arXiv subjects

A. Rebei

Publications and source records attributed to A. Rebei.

16 recordsLinked to original sources

arXiv2009

Athermal All-Optical Femtosecond Magnetization Reversal in GdFeCo

Magnetization reversal in GdFeCo by circularly polarized light is shown to occur at the femtosecond time scale. In contrast to the well known laser-assisted magnetization reversal based on the laser heating, we here demonstrate that this femtosecond all-optical magnetization reversal is more efficient at lower temperatures. The lower the temperatures, the smaller the laser fluence required for the switching. This switching is in agreement with a more recent theoretical prediction $[ \text{Phys. Lett. A \textbf{372}, 1915 (2008)} ] $ and demonstrates the feasibility of the femtosecond athermal magnetization reversal.

cond-mat.mtrl-sci↗

arXiv2007

Dynamics of femtosecond magnetization reversal induced by circularly polarized light in the presence of fluctuations and dissipation

Magnetization reversal by a femtosecond circularly polarized laser pulse has been recently demonstrated in rare-earth doped transition metals (RE-TM). The switching mechanism has been attributed to an inverse Faraday effect and thermal effects. Based on the parameters provided in the experimental work, we show that this claim is unlikely to give rise to femtosecond reversal. Using a hybrid itinerant-localized picture of the RE-TM system, we propose a new mechanism that requires the presence of the rare earth element to reduce the symmetry of the system as well as a strong enhancement of spin-orbit coupling between the d electrons and the f moments in the presence of the laser. Our model does not require the heating close to the Curie temperature of the sample.

cond-mat.mtrl-sci↗

arXiv2007

Current-Induced Motion of Narrow Domain Walls and Dissipation in Ferromagnetic Metals

Spin transport equations in a non-homogeneous ferromagnet are derived in the limit where the sd exchange coupling between the electrons in the conduction band and those in the d band is dominant. It is shown that spin diffusion in ferromagnets assumes a tensor form. The diagonal terms are renormalized with respect to that in normal metals and enhances the dissipation in the magnetic system while the off-diagonal terms renormalize the precessional frequency of the conduction electrons and enhances the non-adiabatic spin torque. To demonstrate the new physics in our theory, we show that self-consistent solutions of the spin diffusion equations and the Landau-Lifshitz equations in the presence of a current lead to a an increase in the terminal velocity of a domain wall which becomes strongly dependent on its width. We also provide a simplified equation that predicts damping due to the conduction electrons.

cond-mat.mes-hall↗

arXiv2007

The magneto-optical Barnett effect and spin momentum transfer:Magnetization switching by circularly polarized light

The interaction of polarized light with a spin in the presence of dissipation is shown to be equivalent to a spin transfer process that can cause switching. In plasmas, the spin transfer is dominated by a spin-spin exchange term while at lower energy densities it is dominated by an optical Barnett-like effect. This latter effect is used in conjunction with optical phonons to predict femtosecond magnetization reversal believed to be recently measured in GdCoFe thin films. Conventional approaches based on the Bloch and the Landau-Lifshitz equations do not reproduce this ultrafast switching.

cond-mat.mtrl-sci↗

arXiv2006

A generalized definition of spin in non-orientable geometries

Non-orientable nanostructures are becoming feasable today. This lead us to the study of spin in these geometries. Hence a physically sound definition of spin is suggested. Using our definition, we study the question of the number of different ways to define spin. We argue that the possibility of having more than one spin structure should be taken into account energetically. The effect of topology on spin is studied in detail using cohomological arguments. We generalize the definition of equivalence among (s)pin structures to include non-orientable spaces.

cond-mat.mtrl-sci↗

arXiv2006

Suppression of telegraph noise in a CPP spin valve by an oscillating spin torque: Numerical study

The phenomenon of stochastic resonance (SR) has been mainly studied in one-dimensional systems with additive noise. We show that in higher dimensional systems and in the presence of multiplicative noise, a non-linear magnetic system with a strongly periodic current can show behavior similar to that of SR but only for frequencies below the ferromagnetic resonance (FMR) frequency of the system. Such a phenomena can provide an effective way to suppress low frequency noise in spin valve magnetic sensors.

