Kerr Microscopy Research Articles

Interpretation of Kerr Microscopic Domain Contrast on Curved Surfaces

In this letter we address a technical peculiarity of wide-field magneto-optical Kerr microscopy, which may easily lead to a misinterpretation of the domain contrast on curved surfaces. On the example of circumferentially magnetized domains in ferromagnetic microwires we show that the checkerboard domain pattern, which typically shows up in polar Kerr sensitivity, is caused by the superposition of longitudinal Kerr contrasts that arise from in-plane magnetization components and inclined light incidence rather than from the magnetization components perpendicular to the focal plane as claimed in the literature. Experimental ways to avoid such peculiar contrasts are demonstrated.

Analysis of Magneto-Optical Hysteresis Loops of Amorphous and Surface-Crystalline Fe-Based Ribbons.

Three Fe-based ribbon-type samples prepared by a conventional planar flow casting process are studied from the viewpoint of the amorphous Fe80Si4B16 and partially surface crystallized Fe80Si10B10, and Fe80.5Nb6.9B12.6, microstructures. Surface magnetic properties are investigated by magneto-optical Kerr microscopy, allowing the measurement of a local hysteresis loop from a selected area on the ribbon surface, and simultaneously, a domain structure corresponding to a definite point at the loop. For an amorphous sample, the changes in the slopes of hysteresis loops are related either to the size of the selected surface area, from which the loop is measured, or to the type, width, and movement of magnetic domains through this area. In the first case, the resizing of the area simulates an effect of changing the diameter of the incident laser beam on the magneto-optical properties of the ribbon. In the latter case, the observed wide-curved and fingerprint domains are responsible for markedly different shapes of the hysteresis loops at lower magnetic fields. If the surface is crystallized, the magnetic properties are more homogenous, showing typical one-jump magnetization reversal with less dependence on the size of the surface area. The magneto-optical experiments are completed by transmission electron microscopy and magnetic force microscopy.

Study of magnetic zigzag domain walls and magnetization reversal process in polycrystalline cobalt thin films: Effect of thickness and crystallographic texturing

The present work reports the effects of crystallographic texturing on the formation of magnetic zigzag domain walls (DWs) and magnetization reversal process in polycrystalline cobalt thin films of different thickness (10 to 91 nm) deposited by magnetron sputtering technique. Grazing incidence x-ray diffraction measurements, carried out in both in-plane and out-of-plane geometry reveals texturing for thinner thickness (≤ 51 nm) films. Magnetic microstructure and magnetization reversal process is studied with Kerr microscopy by utilizing two in-plane orthogonal sensitivities i.e., longitudinal and transverse sensitivity. It is found that with increasing film thickness, the magnetization reversal process changes significantly. Analysis of angular variation of coercivity reveals that for thinner thickness films (≤ 51 nm), the magnetization reversal process follows two-phase model, also substantiated by the observed magnetic domain structure during reversal process. Nevertheless, it is observed that the zigzag DW angle and magnetization ripple microstructure are found to depend on film thickness. The variation of zigzag DWs angle and magnetic ripple variation is explained in terms of the magnetic anisotropy with the thickness of film employing the crystallographic texturing. Further, we have observed crossover in hysteresis loops measured along for angles ≈20∘ away from the hard axis magnetization. The plausible reason could be due to the superposition of the uniaxial anisotropy generated due to magneto-crystalline anisotropy from the hexagonal phase and biaxial anisotropy generated due to the cubic phase of Co.

