Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current (original) (raw)
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Ultrafast and Distinct Spin Dynamics in Magnetic Alloys
Controlling magnetic order on ultrashort timescales is crucial for engineering the next-generation magnetic devices that combine ultrafast data processing with ultrahigh-density data storage. An appealing scenario in this context is the use of femtosecond (fs) laser pulses as an ultrafast, external stimulus to fully set the orientation and the magnetization magnitude of a spin ensemble.Achieving such control on ultrashort timescales, e.g., comparable to the excitation event itself, remains however a challenge due to the lack of understanding the dynamical behavior of the key parameters governing magnetism: The elemental magnetic moments and the exchange interaction. Here, we investigate the fs laser-induced spin dynamics in a variety of multi-component alloys and reveal a dissimilar dynamics of the constituent magnetic moments on ultrashort timescales. Moreover, we show that such distinct dynamics is a general phenomenon that can be exploited to engineer new magnetic media with tailor-made, optimized dynamic properties. Using phenomenological considerations, atomistic modeling and time-resolved X-ray magnetic circular dichroism (XMCD), we demonstrate demagnetization of the constituent sub-lattices on signi¯- cantly di®erent timescales that depend on their magnetic moments and the sign of the exchange interaction. These results can be used as a \recipe" for manipulation and control of magnetization dynamics in a large class of magnetic materials.
Resolving the role of femtosecond heated electrons in ultrafast spin dynamics
Scientific Reports, 2014
Magnetization manipulation is essential for basic research and applications. A fundamental question is, how fast can the magnetization be reversed in nanoscale magnetic storage media. When subject to an ultrafast laser pulse, the speed of the magnetization dynamics depends on the nature of the energy transfer pathway. The order of the spin system can be effectively influenced through spin-flip processes mediated by hot electrons. It has been predicted that as electrons drive spins into the regime close to almost total demagnetization, characterized by a loss of ferromagnetic correlations near criticality, a second slower demagnetization process takes place after the initial fast drop of magnetization. By studying FePt, we unravel the fundamental role of the electronic structure. As the ferromagnet Fe becomes more noble in the FePt compound, the electronic structure is changed and the density of states around the Fermi level is reduced, thereby driving the spin correlations into the limit of critical fluctuations. We demonstrate the impact of the electrons and the ferromagnetic interactions, which allows a general insight into the mechanisms of spin dynamics when the ferromagnetic state is highly excited, and identifies possible recording speed limits in heat-assisted magnetization reversal.
Ultrafast spin transport as key to femtosecond demagnetization
Nature Materials, 2013
Irradiating a ferromagnet with a femtosecond laser pulse is known to induce an ultrafast demagnetization within a few hundred femtoseconds. Here we demonstrate that direct laser irradiation is in fact not essential for ultrafast demagnetization, and that electron cascades caused by hot electron currents accomplish it very efficiently. We optically excite a Au/Ni layered structure in which the 30 nm Au capping layer absorbs the incident laser pump pulse and subsequently use the X-ray magnetic circular dichroism technique to probe the femtosecond demagnetization of the adjacent 15 nm Ni layer. A demagnetization effect corresponding to the scenario in which the laser directly excites the Ni film is observed, but with a slight temporal delay. We explain this unexpected observation by means of the demagnetizing effect of a superdiffusive current of non-equilibrium, non-spin-polarized electrons generated in the Au layer. U ltrafast demagnetization triggered by shining a femtosecond laser pulse onto a ferromagnetic transition-metal sample has been extensively studied since its discovery 1 . In spite of numerous investigations, the mechanism underlying light-induced demagnetization could not yet be clearly identified. A variety of different microscopic models 2-6 has been put forward over the past years to explain how an ultrashort laser pulse could modify the magnetic system within a few hundred femtoseconds after laser excitation 7 . Most are based on ultrafast spin-flip scattering of some kind, such as Elliott-Yafet electron-phonon spin-flip scattering 2 , electron-magnon spin-flip scattering 3 , and Coulomb exchange spin-flip scattering 4 . Other possibilities are direct laserinduced 4 or relativistic electromagnetic-radiation-induced 5 spinflips. Regardless of the diverse nature of these mechanisms, all scenarios adopt the same first step in the excitation process, namely direct absorption of a femtosecond laser pulse in the ferromagnetic film. Conversely, in this work we explore a different process that leads to very efficient ultrafast demagnetization, namely transport of laser-excited non-spin-polarized electrons into the ferromagnet.
