Brillouin Light Scattering Measurements Research Articles

Diagnostic potential of blood plasma longitudinal viscosity measured using Brillouin light scattering

Blood plasma viscosity (PV) is an established biomarker for numerous diseases. Measurement of the shear PV using conventional rheological techniques is, however, time consuming and requires significant plasma volumes. Here, we show that Brillouin light scattering (BLS) and angle-resolved spectroscopy measurements of the longitudinal PV from microliter-sized plasma volumes can serve as a proxy for the shear PV measured using conventional viscometers. This is not trivial given the distinct frequency regime probed and the longitudinal viscosity, a combination of the shear and bulk viscosity, representing a unique material property on account of the latter. We demonstrate this for plasma from healthy persons and patients suffering from different severities of COVID-19 (CoV), which has been associated with an increased shear PV. We further show that the additional information contained in the BLS-measured effective longitudinal PV and its temperature scaling can provide unique insight into the chemical constituents and physical properties of plasma that can be of diagnostic value. In particular, we find that changes in the effective longitudinal viscosity are consistent with an increased suspension concentration in CoV patient samples at elevated temperatures that is correlated with disease severity and progression. This is supported by results from rapid BLS spatial-mapping, angle-resolved BLS measurements, changes in the elastic scattering, and anomalies in the temperature scaling of the shear viscosity. Finally, we introduce a compact BLS probe to rapidly perform measurements in plastic transport tubes. Our results open a broad avenue for PV diagnostics based on the high-frequency effective longitudinal PV and show that BLS can provide a means for its implementation.

Magnonic notch filter based on spin wave caustic beams

Here, we study a magnonic crystal made of low-damping yttrium iron garnet that utilizes pseudo-caustic spin wave beams generated from subwavelength square well features arranged in a two-dimensional array. The lattice symmetry and the angle between the caustic beam propagation direction and the applied magnetic field were tailored to optimize the interaction of spin waves with the engineered defects. A prominent, narrow 3 MHz feature with large rejection efficiency is observed in the spin wave transmission spectrum that could be useful as a narrowband notch filter, and time- and space-resolved Brillouin light scattering (BLS) measurements suggest that both caustic interference and edge effects may contribute to this notch feature. Furthermore, the BLS measurements show that caustics are generated efficiently at the laser ablated wells, and by tuning the frequency by 30 MHz, the caustic beam angles and, hence, the details of how the spin wave caustics hit the ablated wells change sufficiently to add and remove caustic beams, which can be used to create additional device functionality. The generation and conversion of caustic beams show promise for applications that require directional energy transport and for magnonic devices.

Magnetic properties of Pt/Co/Pt trilayers with W insert layer

The results of magnetic investigations of epitaxial trilayers Pt/Co/Pt asymmetrically modified by inserting W layer at the bottom or top Co interfaces in the wide ranges of Co and W layer thicknesses are reported. The samples were epitaxially grown on Pt buffer in double wedge geometry: the dCo thickness gradient of the continuous Co wedge was orthogonal to the dW thickness gradient of non-magnetic W underlayer (overlayer) deposited either as step-like or continuous wedge. Resulting samples have Pt/W/Co/Pt (Pt/Co/W/Pt) stacking sequences. The influence of (dCo, dW) on static magnetization reversal processes were studied using magnetooptical Kerr effect; Brillouin light scattering (BLS) method was applied for dynamical characterization. The magnetic dead layer thickness d0 abruptly increases until dW reaches ∼ 0.5 nm for both orderings. Then its value saturates for Pt/Co/W/Pt, while for Pt/W/Co/Pt samples a slow growth is observed. For dCo corresponding to out-of-plane magnetization the inserting of W layer leads to the strong reduction in coercivity (two orders of magnitude) and transition to in-plane magnetization in the case of Pt/W/Co/Pt ordering, while the Pt/Co/W/Pt samples demonstrates weak changes of coercivity with small change in magnetic anisotropy. The strength of interfacial Dzyaloshinskii-Moriya interaction (iDMI) and spin wave damping for selected dCo thicknesses as a function of dW were derived from BLS measurements. Inserting W with thickness dW ∼ 0.1 nm induces significant iDMI changes, iDMI saturates for dW > 0.4 nm. Our findings demonstrate the efficiency of thin W interlayer on modification of magnetic parameters in Pt/Co/Pt trilayer.

