Electronic excitations in a ferromagnet can trigger ultrafast spin dynamics with potential applications in a speed increase in magnetic recording. The project investigates ultrafast magnetization dynamics, which is driven in metallic layers by ballistic hot electrons. In a ferromagnet these electrons induce a change in the absolute value of the magnetization M through spin-dependent scattering. If the electrons are spin-polarized, scattering at the interface of a noble metal and a ferromagnet results in spin-transfer torque and hence modifies the direction of M. To reveal the underlying mechanisms, we study model systems, which are realized by layers of Au with its large ballistic mean free path and Fe as an itinerant ferromagnet. We aim at understanding ultrafast demagnetization and femtosecond all-optical generation of spin transfer torque effects (excitation of hot electrons with a femtosecond laser). The launched dynamics are probed by magneto-optics in a time-resolved experiment.
The in-plane current profile within multilayers of generic structure Ta/Pt/(CoNi)/Pt/Ta is investigated. A large set of samples where the thickness of each layer is systematically varied was grown, followed by the measurement of the sheet resistance of each sample. The data are analyzed by a series of increasingly elaborate models, from a macroscopic engineering approach to mesoscopic transport theory. Non-negligible variations of the estimated repartition of current between the layers are found. The importance of having additional structural data is highlighted.
We present a structural and high frequency (8.72GHz) electrical characterization of sputter deposited Ti/W, Ti/Ru and Mo/Ti metallic multilayers for potential application as acoustic Bragg reflectors. We prove that all metallic multilayers comprised of different acoustic impedance metals such as Ti, W, Mo are promising candidates for Bragg reflector/bottom electrode in full X-band thin film acoustic resonators. Values for high frequency resistivity of the order of $10^{-8} ohm.m$ are measured by use of a contact-free/non-invasive sheet resistance method.
It is shown that the well-known blue-shift of the fundamental absorption edge in as-deposited compositionally modulated amorphous Si/Ge and As6Se94/Se80Te20 multilayers (with periods of 4-8 nm) is further enhanced due to the thermal or laser-induced intermixing of adjacent layers. The laser-induced intermixing process, as supported by experiments and model calculations, can be attributed to both the local heating and photo-effects in As6Se94/Se80Te20 multilayers, while only the thermal effects were observed for Si/Ge multilayers. Structural transformations, based on this enhanced interdiffusio…
The anomalous Nernst effect (ANE) has been investigated in alternately-stacked multilayer films comprising paramagnetic and ferromagnetic metals. We found that the ANE is enhanced with increasing the number of the paramagnet/ferromagnet interfaces with keeping the total thickness of the films constant, and that the enhancement appears even in the absence of magnetic proximity effects; similar behavior was observed not only in Pt/Fe multilayers but also in Au/Fe and Cu/Fe multilayers free from proximity ferromagnetism. This universal enhancement of the ANE in the metallic multilayers suggests t…
Rhombohedral stacked multilayer graphene displays the occurrence of a magnetic surface state at low temperatures. Recent angular resolved photoemission experiments demonstrate the robustness of the magnetic state in long sequences of ABC graphene. Here, by using first-principles calculations, we show that field-effect doping of these graphene multilayers induces a perfect half-metallic behaviour with 100% of spin current polarization already at dopings attainable in conventional field effect transistors with solid state dielectrics. Our work demonstrates the realisability of a new kind of spin…
Transport properties of magnetic multilayers in current perpendicular to the plane of the layers geometry are defined by diffusive scattering in the bulk of the layers, and diffusive and ballistic scattering across the interfaces. Due to the short screening length in metals, layer by layer treatment of the multilayers is possible, when the macroscopic transport equation (Boltzmann equation or diffusion equation) is solved in each layer, while the details of the interface scattering are taken into account via the boundary conditions. Embedding the ballistic and diffusive interface scattering in…
We study the role of diamagnetic effects on the transport properties of metallic magnetic multilayers to elucidate whether they can explain the Giant Magnetoresistance (GMR) effect observed in those systems. Realistic Fermi surface topologies in layered ferromagnets are taken into account, with the possibilities of different types of orbits depending on the electron spin. Both configurations, with ferromagnetic and anti-ferromagnetic couplings between magnetic layers, are considered and the transmission coefficient for scattering at the interface boundary is modelled to include magnetic and ro…
