Magneto-optical Kerr Spectroscopy Research Articles

Tuning ultrafast demagnetization dynamics in FePdPt ternary alloy films: the role of spin–orbit coupling

The influence of spin–orbit coupling (SOC) on ultrafast demagnetization dynamics is still an obscure issue. Ferromagnetic ternary alloy film containing Platinum element has the advantage that SOC strength can be regulated flexibly by varying the component ratio, thus is an excellent platform for investigating the dynamical effect of SOC. Here the fluence and component dependences of ultrafast demagnetization dynamics in ternary alloy L10-Fe0.5(Pd1-xPtx)0.5 films are studied by using time-resolved magneto-optical Kerr spectroscopy. An excitation-fluence dependent transition of demagnetization dynamics shows the Elliott-Yafet (EY) spin-flip scattering as the dominant mechanism of ultrafast demagnetization. We analyze the dynamics by a compatible fitting model and extract multiple characteristic time constants. It is clearly demonstrated that the gradual enhancement of SOC in fact decreases the spin-flip scattering and slow down the demagnetization process. We further show that the demagnetization time τm scales with the SOC strength ξ approximately under a dependence of τm∝ξ1/3, while a linear dependence exists between the spin–lattice relaxation time τs-l and ξ-6. These findings clarify the role of SOC in EY-dominated demagnetization dynamics and demonstrate an approach for tuning the speeds and duration of ultrafast demagnetization by regulating SOC strength.

Collinear and noncollinear ferrimagnetic phases in Mn4N investigated by magneto-optical Kerr spectroscopy

Ferrimagnetic antiperovskite Mn4N has received growing interest due to room-temperature observation of large perpendicular magnetic anisotropy, low saturation magnetization, and ultrafast response to external magnetic fields. Comprehensive understanding of the underlying magnetic structure is instrumental in design and fabrication of computer memory and logic devices. Magneto-optical spectroscopy provides deeper insight into the magnetic and electronic structure than magnetometry. Simulations of a magneto-optical Kerr effect in biaxially strained Mn4N are performed using density functional theory and linear response theory. We consider three ferrimagnetic phases, two collinear and one noncollinear, which have been investigated separately by earlier studies. The simulated spectra are compared to measured magneto-optical data available in recent literature. One of the collinear ferrimagnetic phases is found to be consistent with the measured spectra. We show that an admixture of the noncollinear phase, which is the ground state of unstrained Mn4N, further improves the agreement with measured spectra, and at the same time, it could explain the lower than predicted net moment and magnetic anisotropy observed in thin films on various substrates.

Ultrafast Laser Modulation of Local Magnetization Orientation in Perpendicularly Exchange-Coupled Bilayer

Laser-induced magnetization dynamics in a perpendicularly exchange-coupled TbFeCo/GdFeCo bilayer film are studied by using pump-probe magneto-optical Kerr spectroscopy. An ultrafast modulation effect on local magnetization orientation is observed. Such ultrafast magnetization reorientation in the GdFeCo layer is revealed to be triggered by the femtosecond laser pulse and driven by the effective exchange field. These processes occur within a timescale of hundreds of picoseconds, in which the field- and fluence-dependent dynamical behaviors are demonstrated. In addition, an atomistic Heisenberg model is proposed for studying the laser-induced magnetization dynamics by using micromagnetic simulation. The simulated results agree with the experimental phenomena and further reveal the underlying mechanism. These results show an approach for ultrafast manipulation of the local magnetization orientation in perpendicularly exchange-coupled structures.

Enhanced Magneto-Optical Kerr Effect of GaAs-Based P-N Junctions in the Terahertz Range

We demonstrate that the magneto-optical Kerr effect at normal incidence in the terahertz (THz) frequency range is useful for evaluating carrier transport properties of particular layers of a p-n junction. Since a single p-type thin film only exhibits a small magneto-optical Kerr effect, magneto-optical Kerr spectroscopy cannot be used to determine the carrier densities of such a film with high sensitivity. However, because the electric field is enhanced at the p-layer in a p-n junction due to the interference between the THz waves that are reflected at the highly doped substrate and the p-layer at the surface, it is possible to conduct magneto-optical Kerr spectroscopy with a higher sensitivity. We numerically calculate and experimentally determine the spectra of the ellipticity and polarization rotation angles for single n- and p-GaAs epitaxial layers and GaAs-based photovoltaic devices with a p-i-n structure and evaluate the carrier densities of the n- and p-layers. At normal incidence, this method has a high spatial resolution, which is beneficial for imaging of large-area devices.

