Magneto-optical Kerr Effect Measurements Research Articles

Ultrafast Magnetization Precession in Perpendicularly Magnetized L10-MnAl Thin Films with Co2MnSi Buffer Layers**Supported by the National Key R&D Program of China (Grant No. 2018YFB0407601), the Key Research Project of Frontier Science of Chinese Academy of Sciences (Grant Nos. QYZDY-SSW-JSC015 and XDPB12), and the National Natural Science Foundation of China (Grant Nos. 11834013, 11874349, and 11774339).

Perpendicularly magnetized L10-MnAl thin films with Co2MnSi buffer layers were prepared on GaAs (001) substrates by molecular-beam epitaxy (MBE). The samples with high crystalline quality show a maximum uniaxial perpendicular magnetic anisotropy constant of 1.4 × 107 erg/cm3. Ultrafast spin dynamics with a magnetization precession frequency up to 200 GHz was investigated by using time-resolved magneto-optical Kerr effect (TRMOKE) measurements, from which the Gilbert damping constant α of epitaxial L10-MnAl thin films is evaluated to be less than 0.0175. This work provides an important reference for analyzing the current-induced magnetization switching process in MnAl-based spintronic devices.

Magneto-optical Kerr effect in a non-collinear antiferromagnet Mn3Ge

Non-collinear antiferromagnet Mn3Sn is a functional material that exhibits the magneto-optical Kerr effect (MOKE) as well as the anomalous Hall effect, arising from the nonvanishing Berry curvature in the broken time-reversal symmetry lattice system likewise the ferromagnets. Mn3Ge, isostructural to Mn3Sn, is also expected to exhibit a similar MOKE but has not been demonstrated yet. In this study, we performed the MOKE measurement in a Mn3Ge single crystal and obtained a large polar MOKE signal (∼8.2 mdeg) and a longitudinal MOKE signal (∼5.6 mdeg). Furthermore, by applying hyperfine polishing and annealing to the surface, we rebuilt a homogenous surface and largely improved the reproducibility of the MOKE signal in Mn3Ge.

Polarization-dependent magnetism of the Ni/BaTiO3 interface

We explore the cross coupling between the ferroelectric and ferromagnetic phases in $\mathrm{Ni}/\mathrm{BaTi}{\mathrm{O}}_{3}$(001) heterostructures and demonstrate the modulation of the magnetism and incidence of exchange bias in the ultrathin metallic Ni overlayer, depending on the ferroelectric state of the bottom layer. We establish that 5-nm-thick monocrystalline Ni film deposited on $\mathrm{BaTi}{\mathrm{O}}_{3}$ with ferroelectric polarization pointing towards the surface (P+) favors the organization of Ni into uniform ferromagnetic domains. Ni grown on $\mathrm{BaTi}{\mathrm{O}}_{3}$ with opposite ferroelectric polarization is featured by emerging exchange-bias coupling between the ferromagnetic Ni top layers and the antiferromagnetic reacted interface, as theoretically explained by first-principles calculations. We explicitly obtain the morphology of the magnetic domains of the crystalline Ni layer in atomic and magnetic force microscopy measurements (AFM/MFM). The resemblance of AFM and MFM images indicate that, although with radically different morphologies, in both cases all spins orient in the Ni plane. Consequently, the distinct signature of the ferroelectric-ferromagnetic coupling extracted from the magneto-optical Kerr effect measurements encodes all the information of sample magnetism. The peculiar magnetic coupling depending on the ferroelectric state indicates new ways of engineering the functionality of metal/ferroelectric interfaces.

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.

