Magneto-optical Measurements Research Articles

Electronic and transport properties of the dual-emitter organometallic compound IrQ(ppy)2

The structure of the IrQ(ppy)2 organometallic dual phosphorescent compound was experimentally and theoretically investigated. The Metal-to-Ligand Charge Transfer toward the phenylpiridine and quinoline ligands explains its dual phosphorescence property. The IrQ(ppy)2 organometallic's electronic properties extend the absorption spectra toward the lower energies than Ir(ppy)3 due to the bandgap reduction induced by the quinoline ligand. The magneto-optical measurements reveal A-terms, which are produced by splitting the triplet states and B-terms, which arise when the electronic levels are mixed in the magnetic field. The IrQ(ppy)2 molecule's transport properties confirm an ambipolar character, consisting of a higher hole transport character induced by the phenyilpiridine ligands and a lower electron transport character induced by the quinoline ligand.

Technical note: A preliminary evaluation of a method for the examination of text substitutions using magneto-optical measurements

Recent developments regarding the detection and measurement of the magnetic heterogeneity between texts produced with two different laser printers were evaluated. The present exploratory study focused on letter- and word-size fragments of texts inserted in a document to simulate the context of printed document alteration by addition. The measured values from genuine samples were used to create model arrays with the replacement or insertion of a letter or a word into the text of a base document. These model arrays were analyzed using newly introduced software features (named “induction at the point”, “target”, and “fragment” functions) enabling the targeted selection and isolation of small subsections of the total sensor area of the magneto-optical measuring device used in this study. It was observed that magnetic measurements of letter- or word-size fragments using the proposed methodology to the tested models allowed detection of text alterations by insertion that could not typically be detected with magnetic measurements over the full field of view of the magneto-optical device.

Resonant enhancement of magnetic damping driven by coherent acoustic phonons in thin Co2FeAl film epitaxied on GaAs

The magnetic dynamics of a thin Co2FeAl film epitaxially grown on GaAs substrate was investigated using the time- resolved magneto-optical Kerr measurement under an out-of-plane external field. The intrinsic magnetic damping constant, which should do not vary with the external magnetic field, exhibits an abnormal huge increase when the precession frequency is tuned to be resonant with that of the coherent longitudinal acoustic phonon in the Co2FeAl/GaAs heterostructure. The experimental finding is suggested to result from the strong coherent energy transfer from spins to acoustic phonons via magnetoelastic effect under a resonant coupling condition, which leads to a huge energy dissipation of spins and a greatly enhanced magnetic damping in Co2FeAl. Our experimental findings provide an experimental evidence of spin pumping-like effect driven by propagating acoustic phonons via magnetoelastic effect, suggesting an alternative approach to the possible long-range spin manipulation via coherent acoustic waves.

Enhancing magneto-optic effects in two-dimensional magnets by thin-film interference

The magneto-optic Kerr effect is a powerful tool for measuring magnetism in thin films at microscopic scales, as was recently demonstrated by the major role it played in the discovery of two-dimensional (2D) ferromagnetism in monolayer CrI3 and Cr2Ge2Te6. These 2D magnets are often stacked with other 2D materials in van der Waals heterostructures on a SiO2/Si substrate, giving rise to thin-film interference. This can strongly affect magneto-optical measurements but is often not taken into account in experiments. Here, we show that thin-film interference can be used to engineer the magneto-optical signals of 2D magnetic materials and optimize them for a given experiment or setup. Using the transfer matrix method, we analyze the magneto-optical signals from realistic systems composed of van der Waals heterostructures on SiO2/Si substrates, using CrI3 as a prototypical 2D magnet, and hexagonal boron nitride to encapsulate this air-sensitive layer. We observe a strong modulation of the Kerr rotation and ellipticity, reaching several tens to hundreds of milliradians, as a function of the illumination wavelength, and the thickness of SiO2 and layers composing the van der Waals heterostructure. Similar results are obtained in heterostructures composed by other 2D magnets, such as CrCl3, CrBr3, and Cr2Ge2Te6. Designing samples for the optimal trade-off between magnitude of the magneto-optical signals and intensity of the reflected light should result in a higher sensitivity and shorter measurement times. Therefore, we expect that careful sample engineering, taking into account thin-film interference effects, will further the knowledge of magnetization in low-dimensional structures.

