Magneto-optical Kerr Effect Signal Research Articles

Combining two-photon lithography with laser ablation of sacrificial layers: A route to isolated 3D magnetic nanostructures

Three-dimensional (3D) nanostructured functional materials are important systems allowing new means for intricate control of electromagnetic properties. A key problem is realising a 3D printing methodology on the nanoscale that can yield a range of functional materials. In this article, it is shown that two-photon lithography, when combined with laser ablation of sacrificial layers, can be used to realise such a vision and produce 3D functional nanomaterials of complex geometry. Proof-of-principle is first shown by fabricating planar magnetic nanowires raised above the substrate that exhibit controlled domain wall injection and propagation. Secondly, 3D artificial spin-ice (3DASI) structures are fabricated, whose complex switching can be probed using optical magnetometry. We show that by careful analysis of the magneto-optical Kerr effect signal and by comparison with micromagnetic simulations, depth dependent switching information can be obtained from the 3DASI lattice. The work paves the way for new materials, which exploit additional physics provided by non-trivial 3D geometries.

Anomalous magnetoresistance and Hall effect in amorphous Pt/TbFeCo thin films

Among the magnetic materials in technological usage, amorphous rare-earth transition metal (RE–TM) alloys are of great interest due to their adjustable magnetic properties and favorable perpendicular magnetic anisotropic (PMA) advantages. In this study, the properties of Tb25Fe61Co14 system with 2, 5, and 10 nm Pt underlayers have been systematically explored by X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature and magnetic field dependence of Hall resistivity, magnetoresistivity, and magneto-optic Kerr Effect (MOKE) measurements. According to the results, whole thin films are amorphous in nature and display an out-of-plane magnetic anisotropy without annealing. Increasing the Pt underlayer thickness helps the TbFeCo layer to reach significantly enhanced perpendicular magnetic anisotropic values and visualize the appearance of anomalous magnetoresistance in the Hall effect measurements. The thin films with 5 and 10 nm Pt underlayer has a ratio of remanence magnetization/saturation magnetization (MR/MS) of around 1, which is the out of plane direction of MOKE signal. Besides, the present study also includes some pieces of evidence that might be related to the Berry phase trend.

Magneto-optical Kerr effect measurement in ultrafast Sagnac interferometry using the Jones matrix approach.

We report the Jones matrix formalism of the magneto-optic Kerr effect (MOKE) for ferromagnets using an ultrafast Sagnac interferometer. Compared to the time-resolved MOKE instrument, the Sagnac interferometer has the advantage of obtaining the real and imaginary parts of the differential MOKE signal as well as the differential reflectivity and the lattice displacement at the same time. In addition, a simple method to obtain the static values of Kerr rotation and ellipticity is presented.

Transitional Faraday and Kerr effect in hybridized topological insulator thin films

We theoretically investigate quantum phase transitions from topologically non-trivial to trivial states in three-dimensional hybridized topological insulator (TI) ultra-thin films. The interplay between the hybridization of the top and bottom surface states (SSs) and Zeeman energy gives rise to topological and normal insulating phases. By tuning the Zeeman energy, we can drive phase transition between these two phases by closing and reopening the band gap. Furthermore, we impinge a Gaussian beam on the surface of the 3D TI to study the Faraday rotation (FR) and magneto-optical Kerr effect (MOKE) in the presence of an external magnetic field, while explicitly taking into account the hybridization between the top and bottom Dirac SSs of the TI film. The FR and MOKE change sign when the polarization is changed from s to p. We demonstrate that giant FR and MOKE can be achieved by tuning the external magnetic field. Furthermore, the MOKE signal takes a sharp anti-phase peak at the charge neutrality point as the system goes from the quantum spin Hall insulator (QSHI) to the semimetallic state. Lastly, the MOKE and FR in the bottom and top SSs can be independently tuned for intra-band and inter-band transitions via n-type and p-type doping respectively.

Anomalous Kerr effect in SrRuO3 thin films

We study the magneto-optical Kerr effect (MOKE) in SrRuO$_3$ thin films, uncovering wide regimes of wavelength, temperature, and magnetic field where the Kerr rotation is not simply proportional to the magnetization but instead displays two-component behavior. One component of the MOKE signal tracks the average magnetization, while the second "anomalous" component bears a resemblance to anomalies in the Hall resistivity which have been previously reported in skyrmion materials. We present a theory showing that the MOKE anomalies arise from the non-monotonic relation between the Kerr angle and the magnetization, when we average over magnetic domains which proliferate near the coercive field. Our results suggest that inhomogeneous domain formation, rather than skyrmions, may provide a common origin for the observed MOKE and Hall resistivity anomalies.

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.

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.

