Laser-induced Magnetization Precession Research Articles

Controlling effective field contributions to laser-induced magnetization precession by heterostructure design

Nanoscale heterostructure design can control laser-induced heat dissipation and strain propagation, as well as their efficiency for driving magnetization precession. Here, we incorporate MgO layers into the experimental platform of metallic Pt-Cu-Ni heterostructures to block the propagation of hot electrons. We show via ultrafast x-ray diffraction the capability of our platform to control the spatio-temporal shape of the transient heat and strain. Time-resolved magneto-optical Kerr experiments with systematic tuning of the magnetization precession frequency showcase control of the magnetization dynamics in the Ni layer. Our experimental analysis highlights the role of quasi-static strain as a driver of precession when the magnetic material is rapidly heated via electrons. The effective magnetic field change originating from demagnetization partially compensates the change induced by quasi-static strain. The strain pulses can be shaped via the nanoscale heterostructure design to efficiently drive the precession, paving the way for opto-magneto-acoustic devices with low heat energy deposited in the magnetic layer.

Magnetization dynamics in layered systems with coexisting bilinear and biquadratic interlayer exchange coupling

Important aspects of exchange-coupled magnetic layered structures are related to the noncollinear arrangement of sublayer magnetizations which can arise from competition between bilinear (BL) and biquadratic (BQ) interlayer exchange coupling (IEC). In this work, the influence of coexisting BL and BQ IEC of different strengths on magnetization precession in layered systems is investigated both experimentally and theoretically. Laser-induced magnetization precession has been studied in the $\text{Fe/Si}({d}_{\text{Si}}$) multilayers (MLS) as a function of the amplitude ($H$) and orientation angle (${\ensuremath{\theta}}_{H}$) of external magnetic field using time-resolved magneto-optical Kerr (TRMOKE) effect. Strongly changing characters of precession frequency dependencies $\ensuremath{\omega}(H,{\ensuremath{\theta}}_{H})$ for Fe sublayer thickness ${d}_{\text{Fe}}=3$ nm and Si spacer-layer thicknesses (${d}_{\text{Si}}$) varying in the range of 0.9--2.4 nm have been observed. Analytical formulas for acoustic and optic mode dispersion relations with coexisting BL and BQ IEC, scaled by ${J}_{1}$ and ${J}_{2}$ parameters, respectively, for the in-plane effective magnetic anisotropy and arbitrary magnetic field direction were derived, and very good agreement with the experimentally observed frequency dependencies has been obtained. It is shown that BQ coexisting with BL IEC significantly influences on the magnitude and form of dispersion relations. From analytical formula derived, it follows that zero-field optical mode frequency tends to zero as $|{J}_{1}|$ approaches $2{J}_{2}$. The acoustic and optic mode-crossing effect has been observed and it is found that values of crossing fields and frequency gaps strongly increase as ${\ensuremath{\theta}}_{H}$ angles decrease and depend on relative BL and BQ IEC strengths. The BL IEC is of ferromagnetic type with ${J}_{1}\ensuremath{\approx}1.6$ mJ/${\mathrm{m}}^{2}$ for the MLS with ${d}_{\text{Si}}=0.9$ nm, and changes to antiferromagnetic one with ${J}_{1}\ensuremath{\approx}\ensuremath{-}0.9$ mJ/${\mathrm{m}}^{2}$ for the MLS with ${d}_{\text{Si}}=1.4$ nm, while the ${J}_{2}$ parameter of BQ IEC decreases from 1.8 to 1.0 mJ/${\mathrm{m}}^{2}$. The coupling strengths decrease by one to two orders of magnitude for the sample with ${d}_{\text{Si}}=2.4$ nm, but both mode frequencies are still observed and well reproduced by the theory. It is shown that ${J}_{1}$ and ${J}_{2}$ parameters obtained in the TRMOKE experiment coincide within the estimated error bars with the determined from independent measurements of magnetization processes in the static magneto-optical Kerr effect and interpreted with the use of analytical formulas derived. Numerical solutions of coupled Landau-Lifshitz-Gilbert (LLG) equations for acoustic and optic modes, with inclusion of BL IEC, intrinsic Gilbert damping, and spin-pumping damping terms, and extended to include BQ IEC, were performed and fitted to experimental data. It is shown that determined effective damping coefficients on $H$ and ${\ensuremath{\theta}}_{H}$ dependencies for acoustic and optic modes are very well simulated with the use of LLG equation solutions with Gilbert damping, spin-pumping-damping--related effective spin-mixing conductance, and spin-diffusion length parameters included. The dependencies of the parameters on ${d}_{\text{Si}}$ spacer-layer thickness are discussed and compared with available data for other systems.