Sensing of temperature through magnetooptical domain wall susceptibility

Temperature measurements are vital in laboratory and industry settings. We demonstrate magnetooptical measurement schemes to relate magnetic and micromagnetic features to temperature sensing using magnetooptical active iron garnet films. A particular focus is on a temperature measurement scheme based on changes in the magnetic domain wall susceptibility, analyzing the resulting magnetooptical signal amplitude in a magnetic modulation field. The range of application is limited by the Néel temperature of the sensing garnet material. Temperatures are perceived with an accuracy of below 0.1 °C from the changes in the magnetooptical domain wall response. The thin film sensor element features additionally high sensitivity magnetic field and electrical current sensing capabilities, allowing for multi-functional sensor applications.

Study of magnetic nanowires of amorphous Co20Fe60B20 prepared by oblique angle deposition on nanorippled substrate

An array of magnetic nanowires of amorphous Co20Fe60B20 alloy has been prepared by oblique angle deposition of material on nanorippled substrate. The angle of incidence is chosen such that the deposition takes place on only one facet of the nanoripples. Grazing incidence small angle x-ray scattering measurement has been done in order to characterize the morphology of the nanowires. It is found that the oblique angle deposition results in columnar growth, and each individual nanowire consists of nanocolumns having average diameter of 7 nm and a tilt angle of 37° with respect to the surface normal. Azimuthal angle dependence of longitudinal magneto-optical Kerr effect measurement shows that the nanowires possess a well-defined uniaxial magnetic anisotropy with the easy axis of magnetization tilted by an angle of 25° with respect to the direction of the ripples. The magnetization follows the Stoner-Wohlfarth model with a very narrow distribution in the direction of easy axis. It is suggested that, tilt of the easy axis can be understood in terms of the interplay of two shape anisotropy terms corresponding to the ensemble of strongly interacting nanocolumns through magnetostatic dipolar interaction, and the overall shape of a nanowire.

The measurement of anisotropic thermal transport using time-resolved magneto-optical Kerr effect

Owing to the increasing demand for the improved management of thermal transport in devices, it is necessary to develop an optimal method for the measurement of anisotropic heat conduction. Although several techniques have been used to measure the heat transport in anisotropic materials, the accurate determination of anisotropic thermal conductivity remains a major challenge. In this study, we present time-resolved magneto-optical Kerr effect measurement using a pump–probe technique to investigate the thermal characterization of various materials. A substrate/ferromagnetic metal (FM) structure is prepared, where FM acts as a transducer to detect the thermal transport in substrates of silicon, sapphire, and highly ordered pyrolytic graphite. By performing experiments under various modulation frequencies and laser spot sizes, we determine both in-plane and through-plane thermal conductivities. We also discuss the limitations of measurement and provide useful guidelines to determine the thermal properties of different materials with high accuracy.

Ultrafast control of lattice strain via magnetic circular dichroism

Using ultrafast x-ray diffraction, we directly monitor the lattice dynamics induced by femtosecond laser pulses in nanoscale thin films of bismuth iron garnet in external magnetic fields ${H}_{\mathrm{ext}}$. We control the ultrafast laser-induced lattice strain amplitude by changing the laser pulse helicity. The strength of ${H}_{\mathrm{ext}}$ is used as an external parameter to switch the helicity dependence on and off, respectively. Based on magneto-optical spectroscopic measurements, we explain these phenomena by magnetic circular dichroism. Our findings highlight an important approach for ultrafast manipulation of lattice strain in magnetic materials, in particular insulators, and open exciting perspectives towards ultrafast control of lattice strain and heat-induced magnetization switching and spin waves in bismuth substituted iron garnets using the polarization of light.