Tracking picosecond strain pulses in heterostructures that exhibit giant magnetostriction.

We combine ultrafast X-ray diffraction (UXRD) and time-resolved Magneto-Optical Kerr Effect (MOKE) measurements to monitor the strain pulses in laser-excited TbFe2/Nb heterostructures. Spatial separation of the Nb detection layer from the laser excitation region allows for a background-free characterization of the laser-generated strain pulses. We clearly observe symmetric bipolar strain pulses if the excited TbFe2 surface terminates the sample and a decomposition of the strain wavepacket into an asymmetric bipolar and a unipolar pulse, if a SiO2 glass capping layer covers the excited TbFe2 layer. The inverse magnetostriction of the temporally separated unipolar strain pulses in this sample leads to a MOKE signal that linearly depends on the strain pulse amplitude measured through UXRD. Linear chain model simulations accurately predict the timing and shape of UXRD and MOKE signals that are caused by the strain reflections from multiple interfaces in the heterostructure.

Giant magneto-optical Kerr enhancement from films on SiC due to the optical properties of the substrate

We report a giant enhancement of the midinfrared magneto-optical complex Kerr angle (polarization change of reflected light) in a variety of materials grown on SiC. In epitaxially grown multilayer graphene, the Kerr angle is enhanced by a factor of 68, which is in good agreement with Kerr signal modeling. Strong Kerr enhancement is also observed in Fe films grown on SiC and Al-doped bulk SiC. Our experiments and modeling indicate that the enhancement occurs at the high-energy edge of the SiC reststrahlen band where the real component of the complex refractive index ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}}_{}$ passes through unity. Furthermore, since the signal is greatly enhanced when $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}=1$, the enhancement is predicted to exist over the entire visible and IR spectrum for a free-standing film. We also predict similar giant enhancement in both Faraday (transmission) and Kerr rotation for thin films on a metamaterial substrate with refractive index ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}}_{}=\ensuremath{-}1$. This paper demonstrates that the substrate used in magneto-optical Kerr effect (MOKE) measurements must be carefully chosen when investigating magneto-optical materials with weak MOKE signals or when designing MOKE-based optoelectronic devices.

The optical cavity enhanced magneto-optical Kerr effect signals of AAO/Al-based CoFeB nanostructure arrays

The designs and fabrications of magnetic nanostructures that are intended to enhance and tailor magneto-optical rotations have been the subject of intense interest for ultrasensitive biosensor and communication applications. In this research study, flexibly regulated longitudinal magneto-optical Kerr effects (L-MOKE) were investigated in CoFeB (Co40Fe40B20) nanostructure arrays patterned on the single pass anodic aluminum oxide template (AAO/Al) using a magnetron sputtering method, showing a great nano-cavity enhancement effect. The optical and L-MOKE properties of the as-prepared nanostructural thin films on the AAO/Al substrate are significantly distinguished from those of the CoFeB granular film on the Al substrate, exhibiting a distinct resonance. The L-MOKE peak gives a clear blue-shift with the increase of the pore diameter of the AAO template, which can be elucidated by the pure optical interference theory. By optimizing the pore diameter of the AAO template, the Kerr rotation of the CoFeB nanoporous arrays can reach 0.153 degree, which is approximately 1.65 times higher than that of the CoFeB granular film. The proposed nanostructural thin film could be potentially applied as highly accurate magneto-optical sensors, conveying new pathways in the construction of advanced magneto-optical devices for optoelectronic and magneto-optical applications.

Electrical control of 2D magnetism in bilayer CrI3.

Controlling magnetism via electric fields addresses fundamental questions of magnetic phenomena and phase transitions1-3, and enables the development of electrically coupled spintronic devices, such as voltage-controlled magnetic memories with low operation energy4-6. Previous studies on dilute magnetic semiconductors such as (Ga,Mn)As and (In,Mn)Sb have demonstrated large modulations of the Curie temperatures and coercive fields by altering the magnetic anisotropy and exchange interaction2,4,7-9. Owing to their unique magnetic properties10-14, the recently reported two-dimensional magnets provide a new system for studying these features15-19. For instance, a bilayer of chromium triiodide (CrI3) behaves as a layered antiferromagnet with a magnetic field-driven metamagnetic transition15,16. Here, we demonstrate electrostatic gate control of magnetism in CrI3 bilayers, probed by magneto-optical Kerr effect (MOKE) microscopy. At fixed magnetic fields near the metamagnetic transition, we realize voltage-controlled switching between antiferromagnetic and ferromagnetic states. At zero magnetic field, we demonstrate a time-reversal pair of layered antiferromagnetic states that exhibit spin-layer locking, leading to a linear dependence of their MOKE signals on gate voltage with opposite slopes. Our results allow for the exploration of new magnetoelectric phenomena and van der Waals spintronics based on 2D materials.