Control of Skyrmion Chirality in Ta/Fe−Co−B/TaOx Trilayers by TaOx Oxidation and Fe−Co−B Thickness

Skyrmions are magnetic bubbles with nontrivial topology envisioned as data bits for ultrafast and power-efficient spintronic memory and logic devices. They may be stabilized in heavy-metal/ferromagnetic/oxide trilayer systems. The skyrmion chirality is then determined by the sign of the interfacial Dzyaloshinskii-Moriya interaction (DMI). Nevertheless, for apparently identical systems, there is some controversy about the DMI sign. Here, we show that the degree of oxidation of the top interface and the thickness of the ferromagnetic layer play a major role. Using Brillouin light-scattering measurements in $\mathrm{Ta}/\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}/{\mathrm{Ta}\mathrm{O}}_{x}$ trilayers, we demonstrate a sign change of the DMI with the degree of oxidation of the $\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}/{\mathrm{Ta}\mathrm{O}}_{x}$ interface. Using polar magneto-optical Kerr effect microscopy, we consistently observe a reversal of the direction of current-induced motion of skyrmions with the oxidation level of ${\mathrm{Ta}\mathrm{O}}_{x}$; this is attributed to their chirality reversal. In addition, a second chirality reversal is observed when changing the $\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}$ thickness, probably due to the proximity of the two $\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}$ interfaces in the ultrathin case. By properly tuning the chirality of the skyrmion, spin-transfer and spin-orbit torques combine constructively to enhance the skyrmion velocity. These observations thus allow us to envision an optimization of the material parameters to produce highly mobile skyrmions. Moreover, this chirality control enables a versatile manipulation of skyrmions and paves the way towards multidirectional devices.

A Brillouin light scattering study of the spin-wave magnetic field dependence in a magnetic hybrid system made of an artificial spin-ice structure and a film underlayer

We present a combined Brillouin light scattering (BLS) and micromagnetic simulation investigation of the magnetic-field-dependent spin-wave spectra in a hybrid structure made of permalloy (NiFe) artificial spin-ice (ASI) systems, composed of stadium-shaped nanoislands, deposited on the top of an unpatterned permalloy film with a nonmagnetic spacer layer. The thermal spin-wave spectra were recorded by BLS as a function of the magnetic field applied along the symmetry direction of the ASI sample. Magneto-optic Kerr effect magnetometry was used to measure the hysteresis loops in the same orientation as the BLS measurements. The frequency and the intensity of several spin-wave modes detected by BLS were measured as a function of the applied magnetic field. Micromagnetic simulations enabled us to identify the modes in terms of their frequency and spatial symmetry and to extract information about the existence and strength of the dynamic coupling, relevant only to a few modes of a given hybrid system. Using this approach, we suggest a way to understand if the dynamic coupling between ASI and film modes is present or not, with interesting implications for the development of future three-dimensional magnonic applications and devices.

Direct probing of strong magnon–photon coupling in a planar geometry

We demonstrate direct probing of strong magnon–photon coupling using Brillouin light scattering (BLS) spectroscopy in a planar geometry. The magnonic hybrid system comprises a split-ring resonator loaded with epitaxial yttrium iron garnet thin films of 200 nm and 2.46 μm thickness. The BLS measurements are combined with microwave spectroscopy measurements where both biasing magnetic field and microwave excitation frequency are varied. The cooperativity for the 200 nm-thick YIG films is 1.1, and larger cooperativity of 29.1 is found for the 2.46 μm-thick YIG film. We show that BLS is advantageous for probing the magnonic character of magnon–photon polaritons, while microwave absorption is more sensitive to the photonic character of the hybrid excitation. A miniaturized, planar device design is imperative for the potential integration of magnonic hybrid systems in future coherent information technologies, and our results are a first stepping stone in this regard. Furthermore, successfully detecting the magnonic hybrid excitation by BLS is an essential step for the up-conversion of quantum signals from the microwave to the optical regime in hybrid quantum systems.

Strong interfacial Dzyaloshinskii–Moriya induced in Co due to contact with NiO

The magnetic properties of NiO/Co/Pt as a function of Co layer thickness were investigated by polar magneto-optical Kerr effect (PMOKE) (magnetometry and microscopy) and Brillouin Light Scattering (BLS) spectroscopy. PMOKE measurements revealed strong surface anisotropy (1.8 mJ/m2) favoring perpendicular magnetic anisotropy and asymmetric domain wall propagation explained by anticlockwise chirality. BLS measurements show that this chirality is induced by strong interfacial Dzyaloshinskii–Moriya interaction (+ 2.0 pJ/m). This is one of the highest values reported so far for Co layers surrounded by different layers. The observed chirality is opposite to what has been found in Co/oxide interfaces. These results and data published earlier, indicate that the strength of interfacial Dzyaloshinskii–Moriya interaction increases with the amount of stoichiometric NiO. Therefore, this work shows that NiO is the source of the interfacial Dzyaloshinskii–Moriya interaction.