Enhanced electrical switching of magnetism in the artificial multiferroic CoFeB/Pb(Mg1/3Nb2/3)O3–PbTiO3 heterostructure

Multiferroic heterostructure CoFeB/Pb(Mg1/3Nb2/3)O3–PbTiO3 is prepared by depositing CoFeB films on Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystal through direct current magnetron sputtering. The electric field induced magnetism change of CoFeB film was studied by magneto-optical Kerr effect measurement. The magnetic hard/easy axis change was investigated under direct current electric field. Alternative current voltage magneto-optical Kerr effect test indicates the different magnetoelectric mechanism with the change of film thickness. The microwave frequency tunability was observed under the electric field and the tunable ferromagnetic resonance field reaches 190 Oe. Our study provides great opportunities for the application of electric-field tunable information storage 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.

Evolutions of acoustic and optical mode resonances in the spin reorientation Permalloy film

We investigate the static and dynamic magnetic properties of a wedge Permalloy film. The spin distribution of the wedge film depends on the film thickness, ranging from the homogeneous in-plane to the out-of-plane component. The spin reorientation region of the wedge film is determined by using a magnetic force microscopy and magneto-optical Kerr effect measurements as well as the micromagnetic simulation. The dynamic properties of the spin distribution depending on the thickness are investigated by the microwave method. The acoustic and optical mode resonances of the wedge film are controlled by changing the direction of the microwave magnetic field and the stripe domains. The resonance frequency could be influenced by its domain structure and reveals the different anisotropies of the film. The results show that the spin distribution of the stripe domain depending on the thickness can be used to control the spin dispersion and precession frequency of the film for high-frequency applications.

Temperature induced interface roughness and spin reorientation transition in Co/Au multilayers thin films

Spin reorientation transition (SRT) due to high temperature has been studied in Co/Au multilayered thin films. The effect of temperature on structural and magnetic properties of the thin films has been investigated. Film thickness and interface roughness has been studied by measuring x-ray reflectivity. Multilayers are thermodynamically stable, and thickness of the layers and interface roughness remains unchanged over longer periods of time as well as up to a temperature of 300 °C. An increase in interface roughness at higher temperatures indicates intermixing of Co and Au into each other. Behavior of coercive field for both the samples is quite different. However, the spin reorientation transition occurred in both the samples around 350 °C. Here we are reporting our study on temperature induced spin reorientation transition in Co/Au multilayers. A spin reorientation transition is observed from out-of-plane to in-plane direction for 12 Å thick Co layer sample at 350 °C and a reverse transition is observed for 25 Å thick Co layer sample. Results were obtained using ex situ and in situ Magneto-Optic Kerr Effect (MOKE) measurements.

Three-dimensional frequency- and phase-multiplexed magneto-optical microscopy.

We describe a new approach to scanning magneto-optical Kerr effect (MOKE) microscopy in which two opto-mechanical choppers modulate the spatial profile of a probe laser beam to separately encode all three magnetization components at different frequencies and phases in one signal. We demonstrate this multiplexed technique in two representative regimes: the equilibrium and non-equilibrium response of a magnetic vortex to a changing magnetic field. We observe the translation of the vortex state in equilibrium and the spiraling gyrotropic trajectory of the vortex position out of equilibrium. We compare the results to a traditional MOKE measurement and to micromagnetic simulations. We find that the multiplexed method presented here provides better agreement with simulation than previous methods and equal or better signal-to-noise ratio.

Hump-like structure in Hall signal from ultra-thin SrRuO3 films without inhomogeneous anomalous Hall effect

A controversy arose over the interpretation of the recently observed hump features in Hall resistivity ρxy from ultra-thin SrRuO3 (SRO) film; it was initially interpreted to be due to topological Hall effect but was later proposed to be from existence of regions with different anomalous Hall effect (AHE). In order to settle down the issue, we performed Hall effect as well as magneto-optic Kerr-effect measurements on 4 unit cell SRO films. Clear hump features are observed in ρxy, whereas neither hump feature nor double hysteresis loop is seen in the Kerr rotation which should be proportional to the magnetization. In addition, magnetization measurement by superconducting quantum interference device shows no sign of multiple coercive fields. These results show that inhomogeneous AHE alone cannot explain the observed hump behavior in ρxy data. We suggest that emergence of the hump structure in ρxy is closely related to the growth condition.