Establishing a Correction Factor for Oblique Angle Deposition and Its Verification by the Magneto-Optical Kerr Effect

Physical vapor deposition (PVD) is one of the major methods for fabricating thin films. Measuring and controlling the thickness of deposited layers are very momentous, especially for optics and photonics applications. Despite regular depositions with substrates situated near the thickness monitor, the oblique angle deposition method requires applying a correction factor for estimating real thicknesses. In this paper, we establish a correction factor formula for different positions of the substrate relative to the deposition source. After numerical simulations, several samples with [Glass/Nickel] and [Glass/Nickel/Zinc Sulfide] structures are fabricated using an oblique angle PVD method. Magneto-optical Kerr measurements and hysteresis loop analysis are obtained from three different points of samples with different thicknesses and compared to the corresponding simulated structures. The results of the correction factor show a high degree of conformity with simulations. Cross-sectional field emission scanning electron microscopic characterization is also used to illustrate the thickness of samples. The thicknesses are then compared to the results obtained from the correction factor, confirming its reliability for practical use in oblique angle deposition methods.

Magneto-optical conductivity in generic Weyl semimetals

Magneto-optical studies of Weyl semimetals have been proposed as a versatile tool for observing low-energy Weyl fermions in candidate materials including the chiral Landau level. However, previous theoretical results have been restricted to the linearized regime around the Weyl node and are at odds with experimental findings. Here, we derive a closed form expression for the magneto-optical conductivity of generic Weyl semimetals in the presence of an external magnetic field aligned with the tilt of the spectrum. The systems are taken to have linear dispersion in two directions, while the tilting direction can consist of any arbitrary continuously differentiable function. This general calculation is then used to analytically evaluate the magneto-optical conductivity of Weyl semimetals expanded to cubic order in momentum. In particular, systems with arbitrary tilt, as well as systems hosting trivial Fermi pockets are investigated. The higher-order terms in momentum close the Fermi pockets in the type-II regime, removing the need for unphysical cutoffs when evaluating the magneto-optical conductivity. Crucially, the ability to take into account closed over-tilted and additional trivial Fermi pockets allows us to treat model systems closer to actual materials and we propose a simple explanation why the presence of parasitic trivial Fermi pockets can mask the characteristic signature of Weyl fermions in magneto-optical conductivity measurements.

Direct Imaging of Current-Induced Antiferromagnetic Switching Revealing a Pure Thermomagnetoelastic Switching Mechanism in NiO.

We unravel the origin of current-induced magnetic switching of insulating antiferromagnet/heavy metal systems. We utilize concurrent transport and magneto-optical measurements to image the switching of antiferromagnetic domains in specially engineered devices of NiO/Pt bilayers. Different electrical pulsing and device geometries reveal different final states of the switching with respect to the current direction. We can explain these through simulations of the temperature-induced strain, and we identify the thermomagnetoelastic switching mechanism combined with thermal excitations as the origin, in which the final state is defined by the strain distributions and heat is required to switch the antiferromagnetic domains. We show that such a potentially very versatile noncontact mechanism can explain the previously reported contradicting observations of the switching final state, which were attributed to spin-orbit torque mechanisms.

Effect of demagnetization factors on spin current transport

The demagnetizing field associated with shape anisotropy originates from the magnetic field generated through magnetization of magnetic materials. The shape and demagnetizing effects must be considered as characteristics of a magnetic object in the presence of a magnetic field. In this study, we demonstrated the strong influence of the demagnetizing field on the spin current transports in Pt/YIG, including the spin Seebeck effect, spin Hall effect, spin Hall magnetoresistance, and planar Hall resistance. The calculated effective demagnetizing factors were closely related to the thickness- and width-dependent anomalous plateau behaviors of the spin-dependent electrical and thermal transports as well as magneto-optical measurements. Moreover, we demonstrated that the width of the plateau manifested from the shape of the YIG sample. Furthermore, through magnetic force microscopy, we directly visualized the evolution of the magnetic domains with an abrupt 90\ifmmode^\circ\else\textdegree\fi{} magnetic rotation, which we further corroborated by the 90\ifmmode^\circ\else\textdegree\fi{} resistance phase shift in the angular-dependent planar Hall effect.