cond-mat.mtrl-sci↗

arXiv2004

Fluctuations of the Magnetization in Thin Films due to Conduction Electrons

A detailed analysis of damping and noise due to a {\it sd}-interaction in a thin ferromagnetic film sandwiched between two large normal metal layers is carried out. The magnetization is shown to obey in general a non-local equation of motion which differs from the the Gilbert equation and is extended to the non-adiabatic regime. To lowest order in the exchange interaction and in the limit where the Gilbert equation applies, we show that the damping term is enhanced due to interfacial effects but it also shows oscillations as a function of the film thickness. The noise calculation is however carried out to all orders in the exchange coupling constant. The ellipticity of the precession of the magnetization is taken into account. The damping is shown to have a Gilbert form only in the adiabatic limit while the relaxation time becomes strongly dependent on the geometry of the thin film. It is also shown that the induced noise characteristic of sd-exchange is inherently colored in character and depends on the symmetry of the Hamiltonian of the magnetization in the film. We show that the sd-noise can be represented in terms of an external stochastic field which is white only in the adiabatic regime. The temperature is also renormalized by the spin accumulation in the system. For large intra-atomic exchange interactions, the Gilbert-Brown equation is no longer valid.

cond-mat.mtrl-sci↗

arXiv2004

sd-type Exchange Interactions in Nonhomogeneous Ferromagnets

Motivated by a need to understand spin-momentum transport in CPP (current perpendicular to the plane) structures, a quantum field theoretical treatment of spin-spin interactions in ferromagnets is presented. The sd interaction of the conduction electrons and the magnetic medium is treated non-perturbatively from first principles in real space. The localized magnetic moments also interact with each other through a Heisenberg exchange potential. To take into account correlation effects, a second quantized formulation is used. The semi-classical limit is taken by using a coherent-state path-integral technique which also allows us to go beyond a linear-response approach. We derive a set of coupled equations of motion for the nonuniform magnetization, the spin current and the two-point correlation functions of the magnetization. The rate of change of the magnetization is shown to obey a generalized Landau-Lifshitz equation that takes into account interaction with the conduction electrons. Within the relaxation time approximation it is shown that the polarization of the conduction electrons obeys a diffusion equation. The diffusion tensor, which has off-diagonal terms due to the sd exchange interaction, is now explicitly dependent on the magnetization of the medium. We also show that the magnetization fluctuations satisfy a diffusion-type equation. The derived equations are used in two illustrative examples.

cond-mat.mtrl-sci↗

arXiv2004

Spin accumulation in ferromagnets

Using a density matrix formulation for the effective action, we obtain a set of macroscopic equations that describe spin accumulation in a non-homogeneous ferromagnet. We give a new expression for the spin current which extends previous work by taking into account the symmetry of the ferromagnetic state through a careful treatment of the exchange term between the conduction electrons and the magnetization, i.e, d-electrons. We consider a simple application which has been discussed previously and show that in this case spin accumulation is an interface effect confirming earlier results arrived at by different methods.

cond-mat.mtrl-sci↗

arXiv2003

Spin momentum transfer in current perpendicular to the plane spin valves

We present experimental and numerical micromagnetic data on the effect of spin momentum transfer in current perpendicular to the plane spin valves. Starting from a configuration with orthogonal free and pinned layer magnetizations, the free layer magnetization exhibits abrupt current induced switching that is qualitatively consistent with the spin torque model. When operating the spin valve as a field sensor, spin transfer can produce a change in resistance that mimics an effective magnetic field and induce magnetic instability that requires a larger bias field in order to stabilize the device.

cond-mat.mtrl-sci↗

arXiv2003

Correlation energy of an electron gas: a functional approach

Correlation effects of an electron gas in an external potential are derived using an Effective Action functional method. Corrections beyond the random phase approximation (RPA) are naturally incorporated by this method. The Effective Action functional is made to depend explicitly on two-point correlation functions. The calculation is carried out at imaginary time. For a homogeneous electron gas, we calculate the effect of exchange on the ring diagrams at zero temperature and show how to include some of the ladder diagrams. Our results agree well with known numerical calculations. We conclude by showing that this method is in fact a variant of the time dependent density functional method and suggest that it is suitable to be applied to the study of correlation effects in the non-homogeneous case.

