18 research outputs found
Dynamic wettability alteration in immiscible two-phase flow in porous media: Effect on transport properties and critical slowing down
The change in contact angles due to the injection of low salinity water or
any other wettability altering agent in an oil-rich porous medium is modeled by
a network model of disordered pores transporting two immiscible fluids. We
introduce a dynamic wettability altering mechanism, where the time dependent
wetting property of each pore is determined by the cumulative flow of water
through it. Simulations are performed to reach steady-state for different
possible alterations in the wetting angle (). We find that deviation
from oil-wet conditions re-mobilizes the stuck clusters and increases the oil
fractional flow. However, the rate of increase in the fractional flow depends
strongly on and as , a critical angle, the system
shows critical slowing down which is characterized by two dynamic critical
exponents.Comment: 8 pages, 9 figure
Describing synchronization and topological excitations in arrays of magnetic spin torque oscillators through the Kuramoto model
The collective dynamics in populations of magnetic spin torque oscillators
(STO) is an intensely studied topic in modern magnetism. Here, we show that
arrays of STO coupled via dipolar fields can be modeled using a variant of the
Kuramoto model, a well-known mathematical model in non-linear dynamics. By
investigating the collective dynamics in arrays of STO we find that the
synchronization in such systems is a finite size effect and show that the
critical coupling-for a complete synchronized state-scales with the number of
oscillators. Using realistic values of the dipolar coupling strength between
STO we show that this imposes an upper limit for the maximum number of
oscillators that can be synchronized. Further, we show that the lack of long
range order is associated with the formation of topological defects in the
phase field similar to the two-dimensional XY model of ferromagnetism. Our
results shed new light on the synchronization of STO, where controlling the
mutual synchronization of several oscillators is considered crucial for
applications.Comment: Accepted for publication in Scientific Reports. Corrected typo in
Eq.(9) from previous versio
Generation of single skyrmions by picosecond magnetic field pulses
We numerically demonstrate an ultrafast method to create
skyrmions in a ferromagnetic sample by applying a
picosecond (effective) magnetic field pulse in the presence of
Dzyaloshinskii-Moriya interaction. For small samples the applied magnetic field
pulse could be either spatially uniform or nonuniform while for large samples a
nonuniform and localized field is more effective. We examine the phase diagram
of pulse width and amplitude for the nucleation. Our finding could ultimately
be used to design future skyrmion-based devices.Comment: 4.5 pages+Supplemental Materia
Tailoring the magnetodynamic properties of nanomagnets using magnetocrystalline and shape anisotropies
Magnetodynamical properties of nanomagnets are affected by the demagnetizing
fields created by the same nanoelements. In addition, magnetocrystalline
anisotropy produces an effective field that also contributes to the spin
dynamics. In this article we show how the dimensions of magnetic elements can
be used to balance crystalline and shape anisotropies, and that this can be
used to tailor the magnetodynamic properties. We study ferromagnetic ellipses
patterned from a 10 nm thick epitaxial Fe film with dimensions ranging from 50
x 150 nm to 150 x 450 nm. The study combines ferromagnetic resonance (FMR)
spectroscopy with analytical calculations and micromagnetic simulations, and
proves that the dynamical properties can be effectively controlled by changing
the size of the nanomagnets. We also show how edge defects in the samples
influence the magnetization dynamics. Dynamical edge modes localized along the
sample edges are strongly influenced by edge defects, and this needs to be
taken into account in understanding the full FMR spectrumComment: Accepted for publication in Phys. Rev.
Thickness and temperature dependence of the magnetodynamic damping of pulsed laser deposited on (111)-oriented SrTi
We have investigated the magnetodynamic properties of
(LSMO) films of thickness 10, 15
and 30 nm grown on (111)-oriented SrTi (STO) substrates by pulsed
laser deposition. Ferromagnetic resonance (FMR) experiments were performed in
the temperature range 100--300 K, and the magnetodynamic damping parameter
was extracted as a function of both film thickness and temperature. We
found that the damping is lowest for the intermediate film thickness of 15 nm
with , where is relatively constant as
a function of temperature well below the Curie temperature of the respective
films.Comment: Accepted for publication in Journal of Magnetism and Magnetic
Material
The impact of wettability alterations on oil release and transport mechanisms in a 2D porous medium
The effects of wettability alterations in a 2D network model of a porous media has been studied. By changing the wetting properties of the reservoir through a developed wettability changing algorithm, previously immobile oil clusters in the network are re-mobilized, leading to significant changes in the steady state flow distribution of the model porous mediaThis caused de-stabilization of percolating and trapped clusters as the wettability was changed from an oil wet to a mixed wet system.A critical transition at a certain wetting angle, depending on the initial saturation and lattice size of the system was found. This indicating a possible phase transition from a percolating flow regime to a more uniform flow distribution through the network model. A link between changes in fractional flow and a percolation transition is also suspected, and using the theoretical framework of percolation theory, critical exponents were estimated
Describing synchronization and topological excitations in arrays of magnetic spin torque oscillators through the Kuramoto model
The collective dynamics in populations of magnetic spin torque oscillators (STO) is an intensely studied topic in modern magnetism. Here, we show that arrays of STO coupled via dipolar fields can be modeled using a variant of the Kuramoto model, a well-known mathematical model in non-linear dynamics. By investigating the collective dynamics in arrays of STO we find that the synchronization in such systems is a finite size effect and show that the critical coupling—for a complete synchronized state—scales with the number of oscillators. Using realistic values of the dipolar coupling strength between STO we show that this imposes an upper limit for the maximum number of oscillators that can be synchronized. Further, we show that the lack of long range order is associated with the formation of topological defects in the phase field similar to the two-dimensional XY model of ferromagnetism. Our results shed new light on the synchronization of STO, where controlling the mutual synchronization of several oscillators is considered crucial for applications