Magneto-optical Kerr Effect Research Articles

Magnetic-Field-Assisted Molecular Beam Epitaxy: Engineering of Fe3O4 Ultrathin Films on MgO(111)

Molecular beam epitaxy is widely used for engineering low-dimensional materials. Here, we present a novel extension of the capabilities of this method by assisting epitaxial growth with the presence of an external magnetic field (MF). MF-assisted epitaxial growth was implemented under ultra-high vacuum conditions thanks to specialized sample holders for generating in-plane or out-of-plane MF and dedicated manipulator stations with heating and cooling options. The significant impact of MF on the magnetic properties was shown for ultra-thin epitaxial magnetite films grown on MgO(111). Using in situ and ex situ characterization methods, scanning tunneling microscopy, conversion electron Mössbauer spectroscopy, and the magneto-optic Kerr effect, we showed that the in-plane MF applied during the reactive deposition of 10 nm Fe3O4(111)/MgO(111) heterostructures influenced the growth morphology of the magnetite films, which affects both in-plane and out-of-plane characteristics of the magnetization process. The observed changes are explained in terms of modification of the effective magnetic anisotropy.

Phonon, electron, and magnon excitations in antiferromagnetic L10 -type MnPt

Antiferromagnetic $L{1}_{0}$-type MnPt is a material with relatively simple crystal and magnetic structures, recently attracting interest due to its high N\'eel temperature and wide usage as a pinning layer in magnetic devices. While it is experimentally well characterized, the theoretical understanding is much less developed, in part due to the challenging accuracy requirements dictated by the small underlying energy scales that govern magnetic ordering in antiferromagnetic metals. In this paper, we use density functional theory, the Korringa-Kohn-Rostoker formalism, and a Heisenberg model to establish a comprehensive theoretical description of antiferromagnetic $L{1}_{0}$-type MnPt, along with accuracy limits, by thoroughly comparing to available literature data. Our simulations show that the contribution of the magnetic dipole interaction to the magnetocrystalline anisotropy energy of ${K}_{1}=1.07\ifmmode\times\else\texttimes\fi{}{10}^{6}$ J/m${}^{3}$ is comparable in magnitude to the spin-orbit contribution. Using our result for the magnetic susceptibility of $5.25\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, a lowest magnon frequency of about 2.02 THz is predicted, confirming THz spin dynamics in this material. From our data for electron, phonon, and magnon dispersion, we compute the individual contributions to the total heat capacity and show that the dominant term at or above 2 K arises from phonons. From the Landau-Lifshitz-Gilbert equation, we compute a N\'eel temperature of 990--1070 K. Finally, we quantify the magnitude of the magneto-optical Kerr effect generated by applying an external magnetic field. Our results provide insight into the underlying physics, which is critical for a deep understanding of fundamental limits of the time scale of spin dynamics, stability of the magnetic ordering, and the possibility of magneto-optical detection of collective spin motion.

Multiple Nonvolatile Skyrmion States in Nanostructured Synthetic Antiferromagnetic Multilayers

Magnetic skyrmions have promising applications as nonvolatile memory for their particlelike and topological-protected features. The recently reported artificial skyrmion has shown a better prospect due to its controllable size and site. Here, we propose and experimentally demonstrate multiple nonvolatile skyrmion states in nanostructured synthetic antiferromagnetic $[\mathrm{Pt}/\mathrm{Co}{]}_{4}/\mathrm{Ru}/[\mathrm{Co}/\mathrm{Pt}{]}_{4}$ multilayers at room temperature. Our magneto-optical Kerr effect measurements of major and minor reversal curves reveal that the skyrmion can exist at zero field with memristive behavior. Using the observed multiple skyrmion states, nonvolatile memory behavior is demonstrated. Our results provide evidence for the concept of skyrmion-based nonvolatile memory, i.e., skyrmion-based memristors and artificial synapses or neurons.

MOKE and MFM on magnetically coated nanofiber mats: Transferring well‐known methods to uncommon samples

AbstractThe hysteresis loops of magnetic samples can be measured by the magneto‐optical Kerr effect (MOKE), while magnetic force microscopy (MFM) is used to depict domain walls or spatially resolved magnetization orientation in magnetic nanostructures or thin films. Both methods are commonly applied on thin films with a perfectly flat surface or on nanostructures. Here, we report MOKE and MFM measurements on a new class of magnetic materials, namely electrospun mats of randomly oriented nanofibers which influence the 30 nm Co coating layer, proving especially the MOKE method utilizable for rough surfaces. Possible interpretations of these first MOKE measurements on highly randomly structured magnetic layers are given.

