Ultrafast Demagnetization Research Articles

Impact of local order and stoichiometry on the ultrafast magnetization dynamics of Heusler compounds

Nowadays, a wealth of information on ultrafast magnetization dynamics of thin ferromagnetic films exists in the literature. Information is, however, scarce on bulk single crystals, which may be especially important for the case of multi-sublattice systems. In Heusler compounds, representing prominent examples for such multi-sublattice systems, off-stoichiometry and degree of order can significantly change the magnetic properties of thin films, while bulk single crystals may be generally produced with a much more well-defined stoichiometry and a higher degree of ordering. A careful characterization of the local structure of thin films versus bulk single crystals combined with ultrafast demagnetization studies can, thus, help to understand the impact of stoichiometry and order on ultrafast spin dynamics.Here, we present a comparative study of the structural ordering and magnetization dynamics for thin films and bulk single crystals of the family of Heusler alloys with composition Co2Fe1 − xMnxSi. The local ordering is studied by 59Co nuclear magnetic resonance (NMR) spectroscopy, while the time-resolved magneto-optical Kerr effect gives access to the ultrafast magnetization dynamics. In the NMR studies we find significant differences between bulk single crystals and thin films, both regarding local ordering and stoichiometry. The ultrafast magnetization dynamics, on the other hand, turns out to be mostly unaffected by the observed structural differences, especially on the time scale of some hundreds of femtoseconds. These results confirm hole-mediated spin-flip processes as the main mechanism for ultrafast demagnetization and the robustness of this demagnetization channel against defect states in the minority band gap as well as against the energetic position of the band gap with respect to the Fermi energy. The very small differences observed in the magnetization dynamics on the picosecond time-scale, on the other hand, can be explained by considering the differences in the electronic structure at the Fermi energy and in the heat diffusion of thin films and bulk crystals.

Laser-induced transient strengthening of coupling in L10-FePt/FeNi exchange-spring film

Laser-induced evolution of the coupling state in a perpendicular exchange-spring L10-FePt/FeNi bilayer film is studied via time-delayed magneto-optical polar Kerr loops. A transient strengthening process of the coupling is found to occur simultaneously with significant reduction of the hard-phase coercivity at ~50 ps after the ultrafast demagnetization. Analysis of the time-dependent magnetization components and the micromagnetic calculation demonstrate that this delayed coupling-strengthening phenomenon originates from the opposite laser-modulation effects respectively on the hard and soft magnetic phases, in which the laser-induced transient reduction of the magnetic anisotropy in FePt phase plays the most important role.

Nonlocal ultrafast demagnetization dynamics of Co/Pt multilayers by optical field enhancement

The influence on ultrafast demagnetization dynamics of metallic nano-structured gratings deposited on thin films of magnetic Co/Pt multilayers is investigated by the time-resolved optical Kerr effect. Depending on the polarization of the pump pulse, a pronounced enhancement of the demagnetization amplitude is found. Calculation of the inhomogeneous optical field distribution due to plasmon interaction and time-dependent solutions of the coupled electron, lattice, and spin temperatures in two dimensions show good agreement with the experimental data, as well as giving evidence of non-local demagnetization dynamics due to electron diffusion.

Irreversible transformation of ferromagnetic ordered stripe domains in single-shot infrared-pump/resonant-x-ray-scattering-probe experiments

The evolution of a magnetic domain structure upon excitation by an intense, femtosecond Infra-Red (IR) laser pulse has been investigated using single-shot based time-resolved resonant X-ray scattering at the X-ray Free Electron laser LCLS. A well-ordered stripe domain pattern as present in a thin CoPd alloy film has been used as prototype magnetic domain structure for this study. The fluence of the IR laser pump pulse was sufficient to lead to an almost complete quenching of the magnetization within the ultrafast demagnetization process taking place within the first few hundreds of femtoseconds following the IR laser pump pulse excitation. On longer time scales this excitation gave rise to subsequent irreversible transformations of the magnetic domain structure. Under our specific experimental conditions, it took about 2 nanoseconds before the magnetization started to recover. After about 5 nanoseconds the previously ordered stripe domain structure had evolved into a disordered labyrinth domain structure. Surprisingly, we observe after about 7 nanoseconds the occurrence of a partially ordered stripe domain structure reoriented into a novel direction. It is this domain structure in which the sample's magnetization stabilizes as revealed by scattering patterns recorded long after the initial pump-probe cycle. Using micro-magnetic simulations we can explain this observation based on changes of the magnetic anisotropy going along with heat dissipation in the film.