Ultrathin Chromia on a Hexagonally-Ordered d0 Ferromagnet: Evidence of Interfacial Exchange Bias at the Cr2O3/TiO2-x Interface.

Heterostructures consisting of 10 Å thick chromia films and 50 Å thick titania films display significant exchange bias at and above room temperature. Chromia films ∼10 Å thick were deposited by molecular beam epitaxy (MBE) of Cr at room temperature in ultrahigh vacuum on 50 Å thick TiO2-x(111) films (x < 0.3) also deposited epitaxially by MBE on Al2O3(0001). Cr deposition yields increased Ti(III) formation in the titania substrate and the formation of a Cr2O3 overlayer, without Cr/Ti interfacial mixing, as determined by in situ photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS). In situ low-energy electron diffraction (LEED) and XPS data indicate that the chromia overlayer is hexagonally ordered and ∼10 Å thick. Longitudinal and polar magneto-optic Kerr effect (MOKE) measurements at 285-315 K provide evidence of strong exchange bias between the boundary layer magnetization of chromia and the ferromagnetic substrate. These data demonstrate the robust room-temperature interaction of the boundary layer magnetization of a multiferroic antiferromagnet with a d0 ferromagnetic substrate.

Enhanced magneto-optical activity of multilayer thin films: a novel platform for magnetic nanoparticle sensing

In this study, we present a novel platform based on the measurement of magneto-optic Kerr effect of a magnetic multilayer thin film sensor with perpendicular magnetic anisotropy, to achieve the quantitative analysis of magnetic nanoparticles (MNPs). Microliter droplets include a varied amount of MNPs located on the sensor surface to perform a systematic investigation of sensing mechanisms. The interaction between the sensor and MNPs, which must be well understood to enhance the sensitivity, is investigated through experiments and micromagnetic simulations. The simulations, in accordance with the experiments, allow us to extract the detection limit of our sensor. We anticipate that our study provides a rational basis for highly sensitive, single-use and precise sensing of MNPs, and will influence future discussions on MNP–magnetic thin film interactions.

The magnetic reversal characteristics of 32-bit composite element magnetic barcodes

Magnetic barcodes containing 32 composite element bits have been produced and measured in order to optimize the design of magnetic microcarriers. Focused magneto-optic Kerr effect measurements allow the determination of the change in magnetic hysteresis when the width of magnetic elements is varied between bits, and the electron beam lithography used in production is confirmed to be accurate to ∼6 nm using scanning electron microscopy. The sharp magnetic switching observed, an important prerequisite for a functioning device, is attributed to the expected dipolar interactions between magnetic elements and the use of magnetically soft Permalloy. A crossover between two magnetic reversal behaviors is discovered when the magnetic elements are ∼200 nm wide. From these measurements, 12 bits were selected on which data can be written with a low probability of error, with the prospect of the other 20 bits being employed for error correction. We have therefore developed a nonvolatile magnetic memory on which 4096 unique codes can be programmed.

Mössbauer and magneto-optical Kerr effect study of polycrystalline gadolinium iron garnet film prepared by spin-coating technique

Polycrystalline gadolinium iron garnet (Gd3Fe5O3) film is prepared by spin-coating technique and subsequent annealing. The structural and magnetic measurements of the prepared films are reported in the present work. Room temperature magneto-optic Kerr effect measurements in longitudinal and polar configuration indicate the oblique magnetic anisotropy. Quantitative information about the oblique magnetic anisotropy is estimated from the conversion electron 57Fe Mössbauer spectroscopy measurements. Signature of magnetic compensation is observed from temperature dependent bulk magnetization measurements.

