Kerr Microscopy Research Articles

Anisotropy and delayed relaxation of the anomalous Hall effect in GdFeCo/Ir/GdFeCo synthetic ferrimagnets

In synthetic ferrimagnets with perpendicular anisotropy GdFeCo/Ir/GdFeCo, the dependence of the hysteresis loops of the anomalous Hall resistance and the characteristics of the loops on the angle between the magnetic field and the plane of the sample are analyzed. The part of the anomalous Hall resistance corresponding to the spin-orbit torque has been identified. The field dependences of the resistance are sensitive to switching between the magnetic states of the two-layer ferrimagnet and they reproduce the shapes of the magnetization hysteresis loops, calculated with the interlayer exchange interaction, crystal anisotropy, and Zeeman energies at different angles between the field and sample. A slow (~30 min) magnetic relaxation of resistivity hysteresis after reorientation of the sample in magnetic field was found. Specific domain dynamics inherent in two-layer samples was revealed by Kerr microscopy. It was found that slow restoration of the resistivity hysteresis loop is due to domain propagation. Keywords: spin Hall effect, spin-orbit torque, magnetic relaxation, domain wall dynamics.

Анизотропия и медленная релаксация аномального эффекта Холла в синтетических ферримагнетиках GdFeCo/Ir/GdFeCo

In synthetic ferrimagnets with perpendicular anisotropy GdFeCo/Ir/GdFeCo, the dependence of the hysteresis loops of the anomalous Hall resistance and the characteristics of the loops on the angle between the magnetic field and the plane of the sample are analyzed. The part of the anomalous Hall resistance corresponding to the spin-orbit torque has been identified. The field dependences of the resistance are sensitive to switching between the magnetic states of the two-layer ferrimagnet and they reproduce the shapes of the magnetization hysteresis loops, calculated with the interlayer exchange interaction, crystal anisotropy, and Zeeman energies at different angles between the field and sample. A slow (~30 min) magnetic relaxation of resistivity hysteresis after reorientation of the sample in magnetic field was found. Specific domain dynamics inherent in two-layer samples was revealed by Kerr microscopy. It was found that slow restoration of the resistivity hysteresis loop is due to domain propagation.

Temperature-dependent Scaling Behavior of Magnetic Hysteresis Loop of CoFeB/Pd Multilayer Film

We report our experimental investigation on the temperature-dependent hysteresis loops and magnetic domain behaviors of CoFeB (0.4 nm)/Pd (1 nm) multilayer on polyimide substrate with a perpendicular magnetic anisotropy, by means of magneto-optical Kerr microscopy. Hysteresis characteristics such as saturation magnetization, coercivity, and hysteresis loop area with respect to the temperature have been analyzed, where powerlaw scaling behaviors are observed with microscopic magnetic domain patterns mediated by numerous nucleation sites.

Effect of TbF3 diffusion on the demagnetization behavior and domain evolution of sintered Nd–Fe–B magnets by electrophoretic deposition

We studied the magnetic properties and domain evolution of annealed and TbF3-diffused sintered Nd–Fe–B magnets using the electrophoretic deposition method. After TbF3 diffusion, the coercivity increased significantly by 9.9 kOe and microstructural analysis suggested that Tb favored the formation of the (Nd, Tb)2Fe14B shell phase in the outer region of the matrix grains. The first magnetization reversal and the dynamic successive domain propagation process were detected with a magneto-optical Kerr microscope. For the TbF3-diffused magnet, the magnetization reversal appeared at a larger applied field and the degree of simultaneous magnetization reversal decreased compared with an annealed magnet. During demagnetization after full magnetization, the occurrence of domain wall motion (DWM) in the reproduced multi-domain regions was observed by the step method. The maximum polarization change resulting from the reproduced DWM was inversely related to the coercivity. The increased coercivity for the diffused magnet was mainly attributed to the more difficult nucleation of the magnetic reversed region owing to the improved magneto-crystalline anisotropy field as a result of Tb diffusion.

Visualization of magnetization reversal processes by dynamic Kerr microscopy

Abstract Dynamic Kerr microscopy enables the analysis of structural defects in magnetic materials and their impact on the demagnetization behavior. Using new visualization techniques, it is possible to image the demagnetization process in the microstructure. It has been found that residual stresses and inclusions in soft magnets, for example, will impede magnetization. In permanent magnets, abnormally large grains, for example, will demagnetize first and then cause an accelerated demagnetization of adjacent grains.

