Session J29: Focus Session: Ultrafast Spin-Dynamics

All-Optical Control of Magnetization in various Ferromagnetic Structures

Until recently, the only class of material that demonstrated all-optical switching (AOS) is a narrow range of rare-earth (RE) and transition-metal (TM) alloy compositions. However, recent experimental investigations have broadened the choice of materials for AOS and showed that optical control of magnetization is a much more general phenomenon. These materials include wide variety of ferrimagnetic RE-TM alloys, RE-free synthetic ferrimagnets and, moreover, ferromagnetic thin films, multi-layers and even their granular films. By employing a magnetization sensitive microscopy technique we investigated the AOS in various ferromagnetic materials like Co/Pt multi-layers as a function of material composition, structure, laser pulse fluence, and multi-layer thickness and also in FePt granular thin films, as a function of grain sizes. Our results show the optimal material conditions in ferromagnets and highlight pathways for reducing possible energy consumption for the AOS in these materials. Moreover, our time-resolved pump-probe measurements on CoPt thin films and their multi-layers reveal the ultrafast magnetization response to the 100 fs laser pulses and its role in AOS.   

Ultrafast Dynamics near the M-edge in Chromium

The exploration of element specific ultrafast spin dynamics in transition metals has been extended by recent advances in table top VUV sources based on high harmonic generation. These sources provide femtosecond time resolution at photon energies that span the magnetism sensitive 3p to 3d band absorption (M-edge) in these materials. The time scale of spin dynamics determines the fundamental limits of magnetic data recording and gives insight into magnetoelectric coupling mechanisms in complex functional materials. Though there have been multiple time-resolved studies on ferromagnetic systems, antiferromagnetic (AFM) dynamics remains largely unexplored. As an AFM test system we choose chromium and measure transient reflectivity for photon energies spanning the chromium M-edge. Picosecond dynamics are measured throughout the spectrum of the VUV probe beam after excitation by an IR laser pulse. A dramatic difference is observed in the transient magnetic linear absorption dichroism of chromium for photon energies above and below the M-edge ($\sim$ 46 eV) as temperature is varied through the AFM transition. While a decrease in reflectivity is seen below the M-edge we find an increase in reflectivity above the edge. We attribute this variation to interplay between electronic and magnetic responses and discuss its relation to ultrafast magnetic ordering dynamics.   

Ultrafast demagnetization: the effect of the pulse length

Upon excitation by a femtosecond laser pulse a ferromagnet can be demagnetized on a sub-picosecond timescale. During the demagnetization both energy and angular momentum are exchanged between the electron gas and the lattice. However, the mechanisms and the characteristic times through which such exchanges occur are still controversial. A strong debate remains on whether bulk spin-flips or spin-currents are the primary cause of momentum transfer. To shed light on this topic two types of experiments are undertaken. First, demagnetization is studied in a pump-probe experiment in Ni for different pump-pulse durations. It is observed that shorter laser pulses demagnetize nickel more effciently than longer pulses. The experiment reveals two processes: a fast process demagnetizes the sample within less than a picosecond and causes a remagnetization on the same timescale. Simultaneously, a slower process causes a magnetization loss that lasts tens of picoseconds. For long pulses only the slow remagnetization process is relevant. Second, a delayed double pulse experiment is performed in which the induced heat from the first pump pulse influences the demagnetization caused by the second through the increase of either the lattice or the electron temperatures.   

Ultrafast demagnetization: A transport effect?

The ultrafast loss of magnetization caused by a femtosecond laser pulse is still not well understood. One of the important contributions is spin flip scattering in the bulk of the magnetic material, mediated by the presence of phonons. The second contribution is the creation of spin currents, which transport spin angular momentum from the magnetic film into the substrate, where spin slips can take place outside of the view of the observer. Spin currents have been predicted by Battiato et al., PRL p.105, 027203 (2010). Here, we discuss a simple thermodynamic model describing laser induced spin currents as a cause of the temperature dependent shift of the chemical potentials. Experiments are discussed, which may be able to separate the effect of the lattice temperature from spin transport effects.   

