Magneto-optical Kerr Measurements Research Articles

Asymmetric Manipulation of Perpendicular Exchange Bias and Programmable Spin Logical Cells by Spin-Orbit Torque in a Ferromagnet/Antiferromagnet System.

Antiferromagnets are competitive candidates for the next generation of spintronic devices owing to their superiority in small-scale and low-power-consumption devices. The electrical manipulation of the magnetization and exchange bias (EB) driven by spin-orbit torque (SOT) in ferromagnetic (FM)/antiferromagnetic (AFM) systems has become focused in spintronics. Here, the realization of a large perpendicular EB field in Co/IrMn and the effective manipulation of the magnetic moments of the magnetic Co layer and EB field by SOT in Pt/Co/IrMn system is reported. During the SOT-driven switching process, an asymmetrically manipulated state is observed. Current pulses with the same amplitude but opposite directions induce different magnetization states. Magneto-optical Kerr measurements reveal that this is due to the coexistence of stable and metastable antiferromagnetic domains in the AFM. Exploiting the asymmetric properties of these FM/AFM structures, five spin logic gates, namely AND, OR, NOR, NAND, and NOT, are realized in a single cell via SOT. This study provides an insight into the special ability of SOT on AFMs and also paves an avenue to construct the logic-in-memory and neuromorphic computing cells based on the AFM spintronic system.

Asymmetric Manipulation of Perpendicular Exchange Bias and Programmable Spin Logical Cells by Spin-Orbit Torque in a Ferromagnet/Antiferromagnet System.

Antiferromagnets are competitive candidates for the next generation of spintronic devices owing to their superiority in small-scale and low-power-consumption devices. The electrical manipulation of the magnetization and exchange bias (EB) driven by spin-orbit torque (SOT) in ferromagnetic (FM)/antiferromagnetic (AFM) systems has become focused in spintronics. Here, the realization of a large perpendicular EB field in Co/IrMn and the effective manipulation of the magnetic moments of the magnetic Co layer and EB field by SOT in Pt/Co/IrMn system is reported. During the SOT-driven switching process, an asymmetrically manipulated state is observed. Current pulses with the same amplitude but opposite directions induce different magnetization states. Magneto-optical Kerr measurements reveal that this is due to the coexistence of stable and metastable antiferromagnetic domains in the AFM. Exploiting the asymmetric properties of these FM/AFM structures, five spin logic gates, namely AND, OR, NOR, NAND, and NOT, are realized in a single cell via SOT. This study provides an insight into the special ability of SOT on AFMs and also paves an avenue to construct the logic-in-memory and neuromorphic computing cells based on the AFM spintronic system.

Verification of ultrafast spin transfer effects in iron-nickel alloys

The optical intersite spin transfer (OISTR) effect was recently verified in Fe50Ni50 using extreme ultraviolet magneto-optical Kerr measurements. However, one of the main experimental signatures analyzed in this work, namely a magnetic moment increase at a specific energy in Ni, was subsequently found also in pure Ni, where no transfer from one element to another is possible. Hence, it is a much-discussed issue whether OISTR in FeNi alloys is real and whether it can be verified experimentally or not. Here, we present a comparative study of spin transfer in Fe50Ni50, Fe19Ni81 and pure Ni. We conclusively show that an increase in the magneto-optical signal is indeed insufficient to verify OISTR. However, we also show how an extended data analysis overcomes this problem and allows to unambiguously identify spin transfer effects. Concomitantly, our work solves the long-standing riddle about the origin of delayed demagnetization behavior of Ni in FeNi alloys.

Fluence and Temperature Dependences of Laser-Induced Ultrafast Demagnetization and Recovery Dynamics in L10-FePt Thin Film.

The fundamental mechanisms of ultrafast demagnetization and magnetization recovery processes in ferromagnetic materials remain incompletely understood. The investigation of different dynamic features which depend on various physical quantities requires a more systematic approach. Here, the femtosecond laser-induced demagnetization and recovery dynamics in L10-Fe0.5Pt0.5 alloy film are studied by utilizing time-resolved magneto-optical Kerr measurements, focusing on their dependences of excitation fluence and ambient temperature over broad ranges. Ultrafast demagnetization dominated by Elliott-Yafet spin-flip scattering, and two-step magnetization recovery processes are found to be involved in all observations. The fast recovery time corresponding to spin-lattice relaxation is much shorter than that of many ferromagnets and increase with excitation fluence. These can be ascribed to the strong spin-orbit coupling (SOC) demonstrated in FePt and the reduction of transient magnetic anisotropy, respectively. Surprisingly, the demagnetization time exhibits no discernible correlation with ambient temperature. Two competitive factors are proposed to account for this phenomenon. On the other hand, the spin-lattice relaxation accelerates as temperature decreases due to enhanced SOC at lower ambient temperature. A semiquantitative analysis is given to get a visualized understanding. These results offer a comprehensive understanding of the dynamic characteristics of ultrafast demagnetization and recovery processes in iron-based materials with strong SOC, highlighting the potential for regulating the magnetization recovery process through temperature and laser fluence adjustments.