Thermally induced all-optical ferromagnetic resonance in thin YIG films

All-optical ferromagnetic resonance (AO-FMR) is a powerful tool for the local detection of micromagnetic parameters, such as magnetic anisotropy, Gilbert damping or spin stiffness. In this work we demonstrate that the AO-FMR method can be used in thin films of yttrium iron garnet (YIG) if a metallic capping layer (Au, Pt) is deposited on top of the film. Magnetization precession is triggered by heating of the metallic layer with femtosecond laser pulses. The heat pulse modifies the magneto-crystalline anisotropy of the YIG film and shifts the quasi-equilibrium orientation of the magnetization, which results in precessional magnetization dynamics. The laser-induced magnetization precession corresponds to a uniform (Kittel) magnon mode, with the precession frequency determined by the magnetic anisotropy of the material as well as the external magnetic field, and the damping time set by a Gilbert damping parameter. The AO-FMR method thus enables measuring local magnetic properties, with a resolution given by the laser spot size.

Single-shot imaging of ultrafast all-optical magnetization dynamics with a spatiotemporal resolution.

We present a laboratory system for single-shot magneto-optical (MO) imaging of ultrafast magnetization dynamics with less than 8fs temporal, micrometer spatial resolutions and a MO Faraday's rotation sensitivity of 4 mdeg/μm. We create a stack of MO images repeatedly employing a single pair of pump and defocused probe pulses to induce and visualize MO changes in the sample. Both laser beams are independently wavelength-tunable, allowing for a flexible, resonant adjustable two-color pump and probe scheme. To increase the MO contrast, the probe beam is spatially filtered through a 50 μm aperture. We performed the all-optical switching experiment in Co-doped yttrium iron garnet films (YIG:Co) to demonstrate the capability of the presented method. We determine the spatiotemporal distribution of the effective field of photo-induced anisotropy, driving the all-optical switching of the magnetization in the YIG:Co film without an external magnetic field. Moreover, using this imaging method, we tracked the process of the laser-induced magnetization precession.

Effect of magnetic anisotropy relaxation on laser-induced magnetization precession in thin galfenol films

The rate and pathways of relaxation of a magnetic medium to its equilibrium following excitation with intense and short laser pulses are the key ingredients of ultrafast optical control of spins. Here we study experimentally the evolution of the magnetization and magnetic anisotropy of thin films of a ferromagnetic metal galfenol (${\mathrm{Fe}}_{0.81}{\mathrm{Ga}}_{0.19}$) resulting from excitation with a femtosecond laser pulse. From the temporal evolution of the hysteresis loops we deduce that the magnetization ${M}_{S}$ and magnetic anisotropy parameters $K$ recover within a nanosecond, and the ratio between $K$ and ${M}_{S}$ satisfies the thermal equilibrium's power law in the whole time range spanning from a few picoseconds to 3 nanoseconds. We further use the experimentally obtained relaxation times of ${M}_{S}$ and $K$ to analyze the laser-induced precession and demonstrate how they contribute to its frequency evolution at the nanosecond timescale.