Vectorization of magneto-optical images of a in plane component of inhomogeneous magnetic fields

A method based on the magneto-optical Kerr effect is demonstrated, which makes it possible to quantitatively determine spatial dependences of the directions of an in-plane component of inhomogeneous magnetic field visualized by soft nanocrystalline films. The method consists in obtaining magneto-optical images of micromagnetic structures that are formed under the influence of an inhomogeneous field. Images in the longitudinal and transverse sensitivity of the Kerr effect are used to calculate the angular dependence of the directions of the unit vectors of this field under the assumption that the components of the field are proportional to the brightness of magneto-optical images. The field direction maps obtained in this way agree with model calculations for our magnets.

Micromorphology and uniaxial magnetic anisotropy of oblique-sputtered Ni80Fe20 films on periodically rippled Al2O3 substrates

Ni80Fe20 (NiFe) films were deposited on periodically rippled Al2O3 substrates by two different template oblique angle deposition (OAD) processes at various oblique deposition angles α (0° ≤ α ≤ 60°). The disparate in-plane uniaxial magnetic anisotropy (UMA) caused by the two template deposition processes of films was revealed by magneto-optical Kerr effect (MOKE) loops and ferromagnetic resonance (FMR) measurements. The analysis of Transmission electron microscopy images indicated that this was related to the different micromorphology of the films due to the two template deposition processes. The angle between substrates’ ripple direction and the material flow incident direction would strongly influence the micromorphology and the induced in-plane UMA field of films. The values of the UMA field of these films can be much larger than those of the films oblique deposited on the plane Al2O3 substrates at the same α by choosing the template deposition direction. The results would help further modulate the UMA of magnetic films deposited on rippled substrates, which would be beneficial for the applications of these films in spintronic, magnetic memory and magnetic microwave devices.

Anisotropic optical and magnetic response in self-assembled TiN–CoFe2 nanocomposites

Transition metal nitrides (e.g., TiN) have shown tremendous promise in optical metamaterials for nanophotonic devices due to their plasmonic properties comparable to noble metals and superior high temperature stability. Vertically aligned nanocomposites (VANs) offer a great platform for combining two dissimilar functional materials with a one-step deposition technique toward multifunctionality integration and strong structural/property anisotropy. Here, we report a two-phase nanocomposite design combining ferromagnetic CoFe2 nanosheets in the plasmonic TiN matrix as a new hybrid plasmonic metamaterial. The hybrid metamaterials exhibit anisotropic optical and magnetic responses, as well as a pronounced magneto-optical coupling response evidenced by Magneto-optic Kerr Effect measurement, owing to the novel vertically aligned structure. This work demonstrates a new TiN-based metamaterial with anisotropic properties and multifunctionality toward light polarization modulation, optical switching, and integrated optics.

Optical Creation of Skyrmions by Spin Reorientation Transition in Ferrimagnetic CoHo Alloys.

Manipulating magnetic skyrmions by means of a femtosecond (fs) laser pulse has attracted great interest due to their promising applications in efficient information-storage devices with ultralow energy consumption. However, the mechanism underlying the creation of skyrmions induced by an fs laser is still lacking. As a result, a key challenge is to reveal the pathway for the massive reorientation of magnetization from trivial to nontrivial topological states. Here, we studied a series of ferrimagnetic CoHo alloys and investigated the effect of a single laser pulse on the magnetic states. Thanks to the time-resolved magneto-optical Kerr effect and imaging techniques, we demonstrate that the laser-induced phase transitions from single domains into a topological skyrmion phase are mediated by the transient in-plane magnetization state, in real time and space domains, respectively. Combining experiments and micromagnetic simulations, we propose a two-step process for creating skyrmions through laser pulse irradiation: (i) the electron temperature enhancement induces a spin reorientation transition on a picosecond (ps) timescale due to the suppression of perpendicular magnetic anisotropy (PMA) and (ii) the PMA slowly restores, accompanied by out-of-plane magnetization recovery, leading to the generation of skyrmions with the help of spin fluctuations. This work provides a route to control skyrmion patterns using an fs laser, thereby establishing the foundation for further exploration of topological magnetism at ultrafast timescales.