Time-resolved detection of spin–orbit torque switching of magnetization and exchange bias

Current-induced magnetization switching driven by spin–orbit torques on sub-nanosecond timescales could be used to create fast and low-power spintronic devices. The time-resolved detection and analysis of switching trajectories in ferromagnet/antiferromagnet exchange-biased structures are the key to designing spin–orbit torque devices with high speed, but insight remains limited. Here we report the time-resolved detection of spin–orbit torque switching of the magnetization and exchange bias in platinum/cobalt/iridium–manganese heterostructures. Using time-resolved magneto-optical Kerr microscopy, combined with micromagnetic simulations, we show that the ferromagnets, as well as interfacial antiferromagnetic spins and exchange bias, can be partially switched by sub-nanosecond current pulses, which allows the switching probabilities to be flexibly controlled at multiple levels. We also show that the spin–orbit-torque-induced switching of the exchange bias, which intimately depends on the current density, can stabilize multilevelled magnetization switching within sub-nanosecond current pulses.

Magnetic triangular bubble lattices in bismuth-doped yttrium iron garnet

Magnetic bubbles have again become a subject of significant attention following the experimental observation of topologically nontrivial magnetic skyrmions. In recent work, tailoring the shape of the bubbles is considered a key factor for their dynamics in spintronic devices. In addition to the reported circular, elliptical, and square bubbles, here we observe triangular bubble domains in bismuth-doped yttrium iron garnet (Bi-YIG) using Kerr microscopy. The bubble domains evolve from discrete circular to latticed triangular and hexagonal shapes. Further, the orientation of the triangular bubbles in the hexagonal lattices can be flipped by decreasing the magnetic field. The sixfold in-plane magnetic anisotropy of Bi-YIG(111) crystal, which is presumably the mechanism underlying the triangular shape of the bubbles, is measured as 1179 erg/cm3. The study of the morphologies of topologically trivial bubbles in YIG offers insight into nontrivial spin textures, which is appealing for future spintronic applications.

Realization of Multi‐Level State and Artificial Synapses Function in Stacked (Ta/CoFeB/MgO)N Structures

AbstractSpintronic devices can realize multi‐state storage and be used to simulate artificial synapses or artificial neurons, which makes them have promising application prospect in the field of artificial neural networks (ANN). This work investigates the current‐induced magnetization reversal in stacked (Ta/CoFeB/MgO)N structures and their application in ANN. It is demonstrated that the complete current‐induced magnetization reversal with large intermediate transition region can be achieved in the sample with N = 2. The magneto‐optical Kerr microscope imaging shows that the large transition region for the sample is ascribed to the “layer‐by‐layer” reversal, owing to the difference of the coercivity of two CoFeB layers. In addition, the simulation of artificial synapses and artificial neurons function based on current‐induced magnetization reversal in the sample is also demonstrated. These results substantiate the stacked (Ta/CoFeB/MgO)N structures as a promising platform for realizing the multi‐level state and artificial synapses function, and its potential application in the field of ANN.

Microstructure, magnetic domains and magnetic properties of Ti/Fe/Ni81Fe19/Fe/Ti multilayer films by obliquely deposition

The metallic magnetic thin films are widely used in high-frequency magnetic devices, the reliability of which greatly depends on the saturation magnetization (Ms) and resonance frequency (fr) of the films. However, increasing the operating frequency of the magnetic thin film is extremely challenging. This study investigates the microstructure, magnetic domains and magnetic properties of obliquely deposited Ti/Fe/Ni81Fe19/Fe/Ti multilayer films. The coercivity distribution of the deposited films is investigated by first order reversal curve (FORC) measurements. The result of static magnetic properties revealed the in-plane uniaxial anisotropy with a high anisotropy field (Hk) up to 495 Oe. A magneto-optical Kerr microscope image displayed the multi-step magnetization reversal processes at different oblique angles. Complex permeability spectra indicated that the resonance frequency increases from 1.27 to 4.71 GHz as the deposition angle is increased from 0° to 25°. Thus, Ti/Fe/Ni81Fe19/Fe/Ti multilayer film with tunable magnetic performances is one of the good candidate materials for microwave devices.