Magnetic ordering in epitaxial ultrathin Pt/W/Co/Pt layers

We discuss the magnetic properties of epitaxial Pt/W(dW)/Co(dCo)/Pt layered films as a function of W(dW) and Co(dCo) layer thicknesses. The samples were investigated by means of: (i) polar magnetooptical Kerr effect based magnetometry and microscopy as well as (ii) Brillouin light scattering (BLS) spectrometry. The bottom W layer exhibits a strong influence on the magnetic anisotropy, dCo-dependent spin reorientation transition and magnetic polarization of atoms forming the interfaces. The areas with (i) in-plane, (ii) out-of-plane, (iii) superparamagnetic with out-of-plane anisotropy, and (iv) nonmagnetic states appear with dCo decrease. The dW driven transition from hard to soft magnetic material with out-of-plane magnetization was found. The observed domain structures (micrometer range bubbles, stripes and dendritic-like) at out-of-plane state are very sensitive to dCo, dW thicknesses and external magnetic field. The presence of Dzyaloshinskii-Moriya interaction was deduced from BLS measurements.

Large Dzyaloshinskii-Moriya interaction and room-temperature nanoscale skyrmions in CoFeB/MgO heterostructures

Magnetic skyrmions in heavy metal (HM)/CoFeB/MgO structures are of particular interest for skyrmion-based magnetic tunnel junction (MTJ) devices because of their reliable generation, stability, and readout through purely electrical methods. To optimize the properties, such as stability, a strong Dzyaloshinskii-Moriya interaction (DMI) is required at room temperature. Here, using first-principles calculations, we demonstrate that huge DMI can be obtained in Ir/CoFe structures with an Fe-terminated configuration. Moreover, Brillouin light-scattering measurements show that indeed Ta/Ir/Co 20 Fe 60 B 20 /MgO thin films with perpendicular magnetic anisotropy exhibit a large DMI value (1.13 mJ/m 2 ) when the thickness of the CoFeB layer is 1.1 nm, which can be attributed to the smooth and Fe-rich interface between Ir and CoFeB layers. Furthermore, we observe stable sub-100 nm magnetic skyrmions at room temperature in the Ir/CoFeB/MgO systems by magnetic force microscope. This work paves the way for promoting the application of skyrmions in CoFeB/MgO-based perpendicular anisotropy MTJ structures. Large DMI in Ir/CoFeB/MgO thin films with perpendicular magnetic anisotropy First-principles calculations combined with Brillouin light-scattering methods Significant role of interfacial configuration on the regulation of DMI Sub-100 nm room-temperature magnetic skyrmions in Ir/CoFeB/MgO multilayers Chen et al. observe a large Dzyaloshinskii-Moriya interaction, combined with the evidence of sub-100-nm room-temperature skyrmions in Ir/CoFeB/MgO structures. This material stack may provide a platform to realize skyrmionics, especially the nucleation and electrical detection of skyrmions in perpendicular anisotropy-based magnetic tunnel junctions.

Spin wave wavevector up-conversion in Y-shaped Permalloy structures

Spin waves in micrometer-sized, patterned Y-shaped Permalloy structures were studied using micro-focus Brillouin light scattering (BLS) with a magnetic field applied in-plane. For in-plane magnetized thin films and microstrips, the dispersion relations depend on the angle of the magnetization with respect to the microstrip axis. BLS measurements show that spin waves generated in the two arms that form the top of the Y structure can be channeled into a longer magnetic microstrip that forms the base when the applied field is oriented perpendicular to the long axis of the base. In this configuration, the base supports surface spin waves. A comparison of the BLS data with micromagnetic simulations reveals that low-k spin waves generated by a microstrip antenna in the arms are converted to higher-k spin waves in the base, which may be useful for nanomagnonic applications.

Evaluation of commercial virtually imaged phase array and Fabry-Pérot based Brillouin spectrometers for applications to biology.