Probing 3D magnetic nanostructures by dark-field magneto-optical Kerr effect

Magneto-optical techniques are key tools for the characterization of magnetic effects at a nanoscale. Here, we present the dark-field magneto-optical Kerr effect (DFMOKE), a technique we have recently developed for the characterization of three-dimensional magnetic nanostructures. We introduce the principles of DFMOKE, based on the separation of an incident beam into multiple reflected beams when focusing on a 3D nano-geometry. We show the key modifications needed in a standard focused MOKE magnetometer to perform these measurements. Finally, we showcase the power of this method by detecting the magnetic switching of a single tilted 3D nanowire, independently from the switching of a magnetic thin film that surrounds it. We obtain independent and simultaneous switching detection of the nanowire and the film for all nanowire dimensions investigated, allowing us to estimate a magnetic sensitivity of 7 × 10−15 A m2 for DFMOKE in the setup used. We conclude the article by providing perspectives of future avenues where DFMOKE can be a very powerful characterization tool in the future investigations of 3D magnetic nanostructures.

A Hybrid Magneto‐Optic Capacitive Memory with Picosecond Writing Time

AbstractThe long‐term future of information storage requires the use of sustainable nanomaterials in architectures operating at high frequencies. Interfaces can play a key role in this pursuit via emergent functionalities that break out from conventional operation methods. Here, spin‐filtering effects and photocurrents are combined at metal‐molecular‐oxide junctions in a hybrid magneto‐capacitive memory. Light exposure of metal‐fullerene‐metal oxide devices results in spin‐polarized charge trapping and the formation of a magnetic interface. Because the magnetism is generated by a photocurrent, the writing time is determined by exciton formation and splitting, electron hopping, and spin‐dependent trapping. Transient absorption spectroscopy measurements show changes in the electronic states as a function of the magnetic history of the device within picoseconds of the optical pumping. The stored information is read using time‐resolved scanning magneto optic Kerr effect measurements during microwave irradiation. The emergence of a magnetic interface in the picosecond timescale opens new paths of research to design hybrid magneto‐optic structures operating at high frequencies for sensing, computing, and information storage.

Key Parameters for Detectivity Improvement of Low Noise Anisotropic Magnetoresistive Sensors Made of La2/3Sr1/3MnO3 Single Layers on Vicinal Substrates

The current trend in magnetoresistive sensors development is to increase the sensitivity of single sensing elements by using multilayer structures and to design them into arrays. Such arrays are designed to compensate the excess low frequency noise of individual elements, which limits their magnetic resolution. Here, we report the modeling, design, and fabrication of single layer anisotropic magnetoresistive (AMR) sensors using low noise epitaxial La2/3Sr1/3MnO3 (LSMO) oxide thin films deposited on vicinal SrTiO3 substrates. The fabrication process is simple, and the operation of the sensor is based on a step-induced uniaxial magnetic anisotropy, described using the Stoner–Wohlfarth model. A coherent magnetization reversal process is observed by magneto-optical Kerr effect imaging. A good agreement between experimental data and the expected sensor response confirms the correct operation of the device. Three main fabrication parameters, namely the vicinal angle of the substrate, the deposition temperature, the thin film thickness, and their effects on film anisotropy field and device detectivity have been studied. Detectivity levels as low as 1.4 nT Hz–1/2 at 1 Hz and 240 pT Hz–1/2 in the white noise region are achieved with a single Wheatstone bridge element operating at 310 K. Compared to GMR and AMR sensors, these results are promising for further development and for their use as single layer LSMO low field AMR sensors, including applications as implantable biomedical devices.

Optically Induced Magnetization Switching in NiCo2O4 Thin Films Using Ultrafast Lasers

All-optical magnetization control has recently been garnering considerable attention to realize next-generation ultrafast magnetic information devices. Here, employing a magneto-optical Kerr effect (MOKE) microscope, we observed the laser-induced magnetization switching of ferrimagnetic oxide NiCo2O4 (NCO) epitaxial thin films with perpendicular magnetic anisotropy, where the sample was pumped by 1030 nm laser pulses, and magnetic domain images were acquired via the MOKE microscope with a white light emitting diode. Laser pulses irradiated the NCO thin film at various temperatures from 300 to 400 K while altering the parameters of pulse interval, fluency, and the number of pulses in the absence of the external magnetic field. We observed accumulative all-optical helicity-dependent switching above 380 K. Our observation of oxide NCO thin films facilitates the realization of magnetization switching using ultrafast lasers without applying a magnetic field.