Wide spectral range ultrafast pump-probe magneto-optical spectrometer at low temperature, high-magnetic and electric fields.

We developed a table-top setup to perform magneto-optical pump-probe measurements with the possibility to independently tune the photon-energy of both pump and probe beams in the 0.5 eV-3.5 eV range. Our apparatus relies on a commercial turn-key amplified laser system, able to generate light pulses with duration shorter than or comparable to 100 fs throughout the whole spectral range. The repetition rate of the source can be modified via the computer in the 1 kHz to 1 MHz range. A commercial balanced detector is connected to a high-frequency digitizer, allowing for a highly-sensitive detection scheme: rotations of the probe polarization as small as 70 μdeg can be measured. Additionally, a DC magnetic field as high as 9 T and voltages in the kV regime can be applied on the sample. A cryostat allows us to precisely set the temperature of the specimen in the 4 K-420 K interval. We prove the performance of our setup by measuring the ultrafast demagnetization of a cobalt crystal as a function of a wide variety of experimental parameters.

Domain Structure and Reversal Mechanisms through Diffracted Magneto-optics in Fe80B20 Microsquare Arrays

In this paper, the predictive power of diffracxtive magneto-optics concerning domain structure and reversal mechanisms in ordered arrays of magnetic elements is demonstrated. A simple theoretical model based on Fraunhoffer diffraction theory is used to predict the magnetisation reversal mechanisms in an array of magnetic elements. Different domain structures and simplified models (or educated guesses) of the associated reversal mechanisms produce marked differences in the spatial distributions of the magnetisation. These differences and the associated magnetisation distribution moments are experimentally accessible through conventional and diffractive magneto-optical Kerr effect measurements. The domain and magnetisation reversal predictions are corroborated with Magnetic Force Microscopy (MFM) measurements.

Experimental artifact in MOKE measurements when using paramagnetic sample holders

We describe here an artifact that may affect to magneto-optical Kerr measurements. When paramagnetic sample holders (SH) with non-negligible susceptibilities are used, the inhomogeneity of the applied magnetic field can induce forces and torques on it, shifting the reflected beam, and altering its intensity at the photodetector. The effect is even and can be avoided using low susceptibility paramagnetic or diamagnetic SH We also present a detailed analytical description of the magnetic forces involved and provide some estimated values of the SH shifting, showing that they might distort the magneto-optical Kerr effect signal. Moreover, in this paper we show how the artifact can be removed from the experimental curves with an appropriated data analysis.

One-dimensional antilocalization of electrons from spin disorder probed by nonlinear Hall effect

In recent years, nonreciprocal transport measurement has emerged as a probe of spin orbit interaction, spin texture, superconductivity, and other fundamental properties of materials. Here, we study the nonlinear Hall effect (NLHE) in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}/\mathrm{CoFeB}$ heterostructures measured by second harmonic voltage method. Magneto-optical Kerr effect measurements demonstrate the origin of the NLHE to be the asymmetric magnon scattering mechanism. Moreover, a linear relation between the nonlinear Hall resistance and the electron phase coherence length of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ is observed between 5 and 50 K. We propose a phenomenological model that explains the enhancement of the NLHE below 50 K as a result of one-dimensional antilocalization of electrons from spin disorder, enabled by spin-momentum locking and conservation of angular momentum.

A New Zinc Phosphate-Tellurite Glass for Magneto-Optical Applications.