Measuring the complex mechanical properties of biological objects has become a necessity to answer key questions in mechanobiology and to propose innovative clinical and therapeutic strategies. In this context, Brillouin light scattering (BLS) has recently come into vogue, offering quantitative imaging of the mechanical properties without labels and with a micrometer resolution. In biological samples, the magnitude of the spectral changes are typically of a few tens of MHz, and the ability of modern spectrometers to monitor such subtle changes needs to be evaluated. Moreover, the multiplicity of variations in optical arrangements, specific to each lab, requires to set a standard for the assessment of the characteristics of BLS systems. In this paper we propose a protocol to evaluate the precision and accuracy of two commercial spectrometers that is reproducible across labs. For a meaningful comparison, we coupled the spectrometers to the same microscope and to the same laser. We first evaluated the optimum acquisition time and laser power. We evaluated the precision using pure water samples. We determined the accuracy by probing water solutions with increasing concentration of salt and comparing it with theory. Following these quantifications, we applied the VIPA-based spectrometer to tumor spheroids engineered from different cell lines that possess different metastatic potentials and resistance to therapies. On these models, we detected significant changes in the linewidth suggesting that BLS measurements of the viscosity could be used as a read-out to distinguish different levels of drug resistance.

Thickness dependence of interfacial Dzyaloshinskii-Moriya interaction, magnetic anisotropy and spin waves damping in Pt/Co/Ir and Ir/Co/Pt trilayers

Asymmetric Ir/Co/Pt and Pt/Co/Ir trilayers with a wedged Co layer (Co thickness d = 0 ÷ 3.6 nm) were deposited by magnetron sputtering on naturally oxidized Si substrates with a Ta/Au buffer. Their magnetic properties have been investigated using magneto-optical polar Kerr effect and Brillouin light scattering (BLS) technique in the Damon-Eshbach geometry. The BLS measurements shows a frequency asymmetry between Stokes and anti-Stokes peaks position, exploited to determine interfacial Dzyaloshinskii-Moriya interaction energy density DS. DS was nearly two times larger for the Ir/Co/Pt trilayer than for the Pt/Co/Ir with opposite chirality. The effective uniaxial magnetic anisotropy and Ds depend non-monotonically on d with a maximum at d ≈ 1.2 nm. The asymmetry in linewidth for Stokes and anti-Stokes peaks is observed for both Ir/Co/Pt and Pt/Co/Ir trilayers for d < 1.6 nm, and it is increasing with decrease of Co thickness.

Enhancement of Brillouin light scattering signal with anti-reflection layers on magnetic thin films

The significant enhancement of Brillouin light scattering (BLS) spectroscopy intensity in a ferromagnetic thin film with an additional dielectric anti-reflection layer is experimentally investigated. The anti-reflection layer thickness dependent BLS measurements on ferromagnetic layers are performed systematically. Consequently, we observe that BLS signals are dramatically enhanced by more than 450% at a specific dielectric layer thickness due to the pure optical effect. Because of the large signal enhancements, the errors of the spin wave resonance peak frequencies are noticeably reduced as well. Since many magnetic properties such as the saturation magnetization, the surface anisotropy, and the exchange stiffness constant are determined by the spin wave resonance frequencies from the BLS spectra, the additional anti-reflection layer can help to improve the reliability of BLS experiments. Especially, the BLS signal improvement plays a crucial role in the precise determination of the interfacial Dzyaloshinskii-Moriya interaction (iDMI) energy density, since the iDMI energy density is calculated from the difference of Stokes and anti-Stokes resonance frequencies, which is typically order of 1 GHz.

Elastic anisotropy and single-crystal moduli of solid argon up to 64 GPa from time-domain Brillouin scattering

Single-crystal elastic moduli ${C}_{ij}$, shear modulus $G$, and Zener anisotropic ratio $A$ of solid argon having the face-centered cubic (fcc) structure were determined at high pressures between 12 and 64 GPa using a combination of experimental and theoretical approaches. The experimental data, namely, the maximal and minimal values of the product of the longitudinal sound velocity and refractive index $n\ifmmode\cdot\else\textperiodcentered\fi{}{V}_{L}$, were obtained from the 3D scanning of elastic inhomogeneities in compressed samples of argon using the technique of time-domain Brillouin scattering. These inhomogeneities, caused by elastic anisotropy of solid argon, were revealed to be about twice as strong as those reported in the earlier experiments using classical Brillouin light scattering (BLS). To derive the ${V}_{L}$ values, we used the refractive index obtained here from ab initio calculations which also permitted us to rule out any contribution to the amplitude of the observed elastic inhomogeneities of the hexagonal close-packed phase of argon, proposed to coexist with the fcc phase at high pressures. From the measured ${C}_{ij}(P)$, we derived pressure dependence of shear modulus of the fcc argon ${G}_{H}(P)$ using the Voigt-Reuss-Hill approximation and found a very good agreement with the earlier $G(P)$ obtained from shear sound velocities in the classical BLS measurements. Our results agree very well with the earlier predictions based on a relatively simple many-body model employing the Buckingham pair potential. Finally, our measurements show a much weaker change of the Cauchy discrepancy $({C}_{12}\ensuremath{-}{C}_{44}\ensuremath{-}2P)$ of the fcc argon with pressure than reported in all earlier experimental and theoretical works.

