67 research outputs found
Dynamics of coupled vortices in a pair of ferromagnetic disks
We here experimentally demonstrate that coupled gyration modes can be
resonantly excited primarily by the ac current in a pair of ferromagnetic disks
with varied separating distance. The sole gyrotropic mode clearly splits into a
higher and a lower frequency modes for different configurations of polarities
via dipolar interaction. These experimental results indicate that the
magnetostatically coupled pair of vortices behaves similar to a diatomic
molecule with bonding and anti-bonding states. These findings lead to the
possible extension of designing the magnonic band structure in a chain or an
array of vortices.Comment: 4 pages, 4 figures, accepte
CMB Fluctuations and String Compactification Scales
We propose a mechanism for the generation of temperature fluctuations of
cosmic microwave background. We consider a large number of fields, such as
Kaluza-Klein modes and string excitations. Each field contributes to the
gravitational potential by a small amount, but an observable level of
temperature fluctuations is achieved by summing up the contribution of
typically of order 10^{14} fields. Tensor fluctuations are hardly affected by
these fields. Our mechanism is based on purely quantum effects, and is
different from the "slow-roll" or "curvaton" scenario. Using the observed data,
we find constraints on the parameters of this model, such as the size of the
extra dimensions and the string scale. Our model predicts a particular pattern
of non-gaussianity with a small magnitude.Comment: 4 pages, 2 figures; v2: Corrected a mistake in eq. (11), which does
not affect the subsequent context. Added a comment on inflaton fluctuations
at the end of the paper. Final version to appear in Phys. Lett.
Simulations on the Effect of Magnetic Anisotropy on Switching of an Easy Cone Magnetized Free Layer
The easy cone state of magnetization that arises as a result of a competition between the second- and fourth-order perpendicular magnetic anisotropies has the advantage of a non-zero electric-field torque without the application of a bias magnetic field and is thus a potential candidate for purely voltage-driven magnetic storage devices. In this article, the onset of the easy cone state of magnetization is simulated in a ferromagnet film. Subsequently, the switching field and time for voltage-controlled magnetization switching process are studied as a function of the inclination angle of the easy cone state from the film normal in the range of 0°- 45°. The switching field is found to decrease with decreasing anisotropy. The switching time is found to become faster for higher inclination angle of the easy cone state due to an increase in its torque
Secondary spin current driven efficient THz spintronic emitters
Femtosecond laser-induced photoexcitation of ferromagnet (FM)/heavy metal
(HM) heterostructures have attracted attention by emitting broadband terahertz
frequencies. The phenomenon relies on the formation of ultrafast spin current,
which is largely attributed to the direct photoexcitation of the FM layer.
However, we reveal that during the process, the FM layer also experiences a
secondary excitation led by the hot electrons from the HM layer that travel
across the FM/HM interface and transfer additional energy in the FM. Thus, the
generated secondary spins enhance the total spin current formation and lead to
amplified spintronic terahertz emission. The results also emphasize the
significance of the secondary spin current, which even exceeds the primary spin
currents when FM/HM heterostructures with thicker HM are used. An analytical
model is developed to provide deeper insights into the microscopic processes
within the individual layers, underlining the generalized ultrafast
superdiffusive spin-transport mechanism.Comment: 20 pages, 3 figure
Modulation of Magnetization Precession Trajectories by Perpendicular Magnetic Anisotropy in CoFeB Thin Films
Precession trajectories during excitation of magnetization dynamics play an important role in determining spin wave propagation, emission power of spin torque oscillator, and spin current generated by spin pumping. However, comparatively little information has emerged on the effect of perpendicular magnetic anisotropy (PMA) on dynamical magnetization trajectories. The effect of PMA on magnetization dynamics has become particularly important since the discovery of voltage control of magnetic anisotropy in CoFeB/MgO junctions. This motivates us to investigate the effect of PMA on dynamical magnetization trajectories of CoFeB thin films using micromagnetic simulations. The ellipticity of the trajectories is found to increase monotonically with PMA. On the other hand, the area of such elliptical trajectories, which determine the spin current generation, shows a non-monotonous change with respect to PMA. This area can be expected to be maximum for the case where ellipticity is ~0.5
Unconventional spin polarization at Argon ion milled SrTiO3 Interfaces
Interfacial two-dimensional electron gas (2DEG) formed at the perovskite-type
oxide, such as SrTiO3, has attracted significant attention due to its
properties of ferromagnetism, superconductivity, and its potential application
in oxide-based low-power consumption electronics. Recent studies have
investigated spin-to-charge conversion at the STO interface with different
materials, which could affect the efficiency of this 2DEG interface. In this
report, we presented an Ar^+ ion milling method to create a 2DEG at STO
directly by inducing oxygen vacancies. To quantify the spin-to-charge
conversion of this interface, we measured the angular-dependent spin-torque
ferromagnetic resonance (ST-FMR) spectra, revealing an unconventional spin
polarization at the interface of Argon ion-milled STO and NiFe. Furthermore, a
micromagnetic simulation for angular-dependent spin-torque ferromagnetic
resonance (ST-FMR) has been performed, confirming the large unconventional spin
polarization at the interface
Spin Pumping in Asymmetric Fe50Pt50/Cu/Fe20Ni80 Trilayer Structure
Herein, spin transport dynamics across asymmetric Fe50Pt50/Cu/Fe20Ni80 softâmagnetic trilayer structure is reported and thereby modulation of magnetic parameters including damping and effective field is determined by means of the angular dependence of broadband ferromagnetic resonance measurements. At distinct precession of individual magnetic layer, spinâpumping is found to be prevalent with expected linewidth increase. Mutual precession for wide range of resonance configuration reveals a collective reduction in anisotropy field of around 200âmT for both Fe50Pt50 and Fe20Ni80 systems. Subsequent observation of noâexcess interface damping shows the possible control of spinâpumping effect by tuning the net flow of spinâcurrent in a multilayer structure. These experimental findings have significance for microwave devices that require tunable anisotropy field in magnetic multilayers
All-Optical Excitation and Detection of Picosecond Dynamics of Ordered Arrays of Nanomagnets with Varying Areal Density
We have demonstrated optical excitation and detection of collective
precessional dynamics in arrays of coupled Ni80Fe20 (permalloy) nanoelements
with systematically varying areal density by an all-optical time-resolved Kerr
microscope. We have applied this technique to precisely determine three
different collective regimes in these arrays. At very high areal density, a
single uniform collective mode is observed where the edge modes of the
constituent elements are suppressed. At intermediate areal densities, three
nonuniform collective modes appear and at very low areal density, we observe
noncollective dynamics and only the centre and edge modes of the constituent
elements appear.Comment: 12 pages, 4 figure
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