Laser-induced Demagnetization Research Articles

Coherent control of magnetization precession by double-pulse activation of effective fields from magnetoacoustics and demagnetization

We demonstrate the coherent optical control of magnetization precession in a thin Ni film by a second excitation pulse, which amplifies or attenuates the precession induced by a first pulse depending on the fluences of the pump-pulses and the pump-pump delay. This control goes beyond the conventional strategy, where the same mechanism drives the precession in-phase or out-of-phase. We balance the magneto-acoustic mechanism driven by quasi-static strain and the shape-anisotropy change triggered by laser-induced demagnetization. These mechanisms tilt the transient effective magnetic field in opposite directions in the case of negative magneto-elastic coupling (b1<0). While the strain response is linear in the fluence, demagnetization is nonlinear near the Curie temperature, enabling fluence-based control scenarios.

Spectral shift in terahertz emission by ultrafast laser-induced demagnetization

We demonstrate a spectral shift in the peak frequency of terahertz (THz) emission by ultrafast laser-induced demagnetization in the Fe20Ni80 (Py) film. Experimentally, red spectral shifts of the THz emission are achieved by increasing the laser fluence or decreasing the Curie temperature of the Py film. Based on the microscopic three-temperature model, it is found that such redshifts are attributed by the slowing down of laser induced demagnetization processes in the Py film. The results not only enrich our understanding of THz emission introduced by ultrafast laser-induced demagnetization but also provide a practical way to detect the timescale of laser-induced ultrafast demagnetization dynamics.

Lattice structure dependence of laser-induced ultrafast magnetization switching in ferrimagnets

The experimental discovery of single-pulse ultrafast magnetization switching in ferrimagnetic alloys, such as GdFeCo and MnRuGa, opened the door to a promising route toward faster and more energy efficient data storage. A recent semi-phenomenological theory has proposed that a fast, laser-induced demagnetization below a threshold value puts the system into a dynamical regime where angular momentum transfer between sublattices dominates. Notably, this threshold scales inversely proportional to the number of exchange-coupled nearest neighbors considered in the model, which in the simplest case is directly linked to the underlying lattice structure. In this work, we study the role of the lattice structure on the laser-induced ultrafast magnetization switching in ferrimagnets by complementing the phenomenological theory with atomistic spin dynamics computer simulations. We consider a spin model of the ferrimagnetic GdFeCo alloy with increasing number of exchange-coupled neighbors. Within this model, we demonstrate that the laser-induced magnetization dynamics and switching depend on the lattice structure. Furthermore, we determine that the critical laser energy for switching reduces for decreasing number of exchange-coupled neighbors.

Ultrafast Spin Dynamics and Photoinduced Insulator-to-Metal Transition in α-RuCl3.

Laser-induced ultrafast demagnetization is a phenomenon of utmost interest and attracts significant attention because it enables potential applications in ultrafast optoelectronics and spintronics. As a spin-orbit coupling assisted magnetic insulator, α-RuCl3 provides an attractive platform to explore the physics of electronic correlations and unconventional magnetism. Using time-dependent density functional theory, we explore the ultrafast laser-induced dynamics of the electronic and magnetic structures in α-RuCl3. Our study unveils that laser pulses can introduce ultrafast demagnetizations, accompanied by an out-of-equilibrium insulator-to-metal transition in a few tens of femtoseconds. The spin response significantly depends on the laser wavelength and polarization on account of the electron correlations, band renormalizations, and charge redistributions. These findings provide physical insights into the coupling between the electronic and magnetic degrees of freedom in α-RuCl3 and shed light on suppressing the long-range magnetic orders and reaching a proximate spin liquid phase for two-dimensional magnets on an ultrafast time scale.

Fluence and Temperature Dependences of Laser-Induced Ultrafast Demagnetization and Recovery Dynamics in L10-FePt Thin Film.

