Session D15: Focus Session: Spins in Metals - Ultra Fast Dynamics

Ultrafast Magnetism of Multi-component Ferromagnets and Ferrimagnets on the Time Scale of the Exchange Interaction

Revealing the ultimate speed limit at which magnetic order can be controlled, is a fundamental challenge of modern magnetism having far reaching implications for the magnetic recording industry [1]. Exchange interaction is the strongest force in magnetism, being ultimately responsible for ferromagnetic or antiferromagnetic spin order. How do spins react after being optically excited on a timescale of or even faster than the exchange interaction? Here, we demonstrate that femtosecond (fs) measurements of ferrimagnetic and ferromagnetic alloys using X-ray magnetic circular dichroism provide revolutionary new insights into the problem of ultrafast magnetism on timescales pertinent to the exchange interaction. In particular, we show that upon fs optical excitation the ultrafast spin reversal of GdFeCo - a material with antiferromagnetic coupling of spins - occurs via a transient ferromagnetic state [2]. The latter emerges due to different dynamics of the Gd and Fe magnetic moments: Gd switches within 1.5 ps while it takes only 300 fs for Fe. Thus, by using a single fs laser pulse one can force the spin system to evolve via an energetically unfavorable way and temporarily switch from an antiferromagnetic to a ferromagnetic type of ordering. In order to understand whether the observation of this temporarily decoupled and element-specific dynamics is a general phenomenon or just something strictly related to the case of ferrimagnetic GdFeCo, we have investigated the demagnetization of the archetypal ferromagnetic NiFe alloys. Essentially, we observe the same distinct magnetization dynamics of the constituent magnetic moments: Ni demagnetizes within $\sim $300 fs being much faster than the demagnetization of Fe of $\sim $800 fs. This distinct demagnetization behavior leads to an apparent decoupling of the Fe and Ni magnetic moments on a few hundreds of fs time scale, despite the strong exchange interaction of 260meV ($\sim $16 fs) that couples them. These observations supported by atomistic simulations, present a novel concept of manipulating magnetic order on different classes of magnetic materials on timescales of the exchange interaction [3]. \\[4pt] [1] A. Kirilyuk, A.V. Kimel and Th. Rasing, \textit{Rev. Mod. Phys.} \textbf{82}, 2731 (2010). \\[0pt] [2] I. Radu \textit{et al.}, \textit{Nature} \textbf{472}, 205 (2011). \\[0pt] [3] I. Radu \textit{et al.}, \textit{submitted} (2011).   

Uncovering the Ultrafast Angular Momentum Transfer Channels on the Nanoscale in GdFeCo

The ultrafast control of electron spins is of both fundamental scientific and technological interest. Recent experiments have shown that femtosecond laser excitation can act as a stimulus to switch the magnetization direction in ferrimagnetic GdFeCo, called all-optical switching. However, how angular momentum is transferred to result in a switched state remains unknown. To further understand this mechanism, we use 80fs x-ray pulses from LCLS to study how angular momentum transfer is triggered in GdFeCo by fs laser excitation using time-, element- and spatially-resolved x-ray resonant magnetic scattering. We present here the first-ever measurement of the fs magnetic response in GdFeCo with spatial resolution down to 10nm. Our results reveal drastically different behaviors on the nanoscale as compared to the bulk and provide insight into the angular momentum transfer channels.   

Ultrafast Demagnetization Measurements using Extreme Ultraviolet Light: Comparison of Electronic and Magnetic Contributions

Ultrashort pulses from high-harmonic generation provide new capabilities for uncovering coupled charge, spin, and phonon dynamics in magnetic materials by combining elemental selectivity with ultrafast time resolution in a tabletop source. In this talk, we address an important question in magneto-optics that has implications for understanding femtosecond magnetism: is the signal from the transverse magneto-optical Kerr effect at the M-shell absorption edges of a magnetic material purely magnetic or perturbed by non-magnetic optical artifacts? We conclusively show that high harmonics sensitively probe the magnetic state, with negligible contributions from electronic effects because of hot-electron dynamics. Finally, our measurements are in excellent agreement with conventional visible-wavelength magneto-optics probes and illustrate the power of high harmonics for probing the dynamics in magnetic materials.   