Laser-induced ultrafast demagnetization time and spin moment in ferromagnets: First-principles calculation

When a laser pulse excites a ferromagnet, its spin undergoes a dramatic change. The initial demagnetization process is very fast. Experimentally, it is found that the demagnetization time is related to the spin moment in the sample. In this study, we employ the first-principles method to directly simulate such a process. We use the fixed spin moment method to change the spin moment in ferromagnetic nickel, and then we employ the Liouville equation to couple the laser pulse to the system. We find that in general the dependence of demagnetization time on the spin moment is nonlinear: It decreases with the spin moment up to a point, after which an increase with the spin moment is observed, followed by a second decrease. To understand this, we employ an extended Heisenberg model, which includes both the exchange interaction and spin-orbit coupling. The model directly links the demagnetization rate to the spin moment itself and demonstrates analytically that the spin relaxes more slowly with a small spin moment. A future experimental test of our predictions is needed.

Theory of Ultrafast Demagnetization after Femtosecond Laser Pulses

Experimentally it has been found that a ferromagnetic metallic lm is strongly demagnetized within few hundred fs after exposure to a fs laser pulse. The theories of this ultrafast demagnetization are reviewed, especially the in uence of scatterings of the excited electrons at phonons and at magnons. The calculations for phonons and magnons are performed by the ab initio spin-density-functional electron theory for Ni and Fe, and they are based on Fermi's golden rule for transition rates. The application of this rule on the fs time scale is critically discussed in view of results from quantum-kinetic density-matrix calculations. It is shown that the experimentally observed demagnetization rates cannot be explained by spinip scatterings of electrons exclusively at phonons or exclusively at magnons. A combination of individual spinip electron phonon and spinip electron magnon processes is shown to be a potential candidate for the explanation of ultrafast demagnetization.

Photoinduced Spin Precession and Ultrafast Demagnetization in Co2FeAl Films With Crossover From In-Plane to Perpendicular Magnetic Easy Axis

Laser-induced spin precession and ultrafast demagnetization were investigated in MgO-capped Co2 FeAl (MO-CFA) films. Both the Gilbert damping parameter α and the demagnetization time τ $_m$ monotonically increased with the increasing perpendicular magnetic anisotropy as CFA thickness $x$ was reduced to 0.6 nm. The increase in α may be explained by a stronger spin-orbit coupling. The τ $_m$ of an $L2_{1}$ Co2FeAl film is larger than that of thick MO-CFA, but smaller than that of MO-CFA with $x \le 0.9$ nm, indicating that the restored orbital angular momentum may have a stronger and opposite effect on τ $_m$ compared to the reduced spin polarization in thin MO-CFA.

Kerr rotation in Cu, Ag, and Au driven by spin accumulation and spin-orbit coupling

We measure transient spin accumulation in Cu, Ag, and Au by time-resolved magneto-optical Kerr effect. The transient spin current is generated by ultrafast demagnetization of a ferromagnetic [Co/Pt] layer, and spin accumulates in an adjacent normal metal, Cu, Ag, or Au by spin diffusion. The magnitude of the Kerr rotation is described by an off-diagonal conductivity tensor that is proportional to spin accumulation and spin-orbit coupling. From comparisons between observed Kerr rotations and calculated spin accumulations, we determine the strength of spin-orbit coupling of conduction electrons in Cu, Ag, and Au to be 0.02, 0.01, and $0.12\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, respectively.

Ultrafast demagnetization of FePt:Cu thin films and the role of magnetic heat capacity

The phenomenon of different time scales of ultrafast demagnetization has attracted much attention. This so-called diversity of ultrafast demagnetization has been explained by the microscopic three temperature model (M3TM) and by the Landau-Lifshitz-Bloch model (LLBM). Here, we revisit the basic three temperature model (3TM) and provide a general criterion for explaining the different time scales observed. We focus on the role of magnetic heat capacity, which we find mainly determines the slowing down of the demagnetization time with increasing ambient temperature and laser fluence. In this context, we clarify the role of magnetic heat capacity in the M3TM and compare the 3TM with the LLBM. To illustrate the role of magnetic heat capacity, we present a simulation of ultrafast demagnetization of Ni. Furthermore, we present time-resolved magneto-optic Kerr effect measurements of ultrafast demagnetization and specific heat of ${\mathrm{Fe}}_{46}{\mathrm{Cu}}_{6}{\mathrm{Pt}}_{48}$ from 300 K to close to its Curie temperature. While most of the prior experimental research used high-fluence laser pulses causing sizable temperature excursions of the sample, our experiments involve small temperature excursions, which are crucial for studying the role of magnetic heat capacity in ultrafast demagnetization. Our experimental results corroborate that the slowing down of ultrafast demagnetization is dominated by the increase of the magnetic heat capacity near the Curie temperature.

