115 research outputs found
Frequency modulation of spin torque oscillator pairs
The current controlled modulation of nano-contact based spin torque
oscillator (STO) pairs is studied in both the synchronized and non-synchronized
states. The synchronized state shows a well behaved modulation and demonstrates
robust mutual locking even under strong modulation. The power distribution of
the modulation sidebands can be quantitatively described by assuming a single
oscillator model. However, in the non-synchronized state, the modulation
sidebands are not well described by the model, indicating interactions between
the two individual nano-contact STOs. These findings are promising for
potential applications requiring the modulation of large synchronized STO
arrays
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
Spin and orbital effects in a 2D electron gas in a random magnetic field
Using the method of superbosonization we consider a model of a random
magnetic field (RMF) acting on both orbital motion and spin of electrons in two
dimensions. The method is based on exact integration over one particle degrees
of freedom and reduction of the problem to a functional integral over
supermatrices . We consider a general case when
both the direction of the RMF and the g-factor of the Zeeman splitting are
arbitrary. Integrating out fast variations of we come to a standard
collisional unitary non-linear -model. The collision term consists of
orbital, spin and effective spin-orbital parts. For a particular problem of a
fixed direction of RMF, we show that additional soft excitations identified
with spin modes should appear. Considering % -correlated weak RMF and
putting g=2 we find the transport time . This time is 2 times
smaller than that for spinless particles.Comment: 9 pages, no figure
Synchronization of spin-torque driven nanooscillators for point contacts on a quasi-1D nanowire: Micromagnetic simulations
In this paper we present detailed numerical simulation studies on the
synchronization of two spin-torque nanooscillators (STNO) in the quasi-1D
geometry: magnetization oscillations are induced in a thin NiFe nanostripe by a
spin polarized current injected via square-shaped CoFe nanomagnets on the top
of this stripe. In a sufficiently large out-of-plane field, a propagating
oscillation mode appears in such a system. Due to the absence of the
geometrically caused wave decay in 1D systems, this mode is expected to enable
a long-distance synchronization between STNOs. Indeed, our simulations predict
that synchronization of two STNOs on a nanowire is possible up to the
intercontact distance 3 mkm (for the nanowire width 50 nm). However, we have
also found several qualitatively new features of the synchronization behaviour
for this system, which make the achievement of a stable synchronization in this
geometry to a highly non-trivial task. In particular, there exist a minimal
distance between the nanocontacts, below which a synchronization of STNOs can
not be achieved. Further, when the current value in the first contact is kept
constant, the amplitude of synchronized oscillations depends non-monotonously
on the current value in the second contact. Finally, for one and the same
currents values through the contacts there might exist several synchronized
states (with different frequencies), depending on the initial conditions.Comment: 13 pages with 4 figurews, recently submitted to PR
Experimental evidence of self-localized and propagating spin wave modes in obliquely magnetized current-driven nanocontacts
Through detailed experimental studies of the angular dependence of spin wave
excitations in nanocontact-based spin-torque oscillators, we demonstrate that
two distinct spin wave modes can be excited, with different frequency,
threshold currents and frequency tuneability. Using analytical theory and
micromagnetic simulations we identify one mode as an exchange-dominated
propagating spin wave, and the other as a self-localized nonlinear spin wave
bullet. Wavelet-based analysis of the simulations indicates that the apparent
simultaneous excitation of both modes results from rapid mode hopping induced
by the Oersted field.Comment: 5 pages, 3 figure
Preparation and structural properties of thin films and multilayers of the Heusler compounds Cu2MnAl, Co2MnSn, Co2MnSi and Co2MnGe
We report on the preparation of thin films and multilayers of the
intermetallic Heusler compound CuMnAl, Co2MnSn, Co2MnSi and Co2MnGe by
rf-sputtering on MgO and Al2O3 substrates. Cu2MnAl can be grown epitaxially
with (100)-orientation on MgO (100) and in (110)-orientation on Al2O3 a-plane.
The Co based Heusler alloys need metallic seedlayers to induce high quality
textured growth. We also have prepared multilayers with smooth interfaces by
combining the Heusler compounds with Au and V. An analysis of the ferromagnetic
saturation magnetization of the films indicates that the Cu2MnAl-compound tends
to grow in the disordered B2-type structure whereas the Co-based Heusler alloy
thin films grow in the ordered L21 structure. All multilayers with thin layers
of the Heusler compounds exhibit a definitely reduced ferromagnetic
magnetization indicating substantial disorder and intermixing at the
interfaces.Comment: 21 pages, 8 figure
Current-driven excitations in magnetic multilayers: a brief review
In 1996, Berger and Slonczewski independently predicted that a large enough
spin-polarized dc current density sent perpendicularly through a ferromagnetic
layer could produce magnetic excitations (spin-waves) or reversal of
magnetization (switching). In the past few years, both current-driven switching
and current-driven excitation of spin-waves have been observed. The switching
is of potential technological interest for direct 'writing' of magnetic random
access memory (MRAM) or magnetic media. The spin-wave generation could provide
a new source of dc generated microwave radiation. We describe what has been
learned experimentally about these two related phenomena, and some models being
tested to explain these observations.Comment: 5 pages, 7 figures, expected to appear in conf. proceeding
Neuromorphic Hebbian learning with magnetic tunnel junction synapses
Neuromorphic computing aims to mimic both the function and structure of
biological neural networks to provide artificial intelligence with extreme
efficiency. Conventional approaches store synaptic weights in non-volatile
memory devices with analog resistance states, permitting in-memory computation
of neural network operations while avoiding the costs associated with
transferring synaptic weights from a memory array. However, the use of analog
resistance states for storing weights in neuromorphic systems is impeded by
stochastic writing, weights drifting over time through stochastic processes,
and limited endurance that reduces the precision of synapse weights. Here we
propose and experimentally demonstrate neuromorphic networks that provide
high-accuracy inference thanks to the binary resistance states of magnetic
tunnel junctions (MTJs), while leveraging the analog nature of their stochastic
spin-transfer torque (STT) switching for unsupervised Hebbian learning. We
performed the first experimental demonstration of a neuromorphic network
directly implemented with MTJ synapses, for both inference and
spike-timing-dependent plasticity learning. We also demonstrated through
simulation that the proposed system for unsupervised Hebbian learning with
stochastic STT-MTJ synapses can achieve competitive accuracies for MNIST
handwritten digit recognition. By appropriately applying neuromorphic
principles through hardware-aware design, the proposed STT-MTJ neuromorphic
learning networks provide a pathway toward artificial intelligence hardware
that learns autonomously with extreme efficiency
Electron localization by a magnetic vortex
We study the problem of an electron in two dimensions in the presence of a
magnetic vortex with a step-like profile. Dependending on the values of the
effective mass and gyromagnetic factor of the electron, it may be trapped by
the vortex. The bound state spectrum is obtained numerically, and some limiting
cases are treated analytically.Comment: 8 pages, latex, 4 figure
Anisotropic magnetoresistance in a 2DEG in a quasi-random magnetic field
We present magnetotransport results for a 2D electron gas (2DEG) subject to
the quasi-random magnetic field produced by randomly positioned sub-micron Co
dots deposited onto the surface of a GaAs/AlGaAs heterostructure. We observe
strong local and non-local anisotropic magnetoresistance for external magnetic
fields in the plane of the 2DEG. Monte-Carlo calculations confirm that this is
due to the changing topology of the quasi-random magnetic field in which
electrons are guided predominantly along contours of zero magnetic field.Comment: 4 pages, 6 figures, submitted to Phys. Rev.
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