Tuning interfacial spin pump in Ta/CoFeB/MgO films by ultrafast laser pulse

The operation speed and the energy-efficiency of magnetic random access memory (MRAM) is largely controlled by Gilbert damping of magnetic layers. The ultrafast laser pulse may offer an opportunity to tune the interfacial spin pumping, which can then control the Gilbert damping. Here, we have investigated the ultrafast laser induced magnetization precession, especially the magnetic damping, of a series of Ta/CoFeB/MgO thin films using the pump–probe time-resolved magneto-optical Kerr effect (TR-MOKE) measurements. The pump fluence dependence of the magnetic damping has been found to vary with the thickness tCoFeB of the nanoscale CoFeB layer. Remarkably, the intrinsic damping constant α0 has been found to decrease with the increase in the pump fluence when the thickness of the CoFeB layer is less than 1.2 nm. This fluence-dependent behavior of α0 is attributed to the fluence-dependent contribution of the Ta/CoFeB interface induced spin pumping effect. The ultrafast laser pulses effectively enhance the interfacial spin pumping effect via tuning the spin diffusion length of the adjacent Ta layer, from 2.4 to 7.1 nm. Our findings provide insights into the ultrafast laser pulse driven magnetic dynamics and interfacial spin manipulation in Ta/CoFeB/MgO structures.

Laser-Induced Magnetization Precession in Individual Magnetoelastic Domains of a Multiferroic Co40Fe40B20/BaTiO3 Composite

Using a magneto-optical pump-probe technique with micrometer spatial resolution we show that magnetization precession can be launched in individual magnetic domains imprinted in a Co$_{40}$Fe$_{40}$B$_{20}$ (CoFeB) layer by elastic coupling to ferroelectric domains in a BaTiO$_{3}$ substrate. The dependence of the precession parameters on external magnetic field strength and orientation reveal that by laser-induced ultrafast partial quenching of the magnetoelastic coupling parameter of CoFeB by $\approx$27% along with 10% ultrafast demagnetization trigger the magnetization precession. The relation between the laser-induced reduction of the magnetoelastic coupling and the demagnetization is approximated by the $n(n+1)/2$-law with n$\approx$2. This correspondence confirms the thermal origin of the laser-induced anisotropy change. Based on the analysis and modeling of the excited precession we find signatures of laser-induced precessional switching, which occurs when the magnetic field is applied along the hard magnetization axis and its value is close to the effective magnetoelastic anisotropy field. The precession excitation process in an individual magnetoelastic domain is found to be unaffected by neighboring domains. This makes laser-induced changes of magnetoelastic anisotropy a promising tool for driving magnetization dynamics and switching in composite multiferroics with spatial selectivity.

All-optical probe of magnetization precession modulated by spin–orbit torque

Laser-induced magnetization precession modulated by an in-plane direct current was investigated in a W/CoFeB/MgO micron-sized strip using an all-optical time-resolved magneto-optical Kerr effect microscope. We observed a relatively large change in the precession frequency, owing to a current-induced spin–orbit torque. The generation efficiency of the spin–orbit torque was evaluated as −0.35 ± 0.03, which was in accordance with that evaluated from the modulation of damping. This technique may become an alternate method for the evaluation of spin–orbit torque.

Influence of magnetic field orientation on laser-induced magnetization precession in Ni-Mn-Sn Heusler alloy film

Laser-induced magnetization precession is investigated in Ni-Mn-Sn Heulser alloy film using pump-probe all-optical technique. The studies are performed as a function of external magnetic field H applied at various angles θH with respect to the sample normal. Strong dependence of precession parameters, frequency f, amplitude A and effective damping αeff on H and θH was found. For H>3 kOe and θH<25°, two precession modes, identified as Damon-Eshbach (DE) and first-order standing spin wave (PSSW) modes were observed. The values of wave vector k=0.37 μm−1 for DE mode and exchange constant Aex=0.22 μerg cm−1 for PSSW mode have been determined. The A(H) increases up to six times as θH decreases from 55° to 5° and exhibits a maximum at around H=1.8 kOe for each θH. From experimentally obtained exponentially decayed αeffH dependencies, intrinsic damping α0θH is estimated to ≈0.025. The dependencies of the magnetization precession parameters have been well simulated in the frame of phenomenological approach, based on Landau-Lifshitz-Gilbert equation.