Physical Review Letters, 2006
The femtosecond magnetization dynamics of a thin cobalt film excited with ultrashort laser pulses has been studied using two complementary pump-probe techniques, namely spin-, energyand time-resolved photoemission and time-resolved magneto-optical Kerr effect. Combining the two methods it is possible to identify the microscopic electron spin-flip mechanisms responsible for the ultrafast macroscopic magnetization dynamics of the cobalt film. In particular, we show that electron-magnon excitation does not affect the overall magnetization even though it is an efficient spin-flip channel on the sub-200 fs timescale. Instead we find experimental evidence for the relevance of Elliott-Yafet type spin-flip processes for the ultrafast demagnetization taking place on a time scale of 300 fs.
Applied Sciences
The vision to manipulate and control magnetism with light is driven on the one hand by fundamental questions of direct and indirect photon-spin interactions, and on the other hand by the necessity to cope with ever growing data volumes, requiring radically new approaches on how to write, read and process information. Here, we present two complementary experimental geometries to access the element-specific magnetization dynamics of complex magnetic systems via ultrafast magneto-optical spectroscopy in the extreme ultraviolet spectral range. First, we employ linearly polarized radiation of a free electron laser facility to demonstrate decoupled dynamics of the two sublattices of an FeGd alloy, a prerequisite for all-optical magnetization switching. Second, we use circularly polarized radiation generated in a laboratory-based high harmonic generation setup to show optical inter-site spin transfer in a CoPt alloy, a mechanism which only very recently has been predicted to mediate ultraf...
Ultrafast Spin Dynamics in Multisublattice Magnets
Physical Review Letters, 2012
We propose a general theoretical framework for ultrafast laser-induced spin dynamics in multisublattice magnets. We distinguish relaxation of relativistic and exchange origin and show that when the former dominates, nonequivalent sublattices have distinct dynamics despite their strong exchange coupling. Even more interesting, in the exchange dominated regime sublattices can show highly counterintuitive transitions between parallel and antiparallel alignment. This allows us to explain recent experiments with antiferromagnetically coupled sublattices, and predict that such transitions are possible with ferromagnetic coupling as well. In addition, we predict that exchange relaxation enhances the demagnetization speed of both sublattices only when they are antiferromagnetically coupled.
High-frequency magnon excitation due to femtosecond spin-transfer torques
Physical review, 2020
Femtosecond laser pulses can induce ultrafast demagnetization as well as generate bursts of hot-electron spin currents. In trilayer spin valves consisting of two metallic ferromagnetic layers separated by a nonmagnetic one, hot-electron spin currents excited by an ultrashort laser pulse propagate from the first ferromagnetic layer through the spacer, reaching the second magnetic layer. When the magnetizations of the two magnetic layers are noncollinear, this spin current exerts a torque on magnetic moments in the second ferromagnet. Since this torque is acting only within the subpicosecond timescale, it excites coherent high-frequency magnons, as recently demonstrated in experiments. Here, we calculate the temporal shape of the hot-electron spin currents using the superdiffusive transport model and simulate the response of the magnetic system to the resulting ultrashort spintransfer torque pulse by means of atomistic spin-dynamics simulations. Our results confirm that the acting spincurrent pulse is short enough to excite magnons with frequencies beyond 1 THz, a frequency range out of reach for current-induced spin-transfer torques. We demonstrate the formation of thickness-dependent standing spin waves during the first picoseconds after laser excitation. In addition, we vary the penetration depth of the spintransfer torque to reveal its influence on the excited magnons. Our simulations clearly show a suppression effect of magnons with short wavelengths already for penetration depths in the range of 1 nm, confirming experimental findings reporting penetration depths below 2 nm.
Nature, 2011
Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism 1-4 . Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. Usually the dynamics are studied by observing the collective response of exchange-coupled spins, that is, spin resonances, after an external perturbation by a pulse of magnetic field, current or light. The periods of the corresponding resonances range from one nanosecond for ferromagnets down to one picosecond for antiferromagnets. However, virtually nothing is known about the behaviour of spins in a magnetic material after being excited on a timescale faster than that corresponding to the exchange interaction (10-100 fs), that is, in a non-adiabatic way.