Time-resolved Magneto-optical Kerr Effect Research Articles

Gilbert Damping in Annealed Perpendicular Exchange Biased CoFeB/Pt/MnIr Multilayers

Exchange bias and intrinsic Gilbert damping [Formula: see text] are two important parameters in magnetic multilayers. To explore the correlation between exchange coupling and damping, the samples Ta (2.6[Formula: see text]nm)/Pt (2.5[Formula: see text]nm)/Co[Formula: see text]Fe[Formula: see text]B[Formula: see text] (0.63[Formula: see text]nm)/Pt (0.5[Formula: see text]nm)/Mn[Formula: see text]Ir[Formula: see text] ([Formula: see text])/Ta (4[Formula: see text]nm) are deposited by using pulsed-DC magnetron sputtering. The static and dynamic magnetic responses with different thickness of antiferromagnetic MnIr are investigated using vibrating sample magnetometer (VSM) and the pump-probe time-resolved magneto-optical Kerr effect (TR-MOKE). The exchange bias initially increases and then decreases with increasing thickness of MnIr layer, which is associated with the interfacial exchange coupling. The dynamic results demonstrate that the Gilbert damping of CoFeB/Pt/MnIr can increase by up to 40% compared with CoFeB/Pt. Based on the fact that the spin pumping has been excluded because of the Pt interlayer, we attribute the enhancement in Gilbert damping to the exchange coupling between AFM and FM.

Nonequilibrium heat transport in Pt and Ru probed by an ultrathin Co thermometer

Non-equilibrium of electrons, phonons, and magnons in metals is a fundamental phenomenon in condensed matter physics and serves as an important driver in the field of ultrafast magnetism. In this work, we demonstrate that the magnetization of a sub-nm-thick Co layer with perpendicular magnetic anisotropy can effectively serve as a thermometer to monitor non-equilibrium dynamics in adjacent metals, Pt and Ru, via time-resolved magneto-optic Kerr effect. The temperature evolutions of the Co thermometer embedded in Pt layers of different thicknesses, 6-46 nm, are adequately described by a phenomenological three temperature model with a consistent set of materials parameters. We do not observe any systematic deviations between the model and the data that can be caused by a non-thermal distribution of electronic excitations. We attribute the consistently good agreement between the model and the data to strong electron-electron interaction in Pt. By using Pt/Co/Pt and Pt/Co/Pt/Ru structures, we determine the electron-phonon coupling parameters of Pt and Ru, g_ep(Pt)=(6+/-1)x10^17 W m-3 K-1 and g_ep(Ru)=(9+/-2)x10^17 W m-3 K-1. We also find that the length scales of non-equilibrium between electrons and phonons are l_ep=({\Lambda}_e/g_ep)^1/2 =9 nm for Pt and 7 nm for Ru, shorter than their optical absorption depths, 11 and 13 nm, respectively. Therefore, the optically thick Pt and Ru layers show two steps of temperature rise: The initial jump of electron temperature that occurs within 1 ps is caused by direct optical excitation and electronic heat transport within a distance l_ep for the Co layer. The second temperature rise is caused by heat transport by electrons and phonons that are near thermal equilibrium. We contrast two-temperature modeling of heat transport in Pt an Ru films to calculations for Cu, which has a much longer non-equilibrium length scale, l_ep=63 nm.

High sensitivity pump-probe measurements of magnetic, thermal, and acoustic phenomena with a spectrally tunable oscillator.

We describe an optical pump/probe system for sensitive measurements of time-resolved optical measurements of material dynamics. The instrument design is optimized for time-resolved magneto-optic Kerr effect (TR-MOKE) measurements of dynamics in magnetic materials. The system also allows for time-domain thermoreflectance (TDTR) measurements of thermal transport properties and picosecond acoustic measurements of film thickness and/or elastic constants. The system has several advantages over the conventional designs for TR-MOKE and/or TDTR systems. Measurements of pump-induced changes to the probe beam intensity are shot-noise limited. The system's design allows for MOKE and/or thermoreflectance measurements of both sides of a sample. Pumping and probing the sample on opposite sides allows nanoscale flash diffusivity measurements of transport properties. The wavelengths of the pump and probe beams are straightforward to tune between 350-525 nm and 690-1050 nm. A tunable wavelength allows for optical resonances in a wide array of materials to be excited and/or probed. Finally, the setup is calibrated to allow for the real and imaginary components of Kerr signals to be separately quantified.