Evolution of magnetic anisotropy in cobalt film on nanopatterned silicon substrate studied in situ using MOKE

Evolution of magnetization behaviour of cobalt film on nano patterned silicon (1 0 0) substrate, with film thickness, has been studied. In situ magneto-optical Kerr effect measurements during film deposition allowed us to study genuine thickness dependence of magnetization behaviour, all other parameters like surface topography, deposition conditions remaining invariant. The nanopatterned substrate had a 8 nm thick amorphous silicon nitride surface layer. Thus, induced magnetic anisotropy in the Co film is expected to be of purely topographical origin, excluding any effect of the single crystalline Si (1 0 0) surface on magnetic anisotropy film due to possible epitaxy or surface step-induced anisoropy. In conformity with earlier results, the film induces a uniaxial magnetic anisotropy in the film, with its magnitude decreasing with increasing thickness. A magnetically dead layer of 2.6 nm forms at the interface with the substrate. Analysis of the magnetic layer thickness dependence of anisotropy shows that it has contributions from both, i) exchange energy which is volume dependent and, ii) stray dipolar fields at the surface/interface. This suggests that local magnetization follows only partially the topography of the rippled surface. As expected from energy considerations, for small film thickness, the local magnetization closely follows the surface contour of the ripples making the volume term as the dominant contribution. With increasing film thickness, the local magnetization gradually deviates from the local slope and approaches towards a uniform magnetization along the macroscopic film plane making the surface term as the dominant contribution. Significant deviation of the observed stray dipolar field contribution from the anisotropy energy calculated on the basis of Schlomann’s theory is observed, which can be attributed to several factors like, deviation of surface topography from an ideal sinusoidal wave, breaks of continuity along the ripple direction, topological defects like, pattern dislocations, and small film thicknesses compared to the correlation length of surface corrugations.

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.

Development of residual stress and uniaxial magnetic anisotropy during growth of polycrystalline Co film

Growth of e-beam evaporated Co films on Si (001)/SiO2 substrate has been studied using real-time multi-beam optical stress sensor (MOSS), four probe resistivity (FPR) and in-situ magneto-optic Kerr effect (MOKE) measurements. While MOSS and FPR provide information about internal stress and film morphology, MOKE provides information about magnetic properties, thus making it possible to correlate the evolution of film stress and morphology with that of uniaxial magnetic anisotropy (UMA) in the film. Film grows via Volmer–Weber mechanism, where islands grow larger to impinge with other islands and eventually coalesce into a continuous film at around 10 nm thickness. Development of tensile stress in the film is found to be associated with the island coalescence process. The film exhibits a well-defined UMA, which varies synchronously with internal tensile stress developed during growth, suggesting that the observed UMA originates from minimization of magnetoelastic and morphological anisotropic energies in the presence of internal stress. Absence of UMA in Co film having Ag underlayer in the form of islands is understood in terms of random short-range stress in the film caused by arbitrary pinning by the Ag islands.

Varying steps in hysteresis loops of Co square nano-frames

Abstract Magnetic nanostructures are investigated nowadays for diverse applications as well as in basic research. They often show unexpected magnetization reversal processes which are not only of high interest for understanding magnetization reversal processes on the nano-scale, but also due to their possible use in magneto-electronic devices, spintronics, neuromorphic computing and other areas. Square nano-frames prepared from iron, e.g., belong to the magnetic nano-particles exhibiting additional stable states at remanence which can be used for quaternary storage devices. The reason for this study is that nano-particles prepared from permalloy or nickel with their very low magneto-crystalline anisotropy do usually not show steps in the hysteresis loop and corresponding stable intermediate state, while the magneto-crystalline anisotropy of cobalt often dominates over the shape-anisotropy, resulting in a large number of steps which are not always stable. Here we report on experimental investigations by magneto-optic Kerr effect (MOKE) and micromagnetic simulations of square Co nano-frames, exhibiting steps along the slopes of the hysteresis loops. The positions and numbers of these steps varied during measurements without intentionally changing the experimental setup. Micromagnetic simulations were carried out to explain the experimental findings which could be attributed to different Co crystallite orientations, resulting in a modified magneto-crystalline anisotropy and thus different hysteresis loops for smallest variations of the laser beam position on the samples during MOKE measurements.