An investigation of magneto-optical Kerr effect signal by optical cavity in [Glass/Co/ZnS] structure

In this paper, using the magneto optical Kerr effect, the magnetic properties of [Glass/Co/ZnS] structure has been investigated. Initially, by co-precipitation method, zinc sulfide was prepared and then by X-ray diffraction pattern, its structure was confirmed. In the structure of [Glass/Co/ZnS], cobalt was used as a magnetic material and zinc sulfide acts as a capping layer. Before fabrication process, the optimums cobalt and zinc sulfide thicknesses were calculated using the MATLAB simulation software and the desired thicknesses were selected. To obtain layers with variable thicknesses, the oblique thermal evaporation technique was used and different thicknesses of about 5–90 nm for cobalt and 45 nm for zinc sulfide on the glass substrate were coated. The longitudinal Kerr signal (LKS) of the [Glass/Co/ZnS] samples were measured in different cobalt thicknesses. By using the magneto-optical Kerr effect (MOKE), the hysteresis loop of samples in the longitudinal geometry have been measured samples and it was observed that the sample has an easy axis in the plane of the film and in the direction of its large diameter. The results have shown that the capping layer with suitable thickness, by creation of an optical cavity, has been able to amplify the Kerr signal by more than 1.5 times, which is noticeable by considering the simplicity of the technique. This structure can be applied as a highly accurate magneto-optical sensor.

Enhancing the magneto-optical Kerr effect through the use of a plasmonic antenna.

We employ an extended finite-element model as a design tool capable of incorporating the interaction between plasmonic antennas and magneto-optical effects, specifically the magneto-optical Kerr effect (MOKE). We first test our model in the absence of an antenna and show that for a semi-infinite thin-film, good agreement is obtained between our finite-element model and analytical calculations. The addition of a plasmonic antenna is shown to yield a wavelength dependent enhancement of the MOKE. The antenna geometry and its separation from the magnetic material are found to impact the strength of the observed MOKE signal, as well as the antenna's resonance wavelength. Through optimization of these parameters we achieved a MOKE enhancement of more than 100 when compared to a magnetic film alone. These initial results show that our modeling methodology offers a tool to guide the future fabrication of hybrid plasmonic magneto-optical devices and plasmonic antennas for magneto-optical sensing.

Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal.

When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation1. This magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties2, 3 as well as for various applications including magneto-optical recording4. Recently, there has been a surge of interest in antiferromagnets (AFMs) as prospective spintronic materials for high-density and ultrafast memory devices, owing to their vanishingly small stray field and orders of magnitude faster spin dynamics compared to their ferromagnetic counterparts5-9. In fact, the MOKE has proven useful for the study and application of the antiferromagnetic (AF) state. Although limited to insulators, certain types of AFMs are known to exhibit a large MOKE, as they are weak ferromagnets due to canting of the otherwise collinear spin structure10-14. Here we report the first observation of a large MOKE signal in an AF metal at room temperature. In particular, we find that despite a vanishingly small magnetization of M ~0.002 µB/Mn, the non-collinear AF metal Mn3Sn15 exhibits a large zero-field MOKE with a polar Kerr rotation angle of 20 milli-degrees, comparable to ferromagnetic metals. Our first-principles calculations have clarified that ferroic ordering of magnetic octupoles in the non-collinear Néel state16 may cause a large MOKE even in its fully compensated AF state without spin magnetization. This large MOKE further allows imaging of the magnetic octupole domains and their reversal induced by magnetic field. The observation of a large MOKE in an AF metal should open new avenues for the study of domain dynamics as well as spintronics using AFMs.

Localized surface plasmon resonance enhanced magneto-optical Kerr effect in Ni80Fe20 thin films coated with Au nanorods

The influence of the localized surface plasmon resonance on the magneto-optical Kerr effect (MOKE) was investigated in Ni80Fe20 thin films coated with gold nanorods. The nanorods are dimensionally tuned to support localized surface plasmon resonance near the incident laser frequency. A significant enhancement of magneto-optical response is observed, in which the MOKE signal is increased by over 40% compared to the reference Ni80Fe20 films. The spectral dependence of MOKE signals exhibits the maximum magneto-optical response centered around the longitudinal surface plasmon resonance wavelength of gold nanorods. Finite-difference time-domain modeling confirms that the excitation of the localized surface plasmons leads to the enhancement of magneto-optical responses.