Theory of spin hot pockets in laser-induced demagnetization in ferromagnetic nickel

We will first review the current theory of the demagnetization process, with a particular focus on two distinctive contributions: (a) the optical dipole interaction (ODI) between a laser field and a magnetic system and (b) the spin expectation value change (SEC) between two transition states. We then introduce a new optical spin operator, a product of SEC and ODI between transition states. In ferromagnetic nickel, our first-principles calculation demonstrates that the larger the value of optical spin operator is, the greater the dynamic spin moment change is. This simple operator is very useful, as it directly links the time-dependent spin moment change $\Delta M_{z}^{\bf k}(t)$ at every crystal-momentum ${\bf k}$ point to its intrinsic electronic structure and magnetic properties. Those hot spin pockets should be the focus of future research.   

Ultrafast Transport of Laser-Excited Spin-Polarized Carriers in Metallic Multilayers

The ultrafast spin dynamics induced by a transport of spin polarized carriers is a hot topic motivated by the fundamental interest in magnetic excitations and applications like spintronics and data storage. To understand underlying elementary processes typically occurring on femtosecond time scales, we have developed a time-of-flight-like approach that probes the spin dynamics induced by hot carriers (HC) and demonstrated a \textit{spin polarized} HC transport through an epitaxial Au/Fe/MgO(001) structure. Using a back pump-front probe configuration, we establish that HC induced in Fe by the pump laser pulse can form a nearly \textit{ballistic} spin current (SC) in Au. Optical second harmonic (SH) generated at the Au surface by the probe pulse monitors the transient surface HC density and spin polarization (SP). Since HC with different SP are excited in Fe to different final energies and consequently have different lifetimes in Au, the HC pulse has steep leading part formed by ballistic HC with the negative SP and shallow trial part formed by diffusive HC with the positive SP. This leads to the SC sign change within the 1~ps overall SC pulse duration. We also make a step towards understanding the origin of laser-induced ultrafast demagnetization overcoming the limited ability of conventional pump-probe schemes to distinguish photon-, electron-, and phonon-mediated effects. Comparing the SH response of Fe to the direct optical excitation with that to the excitation by hot carriers generated in Au, we rule out coherent effects of the pump pulse electromagnetic field and show that the HC-induced spin dynamics is responsible for the ultrafast demagnetization.   

First-principles dynamical calculation of a pump-probe scenario for the spin flip on NiO

Using a fully ab-initio approach we calculate in a dynamic way the time-dependent probe signal of a spin flip scenario on the antiferromagnetic NiO surface. We start from a first-principles calculation of the highly correlated, relativistic, electronic states of a doubly embedded NiO$_{5}^{-8}$ cluster followed by the time-propagation of the system under the influence of the spin-flipping pump pulse \emph{and} the detecting probe pulse. This way we treat both pulses on equal footing and, for the first time, consider the effects of the electronic non-equilibrium due to the concurrent presence of the pulses. Our time-resolved calculations reveal the subtle influence of the probe pulse itself on the detection signal, which cannot be completely treated solely by the time propagation of the pump pulse and the subsequent calculation of the static susceptibility tensor [1,2]. We also analyze the angular-momentum conservation and its distribution among the system and both laser pulses [3].\\ $[1]$ G. P. Zhang, W. H\"{u}bner, G. Lefkidis, Y. Bai, and T. F. George, Nature Physics {\bf 5}, 499 (2009)\\ $[2]$ G. P. Zhang, G. Lefkidis, W. H\"{u}bner and Y. Bai, J. Appl. Phys. {\bf 109}, 07D303 (2011)\\ $[3]$ G. Lefkidis, G. P. Zhang and W. H\"{u}bner, Phys. Rev. Lett. {\bf 103}, 217401 (2009)   

