We show that unlike the bright solitons, the parametrically driven kinks are immune from instabilities for all dampings and forcing amplitudes; they can also form stable bound states. In the undamped case, the two types of stable kinks and their complexes can travel with nonzero velocities.
The parametrically driven Ginsburg-Landau equation has well-known stationary solutions-the so-called Bloch and Ne el, or Ising, walls. In this paper, we construct an explicit stationary solution describing a bound state of two walls. We also demonstrate that stationary complexes of more than two walls do not exist.
We study interactions between the dark solitons of the parametrically driven nonlinear Schrödinger equation, Eq. ͑1͒. When the driving strength, h, is below ͱ ␥ 2 +1/9, two well-separated Néel walls may repel or attract. They repel if their initial separation 2z͑0͒ is larger than the distance 2z u between the constituents in the unstable stationary complex of two walls. They attract and annihilate if 2z͑0͒ is smaller than 2z u . Two Néel walls with h lying between ͱ ␥ 2 +1/9 and a threshold driving strength h sn attract for 2z͑0͒ Ͻ 2z u and evolve into a stable stationary bound state for 2z͑0͒ Ͼ 2z u . Finally, the Néel walls with h greater than h sn attract and annihilate-irrespective of their initial separation. Two Bloch walls of opposite chiralities attract, while Bloch walls of like chiralities repel-except near the critical driving strength, where the difference between the like-handed and oppositely handed walls becomes negligible. In this limit, similarly handed walls at large separations repel while those placed at shorter distances may start moving in the same direction or transmute into an oppositely handed pair and attract. The collision of two Bloch walls or two nondissipative Néel walls typically produces a quiescent or moving breather.
Precession of magnetization via the inverse Faraday effect is investigated with a view of determining the fundamental limit on the precession speed. Such a limit could have important consequences for ultrafast magnetic switching. The angular momentum required for precession is shown to be supplied by the light. This indicates that there is no fundamental obstruction to magnetization reversal on the time scale of a laser pulse provided that a material with a sufficiently strong magneto-optical response can be found. DOI: 10.1103/PhysRevB.79.212412 PACS number͑s͒: 75.60.Jk, 75.40.Gb, 78.20.Ls, 85.70.Li The ability to control magnetization on a subpicosecond time scale is growing in importance as the speed of electronic devices increases. The current generation of technology employs magnetic fields to induce magnetization dynamics. However, due to the difficulty of creating ultrashort magnetic pulses and the recent discovery that magnetic switching by strong magnetic fields can be unpredictable, 1 alternative techniques of controlling magnetization are under intense investigation. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Optical methods 2-13 are particularly promising due to the availability of ultrashort laser pulses. However, the fundamental mechanisms of optically induced demagnetization and magnetic switching are not fully understood. In particular, the question of which reservoir supplies the angular momentum needed for demagnetization remains controversial. This reservoir plays a decisive role in determining the maximum demagnetization speed so resolving this issue is important for technological applications.Most experiments on optical demagnetization and magneto-optical switching employ thermal methods. An optical pulse is absorbed, the electrons are driven far from equilibrium, and the sample is almost completely demagnetized within a few hundred femtoseconds. [3][4][5][6][7][8][9][10] If this is performed in the presence of a magnetic field, the magnetization can be reversed. [2][3][4][5] Some estimates show that thermal demagnetization occurs too rapidly for phonon processes to be relevant and it has been suggested that angular momentum is transferred between the spin and orbital components of the electrons. 9 On the other hand, it is possible that the nonequilibrium electrons experience a spin-phonon interaction that is much stronger than usual. In this case, the phonons could provide the angular momentum. 8,10 Transfer of angular momentum by the absorption of photons has also been considered. 9,13,18 Theoretical arguments based on the number of photons absorbed 9 and experiments using circularly polarized light with nickel 18 indicate that the photon angular momentum is irrelevant, although experiments with GdFeCo yielded the opposite conclusion. 13 Thermomagnetic switching is associated with an increase in temperature, so devices employing these methods will suffer a significant cooling time before new information can be written to them. The inverse Faraday effect ͑IFE͒ offers the p...
The Faraday effect is an extremely useful probe of magnetization dynamics on an ultrafast scale. However, in birefringent materials, interpreting experimental results is nontrivial. We investigate the link between magnetization and Faraday rotation by solving Maxwell's equations in a magneticallyordered, birefringent material. We find that the periodic dependence of the Faraday rotation on the sample thickness (a well-known effect of birefringence) complicates the correspondence between the sample magnetization and the measured rotation; in particular, the normalization constant for comparing magnetization and rotation depends on the sample thickness. Furthermore, sample alignment becomes important. If the incident light is not polarized along a birefringence axis of the sample, then the magnetization can be correctly interpreted only if the dielectric tensor is very accurately known.
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