Magneto-optical Kerr effect of a Ni2.00Mn1.16Ga0.84 single crystal across austenite and intermartensite transitions

We carried out magneto-optical Kerr effect (MOKE) and magnetization measurements on a single crystal of Ni2.00Mn1.16Ga0.84, which is a magnetic shape memory material with application potential for actuator devices or for energy recuperation. Up to the time of our study, there had been reports of MOKE measurements in polar geometry. Against earlier predictions, we show that surface magnetic states of the martensite and the austenite can be also probed efficiently via longitudinal MOKE. A single-variant magnetic state prepared at room temperature is characterized by square-shaped ferromagnetic hysteresis loops yielding coercive fields, which are key material properties for future applications. Temperature dependencies of Kerr rotation were found to be linearly proportional to magnetization for martensitic phases. After passing through an inter-martensitic structural transition below room temperature in zero magnetic field, the coercive fields are more than doubled in comparison with the room temperature values. Above room temperature where an austenite structure is formed, MOKE signals are dominated by quadratic contributions and the magnitude of Kerr rotation drops due to changes in the electronic and magnetic domains structure.

Influence of Nanovoids Depth on the Surface Magneto-Optical Kerr Effect Modulations in Magnetic Photonic Crystal Slab

Magnetic photonic crystal slab with nanovoids buried in Ni films were fabricated using nanosphere lithography combined with electrochemical deposition. One featured structure was dominated the spectra of polar surface magneto-optical Kerr effect (MOKE) of samples with different voids depth. For the shallow ones, surface lattice resonance (SLR) was revealed through angle resolved spectra analysis, where the location of degeneracy points corresponded to that of the featured structure in MOKE signal. For the deeper ones, localized plasmons and degeneracy points were all demonstrated in angle resolved reflection spectra, where localized plasmons dominated the resonant MOKE signal, and the degeneracy points blue-shifted compared with that of shallow ones. Changing the lattice of samples, similar phenomena were presented. The physical mechanism of SLR could be attributed to the diffraction of surface plasmons by the reciprocity vector, and the localized plasmons were attributed to the Mie scattering. The featured structure in polar MOKE spectra demonstrated high sensitivity, which might pave the way to on-chip MO devices.

Enhancement of the magneto-optical Kerr effect signal using bilayer metallic structures consisting of Co/noble metal

The nanostructures of the noble metals Au, Ag, and Cu have attracted much interest because of their unique and tunable optical properties on account of their surface plasmon resonance (SPR) resulting in the strong scattering and absorption of light. The magneto-optical Kerr effect (MOKE) as a function of the angle of incidence in bilayered metallic structures of Co/Au, Co/Ag, and Co/Cu was calculated. The effect of the thickness of the bilayered metallic films was also investigated. The calculation results demonstrated that the MOKE signals of these films were greatly affected by a number of factors, including the angle of incidence, the linear polarization of the incident light, typical films, and the film thickness. By calculating and analyzing the MOKE signals from samples with different conditions, the results suggest that the signals would be enhanced by using the noble metal film due to their properties of surface plasmon polaritons.

Quadratic Magneto-Optic Kerr Effect Investigations of Fe(100) Grown on Ir(100)

Magneto-optic Kerr effect (MOKE) is a widely used tool in surface physics to characterize magnetic thin films and single crystals. In most of its applications, the magneto-optic (MO) coupling is assumed to be linear on magnetization by neglecting the quadratic and higher dependences. However, recent observations have shown that the quadratic dependence cannot be always ignored, particularly in systems like ferromagnetic metal thin films, Heusler alloys, and even in half-metallic ferrimagnets. We have used a rotating field method to extract the QMOKE signal from Fe(100) film grown on (100) surface of a single crystal iridium. Our results show that the QMOKE signal from this system is comparable to the linear MOKE signal. We report the parameters L, b, and c which relate the linear (K) and quadratic (G and $\Delta \text{G}$ ) MO coupling coefficients. The real part of L, b, and c obtained from Fe(100)/Ir(100) are −9.75 ± 0.03 mdeg, 7.28 ± 0.09 mdeg, and −5.0 ± 0.1 mdeg, respectively.

Advanced MOKE magnetometry in wide-field Kerr-microscopy

The measurement of MOKE (Magneto-Optical Kerr Effect) magnetization loops in a wide-field Kerr microscope offers the advantage that the relevant domain images along the loop can be readily recorded. As the microscope's objective lens is exposed to the magnetic field, the loops are usually strongly distorted by non-linear Faraday rotations of the polarized light that occur in the objective lens and that are superimposed to the MOKE signal. In this paper, an experimental method, based on a motorized analyzer, is introduced which allows to compensate the Faraday contributions, thus leading to pure MOKE loops. A wide field Kerr microscope, equipped with this technology, works well as a laser-based MOKE magnetometer, additionally offering domain images and thus providing the basis for loop interpretation.