Quantitative analyses of surface and bulk magnetization in Nd2Fe14B and SmCo5 single crystals: towards understanding the large N eff in nucleation-type magnets

The performance of sintered permanent magnets with nucleation-type coercivity mechanism is largely governed by the magnetic state of thin surface layers of constituent grains, and a deeper insight into magnetization–demagnetization processes occurring in the shell part of the grains is very important for further improvement of hard magnetic materials. In this work, we used Nd2Fe14B and SmCo5 single crystals as model objects. By applying magneto-optical Kerr microscopy and conventional magnetometry, we compare the magnetization–demagnetization processes occurring in the thin surface layer and in the volume of both single crystals. We show that upon magnetization along the c-axis, the volume of the single crystals saturates in the field, rigorously determined by demagnetization factor of the bulk sample, whereas in the surface layer a magnetic domain structure exists up to 1.88 T in Nd2Fe14B and 1.19 T in SmCo5 regardless of their bulk demagnetization factors. This means that the surface layer with orientation perpendicular to c-axis magnetizes as a thin magnetic film and has an effective demagnetization factor N eff ∼ 1. We also show that this effect can be reproduced in the framework of conventional finite element method modeling but the analytical solution of this problem still needs to be found. We believe that our findings can be useful for understanding of the formation of a high coercive state in nucleation-type permanent magnets, where the phenomenological concept of the large effective demagnetization factor N eff plays an important role.

Tuning the interfacial Dzyaloshinskii–Moriya interaction in perpendicularly magnetized CoFeB system

Magnetic field driven domain-wall motion in the creep regime is investigated with Kerr microscopy in out-of-plane magnetized as-deposited and annealed Ta/Pt/CoFeB/Pt multilayers. In addition to the effect of thermal annealing on the quasi-static magnetic properties of Ta (3 nm)/Pt (3 nm)/CoFeB ( nm)/Pt (2 nm) film stacks where , the study focuses on the modified interfacial Dzyaloshinskii–Moriya interaction (iDMI) that governs the dynamics of chiral domain walls. The strength of the iDMI decreases with an increase in both at a fixed annealing temperature and annealing temperature for a fixed . While the former behaviour arises due to the interfacial nature of iDMI, the latter correlates with the annealing induced suppression of magnetization and enhancement of interface roughness that could be, respectively, ascribed to B-segregation and Co-Fe-Pt intermixing.

Tuning magnetic interaction between two identical perpendicularly magnetized layers by a nonmagnetic spacer layer

Tuning magnetic properties of perpendicularly magnetized system is technologically important and have application in modern spintronics devices. In this article, we have considered TbFe based perpendicular trilayer magnetic system by inserting a nonmagnetic spacer layer in between. A systematic variation in thickness of non-magnetic Ti spacer (S) in the trilayer structure, TbFe (40 nm)/ Ti (0, 2, 5, 10, 15, 30 nm)/ TbFe (40 nm) is explored to understand and correlate the structural, magnetic, and microscopic properties of the system. Magnetostatic coupling dominates in these systems, and the strength of the coupling decreases as the S thickness increases. Ti and Fe forms a interfacial TiFe layer, roughness of which increases as the thickness of S increases. The increase in interfacial roughness causes effective magnetization compensation at S = 30 nm, where Hc and Mr are least compared to other spacer thicknesses. However, for S = 2 nm coupling strength was found to be highest compared to other cases, as a result, magnetic domains are compressed. Further increase in thickness of S leads to isolated domains and finally a single domain state is apparent for higher value of S. The experimental findings are supported with in-depth micromagnetic modeling. Kerr microscopy and first-order reversal imaging reveal nucleation and domain wall propagation as the dominated magnetization reversal process in these systems. These microscopic insights and tunability of magnetic properties of the trilayer system may find use in various spintronics applications.

Magnetic domain scanning imaging using phase-sensitive THz-pulse detection

In our study, we determine the alignment of magnetic domains in a CoFeB layer using THz radiation. We generate THz pulses by fs laser pulses in magnetized CoFeB/Pt heterostructures based on spin currents. An LT-GaAs Auston switch detects the radiation phase sensitively and allows us to determine the magnetization alignment. Our scanning technique with motorized stages, with step sizes in the sub-micrometer range, allows us to image two dimensional magnetic structures. Theoretically, the resolution is restricted to half of the wavelength if focusing optics in the far-field limit are used. By applying near-field imaging, the spatial resolution is enhanced to the single digit micrometer range. For this purpose, spintronic emitters in diverse geometric shapes, e.g., circles, triangles, squares, and sizes are prepared to observe the formation of magnetization patterns. The alignment of the emitted THz radiation can be influenced by applying unidirectional external magnetic fields. We demonstrate how magnetic domains with opposite alignment and different shapes divided by domain walls are created by demagnetizing the patterns using minor loops and imaged using phase sensitive THz radiation detection. For analysis, the data are compared to Kerr microscope images. The possibility of combining this method with THz range spectroscopic information of magnetic texture or antiferromagnets in direct vicinity to the spintronic emitter makes this detection method interesting for a much wider range of applications probing THz excitation in spin systems with high resolution beyond the Abbe diffraction limit, limited solely by the laser excitation area.