cond-mat.mtrl-sci↗

arXiv2003

Correlation Functions of the Magnetization in Thin Films

We calculate the correlation functions of uniform magnetization in thin ferromagnetic films for small deviations from equilibrium, by using a functional formalism. To take account of dissipation and fluctuations consistently, the magnetization is coupled to a bosonic heat bath. The correlation functions show strong dependence on the nature of the coupling between the bath and the system. Depending on what coupling we choose, we show how the recent results $(\text{J. Appl. Phys. 90, 5768 (2001); Phys. Rev. B 65, 172417 (2002)}) $ obtained by macroscopic methods can be related to the microscopic treatment adopted here.

cond-mat.mtrl-sci↗

arXiv2002

Fluctuations and Dissipation of Coherent Magnetization

A quantum mechanical model is used to derive a generalized Landau-Lifshitz equation for a magnetic moment, including fluctuations and dissipation. The model reproduces the Gilbert-Brown form of the equation in the classical limit. The magnetic moment is linearly coupled to a reservoir of bosonic degrees of freedom. Use of generalized coherent states makes the semiclassical limit more transparent within a path-integral formulation. A general fluctuation-dissipation theorem is derived. The magnitude of the magnetic moment also fluctuates beyond the Gaussian approximation. We discuss how the approximate stochastic description of the thermal field follows from our result. As an example, we go beyond the linear-response method and show how the thermal fluctuations become anisotropy-dependent even in the uniaxial case.

cond-mat.mtrl-sci↗

arXiv2000

New Method for Phase transitions in diblock copolymers: The Lamellar case

A new mean-field type theory is proposed to study order-disorder transitions (ODT) in block copolymers. The theory applies to both the weak segregation (WS) and the strong segregation (SS) regimes. A new energy functional is proposed without appealing to the random phase approximation (RPA). We find new terms unaccounted for within RPA. We work out in detail transitions to the lamellar state and compare the method to other existing theories of ODT and numerical simulations. We find good agreements with recent experimental results and predict that the intermediate segregation regime may have more than one scaling behavior.

cond-mat.mtrl-sci↗

arXiv1999

Generalized Random Phase Approximation: Zero and Non-Zero Temperature Properties of an Interacting Electron Gas

Correlated systems at both zero and nonzero temperature are treated here from a novel angle using a functional method. This functional method is an extension of the usual effective potential method. Here, however the effective action is made to depend explicitly on the correlation effects that are inherent in the physics involved. This will enable us to obtain new expressions for the free energy, the specific heat and the ground state energy. The new expansion is shown to give the expected results for the homogeneous case at zero temperature. However at non-zero temperature we are able to get new sets of diagrams that have a vanishing effect at zero temperature. To lowest order these diagrams if summed properly will solve a ln T anomaly in the specific heat of an electron gas at low temperature. We are also able to show that this method provides a very clear way to extend the RPA approximation. We calculate the effect of exchange on the ring diagrams at zero temperature and show how to include some of the ladder digrams. Our results agree well with known numerical calculations. We conclude by showing that this method is in fact a variant of the time dependent density functional method and can in principle be applied to study the effects of correlation in the nonhomogeneous case.

cond-mat↗

arXiv1998

The Specific Heat of a Fermi Gas at Low Temperature: A closer look at the TlnT behavior

At finite temperature, a Fermi gas can have sates that hold simultaneously a particle and a hole with a finite probability. This gives rise to a new set of diagrams that are absent at zero temperature. The so called "anomalous" diagram is just one of the new diagrams. We have already studied the contribution of these new diagrams to the thermodynamic potential (Phys. Lett. A224, 127 (1996) ). Here we continue that work and calculate their effect on the specific heat. We will also calculate the finite temperature contribution of the ring diagrams. We conclude that the TlnT behavior due to exchange gets canceled by the new contribution of the new diagrams and screening is not essential to solve this anomaly.

cond-mat.str-el↗