Magnetoelastic coupling to coherent acoustic phonon modes in the ferrimagnetic insulator GdTiO3

In this work we investigate single crystal $\mathrm{Gd}\mathrm{Ti}{\mathrm{O}}_{3}$, a promising candidate material for Floquet engineering and magnetic control, using ultrafast optical pump-probe reflectivity and magneto-optical Kerr spectroscopy. $\mathrm{Gd}\mathrm{Ti}{\mathrm{O}}_{3}$ is a Mott-Hubbard insulator with a ferrimagnetic and orbitally ordered ground state (${T}_{C}$ = 32 K). We observe multiple signatures of the magnetic phase transition in the photoinduced reflectivity signal, in response to above band-gap 660-nm excitation. Magnetic dynamics measured via Kerr spectroscopy reveal optical perturbation of the ferrimagnetic order on spin-lattice coupling timescales, highlighting the competition between the ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Ti}}^{3+}$ magnetic sub-lattices. Furthermore, a strong coherent oscillation is present in the reflection and Kerr dynamics, attributable to an acoustic strain wave launched by the pump pulse. The amplitude of this acoustic mode is highly dependent on the magnetic order of the system, growing sharply in magnitude at ${T}_{C}$, indicative of strong magneto-elastic coupling. The driving mechanism, involving strain-induced modification of the magnetic exchange interaction, implies an indirect method of coupling light to the magnetic degrees of freedom and emphasizes the potential of $\mathrm{Gd}\mathrm{Ti}{\mathrm{O}}_{3}$ as a tunable quantum material.

Field and fluence dependences of laser-induced multiple spin-wave dynamics in Co2FeAl0.5Si0.5 films

Field- and fluence-dependent spin-wave dynamics in full-Heusler Co2FeAl0.5Si0.5 films are studied by using time-resolved magneto-optical polar Kerr spectroscopy. Volume magnetostatic spin-wave (VMSW) and perpendicular standing spin-wave (PSSW) modes are excited in the films with thicknesses of 60 and 100 nm, while only the Kittel mode is observed in the films with thicknesses of 150 and 200 nm. The amplitudes of all three spin-wave modes increase with increasing field and fluence, and the frequencies slightly decrease with increasing fluence as expected. The lifetimes and effective damping are found to be modulated by both the external field and excitation fluence. The effective damping of VMSW and Kittel modes presents strong field dependences that are significantly different from each other and can be attributed to the field-dependent group velocity and magnetic inhomogeneity, respectively. In addition, the effective damping of VMSW and PSSW modes increases with increasing fluence, showing a dominant mechanism of electron–phonon scattering enhancement. However, that of the Kittel mode decreases with increasing fluence and further demonstrates the dominant contribution from magnetic inhomogeneity in this case, which is inferred to suppress the excitation of the VMSW mode.

Magneto-optical Kerr effect in spin split two-dimensional massive Dirac materials

Two-dimensional (2D) massive Dirac electrons possess a finite Berry curvature, with Chern number 1/2, that entails both a quantized dc Hall response and a subgap full-quarter Kerr rotation. The observation of these effects in 2D massive Dirac materials such as gapped graphene, hexagonal boron nitride or transition metal dichalcogenides (TMDs) is obscured by the fact that Dirac cones come in pairs with opposite sign Berry curvatures, leading to a vanishing Chern number. Here, we show that the presence of spin–orbit interactions, combined with an exchange spin splitting induced either by diluted magnetic impurities or by proximity to a ferromagnetic insulator, gives origin to a net magneto-optical Kerr effect in such systems. We focus on the case of TMD monolayers and study the dependence of Kerr rotation on frequency and exchange spin splitting. The role of the substrate is included in the theory and found to critically affect the results. Our calculations indicate that state-of-the-art magneto-optical Kerr spectroscopy can detect a single magnetic impurity in diluted magnetic TMDs.

Ultrafast dynamics of 4f electron spins in TbFeCo film driven by inter-atomic 3d–5d–4f exchange coupling

The ultrafast demagnetization dynamics of 3d and 4f spins, respectively, in FeCo and Tb of TbFeCo alloy film are studied independently by employing a dual-color time-resolved magneto-optical Kerr spectroscopy. The demagnetization dynamics of 3d and 4f spins are independently probed, respectively, by 800 and 400 nm light. Two-step demagnetization dynamics are observed for both the 3d and 4f spins under the excitation of 800 nm laser. In particular, the onset of 4f spin dynamics presents a delayed time with respect to the one of 3d spin dynamics. Those results clearly reveal a strong inter-atomic 3d–5d–4f exchange coupling which drives the first-step subpicosecond ultrafast demagnetization process of 4f spins, and a spin(4f)-lattice coupling which drives the second-step slower demagnetization process of 4f spins. A numerical calculation based on four temperature model reproduces the coupling characteristics in the demagnetization dynamics, and reveals the energy evolution dynamics among the different subsystems. These results provide a direct demonstration of strong coupling dynamics between the two spin subsystems in rare earth-transition metal alloy occurring within subpicosecond timescale, and show a new approach for ultrafast control of 4f spins via an indirect excitation.