Spin relaxation in the ultrathin metallic films of stacked microelectronic devices is investigated on the basis of a modified Landau-Lifshitz equation of micromagnetic dynamics in which the damping torque is treated as originating from the coupling between precessing magnetization-vector and the introduced stress-tensors of intrinsic and extrinsic magnetic anisotropy. Particular attention is given to the time of exponential relaxation and ferromagnetic resonance linewidth which are derived in analytic form from the equation of magnetization energy loss and Gabor uncertainty relation between th…
We measured spin transport in nonferromagnetic (NM) metallic multilayers from the contribution to damping due to spin pumping from a ferromagnetic Co90Fe10 thin film. The multilayer stack consisted of NM1/NM2/Co90Fe10(2 nm)/NM2/NM3 with varying NM materials and thicknesses. Using conventional theory for one dimensional diffusive spin transport in metals, we show that the effective damping due to spin pumping can be strongly affected by the spin transport properties of each NM in the multilayer, which permits the use of damping measurements to accurately determine the spin transport properties …
Single Pulse All Optical Helicity Independent Switching (AO-HIS) represents the ability to reverse the magnetic moment of a nanostructure using a femtosecond single laser pulse. It is an ultrafast method to manipulate magnetization without the use of any applied field. Since the first switching experiments carried on GdFeCo ferrimagnetic systems, single pulse AO-HIS has been restricted for a while to Gd-based alloys or Gd/FM bilayers where FM is a ferromagnetic layer. Only recently has AO-HIS been extended to a few other materials: MnRuGa ferrimagnetic Heusler alloys and Tb/Co multilayers with…
The mechanical properties and deformation mechanisms of Cu/Nb nanoscale metallic multilayers (NMMs) manufactured by accumulative roll bonding (ARB) are studied at 25C and 400C. Cu/Nb NMMs with individual layer thicknesses between 7 and 63 nm were tested by in-situ micropillar compression inside a scanning electron microscope Yield strength, strain-rate sensitivities and activation volumes were obtained from the pillar compression tests. The deformed micropillars were examined under scanning and transmission electron microscopy in order to examine the deformation mechanisms active for different…
We demonstrate that the intrinsic properties of monolayer graphene allow it to act as a more effective saturable absorber for mode-locking fiber lasers compared to multilayer graphene. The absorption of monolayer graphene can be saturated at lower excitation intensity compared to multilayer graphene, graphene with wrinkle-like defects, and functionalized graphene. Monolayer graphene has a remarkable large modulation depth of 95.3%, whereas the modulation depth of multilayer graphene is greatly reduced due to nonsaturable absorption and scattering loss. Picoseconds ultrafast laser pulse (1.23 p…
The irradiation with the 10 ns pulsed infrared Nd:YAG laser was applied to transform FePdCu multilayers into chemically ordered L10 phase. The X-ray diffraction methods (Theta/2Theta scan, psi-scan, omega-scan) were used to trace the presence of L10 phase after laser annealing with different number of pulses. The size and shape of crystallites was determined depending on their orientation with respect to film plane. The (111) oriented crystallites of constituent metals were built as coherent domains spreading through multilayers during deposition of films. Laser annealing induced the transform…
We use density functional theory plus dynamical mean-field theory to demonstrate the emergence of a metallic layer at the interface between the two Mott insulators LaTiO$_3$ and LaVO$_3$. The metallic layer is due to charge transfer across the interface, which alters the valence state of the transition metal cations close to the interface. Somewhat counter-intuitively, the charge is transferred from the Ti cations with formal $d^1$ electron configuration to the the V cations with formal $d^2$ configuration, thereby increasing the occupation difference of the $t_{2g}$ states. This can be unders…
The Cu/Fe/Pd multilayers were transformed into L10-ordered FePdCu alloy by pulsed laser annealing. The initial multilayers were irradiated with 1, 10, 100, and 1000 laser pulses with duration time of 10 ns and energy density of 235 mJ/cm2. The gradual change of the number of laser pulses allowed to investigate the structural and magnetic properties at early stages of the transformation and L10-ordering processes. The measurements were carried out using X-Ray Diffraction, SQUID magnetometry, and Magnetic Force Microscopy. We found that L10 FePdCu (111)-oriented nanograins are formed by ordering…