Integration and characterization of micron-sized YIG structures with very low Gilbert damping on arbitrary substrates

We present a process that allows the transfer of monocrystalline yttrium-iron-garnet microstructures onto virtually any kind of substrate. The process is based on a recently developed method that allows the fabrication of freestanding monocrystalline YIG bridges on gadolinium-gallium-garnet. Here, the bridges' spans are detached from the substrate by a dry etching process and immersed in a watery solution. Using drop-casting, the immersed YIG platelets can be transferred onto the substrate of choice, where the structures finally can be reattached and, thus, be integrated into complex devices or experimental geometries. Using time-resolved scanning Kerr microscopy and inductively measured ferromagnetic resonance, we can demonstrate that the structures retain their excellent magnetic quality. At room temperature, we find a ferromagnetic resonance linewidth of μ0ΔHHWHM≈195 μT and we were even able to inductively measure magnon spectra on a single micrometer-sized yttrium-iron-garnet platelet at a temperature of 5 K. The process is flexible in terms of substrate material and shape of the structure. In the future, this approach will allow for types of spin dynamics experiments until now unthinkable.

Spin–orbit torque driven four-state switching in splicing structure

We prepared a splicing structure by using electron beam lithography, in which Pt and Ta were spliced together as the spin source upon perpendicularly magnetized Pt/Co/Pt heterostructures. It is found that Pt and Ta can modulate both spin–orbit torque and Dzyaloshinskii–Moriya interaction on the same magnetic layer, respectively. The four-state magnetization switching is achieved with the combinations of two spliced parts, which is observed by using a magneto-optical Kerr microscope. The initial nucleation position determines the chirality. The Pt side stabilizes a right-hand Néel domain wall and the Ta side stabilizes a left-hand Néel domain wall. Our study provides a method for further increasing the storage density and studying spin–orbit torque laterally.

Effect of Si/B ratio on glass-forming ability, phase transitions and magnetic properties in (Fe40Ni40SixByCu1)0.97Nb0.03 alloys

The (Fe40Ni40SixByCu1)0.97Nb0.03 (Si/B = 3:1, 2:1, 1:1, 1:2 and 1:3) alloys were fabricated through the melt-spun technique. The effects of Si/B ratio on the glass-forming ability (GFA), phase transitions and soft magnetic properties were investigated. Results showed that when Si/B = 1:1, 1:2 and 1:3, the GFA of the FeNiSiBCuNb alloy is good, and the amorphous structure is homogeneous. We found a phase transition from the amorphous phase to α-Fe (Si) phase and then to γ-(Fe,Ni)Si phase in Si/B = 1:1 alloy by employing a simple heat treatment process. The alloys with Si/B = 1:1 and 1:2 containing a low coercivity of 0.30 A/m and 0.25 A/m domain structures were characterized with magneto-optical Kerr microscope (MOKE) to unveil that the internal stress relief is beneficial to the magnetization process. The three alloys exhibit different crystallization behaviors, and moderate Si can effectively inhibit γ-(FeNi) grains overgrowth.

Direct current tolerant characteristics of FeCuNbSiB nanocrystalline in tensile stress annealing

The effect of relevant factors on the magnetic properties and structures of FeCuNbSiB nanocrystalline alloy in the tensile stress process was investigated. The continuous annealing apparatus was designed to control the tensile stress, annealing temperature, isothermal time, heating rate, and cooling rate. Different levels of relevant factors were performed on the ribbon samples using orthogonal experiments designing and then studied by simple variable controlling. To compare the DC tolerance property, the DC-bias field H0.98 is defined. The results show that during annealing the above relevant factors significantly affects the magnetic property including the DC-bias field and core loss. It is observed that the DC-bias field H0.98 and the unit weight core loss of alloy Pm reach the optimum value in 570 and 590 °C and subsequently deteriorate with the increasing annealing temperature. The DC tolerance property is proportional to the tensile stress and when the tensile stress increasing up to 94 MPa, the DC-bias field H0.98 shows a linear relationship with tensile stress. On the other hand, the DC tolerance property is negligible corresponding to the tensile stress smaller than 94 MPa and conversely, the core loss of alloy decrease significantly. The DC-bias field H0.98 of annealing samples exhibit a comparably stable value in the time range of 90 s to 900 s. In addition, the core loss Pm shows a similarly linear decreasing function with isothermal time from 20 to 900 s. Based on the results, the DC-bias field H0.98 is more than 3500 A/m when the tensile stress exceeds 230 MPa and the unit weight of core loss Pm is about 63 W/kg corresponding to f = 100 kHz and Bm = 300 mT when the isothermal time is 900 s, respectively. Analysis with magneto-optical Kerr microscopy reveals that the width of magnetic domains decreases with an increasing tensile stress and annealing temperature. Besides, when the isothermal time exceed 480 s, the magnetic domain wall shows a transformation from perpendicular to parallel to the direction of magnetization. In addition, the relationship between magnetic properties and relevant factors in the tensile stress process is discussed.