Longitudinal evolution of magnetization of ferromagnets following ultrafast demagnetization: role of finite size and shape of demagnetized region

A dependence of a relaxation rate on the shape of the demagnetized region for a longitudinal evolution of total magnetization to its equilibrium value following the ultrafast demagnetization is demonstrated. This shape-dependence is caused by a motion of the wave front inside the demagnetized region. The contribution of the wave front for spherically symmetric shape of the demagnetized region is up to 3 times and for cylindrically symmetric shape up to 2 times stronger than for one dimensional demagnetized region. This effect can be observed after the demagnetization by a tightly focused femtosecond laser pulse.

Testing spin-flip scattering as a possible mechanism of ultrafast demagnetization in ordered magnetic alloys

We use element-resolved IR-pump/extreme ultraviolet-probe experiments to disentangle the ultrafast interplay of the magnetic sublattices of an ordered crystalline magnetic alloy. As a paradigmatic example, we investigate the case of the FeRh alloy, which shows a delayed response for the different components. Furthermore, a detailed time-resolved magneto-optic study shows that the data can be analyzed by only assuming Elliot-Yafet-like scattering, as the underlying mechanism for ultrafast demagnetization, resulting in an unexpected nonmonotonic dependence of the spin-flip rate, as a function of quenching.

Evolution of the modulus of the total magnetic moment of a ferromagnet after ultrafast demagnetization

The longitudinal relaxation rate of the total magnetization to its equilibrium value after ultrafast demagnetization by a femtosecond laser pulse is governed by two phenomena: the homogeneous evolution of magnetization and the motion of the wavefront into the demagnetized region. If the demagnetized region is sufficiently large, the homogeneous evolution dominates. However, as the diameter of the demagnetized region decreases, the relative contribution of the wavefront increases, and in the case when a femtosecond laser pulse is focused to a diffraction-limited minimum size, the wavefront can significantly increase the longitudinal relaxation rate.

Influence of magnetic fields on spin-mixing in transition metals

In a relativistic description of a real solid the crystal wave functions are never pure but spin-mixed functions. As a direct result of the spin-mixing, scattering mechanisms, which are in principal diagonal in spin-space, as for example the deformation potential part of the electron-phonon scattering potential, may lead to spin flips. Spin-mixing thereby affects damping and lifetimes in the fast and ultrafast regime of magnetization dynamics. The average spin-mixing appears as an important parameter in many models, the most prominent being the Elliott-Yafet model of spin relaxation. The application of an external magnetic field, a laser-field, or a Faraday-field induced by a laser may be considered as a source of magnetic field in a sample. Therefore we will discuss the influence of a homogeneous applied magnetic field on the average spin-mixing calculated in the framework of spin-density-functional theory. It is shown that even high fields do not significantly change the spin-mixing in $3d$ transition metals. Therefore the related dynamics are unaffected, which is in accordance with the experimental data on ultrafast demagnetization's dependence on light polarization.

Exploring laser-induced interlayer spin transfer by an all-optical method

We investigate the influence of spin currents during ultrafast laser-induced demagnetization of magnetic bilayer structures by a new all-optical method to measure material- and/or depth-resolved magnetization dynamics. By describing the magneto-optical response of the bilayers in the complex Kerr plane, it is shown that the material-specific magnetization dynamics of the individual layers can be measured by a marginal adjustment to any conventional time-resolved magneto-optical Kerr effect setup. We use this technique to trace superdiffusive spin currents in magnetic Ni/Fe bilayers, providing new insight on its importance to ultrafast laser-induced demagnetization.

Role of electron-magnon scatterings in ultrafast demagnetization

Possible contributions of electron-magnon scatterings for the ultrafast demagnetization after femtosecond laser irradiation of films of Ni and Fe are investigated by the ab initio spin-density-functional electron theory. The calculations are based on Fermi's golden rule for transition rates, and the nonequilibrium after the action of the laser pulse is modeled by different chemical potentials for electrons which are in a ``dominant spin-up'' or in a ``dominant spin-down'' state. First, it is shown that the experimentally observed demagnetization cannot be described by electron-magnon spin-flip scatterings with concomitant changes of the electronic orbital moments which are immediately quenched by the crystal field, a mechanism which has been suggested by Carpene et al. Second, it is argued that the experimentally observed demagnetization possibly can be explained by a combination of individual spin-flip electron-phonon and spin-flip electron-magnon processes. A precondition for this is (among others) that the magnon emission rate is much larger than the magnon absorption rate. It is shown that this precondition is indeed fulfilled for Ni and especially for Fe.