The mechanism of spin-pumping, described by Tserkovnyak et al., is formally analyzed in the general case of a magnetic multilayer consisting of two or more metallic ferromagnetic (FM) films separated by normal metal (NM) layers. It is shown that the spin-pumping-induced dynamic coupling between FM layers modifies the linearized Gilbert equations in a way that replaces the scalar Gilbert damping constant with a nonlocal matrix of Cartesian damping tensors. The latter are shown to be methodically calculable from a matrix algebra solution of the Valet-Fert transport equations. As an example, expl…
The paper presents a new approach, based on controlled oxidation of nanoscale metallic multilayers, to produce strong and hard oxide/metal nanocomposite coatings with high strength and good thermal stability. The approach is demonstrated by performing long term annealing on sputtered Zr/Nb nanoscale metallic multilayers and investigating the evolution of their microstructure and mechanical properties by combining analytical transmission electron microscopy, nano-mechanical tests and finite element models. As-deposited multilayers were annealed at 350 C in air for times ranging between 1 - 336 …
Real-space first-principles approach to orbitronic phenomena in metallic multilayers
Centro Brasileiro de Pesquisas Físicas (CBPF)（巴西物理研究中心）；School of Science and Technology, Örebro University（欧雷布鲁大学理学院）；Department of Physics and Astronomy, Uppsala University（乌普萨拉大学物理与天文学系）；Faculdade de Física, Universidade Federal do Pará（帕拉联邦大学物理学院）；International Iberian Nanotechnology Laboratory (INL)（伊比利亚纳米技术国际实验室）
The spin-dependent mobility for the lateral transport of the hole gas in a GaMnAs/GaAs heterostructure containing several metallic-like ferromagnetic layers is calculated. The electronic structure is obtained self-consistently taking into account the direct Coulomb Hartree and exchange-correlation terms, besides the sp-d exchange interaction with the Mn magnetic moments.
The Anderson model of local-state conduction electron mixing is applied to the p roblem of interlayer magnetic coupling in metallic multilayered structures with palladium (Pd) spacer layers. An oscillation period of $5$ spacer monolayers and the tendency towards ferromag netic bias of the interlayer magnetic coupling that we obtain are consistent with the experimental data.
Since the first experimental observation of all-optical switching phenomena, intensive research has been focused on finding suitable magnetic systems that can be integrated as storage elements within spintronic devices and whose magnetization can be controlled through ultra-short single laser pulses. We report here atomistic spin simulations of all-optical switching in multilayered structures alternating n monolayers of Tb and m monolayers of Co. By using a two temperature model, we numerically calculate the thermal variation of the magnetization of each sublattice as well as the magnetization…
A self-consistent electronic structure calculation based on the Luttinger-Kohn model is performed on GaMnAs/GaAs multilayers. The Diluted Magnetic Semiconductor layers are assumed to be metallic and ferromagnetic. The high Mn concentration (considered as 5% in our calculation) makes it possible to assume the density of magnetic moments as a continuous distribution, when treating the magnetic interaction between holes and the localized moment on the Mn(++) sites. Our calculation shows the distribution of heavy holes and light holes in the structure. A strong spin-polarization is observed, and t…
We use transient absorption spectroscopy with circularly polarized x-rays to detect laser-excited hole states below the Fermi level and compare their dynamics with that of unoccupied states above the Fermi level in ferromagnetic [Co/Pd] multilayers. While below the Fermi level an instantaneous and significantly stronger demagnetization is observed, above the Fermi level the demagnetization is delayed by 35+/-10 fs. This provides a direct visualization of how ultrafast demagnetization proceeds via initial spin-flip scattering of laser-excited holes to the subsequent formation of spin waves.
We examine the atomic intermixing phenomenon in three distinct amorphous CoB-based multilayer thin film platforms - Pt/CoB/Ir, Ir/CoB/Pt and Pt/CoB/MgO - which are shown to stabilise room-temperature chiral magnetic textures. Intermixing occurs predominantly between adjacent metallic layers. Notably, it is stack-order dependent, and particularly extensive when Ir sits atop CoB. Intermixing induced variations in magnetic properties are ascribed to the formation of magnetic dead layer arising from CoIr alloying in the metallic stacks. It also produces systematic variations in saturation magnetiz…
Simultaneous detection of THz emission and transient magneto-optical response is employed to study ultrafast laser-induced magnetization dynamics in multisublattice magnets NdFeCo and GdFeCo amorphous alloys with in-plane magnetic anisotropy. A satisfactory quantitative agreement between the dynamics revealed with the help of these two techniques is obtained for GdFeCo. For NdFeCo the THz emission reveals faster dynamics than the magneto-optical response. This indicates that in addition to spin dynamics of Fe ultrafast laser excitation triggers faster magnetization dynamics of Nd.