Image analysis applied to Brillouin images of tissue-mimicking collagen gelatins.

Brillouin spectroscopy is an emerging analytical tool in biomedical and biophysical sciences. It probes viscoelasticity through the propagation of thermally induced acoustic waves at gigahertz frequencies. Brillouin light scattering (BLS) measurements have traditionally been performed using multipass Fabry-Pérot interferometers, which have high contrast and resolution, however, as they are scanning spectrometers they often require long acquisition times in poorly scattering media. In the last decade, a new concept of Brillouin spectrometer has emerged, making use of highly angle-dispersive virtually imaged phase array (VIPA) etalons, which enable fast acquisition times for minimally turbid materials, when high contrast is not imperative. The ability to acquire Brillouin spectra rapidly, together with long term system stability, make this system a viable candidate for use in biomedical applications, especially to probe live cells and tissues. While various methods are being developed to improve system contrast and speed, little work has been published discussing the details of imaging data analysis and spectral processing. Here we present a method that we developed for the automated retrieval of Brillouin line shape parameters from imaging data sets acquired with a dual-stage VIPA Brillouin microscope. We applied this method for the first time to BLS measurements of collagen gelatin hydrogels at different hydration levels and cross-linker concentrations. This work demonstrates that it is possible to obtain the relevant information from Brillouin spectra using software for real-time high-accuracy analysis.

Influence of the capping layer material on the interfacial Dzyaloshinskii–Moriya interaction in Pt/Co/capping layer structures probed by Brillouin light scattering

Co ultrathin films, of various thicknesses (0.8 nm ⩽ tCo ⩽ 2.5 nm), have been grown by sputtering on Si substrates, using Pt buffer layers and different capping layers (Cu, Ir, MgO and Pt). The x-ray diffraction revealed that our films have a (1 1 1) out-of-plane texture with various degrees of strains. Their magnetic properties have been studied by vibrating sample magnetometry (VSM) and Brillouin light scattering (BLS) in the Damon–Eshbach geometry. VSM characterizations revealed that films with Co thickness below (above) the spin reorientation transition thickness, which is capping layer dependent, are perpendicularly (in-plane) magnetized, suggesting the existence of an interface anisotropy. The surface anisotropy constant was found to be 1.42 ± 0.02 erg cm−2 and of 1.33 ± 0.02 erg cm−2 for the Pt/Co/Cu and Pt/Co/Ir samples, respectively, suggesting that it is due to the Pt/Co interface and that the top Co/Cu, Co/Pt or Co/Ir interfaces have a minor contribution. A lower value of 1.07 ± 0.02 erg cm−2 has been obtained for Pt/Co/MgO most probably due to over-oxidation of Co at the Co/MgO interface. The BLS measurements revealed a pronounced nonreciprocal spin waves propagation, due to the interfacial Dzyaloshinskii–Moriya interaction (iDMI) induced by Pt interface with Co, which increases with decreasing Co thickness. The magnetic dead layer has been taken into account to precisely determine the surface iDMI constant Ds estimated at −0.8 pJ m−1, −1.05 pJ m−1 and −0.95 pJ m−1, respectively for Pt/Co/Ir, Pt/Co/Cu and Pt/Co/MgO for sample thicknesses where a linear thickness dependence of the effective iDMI constant has been observed.

Well-defined metal-polymer nanocomposites: The interplay of structure, thermoplasmonics, and elastic mechanical properties

In this paper, Brillouin light scattering (BLS) characterization revealed an unusual decrease in the speed of sound in an Ag-polystyrene nanohybrid material with increasing filler content. The hybrid material possesses an isotropic and well-defined nanoparticle distribution based on its particle-brush architecture. Temperature-dependent BLS measurements reveal the unique contribution of local thermoplasmonic heating caused by the Ag nanoparticles exposed to laser irradiation. This reversible thermoplasmonic effect implies a lower apparent glass transition temperature (Tg) measured by means of BLS. Furthermore, irreversible aggregation and redispersion of the Ag nanoparticles were observed at temperatures much higher than Tg. This aggregation-redispersion effect is also reflected in a change of the mechanical properties, demonstrating its intimate interplay with the hybrid composite structure.

