Time-resolved Magneto-optical Kerr Effect Research Articles

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.

The thickness-dependent dynamic magnetic property of Co2FeAl films grown by molecular beam epitaxy

Co2FeAl films with different thickness were prepared at different temperature by molecular beam epitaxy. Their dynamic magnetic property was studied by the time-resolved magneto-optical Kerr effect measurements. It is observed that the intrinsic damping factor of Co2FeAl for [100] orientation is not related to the film's thickness and magnetic anisotropy as well as temperature at high-field regime, but increases with structural disorder of Co2FeAl. The dominant contribution from the inhomogeneous magnetic anisotropy is revealed to be responsible for the observed extremely nonlinear and drastic field-dependent damping factors at low-field regime.

Pump-probe measurements of the thermal conductivity tensor for materials lacking in-plane symmetry.

We previously demonstrated an extension of time-domain thermoreflectance (TDTR) which utilizes offset pump and probe laser locations to measure in-plane thermal transport properties of multilayers. However, the technique was limited to systems of transversely isotropic materials studied using axisymmetric laser intensities. Here, we extend the mathematics so that data reduction can be performed on non-transversely isotropic systems. An analytic solution of the diffusion equation for an N-layer system is given, where each layer has a homogenous but otherwise arbitrary thermal conductivity tensor and the illuminating spots have arbitrary intensity profiles. As a demonstration, we use both TDTR and time-resolved magneto-optic Kerr effect measurements to obtain thermal conductivity tensor elements of <110> α-SiO2. We show that the out-of-phase beam offset sweep has full-width half-maxima that contains nearly independent sensitivity to the in-plane thermal conductivity corresponding to the scanning direction. Also, we demonstrate a Nb-V alloy as a low thermal conductivity TDTR transducer layer that helps improve the accuracy of in-plane measurements.

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.

Tunable magnetization dynamics in disordered FePdPt ternary alloys: Effects of spin orbit coupling

The magnetization dynamics of disordered Fe0.5(Pd1−xPtx)0.5 alloy films was studied by time-resolved magneto-optical Kerr effect and ferromagnetic resonance. The intrinsic Gilbert damping parameter α0 and the resonance linewidth change linearly with the Pt atomic concentration. In particular, the induced in-plane uniaxial anisotropy constant KU also increases for x increasing from 0 to 1. All these results can be attributed to the tuning effect of the spin orbit coupling. For the disordered ternary alloys, an approach is proposed to control the induced in-plane uniaxial anisotropy, different from conventional thermal treat methods, which is helpful to design and fabrications of spintronic devices.

Spin wave non-reciprocity and beating in permalloy by the time-resolved magneto-optical Kerr effect

We studied the propagation characteristics of spin wave modes in a permalloy stripe by time-resolved magneto-optical Kerr effect techniques. We observed a beating interference pattern in the time domain under the influence of an electrical square pulse excitation at the centre of the stripe. We also probed the non-reciprocal behaviour of propagating spin waves with a dependence on the external magnetic field. Spatial dependence studies showed that localized edge mode spin waves have a lower frequency than the spin waves at the centre of the stripe, due to the varying magnetization vector across the width of the stripe.

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.

Local control of magnetic damping in ferromagnetic/non-magnetic bilayers by interfacial intermixing induced by focused ion-beam irradiation

The influence of interfacial intermixing on the picosecond magnetization dynamics of ferromagnetic/non-magnetic thin-film bilayers was studied. Low-dose focused-ion-beam irradiation was used to induce intermixing across the interface between a 10 nm Ni81Fe19 layer and a 2–3 nm capping layer of either Au or Cr. Time-resolved magneto-optical Kerr effect was used to study magnetization dynamics as a function of ion-beam dose. With an Au cap, the damping of the un-irradiated bilayer was comparable with native Ni81Fe19 and increased with increasing ion dose. In contrast, for Ni81Fe19/Cr the damping was higher than that for native Ni81Fe19, but the damping decreased with increasing dose.