Optically Driven Collective Spin Excitations and Magnetization Dynamics in the Néel-type Skyrmion Host GaV_{4}S_{8}.

GaV_{4}S_{8} is a multiferroic semiconductor hosting magnetic cycloid (Cyc) and Néel-type skyrmion lattice (SkL) phases with a broad region of thermal and magnetic stability. Here, we use time-resolved magneto-optical Kerr spectroscopy to show the coherent generation of collective spin excitations in the Cyc and SkL phases. Our micromagnetic simulations reveal that these are driven by an optically induced modulation of uniaxial anisotropy. Our results shed light on spin dynamics in anisotropic materials hosting skyrmions and pave a new pathway for the optical manipulation of their magnetic order.

Magnetic anisotropy manipulation of the femtosecond laser-excited spin wave modes in full-Heusler Co2Fe1−xMnxAl films

Spin-wave dynamics in full-Heusler Co2Fe1−xMnxAl films have been investigated using all-optical pump-probe magneto-optical polar Kerr spectroscopy. We find magnetic damping and anisotropy can be modulated by composition x. Damon–Eshbach (DE) spin wave occurs only in the samples which present intrinsic magnetic damping and have huge uniaxial magnetic anisotropy, implying that intrinsic magnetic damping and huge uniaxial magnetic anisotropy is the necessary conditions to excite coherent DE spin wave. Kittel spin wave appears in low uniaxial magnetic anisotropic samples and presents extrinsic magnetic damping. Therefore, laser-excited spin-wave modes can be manipulated by magnetic anisotropy, whose physical picture is discussed phenomenologically.

Magneto-optical Kerr spectroscopy in ferromagnetic semiconductors: determination of the intrinsic complex magneto-optical Voigt constant

Thin films of ferromagnetic semiconductors of the (Ga, Mn)(As, P) family are investigated by means of magneto-optical Kerr spectroscopy in order to determine the spectral dependence of the complex magneto-optical Voigt constant an intrinsic parameter responsible for Kerr rotation and ellipticity. We obtain the spectral dependence of these two conjugate quantities: the Kerr rotation and ellipticity angles. Contrary to strongly absorbing metallic ferromagnetic layers, it is expected that they vary non-linearly with the layer thickness at a fixed wavelength, as a consequence of the weak absorption and the non-negligible optical phase shift in weakly absorbing semiconductor ferromagnets. We show that, from the Kerr rotation and ellipticity angles and the complex refractive index –obtained by ellipsometric measurements—we are able to determine the spectral dependence of the complex intrinsic Voigt constant, thus avoiding the need for samples with different thicknesses. Incidentally, the use of phosphorus substituted (Ga,Mn)As provides an out-of-plane magnetic easy axis, the appropriate configuration for polar magneto-optical Kerr effect, without the need for a strong external magnetic field.

Simultaneous laser excitation of backward volume and perpendicular standing spin waves in full-Heusler Co2FeAl0.5Si0.5 films

Spin-wave dynamics in full-Heusler Co2FeAl0.5Si0.5 films are studied using all-optical pump-probe magneto-optical polar Kerr spectroscopy. Backward volume magnetostatic spin-wave (BVMSW) mode is observed in films with thickness ranging from 20 to 100 nm besides perpendicular standing spin-wave (PSSW) mode, and found to be excited more efficiently than the PSSW mode. The field dependence of the effective Gilbert damping parameter appears especial extrinsic origin. The relationship between the lifetime and the group velocity of BVMSW mode is revealed. The frequency of BVMSW mode does not obviously depend on the film thickness, but the lifetime and the effective damping appear to do so. The simultaneous excitation of BVMSW and PSSW in Heusler alloy films as well as the characterization of their dynamic behaviors may be of interest for magnonic and spintronic applications.

Spin-Transfer Torque Switching at Ultra Low Current Densities

The influence of the tantalum buffer layer on the magnetic anisotropy of perpendicular Co-Fe-B/MgO based magnetic tunnel junctions is studied using magneto-optical Kerr-spectroscopy. Samples without a tantalum buffer are found to exhibit no perpendicular magnetization. The transport of Boron into the tantalum buffer is considered to play an important role on the switching currents of those devices. With the optimized layer stack of a perpendicular tunnel junction, a minimal critical switching current density of only 9.2 kA/cm$^2$ is observed. As of today, this value is the lowest reported value for current-induced magnetization reversal.

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.

Spin-Wave Modes in Exchange-Coupled FePt/FeNi Bilayer Films

A simple magnetic modulation structure of the exchange-coupling FePt/FeNi bilayer film is fabricated and studied for its magnetization dynamics using time-resolved magneto-optical polar Kerr spectroscopy. It is found that two spin-wave modes can be excited. One is fixed at ∼3.2 GHz in frequency for any external field and may serve as a frequency-stabilized spin-wave filter, while the other is external field dependent. In contrast, only the external field-dependent mode is excited in single-layer FeNi, supporting the localized origin of the mode at ∼3.2 GHz, which is confined to a thin exchange-coupling region. The other external field-dependent mode in frequency is attributed to the Kittel mode.