The fundamental mechanisms of ultrafast demagnetization and magnetization recovery processes in ferromagnetic materials remain incompletely understood. The investigation of different dynamic features which depend on various physical quantities requires a more systematic approach. Here, the femtosecond laser-induced demagnetization and recovery dynamics in L10-Fe0.5Pt0.5 alloy film are studied by utilizing time-resolved magneto-optical Kerr measurements, focusing on their dependences of excitation fluence and ambient temperature over broad ranges. Ultrafast demagnetization dominated by Elliott-Yafet spin-flip scattering, and two-step magnetization recovery processes are found to be involved in all observations. The fast recovery time corresponding to spin-lattice relaxation is much shorter than that of many ferromagnets and increase with excitation fluence. These can be ascribed to the strong spin-orbit coupling (SOC) demonstrated in FePt and the reduction of transient magnetic anisotropy, respectively. Surprisingly, the demagnetization time exhibits no discernible correlation with ambient temperature. Two competitive factors are proposed to account for this phenomenon. On the other hand, the spin-lattice relaxation accelerates as temperature decreases due to enhanced SOC at lower ambient temperature. A semiquantitative analysis is given to get a visualized understanding. These results offer a comprehensive understanding of the dynamic characteristics of ultrafast demagnetization and recovery processes in iron-based materials with strong SOC, highlighting the potential for regulating the magnetization recovery process through temperature and laser fluence adjustments.

Spin current driven by ultrafast magnetization of FeRh

Laser-induced ultrafast demagnetization is an important phenomenon that probes arguably the ultimate limits of the angular momentum dynamics in solid. Unfortunately, many aspects of the dynamics remain unclear except that the demagnetization transfers the angular momentum eventually to the lattice. In particular, the role and origin of electron-carried spin currents in the demagnetization process are debated. Here we experimentally probe the spin current in the opposite phenomenon, i.e., laser-induced ultrafast magnetization of FeRh, where the laser pump pulse initiates the angular momentum build-up rather than its dissipation. Using the time-resolved magneto-optical Kerr effect, we directly measure the ultrafast-magnetization-driven spin current in a FeRh/Cu heterostructure. A strong correlation between the spin current and the magnetization dynamics of FeRh is found even though the spin filter effect is negligible in this opposite process. This result implies that the angular momentum build-up is achieved by an angular momentum transfer from the electron bath (supplier) to the magnon bath (receiver) and followed by the spatial transport of angular momentum (spin current) and dissipation of angular momentum to the phonon bath (spin relaxation).

Néel vector switching and terahertz spin-wave excitation in Mn2Au due to femtosecond spin-transfer torques

Efficient and fast manipulation of antiferromagnets has to date remained a challenging task, hindering their application in spintronic devices. For ultrafast operation of such devices, it is highly desirable to be able to control the antiferromagnetic order within picoseconds - a timescale that is difficult to achieve with electrical circuits. Here, we demonstrate that bursts of spin-polarized hot-electron currents emerging due to laser-induced ultrafast demagnetization are able to efficiently excite spin dynamics in antiferromagnetic Mn$_2$Au by exerting a spin-transfer torque on femtosecond timescales. We combine quantitative superdiffusive transport and atomistic spin-model calculations to describe a spin-valve-type trilayer consisting of Fe$|$Cu$|$Mn$_2$Au. Our results demonstrate that femtosecond spin-transfer torques can switch the Mn$_2$Au layer within a few picoseconds. In addition, we find that spin waves with high frequencies up to several THz can be excited in Mn$_2$Au.

Theory of superdiffusive spin transport in noncollinear magnetic multilayers

Ultrafast demagnetization induced by femtosecond laser pulses in thin metallic layers is caused by the outflow of spin-polarized hot-electron currents describable by the superdiffusive transport model. These laser-generated spin currents can cross the interface into another magnetic layer and give rise to magnetization dynamics in magnetic spin valves with noncollinear magnetizations. To describe ultrafast transport and spin dynamics in such nanostructures, we develop here the superdiffusive theory for general noncollinear magnetic multilayers. Specifically, we introduce an Al/Ni/Ru/Fe/Ru multilayer system with noncollinear Ni and Fe magnetic moments and analyze how the ultrafast demagnetization and spin-transfer torque depend on the noncollinearity. We employ ab initio calculations to compute the spin- and energy-dependent transmissions of hot electrons at the interfaces of the multilayer. Taking into account multiple electron scattering at interfaces and spin mixing in the spacer layer, we find that the laser-induced demagnetization of the Ni layer and the magnetization change of the Fe layer strongly depend on the angle between their magnetizations. Similarly, the spin-transfer torques on the Ni and Fe layers and the total spin momentum absorbed in the Ni and Fe layer are found to vary markedly with the amount of noncollinearity. These results suggest that by changing the amount of noncollinearity in magnetic multilayers, one can efficiently control the hot-electron spin transport, which may open a way toward achieving fast, laser-driven spintronic devices.