Magnetic properties of a Fe-based amorphous alloy with stress gradient

The magnetic properties and domain structure of a Fe78Si9B13 amorphous alloy with a stress gradient from upper to the lower surface were investigated. We found that the magnetic properties of the samples vary significantly upon stress variation. Specifically, the magnetic anisotropy increases linearly with the maximum stress, whereas the coercivity, although higher than the one without stress, remains reasonably small. The phase contrast in magnetic force microscopy image and the fingerprint-like domains in magneto-optical Kerr microscopy images for ribbon surface with compressive stress demonstrate the presence of perpendicular anisotropy due to the magneto-elastic coupling. As a result, the compressive stress hinders the reversible magnetization vector rotation and makes the magnetization hard to be saturated. However, the stress causes no irreversible effects, neither on the domain wall movement nor on the magnetization rotation. Thus it is irrelevant to the coercivity. These results are different from the situation of the inhomogeneously distributed quenched-in stress enlarging the coercivity in Fe-based amorphous alloys.

Impact of growth conditions on the domain nucleation and domain wall propagation in Pt/Co/Pt stacks

Understanding the effect of fabrication conditions on domain wall (DW) motion in thin films with perpendicular magnetization is a mandatory issue in order to tune their properties aiming to design spintronics devices based on such phenomenon. In this context, the present work intends to show how different growth conditions may affect DW motion in the prototypical system Pt/Co/Pt. The trilayers were deposited by dc sputtering, and the parameters varied in this study were the Co thickness, the substrate roughness and the base pressure in the deposition chamber. Magneto-optical Kerr effect-based magnetometry and microscopy combined with x-ray reflectometry, atomic force microscopy and transmission electron microscopy were adopted as experimental techniques. This permitted us to elucidate the impact on the hysteresis loops and on the DW dynamics, produced by different growth conditions. As other authors, we found that Co thickness is strongly determinant for both the coercive field and the DW velocity. On the contrary, the topographic roughness of the substrate and the base pressure of the deposition chamber evidence a selective impact on the nucleation of magnetic domains and on DW propagation, respectively, providing a tool to tune these properties.

Electrodeposited Heusler alloy films with enhanced magneto-optical property

Growth of Heusler alloy films through techniques other than conventional physical vapor deposition presents a challenge for spintronic materials research. In this study an electrochemical deposition approach for these alloys is demonstrated in which sub-micron thickness control can be achieved. By using magneto optic Kerr effect and Kerr microscopy measurements a systematic study of surface magnetization reversal behavior of these films is presented. The results demonstrate that the growth of sub-micron films of Co2FeSn leads to significant enhancement in magneto optic Kerr rotation. A large Kerr rotation value up to 300 mdeg was observed for thin films of thickness ∼200 nm. The modified scheme reported here opens up a path for exploration of electrochemical method as a facile route for fabricating Heusler alloys with improved surface property.

Ultralow Switching-Current Density in All-Amorphous W−Hf/Co−Fe−B/TaOx Films

We study current-induced deterministic magnetization switching and domain wall motion via polar Kerr microscopy in all-amorphous W$_{66}$Hf$_{34}$/CoFeB/TaO$_\text{x}$ with perpendicular magnetic anisotropy and large spin Hall angle. Investigations of magnetization switching as a function of in-plane assist field and current pulse-width yield switching current densities as low as $3\times 10^{9}$ A/m$^2$. We accredit this low switching current density to a low depinning current density, which was obtained from measurements of domain wall displacements upon current injection. This correlation is verified by investigations of a Ta/CoFeB/MgO/Ta reference sample, which showed critical current densities of at least one order of magnitude larger, respectively.