Experimental observation of transition from type I to type II ultrafast demagnetization dynamics in chemically disordered Fe60Al40 thin film, driven by laser fluence

Understanding of the laser-induced ultrafast demagnetization dynamics is one of the most challenging and hot topics in magnetism research due to its potential applications in magnetic storage devices and the field of spintronics. Recently, a laser-induced switching of ferromagnetism, driven by a disorder–order transition on FeAl thin films, has been experimentally demonstrated. The switching of ferromagnetic ordering by ultrafast laser pulses in FeAl thin films may open new possible applications of this material such as magnetic data storage and manipulation. Since the speed of the magnetic switching of magnetic states in thin films is one of the critical parameters for these applications, here we used time resolved magneto-optical Kerr measurements to investigate the demagnetization dynamics of Fe60Al40 thin films at room temperature. We have for the first time observed a clear transition from one-step dynamics (type I) to two-step (type II) dynamics in the same material by increasing pump laser fluence. This experimental observation may give a strong confirmation that the ultrafast demagnetization process can be treated as a thermal process and is driven by the difference between temperatures of the electron and spin systems.

Tunable Exchange-Bias-Like Effect in Bi -Substituted Gadolinium Iron Garnet Film

Using magneto-optical Faraday and Kerr measurements, we investigate the magnetic and magneto-optical properties of a thick $\mathrm{Bi}$-substituted gadolinium iron garnet film over a broad range of wavelengths (250--850 nm) and temperatures (150--300 K), including the magnetization compensation point, ${T}_{M}$. We observe an exchange-bias-like effect in the vicinity of ${T}_{M}$. By slightly changing the sample temperature, we can precisely tune the bias field, which reaches a magnitude 6 times higher than the coercive field. We explain this phenomenon by considering the short-range superexchange interaction and a change in the magnetic behavior when moving from the surface to the bulk of the film. This finding may lead to the development of single-film magneto-optical devices based on the exchange-bias effect.

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

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

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

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

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.

Large exchange splitting in monolayer graphene magnetized by an antiferromagnet

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

Ultrafast demagnetization of Pt magnetic moment in L10-FePt probed by magnetic circular dichroism at a hard x-ray free electron laser

Unraveling the origin of ultrafast demagnetization in multisublattice ferromagnetic materials requires femtosecond x-ray techniques to trace the magnetic moment dynamics on individual elements, but this could not yet be achieved in the hard x-ray regime. We demonstrate here the first ultrafast demagnetization dynamics in the ferromagnetic heavy 5d-transition metal Pt using circularly-polarized hard x-rays at an x-ray free electron laser (XFEL). The decay time of laser-induced demagnetization of L10-FePt is determined to be using time-resolved x-ray magnetic circular dichroism at the Pt L3 edge, whereas magneto-optical Kerr measurements indicate the decay time for the total magnetization as . A transient magnetic state with a photo-modulated ratio of the 3d and 5d magnetic moments is demonstrated for pump–probe delays larger than 1 ps. We explain this distinct photo-modulated transient magnetic state by the induced-moment behavior of the Pt atom and the x-ray probing depth. Our findings pave the way for the future use of XFELs to disentangle atomic spin dynamics contributions.

Switching fields and their distributions in rounded-rectangle [Co/Pd]8 nanodots and nanorings with identical configuration

We present two different types of magnetization reversal behaviors for the identically-configured Co/Pd multilayers with rounded-rectangular nanodots and nanorings for various edge-to-edge distances (s), by using focused magneto-optic Kerr measurements and magnetic force microscopy measurements. For the nanodots with a fixed outer diameter of d = 580 nm, the switching field Hsw and switching field distribution (SFD) are almost invariant with respect to s. However, the Hsw and SFD for the nanodot arrays could be easily modulated by varying d, which is ascribed to the size effect. In contrast, the Hsw and SFD are strongly dependent on s for the nanorings. Compared to the strongly coupled magnetic moments of the closely-packed nanorings, the magnetostatic interaction among nanodots is negligible, which should be responsible for the observed different magnetization reversal behaviors.