Ultrafast spin switching in a canted antiferromagnetic YFeO3 driven by pulsed THz radiations

We investigate a detailed process of the precessional motion of the magnetic moment in the canted antiferromagnetic YFeO3 which is excited by a linearly polarized terahertz (THz) pulse at room temperature. By tuning the spectral component of the input THz pulse around the quasi-ferromagnetic mode located near 0.3 THz, we have experimentally clarified the resonance effect in the THz control of the spin state. We could confirm this result also from the simulation based on the Landau-Lifshitz-Gilbert equation with two sub-lattice model for the canted antiferromagnet. Furthermore, we demonstrate that the spin state can be switched all-optically on a picosecond time-scale using THz pulses of square and oscillating shapes. Whereas the oscillating THz pulse with a spectral component resonant with the magnetic excitations is necessary for an efficient magnetization switching, we check the possibility of a further reduction of the necessary THz field strength by examining influences of variations in the anisotropy energy and Dzyaloshinskii-Moriya interaction upon the switching behaviors.   

Ultrafast magnetization dynamics in heterogeneous granular FePt media

Granular FePt in the L1$_0$ phase is a key material for future magnetic data storage devices, supporting the smallest stable magnetic domains. Precessional eigenmodes have recently been studied with fs laser pump-probe techniques[Becker, et al APL 104, 152412 (2014)]. Large 400$\,$GHz eigenfrequencies are observed due to the huge FePt magnetic anisotropy. However, such optical techniques only measure the collective spin precession of all grains without spatial resolution. Here we investigate the nanoscale aspects of magnetization dynamics in FePt with fs X-ray pulses from the Linac Coherent Light Source at Stanford. We show that optical excitation leads to distinctly different magnetic responses due to the size distribution of FePt grains. While part of the laser-excited magnetic grains follow a small applied magnetic field, others do not. The former display heat assisted magnetic recording. In contrast, the magnetic anisotropy of the latter is not significantly affected by laser excitation. Such grains display the typical high-frequency magnetization precession modes. We show that the interplay of ``weak'' and ``hard'' grains leads to magnetic frustration in the ground state causing heterogeneity that is partially lifted by laser excitation.   

Laser-induced ultrafast spin dynamics in di-, tri- and tetranuclear nickel clusters, and the M process

We present a theoretical investigation of the ultrafast magneto-optical dynamics in clusters with 2, 3 and 4 Ni atoms. We study cooperative effects by increasing both the number of active centers and the multiplicities (up to quintets) in our Hilbert space. In the dinuclear cluster Ni$_2$ a novel spin-flip scenario based on the M process is established\footnote{D. Chaudhuri, H. P. Xiang, G. Lefkidis and W. H\"{u}bner, Phys. Rev. B, in print (2014).}. This process is highly operative in case the standard $\Lambda$ process fails. In the trinuclear cluster Ni$_3$Na$_2$ simultaneous spin-flip and spin-transfer is observed. Local spin-switch is also achieved via a non-linear M process which involves two off-resonant transitions. Finally, in the tetranuclear cluster (Ni$_2$)$_2$ the use of quintets provides an insight into an irreversible demagnetization scenario. We believe that our systematic investigation allows us to establish a relation between the magnetic centers and the multiplicities. Our research represents an important step towards the miniaturization of spintronic devices and functionalization of various logic elements based on molecular structures\footnote{W. H\"{u}bner, S. Kersten and G. Lefkidis, Phys. Rev. B \textbf{79}, 184431 (2009).}.   

A look at the Dynamics of Ultrafast Magnetization Reversal

There is an ever pressing need to figure out ways to store more data at a faster rate. The implementation of All Optical Magnetization Reversal is a potential step in this direction. There is a strong experimental understanding of the phenomenon. However, a clear theoretical model doesn't exist. The theoretical model presented here is a potential step towards an understanding of this phenomenon. Through a computer simulation, we show that magnetization reversal could potentially occur through the interaction of the circular polarization of an ultrafast laser pulse, and the orbital angular momentum of a bound spin particle.   