Gate-Tunable Helical Currents in Commensurate Topological Insulator/Graphene Heterostructures.

van der Waals heterostructures made from graphene and three-dimensional topological insulators promise very high electron mobilities, a nontrivial spin texture, and a gate-tunability of electronic properties. Such a combination of advantageous electronic characteristics can only be achieved through proximity effects in heterostructures, as graphene lacks a large enough spin-orbit interaction. In turn, the heterostructures are promising candidates for all-electrical control of proximity-induced spin phenomena. Here, we explore epitaxially grown interfaces between graphene and the lattice-matched topological insulator Bi2Te2Se. For this heterostructure, spin-orbit coupling proximity has been predicted to impart an anisotropic and electronically tunable spin texture. Polarization-resolved second-harmonic generation, Raman spectroscopy, and time-resolved magneto-optic Kerr microscopy are combined to demonstrate that the atomic interfaces align in a commensurate symmetry with characteristic interlayer vibrations. By polarization-resolved photocurrent measurements, we find a circular photogalvanic effect which is drastically enhanced at the Dirac point of the proximitized graphene. We attribute the peculiar gate-tunability to the proximity-induced interfacial spin structure, which could be exploited for, e.g., spin filters.

Field-free spin-orbit torque-induced perpendicularmagnetization switching in YIG/Ta/CoTb/Pt

<p indent="0mm">Spin-orbit torques (SOTs) provide a physical basis for designing new spintronic memory and logic devices. The key to the application is the realization of field-free SOT-driven magnetization switching. In this study, Ta/CoTb/Pt ferromagnetic multilayers were grown on yttrium iron garnet (YIG) magnetic insulating film. We achieved current-induced CoTb magnetization switching under zero magnetic field by using the magnetic dipole interaction field generated by the YIG magnetic insulating layer applied to the CoTb film with perpendicular magnetic anisotropy and the spin torque generated from Ta and Pt spin source layers. Experimentally, we measured the magnetization reversal behavior induced by pulse current through anomalous Hall effect and anomalous Hall resistance and observed the magnetic domain wall motion characteristics driven by pulse current under the magneto-optical Kerr microscope. Moreover, due to the influence of the magneto-optical Kerr microscope, it was found that for YIG/Ta/CoTb/Pt devices, the magnetic domain of ferromagnetic CoTb moves and completes magnetization switching by forming stripe-like features, which is significantly different from the magnetization switching characteristics of the Si/Ta/CoTb/Pt film. Our results may provide a new degree of freedom for the potential application value of the research of spintronic low-power devices, particularly field-free SOT-driven switching-based devices.

Magnetic properties in soft (CoFeB)/hard (Co) bilayers deposited under different Ar gas pressure

We have studied the effect of deposition pressure on the magnetization reversal, domains, anisotropy and Gilbert damping constant in the ferromagnetic (CoFeB and Co) single and bilayer samples. Hysteresis measured by magneto-optic Kerr microscopy for the single layer films prepared at higher deposition pressure indicate no change of loop shape i.e. isotropic behaviour. An enhancement of anisotropy has been observed in the bilayer samples than the single layer samples prepared at a particular deposition condition. However, increasing the deposition pressure to 50 sccm for the bilayer samples, anisotropy gets reduced. For single layer Co film deposited at 10 sccm exhibits branch and patch like domains for different angle between the easy axis and the external magnetic field. However, the Co film deposited at 50 sccm exhibits ripple like domains. In the case of single layer CoFeB, branch and patch like domains are observed deposited at 10 sccm. Patch like domains are found for the CoFeB films deposited at 50 sccm. Pinned labyrinth and ripple kind of magnetic domains along with the big branch domains are found in the bilayer samples. The pinned domains may be due to the interfacial exchange coupling. Similar values of damping constants have been observed for different thin films prepared at different deposition pressure.