Laser-induced magnetization precession in the magnetite Fe3O4 in the vicinity of a spin-reorientation transition

Using time-resolved magneto-optical pump-probe technique we demonstrate excitation of magnetization precession in a single crystalline bulk magnetite Fe3O4 below and in the vicinity of the Verwey and spin-reorientation (SR) phase transitions. Pronounced temperature dependence of the precession amplitude is observed suggesting that the excitation occurs via laser-driven spin-reorientation transition. Similarity observed between the characteristic features of the laser-induced ultrafast SR and Verwey transitions suggests that they both rely on the same microscopic processes.

Modulation of ultrafast laser-induced magnetization precession in BiFeO3-coated La0.67Sr0.33MnO3 thin films

The ultrafast laser-excited magnetization dynamics of ferromagnetic (FM) La0.67Sr0.33MnO3 (LSMO) thin films with BiFeO3 (BFO) coating layers grown by laser molecular beam epitaxy are investigated using the optical pump-probe technique. Uniform magnetization precessions are observed in the films under an applied external magnetic field by measuring the time-resolved magneto-optical Kerr effect. The magnetization precession frequencies of the LSMO thin films with the BFO coating layers are lower than those of uncoated LSMO films, which is attributed to the suppression of the anisotropy field induced by the exchange interaction at the interface between the antiferromagnetic order of BFO and the FM order of LSMO.

All-optical detection of magnetization precession in tunnel junctions under applied voltage

An all-optical time-resolved magneto-optical Kerr effect measurement of a micron-sized tunnel junction with a CoFeB electrode was performed. The femtosecond (fs) laser-induced magnetization precession was clearly observed at various magnetic field angles. The frequency f and relaxation time τ of the magnetization precession varied with the voltage applied via a MgO barrier. The precession dynamics were in accordance with Kittel’s ferromagnetic resonance mode, and the voltage-induced changes in f and τ were well explained by the voltage-induced change in the perpendicular magnetic anisotropy of −36 fJ/Vm.

Distinguishing the laser-induced spin precession excitation mechanism in Fe/MgO(001) through field orientation dependent measurements

Rotational field dependence of laser-induced magnetization precession in a single-crystal Fe/MgO(001) sample was studied by the time resolved magneto-optical Kerr effect. Polar and longitudinal magnetization components were separated by measuring precession dynamics under opposite fields. When the applied field is weaker than the anisotropy field of an Fe film, the precession amplitude is small for the field direction near the easy axis and becomes larger as the field rotates towards the hard axis, showing a four-fold symmetry in agreement with the in-plane magnetic anisotropy; whereas at higher fields, the amplitude displays a drop near the hard axis. Such precession behavior can be well reproduced using an excitation model with rapidly modified but slowly recovered magnetic anisotropy and considering the elliptical precession trajectory. Our results indicate that the dominant mechanism for triggering Fe spin precession is the anisotropy modulation correlating with the lattice thermalization, rather than the transient anisotropy modulation due to the high electron temperature within 1 ps.

Experimental Investigation of Controlling Light Polarization in Optical Waveguides with Magneto-optical Materials GdFe

With a vision that the technology for projecting spins/magnetic state on the polarization of light in a waveguide is worth developing, we have studied experimentally optical controllability of the dynamic state of magnetization, and the optical coupling between a magnetic medium and a waveguide. We describe experimental results on two-pulse control of the laser-induced magnetization precession in a ferrimagnetic GdFe thin film, as well as the evaluation of magneto-optical effect with oblique incident light through free space and a quartz prism.

Ultrafast laser-induced magnetization precession dynamics in FePt/CoFe exchange-coupled films

Femtosecond laser-triggered magnetization precession dynamics in exchange-coupled hard FePt/soft CoFe films has been studied by the time-resolved magneto-optical Kerr effect. The precession frequency is shown to be independent of the CoFe thickness tCoFe, while linearly proportional to the external field for the sample with tCoFe=7 nm, implying that the magnetization relaxation is governed by a uniform precession mode. The result agrees well with the theoretical prediction based on the Landau–Lifshitz–Gilbert equation. The obtained damping factor α∼0.13 is quite large and is thought to be dominated by the FePt layer. Additionally, the oscillation amplitude strongly depends on the CoFe thickness with a maximum value occurred at tCoFe=7 nm.