Absence of spontaneous time-reversal symmetry breaking and ferromagnetism in superconducting NiBi3 single crystal

Recent experiments have pointed to chiral $p$-wave-like superconductivity in epitaxial Bi/Ni bilayers that are spontaneously time-reversal symmetry breaking (TRSB), making it a promising platform for exploring physics useful for topologically protected quantum computing. Quite intriguingly, evidence has emerged that, in nonepitaxial Bi/Ni bilayers, superconductivity arises due to the formation of $\mathrm{Ni}{\mathrm{Bi}}_{3}$, which has been reported to host coexisting ferromagnetic and superconducting orders at the surface. We perform high-resolution surface magneto-optic Kerr effect measurements using a Sagnac interferometer on single-crystal $\mathrm{Ni}{\mathrm{Bi}}_{3}$ and find no sign of any spontaneous Kerr signal except for contributions from trapped vortices. This strongly indicates the absence of TRSB in $\mathrm{Ni}{\mathrm{Bi}}_{3}$, whether due to TRSB in the superconducting state or any coexisting ferromagnetism, and we conclude that the superconductivity found in nonepitaxial Bi/Ni is distinctively different from that in epitaxial Bi/Ni.

Room-temperature weak collinear ferrimagnet with symmetry-driven large intrinsic magneto-optic signatures

Here we present a magnetic thin film with a weak ferrimagnetic (FIM) phase above the N\'eel temperature (${T}_{N}=240\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) and a noncollinear antiferromagnetic (AFM) phase below, exhibiting a small net magnetization due to strain-associated canting of the magnetic moments. A long-range ordered FIM phase has been predicted in related materials, but without symmetry analysis. We now perform this analysis and use it to calculate the magneto-optical Kerr effect (MOKE) spectra in the AFM and FIM phases. From the good agreement between the form of the measured and predicted MOKE spectra, we propose the AFM and FIM phases share the magnetic space group $C{2}^{\ensuremath{'}}/{m}^{\ensuremath{'}}$ and that the symmetry-driven magneto-optic and magneto-transport properties are maximized at room temperature in the FIM phase due to the nonzero intrinsic Berry phase contribution present in these materials. A room temperature FIM with large optical and transport signatures, as well as sensitivity to lattice strain and magnetic field, has useful prospects for high-speed spintronic applications.

Modeling of Enhanced Polar Magneto-Optic Kerr Effect by Surface Plasmons in Au Bowtie Arrays

The weak magneto-optical (MO) signal of traditional MO materials is indeed an important issue for their further practical applications. Although many strategies have been proposed to improve the MO effect, hybridization with noble metal nanostructures is a promising route in recent years due to the high localized-surface plasmon resonances (LSPR) effect. A new magneto-optical surface plasmon resonance (MOSPR) structure hybrid with Au bowtie arrays is proposed to increase the measuring range of the polar magneto-optical Kerr effect (PMOKE) and the quality factor through the LSPR effect. It is verified by a numerical simulation of the finite element method (FEM). The optimized parameters were found by modulating the shape and geometric dimensions. Owing to the significant LSPR from the Au bowties, a PMOKE amplification signal spectrum with narrow linewidth, and a high amplitude with high-sensing performance was achieved. Compared with the bare magnetic film alone, by optimizing the relevant parameters of the LSPR structure, the maximum signal increases 3255 times, and the quality factor can be greatly improved, which would provide important guidance and help for the practical application of MO devices.

Modified magnetism in heterostructures of Cr2Ge2Te6 and oxides

We study the effects of interfacing the van der Waals ferromagnet Cr2Ge2Te6 with oxide layers, such as NiO and MgO, in a few different configurations. For this, the magnetic hysteresis curves, obtained by magneto optical Kerr effect, of Cr2Ge2Te6 flakes of similar thicknesses were evaluated. Compared to the properties of a bare Cr2Ge2Te6, we observed an increased perpendicular magnetic anisotropy and enhanced Curie temperature in the Cr2Ge2Te6/NiO heterostructure. In contrast, we report little or no change of the magnetic properties in a Cr2Ge2Te6/NiO and Cr2Ge2Te6/MgO where the Cr2Ge2Te6 was exfoliated on top of the oxide layer. Finally, strain is considered as the mechanism of the enhancement, as we observed the formation of wrinkles in thin Cr2Ge2Te6/NiO heterostructure and the Raman characteristics close to the wrinkles indicated a tensile strain, which is consistent with stronger ferromagnetism in Cr2Ge2Te6.