All-optical investigation of anisotropic spin pumping in W/CoFeB/W heterostructure

Pure spin current generation and transport in magnetic heterostructures comprising of normal metal (NM)/ferromagnet (FM) interfaces are imperative for future energy efficient spintronic devices. The above properties are considered to be equivalent in terms of spin-mixing conductance and spin pumping efficiency. Correspondingly, the enhancement in Gilbert damping should be equal for every interface with same NM and FM layers. However, this equivalency may break down under certain circumstances, giving rise to anisotropy. Here, we have shown by time-resolved magneto-optical Kerr effect measurements that for W/CoFeB interface, the enhancement in Gilbert damping is dissimilar when the position of W and CoFeB are interchanged. The damping coefficient (α) is 0.010 for CoFeB/W, whereas it is 0.0135 for W/CoFeB. Our in-depth investigation reveals that the precessional trajectory for W/CoFeB system is strongly affected by the induced magnetic anisotropy which is not present in CoFeB/W, leading towards the observed directional spin pumping. This is clearly demonstrated in W/CoFeB/W trilayer where the spin current generated by spin pumping diffuses more towards W underlayer in comparison to W overlayer. Our findings add new insights in the role of ferromagnet/nonmagnet interfaces for controlled generation and utilization of pure spin current for application in advanced spintronic devices.

Phonon and Thermal Properties of Quasi-Two-Dimensional FePS3 and MnPS3 Antiferromagnetic Semiconductors.

We report results of investigation of the phonon and thermal properties of the exfoliated films of layered single crystals of antiferromagnetic FePS3 and MnPS3 semiconductors. Raman spectroscopy was conducted using three different excitation lasers with wavelengths of 325 nm (UV), 488 nm (blue), and 633 nm (red). UV-Raman spectroscopy reveals spectral features which are not detectable via visible Raman light scattering. The thermal conductivity of FePS3 and MnPS3 thin films was measured by two different techniques: the steady-state Raman optothermal and transient time-resolved magneto-optical Kerr effect. The Raman optothermal measurements provided the orientation-average thermal conductivity of FePS3 to be 1.35 ± 0.32 W m-1 K-1 at room temperature. The transient measurements revealed that the through-plane and in-plane thermal conductivity of FePS3 are 0.85 ± 0.15 and 2.7 ± 0.3 W m-1 K-1, respectively. The films of MnPS3 have higher thermal conductivity of 1.1 ± 0.2 W m-1 K-1 through-plane and 6.3 ± 1.7 W m-1 K-1 in-plane. The data obtained by the two techniques are in agreement and reveal strong thermal anisotropy of the films and the dominance of phonon contribution to heat conduction. The obtained results are important for the interpretation of electric switching experiments with antiferromagnetic materials as well as for the proposed applications of the antiferromagnetic semiconductors in spintronic devices.

Observation of spectral narrowing and mode conversion in two-dimensional binary magnonic crystal

We study binary magnonic crystal, where Ni80Fe20 nanodots of two different sizes are diagonally connected forming a unit and those units are arranged in a square lattice. The magnetization dynamics of the sample is measured by using time-resolved magneto-optical Kerr effect microscope with varying magnitude and orientation (ϕ) of the bias magnetic field, which is applied at about 10° angle from the sample plane. Interestingly, at ϕ = 0°, the spin-wave mode profiles show frequency selective spatial localization of spin-wave power within the array. With the variation of ϕ in the range 0° < ϕ ≤ 45°, we observe spectral narrowing due to localized to extended spin-wave mode conversion. Upon further increase of ϕ, the spin-wave modes slowly lose the extended nature and become fully localized again at 90°. We have extensively demonstrated the role of magnetostatic stray field distribution on the rotational symmetries obtained for the spin-wave modes. From micromagnetic simulations, we find that the dipole-exchange coupling between the nanodots leads to remarkable modifications of the spin-wave mode profiles when compared with arrays of individual small and large dots. Numerically, we also show that the physical connection between the nanodots provides more control points over the spin-wave propagation in the lattice at different orientations of bias magnetic field. In particular, the directional propagation of spin-wave is crucial for the application in coupler. Overall, we envision this binary magnonic crystal to have potential applications in magnonic devices such as spin-wave waveguide, filter, coupler, and other on-chip microwave communication devices.

Large anisotropy of magnetic damping in ultrathin epitaxial Fe/GaAs (0 0 1) film

The magnetization dynamics of ultrathin epitaxial Fe films on GaAs (0 0 1) with different thicknesses have been investigated by all-optical time-resolved magneto-optical Kerr effect. For the Fe film with thickness of 8 monolayers, the magnetic damping constant along orientation increases by 66% compared with that along orientation, showing the uniaxial anisotropy of magnetic damping. By the measurement of angular dependent time-resolved magneto-optical Kerr effect, the uniaxial magnetic damping is clearly correlated to the in-plane uniaxial anisotropy field of the Fe film. In addition, the anisotropy of the damping constants is found to disappear for the Fe films thicker than 15 monolayers, suggesting that the anisotropic damping originates from the interfacial effect between Fe and GaAs.