We study the laser-induced torques in the antiferromagnet (AFM) Mn$_2$Au. We find that even linearly polarized light may induce laser-induced torques in Mn$_2$Au, i.e., the light does not have to be circularly polarized. The laser-induced torques in Mn$_2$Au are comparable in magnitude to those in the ferromagnets Fe, Co and FePt at optical frequencies. We also compute the laser-induced torques at terahertz (THz) frequencies and compare them to the spin-orbit torques (SOTs) excited by THz laser-pulses. We find the SOTs to be dominant at THz frequencies for the laser-field strengths used in exp…
Strong two-photon absorption (TPA) in monolayer MoS2 is demonstrated in contrast to saturable absorption (SA) in multilayer MoS2 under the excitation of femtosecond laser pulses in the near infrared region. MoS2 in the forms of monolayer single crystal and multilayer triangular islands are grown on either quartz or SiO2/Si by employing the seeding method through chemistry vapor deposition. The nonlinear transmission measurements reveal that monolayer MoS2 possesses a giant nonsaturation TPA coefficient, larger than that of conventional semiconductors. As a result of TPA, two-photon pumped freq…
We demonstrate that combining standing-wave (SW) excitation with resonant inelastic x-ray scattering (RIXS) can lead to depth resolution and interface sensitivity for studying orbital and magnetic excitations in correlated oxide heterostructures. SW-RIXS has been applied to multilayer heterostructures consisting of a superconductor La$_{1.85}$Sr$_{0.15}$CuO$_{4}$(LSCO) and a half-metallic ferromagnet La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO). Easily observable SW effects on the RIXS excitations were found in these LSCO/LSMO multilayers. In addition, we observe different depth distribution of the …
Femtosecond pulsed lasers have been widely used over the past decades due to their capability to fabricate precise patterns at the micro- and nano- lengths scales. A key issue for efficient material processing is the determination of the laser parameters used in the experimental set ups. Despite a systematic investigation that has been performed to highlight the impact of every parameter independently, little attention has been drawn on the role of the substrate material on which the irradiated solid is placed. In this work, the influence of the substrate is emphasised for films of various thi…
We study laser-induced torques in bcc Fe, hcp Co and $L1_0$ FePt based on first-principles electronic structure calculations and the Keldysh nonequilibrium formalism. We find that the torques have two contributions, one from the inverse Faraday effect (IFE) and one from the optical spin-transfer torque (OSTT). Depending on the ferromagnet at hand and on the quasiparticle broadening the two contributions may be of similar magnitude or one contribution may dominate over the other. Additionally, we determine the nonequilibrium spin polarization in order to investigate its relation to the torque. …
We present a unidimensional discrete solid-on-solid model evolving in time using a kinetic Monte Carlo method to simulate micro-structuring of kerfs on metallic surfaces by means of laser-induced jet-chemical etching. The precise control of the passivation layer achieved by this technique is responsible for the high resolution of the structures. However, within a certain range of experimental parameters, the microstructuring of kerfs on stainless steel surfaces with a solution of $\mathrm{H}_3\mathrm{PO}_4$ shows periodic ripples, which are considered to originate from an intrinsic dynamics. T…
The majority of studies performed on the formation of surface features by femtosecond laser radiation focuses on single scan procedures, i.e. solely manipulating the laser beam once over the target area to fabricate different surface topographies. In this work, the effect of scanning stainless steel 304 multiple times with femtosecond laser pulses is thoroughly investigated over a wide range of fluences. The resultant laser-induced surface topographies can be categorized into two different regimes. In the low fluence regime $(F_{Σline,max} < 130 ~J/cm^2)$, ellipsoidal cones (randomly distribut…
Museum curators and archaeologists use analytical science to provide important information on artworks and objects. For example, scientific techniques provide information on artwork elemental composition, origin and authenticity, and corrosion products, while also finding use in the day-to-day conservation of many historical objects in museums and archaeological sites around the world. In this work two special cases are being discussed. In the first part of our work, physicochemical studies of an icon on a metal substrate were carried out using non-destructive, qualitative analysis of pigments…