Non-equilibrium coupling and femto-second oscillatory spin torque in non-collinear F/N/F magnetic multilayers

By employing time-dependent diffusion theory, we study time-evolution behaviors of spin torque and spin current in non-collinear Ferromagnetic/Normal/Ferromagnetic tri-layers structure. We find in ferromagnetic layer spin toque has femto-second oscillatory in its initio building-up process, which indicates that excited itinerant electron's spin process alone background magnetization in femto-second period near the interfacial region. We also find even mismatch of background magnetization at each F/N interfaces is not enough to compensate discontinuity of spin current near interface, however, by introducing non-equilibrium coupling between two F/N interfaces, one can produce a continuous spin current state across both two interfaces without mandating any boundary conditions. In our study, we find a new universal time-dependent propagator to generate all directions continuous spin current across two F/N interfaces. This new spin propagator is also closely related to spin flip scatter at interface. In addition, we also find the coupling of two N/F interfaces enlarges transverse spin diffusion channels into magnetic layer. Finally, our time-dependent spin current and spin torque states also match our pervious self-consistent steady state results.   

Probing the timescale of the exchange interaction in a ferromagnetic alloy

The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (Permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then, as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the Permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies.   

Driving coherent spin reorientation transition with femtosecond laser pulses

Ultrafast studies of spin reorientation transition provide insight in magnetization switching processes and are important for the magnetic recording technology. Using femtosecond optical techniques, we demonstrate coherent control of the magnetization vector in epitaxial Fe films. These films feature uniaxial anisotropy that is thermally modulated by an optical pulse. We observe an optically-induced spin reorientation transition of first-order that provides an efficient route to ultrafast coherent magnetization switching. The switching is found to be a three-step temporal process: a coherent reorientation ($\sim $ 100 ps) is followed by a spin precession in a newly created metastable state ($\sim $ 300 ps), which evolves into a dual domain state that undergoes relaxation within $\sim $ 2 - 4 ns. We provide a model to explain the experimental data and predict further applications of this technique. The details of the experiments compare favorably with the simulated magnetization trajectories, opening new pathways for coherent control of magnetic dynamics with pulsed lasers.   

Theory of the inverse Faraday effect driven by non-coplanar spin structures

We show theoretically that a new mechanism of the inverse Faraday effect exists in the presence of non-coplanar spin structures in metals even without the spin-orbit interaction. The spin density generated by the effect is proportional to the circular polarization of the light, ($\mathbf{\mathcal{E}}\times \mathbf{\mathcal{E}}^*$) ($\mathbf{\mathcal{E}}$ is the complex amplitude vector of the electric field of the circular light), and $\nabla \times \mathbf{j}_{\rm{s}}^\alpha$, where $\mathbf{j}_{\rm{s}}^\alpha \equiv (\mathbf{n}\times \mathbf{\nabla}\mathbf{n})^\alpha$ is the spin current carried by the spin structure ($\mathbf{n}$ is the unit vector of the localized spin). The effect turns out to be larger than the conventional inverse Faraday effect, and the present mechanism is expected to be useful for the magnetization flip of the Skyrmion.   

Examination of spin waves in a two-dimensional magnetic superlattice

We have studied the properties of spin waves in two-dimensional periodic superlattices of magnetic materials.[1] Frequencies and linewidths are calculated for square and hexagonal symmetry superlattices from the Landau-Lifshitz-Gilbert equations. Large differences in the saturation magnetizations and exchange stiffness constants of the materials are shown to be capable of producing gaps in the magnonic spectrum across the entire superlattice Brillouin zone. For example, with a hexagonal superlattice of Fe and YIG we find gaps of 0.5THz and 1THz within the lowest four bands. Additionally, calculations of the system's Green's functions are used to examine the superlattice's response to pulse excitations, such as from a spin torque oscillator.\\[4pt][1]arXiv:1111.2506v1