Edge localization of spin waves in antidot multilayers with perpendicular magnetic anisotropy

We study the spin-wave dynamics in nanoscale antidot lattices based on Co/Pd multilayers with perpendicular magnetic anisotropy. Using time-resolved magneto-optical Kerr effect measurements we demonstrate that the variation of the antidot shape introduces significant change in the spin-wave spectra, especially in the lower frequency range. By employing micromagnetic simulations we show that additional peaks observed in the measured spectra are related to narrow shell regions around the antidots, where the magnetic anisotropy is reduced due to the Ga+ ion irradiation during the focused ion beam milling process of the antidot fabrication. The results point at new possibilities for exploitation of localized spin waves in out-of-plane magnetized thin films, which are easily tunable and suitable for magnonics applications.

Role of magnetostrictive effect in magnetization dynamics of SmFe thin films

Laser induced magnetization dynamic behaviors of SmxFe1–x thin films with tunable x (0.25 ≤ x ≤ 0.5) are investigated by the time-resolved magneto-optical Kerr effect. It is found that the demagnetization process is accomplished by a two-stage process, the demagnetization time of stage I is within 1.0 ps (t1) for all the samples with different x, while it increases from 0 to 16 ps at stage II (t2) due to the reduced exchange coupling. By introducing an additional magneto-elastic energy contribution, the dispersion relation of precession frequency is deduced, by which the field-dependent frequency curves are well fitted.

Damping of Standing Spin Waves in Bismuth-Substituted Yttrium Iron Garnet as Seen via the Time-Resolved Magneto-Optical Kerr Effect

We investigate spin-wave resonance modes and their damping in insulating thin films of bismuth-substituted yttrium iron garnet by performing femtosecond magneto-optical pump-probe experiments. For large magnetic fields in the range below the magnetization saturation, we find that the damping of high-order standing spin-wave (SSW) modes is about 40 times lower than that for the fundamental one. The observed phenomenon can be explained by considering different features of magnetic anisotropy and exchange fields that, respectively, define the precession frequency for fundamental and high-order SSWs. These results provide further insight into SSWs in iron garnets and may be exploited in many new photomagnonic devices.

Direct observation of terahertz emission from ultrafast spin dynamics in thick ferromagnetic films

We have experimentally investigated the field- and fluence-dependent terahertz (THz) emission behaviors by a femtosecond optical pump in Co ferromagnetic films, together with the time-resolved magneto-optical Kerr effect measurement. The thickness of Co and Ta capping layers has been systematically varied. For thick films, THz emission behavior is found to be directly matched with the photoinduced ultrafast demagnetization and remagnetization dynamics. The angle-dependent THz emission measurement also confirms a direct relation between the THz emission and the photoinduced demagnetization/remagnetization dynamics in the case of thick films, while THz emission from the inverse spin Hall effect also plays a role in the case of thin Co films.

Role of magnetic anisotropy in the ultrafast magnetization dynamics of Gd-Fe thin films of different thicknesses

The time-resolved magneto-optical Kerr effect technique has been exploited to study the magnetization dynamics over femtosecond to nanosecond timescale for Gd-Fe thin films with different thicknesses. The precessional magnetization dynamics is studied by varying the magnitude and orientation of the bias magnetic field from nearly in-plane to nearly out-of-plane (OOP) direction. The analysis of bias magnetic field dependence of precessional frequency has revealed the development of OOP anisotropy when the film thickness was increased from 20 to 100 nm and this is in good agreement with the static magnetic properties of the films. Moreover, the 100-nm-thick film has shown a signature of perpendicular standing spin wave modes along with the uniform Kittel mode. The evolution of their frequency, decay time, and damping with the magnitude and orientation of bias magnetic field is analyzed in detail, showing possible energy transfer between the perpendicular standing spin wave modes and the uniform Kittel mode. Interestingly, the effective damping could be tuned over a broader range of 0.02 to 0.15, which shows the potential for applications in miniaturized and fast magnetic storage, magnetic memory, and magnonic devices.

The unique magnetic damping enhancement in epitaxial Co2Fe1−xMnxAl films**Project supported by the Natural Science Foundation of Guangdong Province, China (Grant No. 2015A030310003) and the Program for Scientific Research Start-up Funds of Guangdong Ocean University, China.