Magnetization at the interface of Cr2O3 and paramagnets with large stoner susceptibility

From the Cr 2p3/2 x-ray magnetic circular dichroism signal, there is clear evidence of interface polarization with overlayers of both Pd and Pt on chromia (Cr2O3). The residual boundary polarization of chomia is stronger for a Pt overlayer than in the case of a Pd overlayer. The reduction of chromia boundary magnetization with a paramagnetic metal overlayer, compared to the free surface, is interpreted as a response to the induced spin polarization in Pt and Pd. Magnetization induced in a Pt overlayer, via proximity to the chromia boundary magnetization, is evident in the polar magneto-optical Kerr measurements. These results are essential to explainations why Pt and Pd are excellent spacer layers for voltage controlled exchange bias, in the [Pd/Co]n/Pd/Cr2O3 and [Pt/Co]n/Pt/Cr2O3 perpendicular magneto-electric exchange bias systems. The findings pave the way to realize ultra-fast reversal of induced magnetization in a free moment paramagnetic layer, with possible application in voltage-controlled magnetic random access memory.

Verification of antiferromagnetic exchange coupling at room temperature using polar magneto-optic Kerr effect in thin EuS/Co multilayers with perpendicular magnetic anisotropy

We report on magneto-optic Kerr measurements in polar geometry carried out on a series of thin Co/EuS multilayers on suitable Co/Pd-multilayer substrates. Thin Co/EuS multilayers of a few nanometers individual layer thickness usually have their magnetization in plane. Co/Pd multilayers introduce a perpendicular magnetic anisotropy in the Co/EuS layers deposited on top, thus making it possible to measure magneto-optic signals in the polar geometry in remanence in order to study exchange coupling. Magneto-optic Kerr-effect spectra and hysteresis loops were recorded in the visible and ultraviolet photon-energy range at room temperature. The EuS contribution to the magneto-optic signal is extracted at 4.1 eV by combining hysteresis loops measured at different photon energies with polar magneto-optic Kerr-effect spectra recorded in remanence and in an applied magnetic field of 2.2 T. The extracted EuS signal shows clear signs of antiferromagnetic coupling of the Eu magnetic moments to the Co layers. This implies that the ordering temperature of at least a fraction of the EuS layers is above room temperature proving that magneto-optic Kerr-effect spectroscopy can be used here as a quasi-element-specific method.

Structural and magnetic properties of the transition metals (TM[dbnd]Co, Ni) and Nb co-doped SrTiO3 thin films

In this article, we report the fabrication of transition metal (TMCo/Ni) and Nb co-doped SrTiO3 (Ni0.06Nb0.05Sr0.12Ti0.16O0.61 and Co0.08Nb0.05Sr0.11Ti0.15O0.61) thin films of thickness around 1.1μm on monocrystalline Si (100) substrate by RF sputtering at 350°C. The as-deposited amorphous films were annealed at 800°C in vacuum to obtain crystalline structure. The X-ray diffraction studies revealed that the annealed polycrystalline films are having a cubic perovskite structure. The surface and cross-sectional morphologies, grain size, elements and structure of the films were studied by atomic force microscope, scanning and transmission electron microscopes. The microstructure and selected area electron diffraction (SAED processed pattern) also confirms the perovskite structure of the SrTiO3. The magnetic behaviour of the films was studied by vibrating sample magnetometry and magneto-optic Kerr measurements. The ferromagnetic responses were observed in both materials at room temperature.

Enhancement of crystallization in Fe[sbnd]Mo[sbnd]Cu[sbnd]B metallic glass by deposition of Co layer

This study aims at a crystallization process of metallic glass ribbon. An impact of the magnetic interaction upon the crystallization process is reported. Amorphous ribbon composed of Fe76 Mo8 Cu1 B15 has been covered by sputtered Co layer in the range 0–100 nm. Deposited Co layers were investigated by magneto-optical Kerr measurements which confirm their ferromagnetic character. The amorphous ribbons with and without Co cover layer were thermally treated under the elevated temperature. Afterwards, the degree of crystallization was determined using transmission Mössbauer spectroscopy and conversion electron Mössbauer spectroscopy measurements at room temperature. Sputtered cobalt layer significantly boosts development of nanograins in the whole amorphous matrix. It was revealed that the crystallization enhancement was caused by magnetic impingement of ferromagnetic Co layer with the thickness higher than 30 nm.