Femtosecond laser excitation of multiple spin waves and composition dependence of Gilbert damping in full-Heusler Co2Fe1−xMnxAl films

Spin-wave dynamics in 30 nm thick Co2Fe1−xMnxAl full-Heusler films is investigated using time-resolved magneto-optical polar Kerr spectroscopy under an external field perpendicular to films. Damon-Eshbach (DE) and the first-order perpendicular standing spin-wave (PSSW) modes are observed simultaneously in four samples with x = 0, 0.3, 0.7, and 1. The frequency of DE and PSSW modes does not apparently depend on composition x, but damping of DE mode significantly on x and reaches the minimum as x = 0.7. The efficient coherent excitation of DE spin wave exhibits the promising application of Co2Fe0.3Mn0.7Al films in magnonic devices.

Complete Permittivity Tensor in Sputtered CuFe₂O₄ Thin Films at Photon Energies between 2 and 5 eV.

This work is devoted to the systematic study of the optical and magneto-optical properties of sputter deposited CuFeO thin films in the photon energy region between 2 and 5 eV using spectroscopic ellipsometry and magneto-optical Kerr spectroscopy. The spectral dependence of both the diagonal and off-diagonal elements of the permittivity tensor is determined. A complete picture about the electron transitions in CuFeO is suggested in the frame of intervalence charge transfer and intersublattice charge transfer transitions. The effect of deposition conditions and post-deposition treatment in CuFeO films upon the optical and magneto-optical properties is discussed.

Spin waves and small intrinsic damping in an in-plane magnetized FePt film

Laser-induced spin-wave dynamics in an in-plane magnetized FePt film is studied using all-optical pump-probe magneto-optical Kerr spectroscopy under different external fields and pump fluences. Uniform precession spin wave is observed. Nonlinear external field dependence of its frequency is found and well explained by the macrospin model of uniform precession. The effective damping shows a significant external-field dependence. Calculation taking account for magnetic inhomogeneity fits the external-field dependence well, revealing main magnetic inhomogeneity origin of the extrinsic damping. An intrinsic Gilbert damping parameter of <0.028 is inferred and shows potential applications of this film in magnonics.

Structure and property changes and femtosecond laser-induced magnetization dynamics of two-year-old high coercive TbFeCo films

An uncoated TbFeCo film on glass substrate exposed to dry air for 2 years is studied using magneto-optical polar Kerr spectroscopy and vibrating sample magnetometry (VSM). The out-of-plane hysteresis loop by VSM is found to become slanted and meanwhile, an in-plane loop is also observable. The Kerr loop measured from the exposed surface is also slanted compared to that measured from the substrate-contacted surface, which indicates that the exposed surface is oxidized. So a bilayer structure of the film is proposed. When a pump laser is switched on, the Kerr loops measured from both surfaces at the delay time of −5ps become anomalous, showing the occurrence of magnetization reversal across magnetization compensation temperature. Unlike that measured from the substrate-contacted surface, femtosecond laser-induced magnetization dynamics measured from the exposed surface does not show magnetization reversal crossing zero magnetization. This can be explained by the bilayer structure as the compensation effect of demagnetization recovery of the oxidized layer on magnetization reversal of perpendicularly anisotropic TbFeCo layer across magnetization compensation temperature. Above experimental results show that the uncoated TbFeCo film cannot resist oxidation in dry air at room temperature for 2 years while the SiO2-coated surface can do so for over 2 years.

Effect of postgrowth oxygen exposure on the magnetic properties of Ni on the Cu-CuO stripe phase

The magnetism and morphology of thin Ni films deposited on the Cu(110)-Cu(110)-$(2\ifmmode\times\else\texttimes\fi{}1)$O surface (Cu-CuO stripe phase) have been studied by scanning tunneling microscopy (STM) and reflectance difference magneto-optical Kerr spectroscopy (RD-MOKE). The magnetic easy axis of the Ni films lies completely in plane up to a coverage of 22.5 ML. Exceeding this coverage, a small remanent magnetization component pointing out of plane evolves. Upon postgrowth oxygen exposure the Ni film becomes completely out-of-plane magnetized and the out-of-plane remanence and coercive field strongly increase during exposure. STM images reveal a fully $(2\ifmmode\times\else\texttimes\fi{}1)$O reconstructed topmost Ni layer after the oxygen exposure, but no morphological changes of the Ni film. We thus conclude that the oxygen-induced surface reconstruction strongly modifies the magnetic properties of the Ni film by enhancing the surface magnetic anisotropy.