Local anisotropy control of Pt/Co/Ir thin film with perpendicular magnetic anisotropy by surface acoustic waves

The control of perpendicular magnetic anisotropy (PMA) in thin films by strain has considerable potential for energy-efficient information storage and data processing. Here, we report on the control of PMA in Pt/Co/Ir thin films by the strain produced by standing surface acoustic waves (SAWs). A significant (∼21%) coercivity reduction (from 4.80 ± 0.03 to 3.80 ± 0.02 mT) can be obtained by applying a standing SAW with a center frequency of 93.35 MHz. Furthermore, the standing SAWs induce a greater-than 11-fold increase in magnetization reversal speed (from 168 ± 3 to up to 2100 ± 80 μm2/s) at 3.2 mT for a total applied RF power of 22.5 dBm. During application of SAWs, wide-field Kerr microscopy reveals the formation of domains in stripes with a periodicity of half of the SAW wavelength. Micromagnetic simulations indicate that the anti-nodes of the standing SAW locally lower the anisotropy due to the magneto-elastic coupling effect, decreasing domain nucleation field while promoting magnetization reversal. Our study suggests the possibility of remote and energy-efficient control of magnetization switching using SAWs.

Kinetics of Domain Structure in Co/Pt/Co Ultrathin Films with Ferromagnetic Interlayer Exchange Interaction: Dependence on Interlayer Thickness

We studied the in-field evolution of the domain structure in ultrathin Co(0.6 nm)/Pt(t)/Co(0.6) nm trilayers with perpendicular magnetic anisotropy for 5 nm &lt; t &lt; 6 nm using polar Kerr microscopy. The critical interlayer thickness tcr = 5.3 nm was found to separate two principal patterns of domain behavior including interlayer correlations and motility of the domain walls. It is shown that magnetization in both Co layers is coupled with strong ferromagnetic interaction for small Pt thickness (t &lt; tcr), while this coupling is weak for thicker (t &gt; tcr) Pt layers. Nonlinear dependence of the wall displacement on the field value is observed. The established final position of domain walls after relaxation depends on the Pt layer thickness. It is determined by balance of the interlayer exchange and energy gain due to the field. The mechanism of wall stabilization is considered in the case of independent wall motion. In the region with weak coupling, dependence of the interlayer interaction energy on Pt thickness was measured.

Berry curvature-induced local spin polarisation in gated graphene/WTe2 heterostructures

Experimental control of local spin-charge interconversion is of primary interest for spintronics. Van der Waals (vdW) heterostructures combining graphene with a strongly spin-orbit coupled two-dimensional (2D) material enable such functionality by design. Electric spin valve experiments have thus far provided global information on such devices, while leaving the local interplay between symmetry breaking, charge flow across the heterointerface and aspects of topology unexplored. Here, we probe the gate-tunable local spin polarisation in current-driven graphene/WTe2 heterostructures through magneto-optical Kerr microscopy. Even for a nominal in-plane transport, substantial out-of-plane spin accumulation is induced by a corresponding out-of-plane current flow. We present a theoretical model which fully explains the gate- and bias-dependent onset and spatial distribution of the intense Kerr signal as a result of a non-linear anomalous Hall effect in the heterostructure, which is enabled by its reduced point group symmetry. Our findings unravel the potential of 2D heterostructure engineering for harnessing topological phenomena for spintronics, and constitute an important step toward nanoscale, electrical spin control.

Tuning the magnetic properties of Fe thin films with RF-sputtered amorphous carbon

RF-sputtered amorphous carbon (a-C) offers a simple and cheap pathway to tune the magnetic properties of transition metal thin films for magnetic memories and different spintronic applications. This paper describes changes in the magnetic properties of iron thin films with a-C overlayers. In as-deposited samples, hybridisation and intermixing at the Fe/a-C interface leads to magnetic softening (Liu et al., 2006) [1], with a reduction in the coercive field (Hc) up to a factor of five for Fe/a-C/Fe trilayers, and a 10–30% lower saturation magnetization as a function of the metal film thickness. After annealing at 500 °C, inter-diffusion and graphitization of the carbon layer results in up to a factor five increased coercivity due to increased pinning as shown via Kerr microscopy. Therefore, RF-sputtered carbon overlayers and post-processing can tune the anisotropy and domain configuration of metallic thin films in a synthesis methodology that is simple, cheap and sustainable.