Magneto-Optical Tools to Study Effects in Dirac and Weyl Semimetals

Research regarding topological Dirac and Weyl semimetals contributes to our understanding not only of the field of solid-state physics, but also the field of high-energy physics as the physics of Dirac and Weyl semimetals resembles the physics of Dirac and Weyl massless fermions. In condensed matter physics, the Weyl nodes are detached in momentum space and may be realized as emergent quasiparticles with a distinct chirality, left-handed or right-handed. These states lead to phenomena like the chiral anomaly and the anomalous Hall effect (AHE). Furthermore, the combination of quantum effects and magnetic effects in magnetic Weyl semimetals is very intriguing. Magneto-optical tools, which are usually used to study magnetic phenomena, also contribute to magnetic Weyl semimetals. Moreover, with the magneto-optical technique, it is possible to follow the dynamics of the processes and to study the lifetime of the Weyl states. In this work, we review and discuss the effects of using magneto-optical tools for studying quantum effects like the chiral anomaly or magnetic effects in magnetic Weyl and Dirac systems using the magneto-optical Kerr effect (MOKE) or Faraday systems including a single detection and imaging. Examples of using magneto-optical systems in the research of ultrafast magnetic dynamics of thin polycrystalline nickel and permaloy are reviewed as are the magnetic spatial dynamics by employing magneto-optical Kerr or Faraday microscopy tools with ferromagnetic thin films. Interestingly, the excitation of a circularly polarized femtosecond laser pulse could lead to the breakage of time-reversal symmetry and to the transformation of the Dirac state to the Floquet–Weyl semimetal state. The development of a suitable ultrafast magneto-optical system for Weyl systems is discussed, and the practical difficulties for the realization of such a system are considered.

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.

Exchange bias effect in Fe/LaAlO3: An interface induced effect

The present work reports the appearance of the exchange bias (EB) effect at low temperatures in epitaxial Fe thin film deposited on single crystalline LaAlO3(001) substrate. The observed EB effect vanishes around 30 K indicating that it could be due to the formation of antiferromagnetic FeO layer at the interface between Fe and LaAlO3 substrate. The EB in Fe/LaAlO3(001) is further substantiated by comparing the observations with polycrystalline Fe film deposited on silicon substrate, kept adjacent to LaAlO3 substrate during the deposition. Magnetic microstructure measured by Kerr microscopy corroborates the structural data of Fe film i.e., cubic four-fold anisotropy and isotropic nature for Fe film deposited on LaAlO3 and silicon substrates, respectively. Further, low temperature Kerr microscopy results reveal that the magnetization reversal process significantly gets modified due to the induced unidirectional anisotropy as a consequence of FeO layer formation at Fe/LaAlO3 interface.

Anomalous Behavior of Magnetic Anisotropy of Amorphous Co40Fe43B17 Thin Film Sandwiched Between Mo Layers

Anomalous behavior of magnetic anisotropy in Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">43</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sub> film sandwiched between Mo layers has been observed as a function of thermal annealing at different temperatures. The as-deposited amorphous film exhibits a uniaxial magnetic anisotropy, the origin of which can be traced to quenched-in stresses. With thermal annealing up to 300 °C, anisotropy remains unchanged. We observe an anomalous magnetic reversal near the hard axis (HA) in the samples annealed at 350 °C and above. The anomaly of the occurrence of nonuniform magnetization near HA can be explained in terms of distribution of easy magnetization direction, as evidenced by the ripple domain structure in Kerr microscopy images.