Ultrafast demagnetization dynamics in the epitaxial FeGe(111) film chiral magnet

The exploring of ultrafast demagnetization is a significant subject for understanding the versatility of spin dynamics. However, the laser-induced ultrafast demagnetization process in chiral magnet FeGe has remained elusive. Here, we report the ultrafast demagnetization processes of FeGe film with various magnetic structures using time-resolved magneto-optical Kerr effect (TRMOKE) measurements. Our work reveals a transition from one-step ultrafast demagnetization (type I) to two-step demagnetization (type II) process by changing ambient temperature, laser fluence, and magnetic field. We confirm that the transition from one-step to two-step ultrafast demagnetization mainly results from the weak electron-phonon (e-p) coupling. In the skyrmion phase region, TRMOKE curves demonstrate a two-step ultrafast demagnetization process. The two-step ultrafast demagnetization will further enhance the magnetization loss, and consequently facilitate the magnetization switching and pave a pathway to engineer chiral magnetic devices on picosecond timescales.

Different spin relaxation properties observed in linearly and circularly polarized laser induced terahertz emission from a Bi/Co bilayer

Recently, helicity-dependent photocurrent was reported in Bi single thin fi lms. It is proposed that the origin of this photocurrent is the combination of photo-spin conversion and spin-charge conversion effects in Bi and efficient spin conversion in Bi is expected. In this study, we measured two types of terahertz (THz) emissions from Bi/Co bilayer films induced by spin current generation using laser-induced demagnetization of the Co layer and photo-spin conversion effect in the Bi layer to investigate the spin current induced by the two mechanisms simultaneously. We clearly observed diff erent Bi thickness dependence of peak intensity and that of bandwidth for THz spin current in two experiments, i.e., spin current induced by demagnetization of Co and that by photo-spin conversion in Bi. The different Bi thickness dependence of spin current intensity and bandwidth in two experiments is caused by different spin relaxation properties of optically excited spin currents in Bi layers.

Investigation of ultrafast demagnetization and Gilbert damping and their correlation in different ferromagnetic thin films grown under identical conditions

Following the demonstration of laser-induced ultrafast demagnetization in ferromagnetic nickel, several theoretical and phenomenological propositions have sought to uncover its underlying physics. In this work we revisit the three temperature model (3TM) and the microscopic three temperature model (M3TM) to perform a comparative analysis of ultrafast demagnetization in 20 nm thick cobalt, nickel and permalloy thin films measured using an all-optical pump-probe technique. In addition to the ultrafast dynamics at the femtosecond timescales, the nanosecond magnetization precession and damping are recorded at various pump excitation fluences revealing a fluence-dependent enhancement in both the demagnetization times and the damping factors. We confirm that the Curie temperature to magnetic moment ratio of a given system acts as a figure of merit for the demagnetization time, while the demagnetization times and damping factors show an apparent sensitivity to the density of states at the Fermi level for a given system. Further, from numerical simulations of the ultrafast demagnetization based on both the 3TM and the M3TM, we extract the reservoir coupling parameters that best reproduce the experimental data and estimate the value of the spin flip scattering probability for each system. We discuss how the fluence-dependence of inter-reservoir coupling parameters so extracted may reflect a role played by nonthermal electrons in the magnetization dynamics at low laser fluences.

Pump-probe x-ray microscopy of photo-induced magnetization dynamics at MHz repetition rates.

