Fractional Matching in Thin Nb Films with Rectangular Arrays of Nanoscaled Magnetic Dots
We have investigated the periodic pinning of magnetic flux quanta in thin Nb
films with rectangular arrays of magnetic dots. In this type of pinning
geometry, a change in the periodicity and shape of the minima in the
magnetoresistance occurs for magnetic fields exceeding a certain threshold
value. This has been explained recently in terms of a reconfiguration
transition of the vortex lattice due to an increasing vortex-vortex interaction
with increasing magnetic field. In this picture the dominating elastic energy
at high fields forces the vortex lattice to form a square symmetry rather than
being commensurate to the rectangular geometry of the pinning array. In this
paper we present a comparative study of rectangular arrays with Ni-dots,
Co-dots and holes. In the magnetic dot arrays, we found a strong fractional
matching effect up to the second order matching field. In contrast, no clear
fractional matching is seen after the reconfiguration. Additionally, we
discovered the existence of hysteresis in the magnetoresistance in the
crossover between the low and the high field regime. We found evidence that
this effect is correlated to the reconfiguration phenomenon rather than to the
magnetic state of the dots. The temperature and angular dependences of the
effect have been measured and possible models are discussed to explain this
behavior.Comment: 1 Table, 5 Figure
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