Exploring the structural, electronic, magnetic, and magneto-optical properties of double perovskites Ca2TMIrO6 (TM = Fe, Co) through first principles study

This study is aimed at exploring the electronic, magnetic, and magneto-optical properties of double perovskites Ca2FeIrO6 and Ca2CoIrO6 with monoclinic structure (space group P21/c) in order to examine their potential applications in spintronic and photovoltaic devices. The calculations were done using the full-potential linearized augmented plane wave within the density functional theory. For the electronic exchange-correlation function, we used the generalized gradient approximation (GGA) and GGA+U (Hubbard potential), and spin-orbit coupling (SOC). The study showed that Ca2FeIrO6 and Ca2CoIrO6 exhibit a monoclinic structure (space group P21/c). The structure relaxation shows an antiferromagnetic behavior in both systems with a magnetic moment of about 6.00 μB for Ca2FeIrO6 and 4.00 μB for Ca2CoIrO6 by using GGA+U approximation. The results of GGA and GGA+U predict the half-metallic behavior of Ca2FeIrO6 and Ca2CoIrO6. The magneto-optical polar Kerr effect (MOKE) was examined by studying the variation of Kerr and ellipticity rotation. The Kerr rotation angle is 1.3º at 4.82 eV and -1.21º at 4.3 eV, and the ellipticity angle is -1.21º at 4.3 eV for Ca2FeIrO6. In the case of Ca2CoIrO6, the Kerr rotation angle is -1.04º at 4.05 eV; the significant Kerr rotation in both compounds may suggest the application of these materials in optoelectronics bias. The named compounds have a potential application in the field of spintronics and its devices, such as in optoelectronics technologies.

Enhanced transverse magneto-optical Kerr effect using ferromagnetic metal perforated with nanopore arrays.

Enhancement of transverse magneto-optical Kerr effects (T-MOKEs) based on surface plasmon resonance has attracted wide attention because of their high sensing performance. Most studies, however, mainly focus on prism-based magnetoplasmonic structures or architectures that incorporate a noble metal lattice and ferromagnetic layer, hindering device fabrication, miniaturization, and integration into a microfluidic sensing configuration. Herein, we propose a single Co6Ag94 ferromagnetic-metal layer perforated with a square nanopore array system with a pronounced T-MOKE magnitude 51 times higher than that of a smooth film, and with potential to detect gaseous analytes. The proposed system is sufficient to detect gaseous media for early environmental monitoring and enables an accessible pathway for magneto-optical sensor design.

Transverse magneto-optical behavior of nanoporous ferromagnetic film based on anodic aluminum oxide/aluminum template and its sensing application.

The transverse magneto-optical Kerr effect (TMOKE) has attracted widespread scientific interest because of its potential applications in biosensor technology, data storage, optical isolation, and telecommunications. More conventional architectures, including prism-based metal/magnetic multilayers and nanoarrays that integrate plasmonic and magnetic materials, are commonly used to amplify TMOKE through the high-quality propagation of the surface plasmon resonance optical mode. However, the main disadvantages of these architectures are their large ohmic losses and radiation damping, resulting in a large optical spectrum linewidth, which hinders the sensing performance. Here, we use a theoretical approach to show that it is possible to employ a low-loss Fabry-Perot optical mode on a magneto-optical platform for TMOKE signal and gaseous sensing enhancement by means of a single CoFeB ferromagnetic film directly overlayed on an already industrial anodic aluminum oxide/aluminum template. The proposed strategy can therefore potentially be exploited for high-precision and low-loss magneto-optical sensors.

Rapid and High Sensitivity Temperature Measurement Based on Near-Extinction Photoelastic Modulated Magneto-Optical Kerr Effect of Fe-Gd Nanofilm

Rapid and High Sensitivity Temperature Measurement Based on Near-Extinction Photoelastic Modulated Magneto-Optical Kerr Effect of Fe-Gd Nanofilm