Uniform precession dynamics and its magnetic damping are investigated in epitaxial Co2Fe1−xMnxAl films by using the time-resolved magneto–optical Kerr effect under out-of-plane configuration. The decay time of uniform precession mode decreases, and thus the magnetic damping increases with the increase of external field. Moreover, the decay time decreases as x decreases, so that the enhancement of magnetic damping occurs in Fe-rich sample. Furthermore, the decay time decreases as the excitation fluence increases, which drops rapidly at low magnetic field comparing with the slow reduction at high magnetic field. This unique magnetic damping enhancement is attributed to the enhancement of homogeneous magnetization.

Piezostrain modulation of magnetic damping in MBE-grown epitaxial Co2FeAl/GaAs heterostructure

The electric voltage manipulated magnetic dynamic properties of Co2FeAl/GaAs heterostructure bonded on PbZr1−xTixO3 piezoelectric substrate were investigated using the time-resolved magneto-optical Kerr effect technique at room temperature. The butterfly-like response of precession frequency dependence on the applied electric field was observed with no external magnetic field applied, indicating efficient manipulation of precession frequency solely by piezostrain. The magnetic damping of Co2FeAl was observed to be enhanced (decreased) under the strengthened compressive (tensile) strain modulation. The observed efficient tuning of both precession frequency and extrinsic damping constant at low magnetic fields is attributed to piezostrain modulated magnetic anisotropy value and its inhomogeneous distribution. When the magnetic dynamics is governed by the external magnetic field at high-field regime, electric field tunability of precession frequency and damping constant becomes weaker and the intrinsic damping constant shows no modulation. The electric voltage tuning of magnetic damping and the associated physical mechanism presented in this work can help designing strain-mediated low-energy cost spintronic devices.

Interlayer modulation on the dynamic magnetic properties of L1-FePt/NM/[CoNi]5 composite film structures

The magnetization precession dynamics is systematically studied in L10-FePt/NM/[CoNi]5 exchange-coupled films using the time-resolved magneto-optical Kerr effect technique, where the nonmagnetic interlayer is Ta or Pt. It is demonstrated that the precession frequency f decreases dramatically with the increasing interlayer thickness of Ta (tTa) or Pt (tPt). By fitting the f ∼ H curves of different tTa with the coherent precession model including the interfacial coupling interaction, the exchange coupling strength as well as the magnetization orientations of FePt and [CoNi]5 are determined. Meanwhile, it is found the magnetic damping constant α can be modulated considerably as well by adjusting the interlayer thickness. For the Ta interlayer, the α value decreases gradually with the increase in tTa as the result of decreased coupling strength, while it shows a monotonic increasing behavior for the Pt layer. The increased α with tPt can be attributed to the enhanced spin pumping contribution from Pt, which greatly exceeds the decreasing tendency caused by the reduced interfacial coupling. This study provides deep insights into the dynamic behaviors of the exchange-coupled magnetic system, which are valuable to achieve effective control of the ultrafast magnetization dynamics for spintronic applications.

Tuning of the intrinsic magnetic damping parameter in epitaxial CoNi(001) films : Role of the band-filling effect

The band-filling effect, which is due to tuning the electron occupation number near the Fermi level, on the intrinsic magnetic damping parameter is demonstrated by employing high-quality epitaxial ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}$ alloy films, in which the Fermi level and the density of states can be continuously tuned by varying the Ni concentration $x$. The intrinsic magnetic damping parameter, measured by the time-resolved magneto-optical Kerr effect, changes weakly at small $x$ and increases sharply for $x&gt;0.80$. The experimental results are well reproduced by the density functional theory calculation. More interestingly, the magnetic damping parameter and the density of states near the Fermi level share similar variation trends, demonstrating their correlation. The band-filling effect in $3d$ magnetic transition metal alloys provides a way to tune the magnetic damping parameter as a key element in the controlg of energy loss and speed of spintronic devices.

Anisotropic spin waves in two-dimensional triangular shaped bi-component magnonic crystal

Abstract Bi-component magnonic crystals with a strong dipole-exchange interaction across the interface of the constituent magnetic elements have shown promising potentials in magnonics and magnon-spintronics. Here, we have reported an all-optical investigation of spin wave dynamics in an array of periodically arranged bi-component magnonic crystal in the form of triangular-shaped Ni 80 Fe 20 nanoelements embedded in Co 50 Fe 50 matrix using time-resolved magneto-optical Kerr effect magnetometry. The spin wave spectra obtained from the sample reveal a broad band of spin wave modes where they possess a strong and systematic bias magnetic field tunability which is crucial for active control over such system in device applications. Further, the spin wave modes show a six-fold and four-fold rotational anisotropy with the bias field orientation due to combined effects of element shape and lattice symmetry. Micromagnetic simulations reproduce the experimental results qualitatively where the simulated mode profiles unravel the spatial distribution of spin wave frequencies inside both constituent elements while the internal magnetic fields play a crucial role for the observed tunability of spin wave dynamics. Development of such magnetically coupled embedded magnetic nanostructures can pave a new pathway in designing the future magnonic devices and faster microwave communication systems.