Detecting the phonon spin in magnon–phonon conversion experiments

Recent advances in the emerging field of magnon spintronics have stimulated renewed interest in phenomena involving the interaction between spin waves, the collective excitations of spins in magnetic materials that quantize as magnons, and the elastic waves that arise from excitations in the crystal lattice, which quantize as phonons. In magnetic insulators, owing to the magnetostrictive properties of materials, spin waves can become strongly coupled to elastic waves, forming magnetoelastic waves—a hybridized magnon–phonon excitation. While several aspects of this interaction have been subject to recent scrutiny, it remains unclear whether or not phonons can carry spin. Here we report experiments on a film of the ferrimagnetic insulator yttrium iron garnet under a non-uniform magnetic field demonstrating the conversion of coherent magnons generated by a microwave field into phonons that have spin. While it is well established that photons in circularly polarized light carry a spin, the spin of phonons has had little attention in the literature. By means of wavevector-resolved Brillouin light-scattering measurements, we show that the magnon–phonon conversion occurs with constant energy and varying linear momentum, and that the light scattered by the phonons is circularly polarized, thus demonstrating that the phonons have spin. Experiments on a magnetostrictive material reveal the conversion between coherent magnons and phonons that have spin.

Interfacial Dzyaloshinskii-Moriya interaction sign in Ir/Co2FeAl systems investigated by Brillouin light scattering

Co2FeAl (CFA) ultrathin films, of various thicknesses (0.9 nm<tCFA<1.8 nm), have been grown by sputtering on Si substrates, using Ir as a buffer layer. The magnetic properties of the structures have been studied by vibrating sample magnetometry (VSM), miscrostrip ferromagnetic resonance (MS-FMR) and Brillouin light scattering (BLS) in the Damon-Eshbach geometry. VSM characterizations show that films are mostly in-plane magnetized and the perpendicular saturating field increases with decreasing CFA thickness suggesting the existence of interface anisotropy. The presence of magnetic dead layers of 0.44 nm has been detected by VSM. The MS-FMR with perpendicular applied magnetic field has been used to determine the gyromagnetic factor. The BLS measurements reveal a pronounced nonreciprocal spin waves propagation, due to the interfacial Dzyaloshinskii-Moriya interaction (DMI) induced by Ir interface with CFA, which increases with decreasing CFA thickness. The DMI sign has been found to be the same (negative) as that of Pt/Co, in contrast to the ab-initio calculation on Ir/Co. The thickness dependence of the effective DMI constant shows the existence of two regimes similarly to that of the perpendicular anisotropy constant. The DMI constant Ds was estimated to be -0.37 pJ/m for the thickest samples where a linear thickness dependence of the effective DMI constant has been observed.

Characterization of the Interfacial Dzyaloshinskii–Moriya Interaction in Pt/Co2FeAl0.5Si0.5Ultrathin Films by Brillouin Light Scattering

Brillouin light scattering (BLS) combined with vibrating sample magnetometry (VSM) have been used to investigate the Dzyaloshinskii–Moriya interaction (DMI) in Pt-buffered Co2FeAl0.5Si0.5 ultrathin films of various thicknesses. VSM measurements of the Co2FeAl0.5Si0.5 (CFAS) thickness dependence of the saturation magnetic moment per unit area revealed a magnetization at saturation of 1286 emu/cm3 and 0.31 nm magnetic dead layer. Furthermore, thickest films ( $t_{{\text {CFAS}}} >1$ nm) are in-plane magnetized, while the thinner ones are perpendicular magnetized. BLS measurements, in the Damon–Eshbach geometry, under an in-plane applied magnetic field revealed the non-reciprocity of the spin waves (SWs) propagating in opposite directions (Stokes and anti-Stokes lines) due to the Pt-induced DMI. Stokes and anti-Stokes lines’ frequency mismatch varies linearly as function of the SW vector allowing the determination of the effective DMI constant. Its thickness dependence leads to determine a value of −0.42 pJ/m for the DMI interface constant, which is significantly lower than that of Pt/Co/AlO x films. Moreover, BLS measurements revealed that the effective magnetization varies linearly with the reciprocal effective CFAS thickness due to the perpendicular interface anisotropy, estimated to be 0.49 mJ/m2, which reinforces the perpendicular magnetization easy axis.