This work investigates the structural, magnetic and magneto-optical properties of a new zinc phosphate-tellurite glass belonging to the 45ZnO-10Al2O3-40P2O5-5TeO2 system. The glass was prepared by a wet method of processing the starting reagents followed by suitable melting–stirring–quenching–annealing steps. Specific parameters such as density, average molecular mass, molar volume, oxygen packaging density, refractive index, molar refractivity, electronic polarizability, reflection loss, optical transmission, band gap and optical basicity have been reported together with thermal, magnetic and magneto-optical characteristics. Absorption bands appear in the blue and red visible region, while over 600 nm the glass becomes more transparent. FTIR and Raman spectra evidenced phosphate-tellurite vibration modes proving the P2O5 and TeO2 network forming role. Magnetic measurements reveal the diamagnetic character of the Te-doped glass with an additional weak ferromagnetic signal, specific to diluted ferromagnetic oxides. Positive Faraday rotation angle with monotonous decreasing value at increasing wavelength was evidenced from magneto-optical measurements. The final product is a composite material comprising of a non-crystalline vitreous phase and Te-based nanoclusters accompanied by oxygen vacancies. The metallic-like Te colloids are responsible for the dark reddish color of the glass whereas the accompanying oxygen vacancies might be responsible for the weak ferromagnetic signal persisting up to room temperature.

Ultrafast Demagnetization Dominates Fluence Dependence of Magnetic Scattering at Co M Edges.

We systematically study the fluence dependence of the resonant scattering cross-section from magnetic domains in Co/Pd-based multilayers. Samples are probed with single extreme ultraviolet (XUV) pulses of femtosecond duration tuned to the Co M_{3,2} absorption resonances using the FERMI@Elettra free-electron laser. We report quantitative data over 3 orders of magnitude in fluence, covering 16 mJ/cm^{2}/pulse to 10 000 mJ/cm^{2}/pulse with pulse lengths of 70fs and 120fs. A progressive quenching of the diffraction cross-section with fluence is observed. Compression of the same pulse energy into a shorter pulse-implying an increased XUV peak electric field-results in a reduced quenching of the resonant diffraction at the Co M_{3,2} edge. We conclude that the quenching effect observed for resonant scattering involving the short-lived Co 3p core vacancies is noncoherent in nature. This finding is in contrast to previous reports investigating resonant scattering involving the longer-lived Co 2p states, where stimulated emission has been found to be important. A phenomenological model based on XUV-induced ultrafast demagnetization is able to reproduce our entire set of experimental data and is found to be consistent with independent magneto-optical measurements of the demagnetization dynamics on the same samples.

Concurrent magneto-optical imaging and magneto-transport readout of electrical switching of insulating antiferromagnetic thin films

We demonstrate stable and reversible current induced switching of large-area (>100 μm2) antiferromagnetic domains in NiO/Pt by performing concurrent transport and magneto-optical imaging measurements in an adapted Kerr microscope. By correlating the magnetic images of the antiferromagnetic domain changes and magneto-transport signal response in these current-induced switching experiments, we disentangle magnetic and non-magnetic contributions to the transport signal. Our table-top approach establishes a robust procedure to subtract the non-magnetic contributions in the transport signal and extract the spin-Hall magnetoresistance response associated with the switching of the antiferromagnetic domains, enabling one to deduce details of the antiferromagnetic switching from simple transport measurements.

Large exchange splitting in monolayer graphene magnetized by an antiferromagnet

Spin splitting in graphene is required to develop graphene-based multifunctional spintronic devices with low dissipation and long-distance spin transport. Magnetic proximity effects are a promising route to realize exchange splitting in the material, which is otherwise intrinsically non-spin-polarized. Here, we show that monolayer graphene can be magnetized by coupling to an antiferromagnetic thin film of chromium selenide, resulting in an exchange splitting energy as high as 134 meV at 2 K. This exchange splitting is shown through shifts in the quantum Hall plateau and quantum oscillations in the graphene, and its energy can be modulated through field cooling, with the exchange splitting energy increasing with positive field cooling and decreasing with negative field cooling. Our experimental demonstration of magnetism in graphene at low temperatures is supported by measurements of resistivity dependence on temperature and magneto-optic Kerr measurements. Monolayer graphene can be magnetized by coupling to an antiferromagnetic thin film of chromium selenide, resulting in an exchange splitting energy as high as 134 meV at 2 K.