Ultrafast magnetization switching in nanoscale magnetic dots

Ultrafast magnetization switching at picosecond and sub-picosecond time scales has tremendous technological potential but still poses numerous questions regarding the underlying quantum mechanical phenomena, including the roles of and interactions between the electrons, spins, and phonons (lattice). At the nanometer-scale dimensions relevant for modern applications, these phenomena become increasingly more pronounced. Until now, helicity-independent all-optical switching (HI-AOS) has been largely limited to amorphous Gd-Fe-Co alloys, for which scaling was challenging due to their relatively low anisotropies. In this work, we demonstrate HI-AOS in amorphous GdCo and scale it to nanometer dimensions while still maintaining uniform out-of-plane magnetization. Single shot HI-AOS is demonstrated in these patterned samples down to a minimum optically detectable magnetic dot size of 200 nm. The ultrafast switching behavior was also confirmed using time-resolved magneto-optic Kerr effect measurements and found to settle to its opposite magnetization state at faster rates for smaller dot diameters, passing a threshold of 75% magnetization reversal within approximately 2 ps for a 200 nm dot compared to approximately 40 ps for a 15 μm pattern. The size dependence of the ultrafast switching is explained in terms of the electron-phonon and spin-lattice interactions.

Ultrafast magnetization dynamics in an epitaxialNi54.3Mn31.9Sn13.8Heusler-alloy film close to the Curie temperature

The influence of the amplitude of an external magnetic field ($H$) and femtosecond laser pulse fluence ($F$) on ultrafast magnetization dynamics has been investigated in a ferromagnetic ${\mathrm{Ni}}_{54.3}{\mathrm{Mn}}_{31.9}{\mathrm{Sn}}_{13.8}$ Heusler-alloy film using the time-resolved magneto-optical Kerr effect. A large slowing down of the demagnetization process was observed and characteristic parameters of magnetization precession were determined for a wide range of $H$ and $F$ values. Long demagnetization times of the order of hundreds of picoseconds have been found and explained as a result of the Curie temperature (${T}_{\mathrm{C}}$) proximity in the alloy film studied. Effective magnetic anisotropy field (${H}_{\text{k}}^{\text{eff}}$) and Gilbert damping parameter dependencies were determined. A significant reduction of the precession frequency versus $F$ of the uniform Kittel mode was found. A strong decrease of ${H}_{\text{k}}^{\text{eff}}$ with $F$ was well simulated in the frame of an extended version of the microscopic three-temperature model (eM3TM), and explained by the ${T}_{\mathrm{C}}$ proximity effect. The estimated low values of the eM3TM model parameters, the demagnetization rate and electron-lattice coupling constant, appeared essential to explain the slowing down effect of demagnetization. Precession amplitude dependencies were explained by a phenomenological approach taking into account ${H}_{\text{k}}^{\text{eff}}$ and changes of the equilibrium magnetization angles induced by pump-pulse excitation.

Field strength- and pump fluence-dependent magnetization dynamics of the Co2FeAl films epitaxially grown on the MgO substrates

The pump fluence- and amplitude of magnetic field-dependent demagnetization and dynamic processes are well studied in the Co2FeAl(CFA) films prepared on the MgO substrates at different temperatures by the time resolved magneto-optical Kerr effect (TR-MOKE). It is found that the demagnetization and the damping processes are strongly dependent on both the amplitude of the external fields and the pump fluences in low fields, resulting in a complex spin relaxation and damping behavior, which is mainly attributed to the non-uniform precession mode induced by the inhomogeneous magnetic anisotropy distribution and the anisotropy field competition. However, with increasing magnetic field gradually, the demagnetization and dynamic processes start to be dominated by the external field, and meanwhile the uniform spin-wave arises. More importantly, the dynamic properties are nearly independent of the amplitude of the magnetic fields and the pump fluences again. The constant damping parameter, which decreases slightly from 4.02 × 10−3 to 1.55 × 10−3 with grown temperature changing from 300 °C to 700 °C, is suggested to be an intrinsic characteristic of the CFA films, and can be attributed to the improvement of the atomic ordering and the crystal quality.