We present time-resolved scanning x-ray microscopy measurements with picosecond photo-excitation via a tailored infrared pump laser at a scanning transmission x-ray microscope. Specifically, we image the laser-induced demagnetization and remagnetization of thin ferrimagnetic GdFe films proceeding on a few nanoseconds timescale. Controlling the heat load on the sample via additional reflector and heatsink layers allows us to conduct destruction-free measurements at a repetition rate of 50 MHz. Near-field enhancement of the photo-excitation and controlled annealing effects lead to laterally heterogeneous magnetization dynamics which we trace with 30 nm spatial resolution. Our work opens new opportunities to study photo-induced dynamics on the nanometer scale, with access to picosecond to nanosecond time scales, which is of technological relevance, especially in the field of magnetism.

Energy relaxation in (La0.6Pr0.4)0.7Ca0.3MnO3 films across the metal-insulator transition

The colossal magnetoresistive manganite ${({\mathrm{La}}_{0.6}{\mathrm{Pr}}_{0.4})}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ (LPCMO) undergoes a ferromagnetic (FM) metal to paramagnetic insulator phase transition at around 195 K, which develops via an intermediate polaronic state strongly susceptible to external magnetic fields. Transient reflectivity studies of LPCMO on timescales from sub-picoseconds to nanoseconds reveal that the overall system dynamics are strongly influenced by the phase transition with a sharp and strong decrease in the relaxation time while crossing from the ferromagnetic metallic phase into the paramagnetic insulating phase. We show that the long relaxation times of the reflectivity after nonequilibrium excitation within the FM phase are caused by the slow recovery of the ordered FM state after laser-induced demagnetization. Control of the nonequilibrium dynamics close to the phase transition is demonstrated by applying moderate magnetic fields below 1 T.

In-vacuum Helmholtz coils for pulsed magnetic fields studying ultrafast demagnetization

The availability of sub 100 fs short and highly intense free-electron laser (FEL) pulses allows for new insights in laser-induced ultrafast demagnetization (LID) of ferromagnetic thin films on nanometer length scales. We designed a pair of in-vacuum Helmholtz coils, providing pulsed magnetic fields up to µ 0 Hz = ±45 mT, for time-resolved experiments at FEL sources in transmission geometry. We report on the implementation of the Helmholtz coils in an optical-pump–resonant-magnetic-scattering (tr-XRMS) experiment at the FEL FERMI (Elettra, Trieste) to study LID in different magnetic domain networks. We discuss the limitations for multi-shot measurements, that rely on the full reversibility of the demagnetization process in-between two pump–probe events, and emphasize the importance of reference–pump–probe schemes, especially in tr-XRMS experiments that employ external Hz fields.

Laser-induced terahertz spin transport in magnetic nanostructures arises from the same force as ultrafast demagnetization

The authors observe here that (a) laser-induced ultrafast demagnetization (UDM) of a metallic ferromagnet $F$ has the same time evolution as (b) terahertz spin transport (TST) from $F$ into an adjacent normal metal $N$. They conclude that UDM in $F$ and TST in $F$|$N$ samples are driven by the same force: (c) a generalized spin voltage, i.e., an excess magnetization of $F$. One can now apply the vast knowledge of UDM to TST to significantly increase spin-current amplitudes for ultrafast spintronic applications.

Impact of electron correlation on the light-induced demagnetization of elemental ferromagnetic metals

The local spin-density approximation (LSDA) is known to describe poorly the electronic structure of 3d transition metals, yet most density-functional-based ab-initio studies of ultra-fast demagnetization rely on it. One way to account for Coulomb correlations among the localized d electrons and go beyond LSDA is to include the effective correlation energy (or Hubbard) U. By doing so, we show here that electronic correlations lead to sizable changes of the laser-induced demagnetization of iron, cobalt, and nickel. We study how the various laser parameters, such as pulse duration or intensity, change the magnetization dynamics. It turns out that the total laser fluence is not suitable to quantify how much a laser pulse demagnetizes a material, as changes in pulse duration and shape influence significantly the outcome. The findings are traced back to the electronic structure of the material, and explained based on phase space for optical transitions.Graphic abstract