Antiferromagnetic domain switching modulated by an ultrathin Co interlayer in the Fe/Co/CoO/MgO(001) system

Using a combination of hysteresis loop, Kerr microscope, and x-ray magnetic circular dichroism measurements, we investigated the antiferromagnetic (AFM) domain switching process modulated by the sub-nm thick Co inserting layer in a single crystalline Fe/Co/CoO/MgO(001). The CoO AFM domain switching occurs at lower temperature for the thicker Co interlayer, and the activation energy barrier of CoO AFM domain switching decreases as the Co interlayer thickness increases. The exchange coupling strength between the AFM spins in CoO layer and the ferromagnetic spins in the Fe/Co bilayer is found to be independent of the Co layer thickness. Our results suggest an approach to modulate the dynamic properties of AFM domains with an interfacial modification.

Creating, probing and confirming tetragonality in bulk FeNi alloys

Effects derived from the simultaneous application of passive, uniform saturating magnetic field and tensile stress during long-time annealing utilizing a unique and simple processing tool, the MultiDriver furnace, are reported. In particular, the evolution of the crystalline structure and the magnetic domain configuration of severely deformed and subsequently annealed equiatomic FeNi alloys were revealed by high-energy synchrotron X-Ray diffraction and Magneto-Optic Kerr Microscopy. These alloys are known to undergo a first-order magnetic phase transformation to a chemically ordered tetragonal (L10) structure in astronomical timeframes. MultiDriver-annealed specimens are found to retain a tetragonal crystallographic state and texture that is similar to that of the precursor deformed state, with a smaller unit cell volume and a larger c/a ratio relative to those of control samples that were annealed under the same conditions but without magnetic field and stress drivers. Magnetic domain images echo these observations: while as-cast FeNi alloys exhibit domain patterns consistent with the presence of a stress-modified cubic magnetocrystalline anisotropy, large areas of domains with uniaxial anisotropy are present in MultiDriver-annealed samples. While no chemical order was demonstrated in these samples, a strategy is proposed for achieving the ordered state by employing a passive magnetic gradient in the MultiDriver furnace to enhance diffusion.

Demagnetization Processes in Multilayered Permalloy-Based Film Structures

By examples of multilayered film structure Cu/[FeNi/Cu]10 deposited by the method of ion-plasma sputtering, a comparative analysis of the magnetic hysteresis loops measured on a vibrating magnetometer, a magnetic measuring complex with primary converter on the basis of SQUID (SQUID magnetometer), and a magneto-optical Kerr microscope is carried out. The use of a SQUID magnetometer made it possible to detect the step-by-step nature of remagnetization of the film structure, the presence of which was confirmed by observations on the Kerr microscope in the quasi-static remagnetization mode.

Electric field control of interfacial Dzyaloshinskii-Moriya interaction in Pt/Co/AlOx thin films

We studied electric field modification of magnetic properties in a Pt/Co/AlOx trilayer via magneto-optical Kerr microscopy. We observed the spontaneous formation of labyrinthine magnetic domain structure due to thermally activated domain nucleation and propagation under zero applied magnetic field. A variation of the period of the labyrinthine structure under electric field is observed as well as saturation magnetization and magnetic anisotropy variations. Using an analytical formula of the stripe equilibrium width we estimate the variation of the interfacial Dzyaloshinskii-Moriya interaction under electric field as function of the exchange stiffness constant.

Spin–orbit torque driven multi-level switching in He+ irradiated W–CoFeB–MgO Hall bars with perpendicular anisotropy

We have investigated the spin–orbit torque-driven magnetization switching in W/CoFeB/MgO Hall bars with perpendicular magnetic anisotropy. He+ ion irradiation through a mask has been used to reduce locally the effective perpendicular anisotropy at a Hall cross. Anomalous Hall effect measurements combined with Kerr microscopy indicate that the switching process is dominated by domain wall (DW) nucleation in the irradiated region followed by rapid domain propagation at a current density as low as 0.8 MA/cm2 with an assisting in-plane magnetic field. Thanks to the implemented strong pinning of the DW at the transition between the irradiated and the non-irradiated region, an intermediate Hall resistance state is induced, which is further verified by finite element simulations. Such a method to control electrically multi-level resistances using He+ ion irradiation shows great potential in realizing neuromorphic and memristor devices.