Ultrafast magnetization dynamics in Co-based Heusler compounds with tuned chemical ordering

We have studied thin film samples of FeSi and MnSi with different degrees of chemical ordering using the time-resolved magneto-optical Kerr effect to elucidate the influence of defects in the crystal structure on magnetization dynamics. Surprisingly, we find that the presence of defects does not influence the optically induced magnetization dynamics on the ultrashort timescale (some 100 fs). However, we observe a second demagnetization stage with a timescale of tens of picoseconds in MnSi for low chemical ordering; that is, a large number of defects. We interpret this second demagnetization step as originating from scattering of mostly thermalized majority electrons into unoccupied minority defect states.

Spin-orbit interaction tuning of perpendicular magnetic anisotropy in L1 FePdPt films

The dependence of perpendicular magnetic anisotropy Ku on spin-orbit coupling strength ξ is investigated in L10 ordered FePd1−xPtx films by time-resolved magneto-optical Kerr effect measurements and ab initio density functional calculations. Continuous tuning of Ku over a wide range of magnitude is realized by changing the Pt/Pd concentration ratio, which strongly modifies ξ but keeps other leading parameters affecting Ku nearly unchanged. Ab initio calculations predict a nearly quadratic dependence of Ku on ξ, consistent with experimental data. Ku increases with increasing chemical order and decreasing thermal spin fluctuations, which becomes more significant for samples with higher Pt concentration. The results demonstrate an effective method to tune Ku utilizing its sensitivity on ξ, which will help fabricate magnetic systems with desirable magnetic anisotropy.

Annealing effect on the electron spin dynamics in heavily Mn-doped (Ga,Mn)As

A temperature- and photo-excitation-density-dependent electron spin dephasing process has been studied by time-resolved magneto-optical Kerr effect (TR-MOKE) measurements for heavily-doped (Ga,Mn)As after appropriate annealing treatment. Unlike the as-grown (Ga,Mn)As in which s-d scattering is dominant for spin dephasing at low temperature, the exchange scattering induced by Mn ions is efficiently suppressed after annealing. For annealed (Ga,Mn)As, the p-d exchange coupling proves to be important for the electron spin dephasing process. Moreover, the Coulomb scattering arising from the weakly-localized holes and Mn impurities is revealed to play an important role in the electron spin dephasing after annealing. Our results demonstrate that the impurityinduced disorder plays a significant role in the electron spin-dephasing process in (Ga,Mn)As when Mn is over doped by as much as 15%, which is a critical issue that needs to be considered to achieve high-quality (Ga,Mn)As thin films with a higher Curie temperature TC and better functionality.

Dynamic separation of nanomagnet sublattices by orientation of elliptical elements

We report the separation of the magnetization dynamics of densely packed nanomagnets depending on their orientation. The arrays consist of interleaved sublattices of identical nickel elliptical disks. By controlling the orientation of the elliptic disks relative to the external field in each sublattice, we simultaneously analyzed the magnetization dynamics in each sublattice using a time-resolved magnetooptic Kerr effect (TR-MOKE) microscopy system. The Fourier spectra showed clearly separated precession modes for sublattices with different orientations. The spectra were shown to be robust against the error in applied field orientation. The sublattice response can be tuned to a single collective frequency by choosing a symmetric field orientation. We analyzed the effect of the interelement coupling with various spacing between nanomagnets and found a relatively weak dependence on dipolar interactions in good agreement with micromagnetic simulations.

Laser induced spin precession in highly anisotropic granular L1 FePt

The dynamic magnetic properties of a highly anisotropic, granular L10 FePt thin film in magnetic fields up to 7 T are investigated using time-resolved magneto-optical Kerr effect measurements. We find that ultrashort laser pulses induce coherent spin precession in the granular FePt sample. Frequencies of spin precession up to over 400 GHz are observed, which are strongly field and temperature dependent. The high frequencies can be ascribed to the high value of the magnetocrystalline anisotropy constant Ku leading to large anisotropy fields Ha of up to 10.7 T at 170 K. A Gilbert damping parameter of α ∼ 0.1 was derived from the lifetimes of the oscillations.

Photoinduced spin angular momentum transfer into an antiferromagnetic insulator

Spin angular momentum transfer into an antiferromagnetic (AFM) insulator is observed in a single-crystalline Fe/CoO/MgO(001) heterostructure by time-resolved magneto-optical Kerr effect. The transfer process is mediated by the Heisenberg exchange coupling between Fe and CoO spins. Spin angular momentum transfer to ordered AFM spins is independent of the external magnetic field and enhances the spin precession damping in Fe, which remains nearly invariant with temperature.