Laser-induced forces on atoms during ultrafast demagnetization

Laser-induced femtosecond demagnetization has attracted a broad attention as a possible candidate for information storage technology. However, whether or not lattice vibration directly participates in demagnetization has been highly controversial over a decade. A recent electron diffraction experiment attributed the demagnetization to the polarized phonon effect, but a similar x-ray diffraction experiment attributed it to the Einstein–de Haas effect. Common to both experiments is that neither the angular momentum of the lattice nor the rotation of the sample was directly probed. Here, we report our first first-principles calculation of forces on atoms induced by an ultrafast laser during ultrafast demagnetization. We employ two complementary methods: (i) the frozen lattice with electronic excitation and (ii) frozen excitation but moving the lattice. We find that the forces on atoms start at −50 fs and peak around 30 fs. The magnitude of the force is far smaller than the empirical estimates. Within the limit of our theory, our results suggest that the polarized phonon effect and the Einstein–de Haas effect are unlikely to be the main course of demagnetization. We expect that our finding has a profound impact on the future direction of laser-induced dynamics in magnetic and quantum materials.

Analysis and control of ultrafast demagnetization dynamics in ferrimagnetic Gdx(CoFe)1-x alloys

Deep understanding and powerful control of the magnetization dynamics is of great importance for developing high speed spintronic devices. In this work, laser-induced ultrafast demagnetization process of Gdx(CoFe)1-x alloy films has been extensively studied by using the fs-laser pump-probe technique. Interestingly, it is found that by adjusting the Gd content x and hence the intersublattice 3d-4f exchange coupling strength, the demagnetization dynamics varies from a fast one-stage decay to a continuous or a discrete two-stage decay, which is mainly resulted from the different energy transfer rates via the intersublattice exchange coupling and has been well simulated by considering the Elliott-Yafet type spin-flip scattering theory and the 4-Temperature model. Moreover, we observed that the second-stage decay time is strongly dependent on the external magnetic field, which is analyzed theoretically and suggests an origin of ultrafast giant magnetic cooling effect via the modulation of lattice-spin coupling. These studies are quite helpful for achieving the efficient manipulation of ultrafast magnetization behaviors.

Magnetic Configuration Driven Femtosecond Spin Dynamics in Synthetic Antiferromagnets.

Ultrafast demagnetization in diverse materials has sparked immense research activities due to its captivating richness and contested underlying mechanisms. Among these, the two most celebrated mechanisms have been the spin-flip scattering (SFS) and spin transport (ST) of optically excited carriers. In this work, we have investigated femtosecond laser-induced ultrafast demagnetization in perpendicular magnetic anisotropy-based synthetic antiferromagnets (p-SAFs) where [Co/Pt]n-1/Co multilayer blocks are separated by Ru or Ir spacers. Our investigation conclusively shows that the ST of optically excited carriers can have a significant contribution to the ultrafast demagnetization in addition to SFS processes. Moreover, we have also achieved an active control over the individual mechanisms by specially designing the SAF samples and altering the external magnetic field and excitation fluence. Our study provides a vital understanding of the underlying mechanism of ultrafast demagnetization in synthetic antiferromagnets, which will be crucial in future research and applications of antiferromagnetic spintronics.

Experimental observation of transition from type I to type II ultrafast demagnetization dynamics in chemically disordered Fe60Al40 thin film, driven by laser fluence

Understanding of the laser-induced ultrafast demagnetization dynamics is one of the most challenging and hot topics in magnetism research due to its potential applications in magnetic storage devices and the field of spintronics. Recently, a laser-induced switching of ferromagnetism, driven by a disorder–order transition on FeAl thin films, has been experimentally demonstrated. The switching of ferromagnetic ordering by ultrafast laser pulses in FeAl thin films may open new possible applications of this material such as magnetic data storage and manipulation. Since the speed of the magnetic switching of magnetic states in thin films is one of the critical parameters for these applications, here we used time resolved magneto-optical Kerr measurements to investigate the demagnetization dynamics of Fe60Al40 thin films at room temperature. We have for the first time observed a clear transition from one-step dynamics (type I) to two-step (type II) dynamics in the same material by increasing pump laser fluence. This experimental observation may give a strong confirmation that the ultrafast demagnetization process can be treated as a thermal process and is driven by the difference between temperatures of the electron and spin systems.