We show evidence that the competition between the antiferromagetic metallic phase and the charge- and orbital-ordered insulating phase at the reentrant phase boundary of a layered manganite, LaSr2Mn2O7, can be manipulated using ultrafast optical excitation. The time-dependent evolution of the Jahn-Teller superlattice reflection, which indicates the formation of the charge and orbital order, was measured at different laser fluences. The laser-induced enhancement and reduction the Jahn-Teller reflection intensity shows a reversal of sign between earlier (~10 ns) and later (~150 ns) time delays d…
From density functional theory investigations helped with crystal chemistry rationale, single-atom C, embedded in layered hexagonal CCn with n = 6, 12, 18 networks, is stable in a magnetic state with M(C)= 2 Bohr Magneton (BM). The examined compositions, all inscribed within the P6/mmm 191 space group are characterized as increasingly cohesive with n, figuring mono, bi and tri layered honeycomb-like C networks respectively. The spin projected total density of states shows a closely half-metallic behavior with a gap at minority spins and metallic majority spins. Such results together with the l…
Our analysis of the experimental data related to nonphotochemical laser induced nucleation in solutions leads to the inevitable conclusion that the phase transformation is initiated by particles that are metallic in nature. This conclusion appears paradoxical because the final products are dielectric crystals. We show that the experimental results are well accounted for by the theory of electric field induced nucleation of metallic particles that are elongated in the direction of the field. However, new physical and chemical insights are required to understand the structure of the metallic pre…
We demonstrate the structural sensitivity and accuracy of the standing wave technique at a high repetition rate free-electron laser, FLASH at DESY in Hamburg, by measuring the photoelectron yield from the surface SiO2 of Mo/Si multilayers. These experiments open up the possibility to obtain unprecedented structural information of adsorbate and surface atoms with picometer spatial accuracy and femtosecond temporal resolution. This technique will substantially contribute to a fundamental understanding of chemical reactions at catalytic surfaces and the structural dynamics of superconductors.
Three-dimensional metallic microstructures find applications as stents in medicine, as ultrabroadband antennas in communications, in micromechanical parts or as structures of more fundamental interest in photonics like metamaterials. Direct metal printing of such structures using three-dimensional laser lithography is a promising approach, which is not extensively applied yet, as fabrication speed, surface quality, and stability of the resulting structures are limited so far. In order to identify the limiting factors, we investigate the influence of light-particle interactions and varying scan…
Dirac points in two-dimensional electronic structures are a source for topological electronic states due to the $\pm π$ Berry phase that they sustain. Here we show that two rutile multilayers (namely (WO$_2$)$_2$/(ZrO$_2$)$_n$ and (PtO$_2$)$_2$/(ZrO$_2$)$_n$, where an active bilayer is sandwiched by a thick enough (n=6 is sufficient) band insulating substrate, show semi-metallic Dirac dispersions with a total of four Dirac cones along the $Γ-M$ direction. These become gapped upon the introduction of spin-orbit coupling, giving rise to an insulating ground state comprising four edge states. We …
Metals in amorphous state are promising for the use as structural materials due to their superior properties related to the absence of grain boundaries and dislocations.Obtaining the amorphous state requires quenching with a high cooling rate.In conventional technologies,this is the reason for a considerable size limitation hindering application of amorphous alloys.Additive manufacturing (AM) is free of the size limitation.The so-called bulk metallic glass (BMG) alloys have extremely low critical cooling rates and can be used in AM. Recent studies on AM from Zr-based BMGs by selective laser me…
The employment of femtosecond pulsed lasers has received significant attention due to its capability to facilitate fabrication of precise patterns at the micro- and nano- lengths scales. A key issue for efficient material processing is the accurate determination of the damage threshold that is associated with the laser peak fluence at which minimal damage occurs on the surface of the irradiated solid. Despite a wealth of previous reports that focused on the evaluation of the laser conditions that lead to the onset of damage, the investigation of both the optical and thermal response of thin fi…