Spin current generated by thermally driven ultrafast demagnetization.

Spin current is the key element for nanoscale spintronic devices. For ultrafast operation of such nano-devices, generation of spin current in picoseconds, a timescale that is difficult to achieve using electrical circuits, is highly desired. Here we show thermally driven ultrafast demagnetization of a perpendicular ferromagnet leads to spin accumulation in a normal metal and spin transfer torque in an in-plane ferromagnet. The data are well described by models of spin generation and transport based on differences and gradients of thermodynamic parameters. The temperature difference between electrons and magnons is the driving force for spin current generation by ultrafast demagnetization. On longer timescales, a few picoseconds following laser excitation, we also observe a small contribution to spin current by a temperature gradient and the spin-dependent Seebeck effect.

Self-assembly and plasmon-enhanced ultrafast magnetization of Ag–Co hybrid nanoparticles

Ultrafast demagnetization in magnetic nanoparticles has attracted considerable attention because of its potential applications such as spintronics. For such applications, it is important to develop materials in which ultrafast magnetization can be controlled by lower-power laser pulses. We developed a new method for fabricating Ag–Co hybrid nanoparticles in a TiO2 film using pulsed laser deposition, and elucidated ultrafast magnetization dynamics of these films by employing the pump–probe time-resolved Faraday rotation measurement. The measurements showed that localized surface plasmon resonance of the nanoparticles resulted in an enhancement of the ultrafast demagnetization of the films.

The study of ultrafast magnetization dynamics induced by femtosecond laser pulses in GdFeCo amorphous film

The time-resolved magneto-optical Kerr spectroscopy technique is used to study the ultrafast magnetization dynamics induced by femtosecond laser pulses in GdFeCo amorphous film. We study concretely the influence of the different pump fluence and the different external magnetic field on magnetization dynamics of ultrafast demagnetization, magnetization reversal and magnetization recovery. The pump fluence dependence magnetization dynamics shows that the degree of demagnetization, the degree of magnetization reversal and the time of magnetization recovery increase with pump fluence, which can be interpreted by the “three-temperature” model. The external magnetic field dependence magnetization dynamics shows that the rate of magnetization reversal increases with the external field, which accord with the magnetization reversal mechanism based on the reversed magnetic domain nucleation and domain-wall motion.

Role of spin-lattice coupling in the ultrafast demagnetization ofGd1−xTbxalloys

After excitation by femtosecond laser pulses, Gd and Tb exhibit ultrafast demagnetization in two steps, with the time constant of the second step linked to the coupling strength of the $4f$ magnetic moments to the lattice. In time-resolved magneto-optical Kerr effect measurements of ${\mathrm{Gd}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ alloys, we observe a decrease in this time constant from 33 to 9 ps with Tb content $x$ increasing from 0 to 0.7. We explain this behavior by the stronger spin-lattice coupling of Tb compared to Gd, which increases the effective spin-lattice coupling in ${\mathrm{Gd}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ with $x$. In contrast, the faster time constant of the first demagnetization step exhibits no dependence on $x$. Additional time- and element-resolved x-ray magnetic circular dichroism measurements show a two-step demagnetization of Gd and Tb in ${\mathrm{Gd}}_{0.6}{\mathrm{Tb}}_{0.4}$ with an equivalent time scale of the second step but a different magnitude of demagnetization which persists for 15 ps. We explain this by an increased coupling of the Gd $4f$ magnetic moments to the lattice compared to pure Gd, via interatomic exchange coupling to the neighboring Tb $4f$ moments mediated by $5d$ electrons, which has limited efficiency and allows an estimation of a characteristic time scale of the interatomic exchange coupling. We assign the first demagnetization step to the dynamics of the laser-excited $5d$ electrons, while the second demagnetization step is dominated by the strength of spin-lattice coupling of the $4f$ electrons.

Formation of the front of longitudinal evolution of the magnetic moment of a ferromagnet under strong inhomogeneous excitation

The longitudinal evolution of the initial magnetization wave front (formed, for example, due to the ultrafast demagnetization of a part of the ferromagnetic material by a femtosecond laser pulse) toward the wave front with a minimum velocity occurs through an intermediate fast wave front. This fast wave front can significantly increase the relaxation rate of the net magnetization in finite-size samples.