Optimal Control of Magnetization Dynamics in Ferromagnetic Materials using TDDFT

Recently [1] intense laser-field induced ultrafast demagnetization was observed in ab-initio simulations using Time-Dependent Density Functional Theory (TDDFT) for various ferromagnetic materials (Fe,Co,Ni). From a practical and technological viewpoint, it is useful if the induced dynamics (e.g. of the total magnetic moment) are controllable. In this talk we apply optimal control theory together with TDDFT calculations to tailor the intense laser pulses so as to achieve a particular outcome (e.g. maximize the total moment lost) while also including any required constraints (e.g pulse duration, pulse frequencies, maximum fluence, etc). \\[4pt] [1] Laser induced ultrafast demagnetization: an ab-initio perspective, K. Krieger, J.K. Dewhurst, P. Elliott, S. Sharma, E.K.U. Gross, submitted (2014).   

Thermal spin transfer torque driven by ultrafast heat current in metallic spin-valve structures

Spin transfer torque (STT), coupling of the angular momentum of the spin of electrons and the magnetization of a ferromagnet, enables the manipulation of nanomagnets with spin currents rather than magnetic fields. STT has been most often realized by passing electrical currents through magnetic layers. Generation of STT by passing a heat current through magnetic layers has been theoretically predicted. This so-called ``thermal STT'' relies on the transport of thermal energy, as opposed to the transport of electrical charge, and provides new functionality for device applications. Here, we provide direct evidence of thermal STT generated by ps time-scale heat currents on the order of 100 GW m$^{\mathrm{-2}}$. In metallic spin valve structures, the physical mechanism for thermal STT is the spin-dependent Seebeck effect (SDSE). We create ultrafast heat currents using ps duration pulses of laser light in the NM1/FM1/NM2/FM2 structure: FM1 acts as a spin generation layer by SDSE and FM2 acts as a spin detection layer by STT; NM1 acts as a heat absorbing layer and NM2 acts as a heat sink layer. The magnetization dynamics of FM2 are probed by time-resolved magneto-optic Kerr effect with a time resolution of 1 ps. By incorporating different ferromagnetic layers, which have different signs for SDSE, and varying the thickness of the heat sink layer, we are able to control the sign and magnitude of thermal spin torque.   

Spin accumulation in out of equilibrium mesoscopic superconductors

We study the spin accumulation in a junction between a superconductor and a ferromagnet or a normal metal in presence of a Zeeman magnetic field applied to the superconductor, and when the junction is taken out of equilibrium by applying a voltage bias. We first apply a DC voltage on the junction and show that the spin relaxation time (~ns) is larger than the charge relaxation time ($\sim$ps) inducing a spin-charge separation in the superconductor. Then we calculate the time-dependence of the spin accumulation for an applied AC voltage. We find that the measured spin accumulation depends on the frequency of the applied bias. This dependence allows one to extract directly the spin relaxation time in the superconductor which is in complete agreement with the experimental result.   

Spin-dependent Otto quantum heat engine based on a molecular substance

We explore the potential of single molecules for thermodynamic cycles\footnote{W. H\"{u}bner, G. Lefkidis, C. D. Dong, D. Chaudhuri, L. Chotorlishvili, and J. Berakdar, Phys. Rev. B {\bf 90}, 024401 (2014)}. To this end we propose two molecular heat engines based on the realistic Ni$_2$ dimer: a quantum Otto engine and a modified quantum Otto engine for which laser-induced optical excitations substitute for one of the heat-exchange points. For reliable predictions and to inspect the role of spin and electronic correlations we perform fully correlated {\emph ab initio} calculations of the electronic structure and the excited states. We analyze the efficiency and the word output of the derived engines and find an enhancement when the spin degree of freedom is included. We also use the von Neumann entropy to describe correlations and entanglement of the engines during the cycles. Furthermore, we link our results to previous results regarding an isobaric stroke\footnote{C. D. Dong, G. Lefkidis and W. H\"{u}bner, J. Supercond. Nov. Magn. {\bf 26}, 1589 (2013)} and a magnetic quantum Diesel engine on the same substance\footnote{C. D. Dong, G. Lefkidis and W. H\"{u}bner, Phys. Rev. B {\bf 88}, 214421 (2013)}.