Magnetization dynamics for L1 MnGa/Fe exchange coupled bilayers

The precessional magnetization dynamics of L10 MnGa/Fe bilayers with a large perpendicular magnetic anisotropy are investigated using vector network analyzer ferromagnetic resonance (VNA-FMR) and time-resolved magneto-optical Kerr effect (TRMOKE). The MnGa/Fe(1 nm) bilayer exhibited perpendicular magnetization and MnGa/Fe(2 and 5 nm) bilayers showed in-plane magnetizations. The VNA-FMR and TRMOKE data for these bilayers are well explained from calculations, based on the coupled Landau-Lifshitz equations, taking into account the interfacial exchange coupling of 2.4 erg/cm2.

Ultrafast demagnetization enhancement in CoFeB/MgO/CoFeB magnetic tunneling junction driven by spin tunneling current

The laser-induced ultrafast demagnetization of CoFeB/MgO/CoFeB magnetic tunneling junction is exploited by time-resolved magneto-optical Kerr effect (TRMOKE) for both the parallel state (P state) and the antiparallel state (AP state) of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two CoFeB layers via the tunneling of hot electrons through the MgO barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electrons tunneling current. It opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions.

Magnetic and Gilbert damping properties of L21-Co2FeAl film grown by molecular beam epitaxy

Co2FeAl film with L21 structure was prepared. Its magnetic and Gilbert damping properties were studied by ferromagnetic resonance (FMR) and time-resolved magneto-optical Kerr effect (TR-MOKE), respectively. It is observed that the apparent damping parameter decreases drastically with increasing magnetic field at low field regime and eventually becomes a constant value of 0.004 at high field regime by TR-MOKE measurements. A Gilbert damping parameter of 0.008 in the hard axis by FMR measurement has also been obtained, which is comparable with that extracted from TR-MOKE measurements at low external field, indicating the extrinsic damping processes involved in the low field regime.

Fast magnetization precession for perpendicularly magnetized MnAlGe epitaxial films with atomic layered structures

Epitaxial growth and magnetization precessional dynamics for tetragonal MnAlGe films are investigated. The films are grown on MgO (100) with c axis parallel to the film normal and well-ordered layered structures. The film exhibits rectangular hysteresis loop with perpendicular magnetic anisotropy constant of 4.7 Merg/cm3 and saturation magnetization of 250 emu/cm3. Magnetization precession with precession frequency of ∼100 GHz is observed by time-resolved magneto-optical Kerr effect. Further, the Gilbert damping constant is found to be less than ∼0.05, which is much larger than that obtained using the first principles calculations.

The Study of the Delay Kerr Loops Induced by Femtosecond Laser Pulses in GdFeCo Film

To study the evolution of the coercivity and magnetization induced by femtosecond laser pulses in GdFeCo film at distinct pump–probe delays and reveal the laser-induced magnetization reversal process, the static delay Kerr loops were measured by the traditional time-resolved magneto-optical Kerr effect technique. The results showed that the loops contained memory effect from external field history and could not reflect the real change of coercivity and magnetization after laser pump. To eliminate the memory effect, a home-made experimental setup of photomagnetic synchronized time-resolved magneto-optical Kerr effect technology was used to measure the dynamic delay Kerr loops and reveal the real evolution of the coercivity and magnetization after laser pump, whose results are of great importance in real magneto-optical recording application.

Correlation Between Ultrafast Demagnetization Process and Gilbert Damping in Amorphous TbFeCo Films

Ultrafast spin dynamics in Tbx(FeCo)1-x films is studied using the time-resolved magneto-optical Kerr effect. At pump fluence of 1.0 mJ/cm2 the ultrafast demagnetization processes are accomplished by one-step and two-step decays (the first fast and second slow decays) for low and high Tb contents, respectively. For the low Tb contents (x <; 0.08), the demagnetization time is almost a constant (~ 600 fs), which is similar with the demagnetization time (τ1) of the first fast decay for the high Tb contents due to bleaching effect. However, the following slow demagnetization time (τ2) for the high Tb contents strongly depends on the Tb contents. It increases with increasing Tb contents, and reaches a maximum of 3.0 ps at x=0.21, then decreases with Tb content further increasing. The distinguished slow decay process is suggested to ascribe to the Gilbert damping factor.