Magnetocrystalline anisotropy and Gilbert damping of Co2MnAl films epitaxially grown on GaAs

The thickness dependence of both the magnetocrystalline anisotropy and Gilbert damping are investigated for L21-ordered Co2MnAl films by time-resolved magneto-optical Kerr (TR-MOKE) measurements. The intrinsic damping parameter of 0.0039 is evaluated by applying both the in-plane and out-of-plane external magnetic field. The magnetocrystalline anisotropy and intrinsic damping parameter are found to show the similar dependence on film’s thickness, revealing their same physical origination related to the spin–orbit coupling. Our experimental findings provide essential information for the dynamic magnetization property of Co2MnAl films for its promising application in spintronic devices.

Gilbert damping in asymmetric MgO(Ta)/CoFeB/Ta(MgO) structures with perpendicular magnetic anisotropy

We investigate the ultrafast magnetization dynamics of perpendicularly magnetized MgO/CoFeB/Ta and Ta/CoFeB/MgO stack structures using an all-optical time-resolved magneto-optical Kerr measurement technique. We find that the effective Gilbert damping parameter αeff converges to a low value of ∼0.02 in the high external field regime, and it stays unchanged at ∼0.02 regardless of the pump fluence. Two asymmetric stack structures of CoFeB show identical dynamic behavior and Gilbert damping, implying that the influence of the under and top interfaces is not distinct.

Spatial mapping of focused surface acoustic waves in the investigation of high frequency strain induced changes

The field of straintronics, in which strain is used to drive phase transitions, ordering and structural changes, has conventionally been limited to dc or low frequency strain. High frequency large strains, which have the potential to serve as a high frequency trigger of strain sensitive physical phenomena, can be generated using focused surface acoustic waves, which produce two dimensional standing strain waves with very high strain at the elliptical focus. Here, the strain standing wave pattern generated by a focused surface acoustic wave is mapped and quantified as a function of voltage and frequency with high spatial resolution. A knife-edge optical reflection method is used to map the strain standing wave pattern generated by a 87.95 MHz annular interdigital transducer on 128° Y-Cut LiNbO3. Subsequent to strain mapping, ferromagnetic Co/Pt multilayers nanostructures are lithographically patterned within the high strain region for preliminary measurements of magnetization changes arising from high frequency fast strain. The knife edge technique is simple, results in excellent spatial resolution and is fully compatible with other optical measurements, such as focused magneto-optic Kerr measurements, while maintaining spatial information. This ability to accurately and reproducibly determine the position of maximum strain and to lock onto a specific strain region is an important step in the investigation of the effects of high frequency strain on thin film materials, which range from magnetic reorientations to strain induced phase transitions.

Magnetic anisotropy related to strain and thickness of ultrathin iron oxide films on MgO(001)

Iron oxide films with different thicknesses (7.6–30 nm) were grown on clean MgO(001) substrates using reactive molecular beam epitaxy at 250 °C depositing Fe in a 5 × 10−5 mbar oxygen atmosphere. X-ray photoelectron spectra and low energy electron diffraction experiments indicate the stoichiometry and the surface structure of magnetite (Fe3O4). Film thicknesses and the lattice constants were analyzed ex situ by x-ray reflectometry and x-ray diffraction, respectively. These experiments reveal the single crystalline and epitactic state of the iron oxide films. However, the obtained vertical layer distances are too small to be strained magnetite and would rather suit to maghemite. Although Raman spectroscopy carried out to analyze the present iron oxide phase showed that the films might have slightly been oxidized in ambient conditions, a posteriori performed XPS measurements exclude a strong oxidation of the surface. Therefore we consider the presence of anti phase boundaries to explain the low vertical layer distances of the magnetite films. Further magnetooptic Kerr measurements were performed to investigate the magnetic properties. While the thinnest film shows a magnetic isotropic behavior, the thicker films exhibit a fourfold magnetic in-plane anisotropy. The magnetic easy axes are in the Fe3O4 directions. We propose that the magnetocrystalline anisotropy is too weak for very thin iron oxide films to form fourfold anisotropy related to the cubic crystal structure.