Andrzej Janutka

Externally driven transmission and collisions of domain walls in ferromagnetic wires

Analytical multidomain solutions to the dynamical (Landau-Lifshitz-Gilbert) equation of a one-dimensional ferromagnet including an external magnetic field and spin-polarized electric current are found using the Hirota bilinearization method. A standard approach to solve the Landau-Lifshitz equation (without the Gilbert term) is modified in order to treat the dissipative dynamics. I establish the relations between the spin interaction parameters (the constants of exchange, anisotropy, dissipation, external-field intensity, and electric-current intensity) and the domain-wall parameters (width and velocity) and compare them to the results of the Walker approximation and micromagnetic simulations. The domain-wall motion driven by a longitudinal external field is analyzed with especial relevance to the field-induced collision of two domain walls. I determine the result of such a collision (which is found to be an elastic one) on the domain-wall parameters below and above the Walker breakdown (in weak- and strong-field regimes). Single-domain-wall dynamics in the presence of an external transverse field is studied with relevance to the challenge of increasing the domain-wall velocity below the breakdown.

Collisions of optical ultra-short vector pulses

Asymptotic two-pulse solutions of a modified (via attaching a nonlinear derivative term) vector nonlinear Schrodinger equation (MVNSE) are analyzed using the Hirota (bilinearization) method. This equation is known to describe the propagation of ultra-short optical pulses in nonlinear fibers (in particular, in the optical-crystal fibers). Some pulse-parameter regimes of applicability of the solutions to study the pulse collisions are estimated. It is found, for these regimes, that the collision of ultra-short vector pulses of almost equal velocities results in a transformation of their polarizations similar as that of the Manakov solitons whenever the pulse-interaction effects can be neglected.

Structure of Magnetic Domain Wall in Cylindrical Microwire

Within a simple model, we study magnetic domain walls (DWs) inside the inner core of the amorphous ferromagnetic microwire whose spin ordering is a core-shell structure. The interaction of the (internal stress created) outer shell of the wire on the inner core is included into the Landau-Lifshitz equation via an effective Dzyaloshinskii-Moriya-like anisotropy. The resulting DW textures are classified. The model is applicable to the nanowires of a modulated diameter (periodically constricted nanowires) as well. In that case, a core-shell magnetic structure is of the purely magnetostatic origin. Because the micromagnetic simulations of the microwires are extremely challenging, the simulations of the structured nanowires are performed with the purpose of verifying analytical predictions on the shape of the DW.

Domain Walls in Nanostripes of Cubic-Anisotropy Ferromagnetic Materials

We discuss the structure of the domain walls (DWs) in ferromagnetic nanostripes formed of cubic-anisotropy materials. Within certain approximations, such stationary textures are described by soliton solutions to the sine-Gordon equation in 2-D that is obtained with a constraint on the Landau-Lifshitz equation. Preferable stable and metastable types of the DWs (transverse DWs, vortex DWs, and others) are established with dependence on the stripe width and thickness using the micromagnetic simulations for a set of the so-called spintronic materials, which are of the current interest with regard to controlling the DW state. The simulated DW textures are classified on the basis of the stationary multisoliton states of the sine-Gordon model in 2-D.

Short-Range Interactions of Domain Walls in Ferromagnetic Nanostripes

Collisions of slowly moving domain walls (DWs) of the transverse and vortex types are analytically studied within the XY approximation. The previously reported effects of the mutual DW annihilation and of the formation of stable 2 π-DWs (a “topological repulsion”) are explained on the basis of a very general mechanism connected to the creation or annihilation the vortex-antivortex pairs.

Driving large-velocity propagation of ferromagnetic π/2 domain walls in nanostripes of cubic-anisotropy materials

We study the externally-driven motion of the domain walls (DWs) of the type in (in-the-plane ordered) nanostripes with crystalline cubic anisotropy. Such DWs are much narrower than the transverse and vortex π DWs in the soft-magnetic nanostripes while they propagate much faster, thus enabling dense packing of magnetization domains and high-speed processing of the many-domain states. The viscous current-driven motion of the DW with velocity above 1000 m s−1 under the electric current of the density ~1012 A m−2 is predicted to take place in the nanostripes of the magnetite. Also, the viscous motion with the velocity above 700 m s−1 can be driven by the magnetic field according to our solution to a 1D analytical model and micromagnetc simulations. Such huge velocities are achievable in the nanostripes of very small cross-sections (only 100 nm width and 10 nm thickness). The fully stress-driven propagation of the DW in the nanostripes of cubic magnetostrictive materials is predicted as well. The strength of the DW pinning to the stripe notches and the thermal stability of the magnetization during the current flow are addressed.

Complexes of Domain Walls in Ferromagnetic Stripes

Interaction of domain walls (DWs) in ferromagnetic stripes is studied with relevance to the formation of stable complexes of many domains. Two DW system is described with the Landau-LifshitzGilbert equation including regimes of narrow and wide stripes which correspond the presence of transverse and vortex DWs. The DWs of both kinds are characterized with their chiralities (the direction of the magnetization rotation in the stripe plane) and polarities (the magnetization orientation in the center of a vortex and/or halfvortices), hence, their interactions are analyzed with dependence on these properties. In particular, pairs of the DWs of opposite or like chiralities and polarities are investigated as well as pairs of opposite (like) chiralities and of like (opposite) polarities. Conditions of the creation of stationary magnetic bubbles built of two interacting DWs are formulated with relevance to the situations of presence and absence of the external magnetic field.

Quantum-like information processing using vector solitons

The concept of simulating the quantum logic via collisions of vector solitons (Janutka 2006 J. Phys. A: Math. Gen. 39 12505, 2007 J. Phys. A: Math. Theor. 40 10813) is developed in the direction of designing a true quantum-information processor that is based on mesoscopic objects, solitons. In this concept, quantum-like information is encoded in the vector-soliton (polarization) parameters. An exponential increase of the logical-operating speed compared to that achievable in the earlier simulation schemes is found to be possible due to a coherent conversion of a 2n-component vector soliton that carries an n-cebit of information into an ensemble of 2n−1 two-component and 2n−2 four-component vector pulses. Two solid-state circuits (transmitting magnetic solitons or fluxons of long Josephson junctions) which enable such a pulse conversion are proposed.

Simulation of quantum logic via collisions of vector solitons

The polarization of a multi-component vector soliton (of the Manakov type) can be thought of as a state vector of a system of qubits (a register of quantum information). A change of this state on demand via colliding the register pulse with other solitons is shown to be possible with arbitrary accuracy. The parameters (polarization, pulse width, velocity) of the register-switching solitons corresponding to the computationally universal set of quantum gates are found. Physical realizations of information processing using effects of the self-focusing of optical media or of the self-induced transparency are considered.

Damping of multiphoton Rabi oscillations and multiphoton Purcell effect

Damping of multiphoton Rabi oscillations due to a finite lifetime of photons is investigated using an algebraic method of the solution of the master equation. The time dependence of the probability of electron transitions between atomic levels is evaluated. In a regime of weak atom–photon coupling (a coupling constant smaller than the photon decay rate), the spontaneous emission of many photons from a single electron transition due to the interaction with a resonant light is found. Thus, a multiphoton counterpart of the Purcell effect is predicted.