Magneto-optical Kerr Effect Hysteresis Research Articles

Effect of growth conditions on magnetization reversal and magnetic anisotropy in Co2Fe0.5Ti0.5Si quaternary Heusler alloy thin films

Ferromagnetic resonance and magneto-optic Kerr effect (MOKE) techniques are employed to unravel the nature of ‘in-plane’ (IP) magnetic anisotropy and magnetization reversal (MR) processes in magnetron-sputtered 100 nm Co2Fe0.5Ti0.5Si (CFTS) thin films, deposited (and subsequently annealed) at different substrate temperatures (Ts) ranging from 200 °C to 550 °C. By varying TS, the CFTS films are produced with different amounts of anti-site (AS) atomic disorder. Irrespective of the degree of AS disorder, the IP uniaxial magnetic anisotropy (UMA) is prevalent in all the CFTS films. The TS450 and TS500 films, deposited at T S = 450 °C and 500 °C, stand out as they have (i) the least AS disorder, (ii) lowest value (α = 0.0055) of the Gilbert damping constant, (iii) high saturation magnetization ( ≅770 G ) at 300 K, (iv) UMA energy density as high as ≅1.6×104erg/cc at 120 K dropping to ≅1.0×104erg/cc at 300 K, (v) spin-wave stiffness ( D ) at T = 0 K, D0≅175 meVÅ2 and, as in other CFTS films, the electron–magnon interaction is primarily responsible for the thermal renormalization of D . Furthermore, in the TS450 and TS500 films, MOKE hysteresis loops at various angles ( ψH ) that the field makes with the easy axis in the film plane, reveals two mutually exclusive UMAs, UMA1≫UMA2 , with easy axes perpendicular to each other. By contrast, only a single UMA is observed in all the remaining films. When ψH=0∘ , MOKE domain images reveal that, consistent with the uniaxial nature of magnetic anisotropy, an abrupt reversal of magnetization is accompanied by the formation of 180° domains. When ψH = 90°, due to the presence of two UMAs in TS450 and TS500 films, following the field reversal, first the reverse domains nucleate and then grow at the expense of the fundamental domains through field-induced domain wall motion.

Influence of twin-crystal structures on the temperature dependence of magneto-optic Kerr effect and magnetic anisotropy in epitaxial Ni thin films

We report on the enhanced Curie temperature and magnetic anisotropy observed in the temperature dependence of magneto-optic Kerr effect (MOKE) in epitaxial Ni films grown on single-crystal c-cut sapphire Al2O3 (0001) substrates. X-ray diffraction (XRD) revealed the epitaxial growth of Ni (111) on Al2O3 (0001) substrate while XRD azimuthal scans indicated the presence of 60°-rotated twin-crystal structures of fcc Ni in ABC…and ACB… stacking in the Ni/Al2O3 (0001) film. Twin-crystal structures were also revealed from the surface morphology of the Ni (111) layer in the Ni/Al2O3 (0001) film. MOKE hysteresis loops indicated higher Kerr remanence with lower coercivity for the Ni/Al2O3 (0001) film as compared to the polycrystalline Ni film grown on Si (100) substrate under the same conditions. Azimuthal MOKE hysteresis loops confirmed the presence of 2-fold in-plane magnetic anisotropy in the Ni/Al2O3 (0001) film. Temperature dependent MOKE measurements in the range of 300– 680 K revealed a power-law dependence of the Kerr remanence similar to magnetization. The Curie temperature (TC) of the Ni/Al2O3 film was found to be almost 10 K higher than bulk Ni. In- and out-of-plane magnetization revealed a uniaxial perpendicular magnetic anisotropy with the easy axis along the Ni/Al2O3 film plane. The calculated values of the anisotropy constants revealed a high degree of magnetocrystalline anisotropy in the epitaxial Ni/Al2O3 (0001) film. A close correlation of MOKE and magnetization hysteresis loops indicated similar surface and volume magnetic behaviors in the thin films. The enhanced temperature dependent MOKE, magnetocrystalline anisotropy and magnetization could be attributed to the twin-crystal structures in the epitaxial Ni/Al2O3 film.

Longitudinal Magneto-Optical Kerr Effect of Nanoporous CoFeB and W/CoFeB/W Thin Films

Nanoporous Co40Fe40B20 (CoFeB) and sandwich tungsten (W)/CoFeB/W thin films were fabricated via an anodic aluminum oxide (AAO) template-assisted magneto sputtering process. Their thickness-dependent magneto-optical Kerr effect (MOKE) hysteresis loops were investigated for enhanced Kerr rotation. Control of the Kerr null points of the polarized reflected light can be realized via the thicknesses of the CoFeB layers and W layers. Simulation of the thickness-dependent phase difference change by the finite element method reveals the existence of the two Kerr null points for W/CoFeB/W thin films, matching the experimental result very well. However, there are two additional Kerr null points for pure CoFeB thin films according to the simulation by comparing with the experimental result (only one). Theoretical analysis indicates that the different Kerr null points between the experimental result and the simulation are mainly due to the enhanced inner magnetization in the ferromagnetic CoFeB layer with the increased thickness, which is usually omitted in the simulation. Clearly, the introduction of non-ferromagnetic W layers can experimentally regulate the Kerr null points of ferromagnetic thin films. Moreover, construction of W/CoFeB/W sandwich thin films can greatly increase the highest magneto-optical susceptibility and the saturated Kerr rotation angle when compared with CoFeB thin films of the same thickness.

Rapid Kerr imaging characterization of the magnetic properties of two-dimensional ferromagnetic Fe3GeTe2

Van der Waals (vdW) ferromagnetic materials have attracted considerable attention in the nanomaterial community, which could provide a unique platform to study magnetism at the nanoscale. Along this direction, many interesting results have been reported, including the electric field control of magnetism and topological spin textures. In this report, we present a rapid and spatially resolved imaging method to study the dimensionality-dependent magnetic properties of Fe3GeTe2 (FGT) nanoflakes. Our method is named as polar magneto-optical Kerr imaging microscopy magnetometry (p-MIMM), which is made possible by analyzing the intensity evolution of wide-field polar magneto-optical Kerr effect (MOKE) images that were collected by varying magnetic fields, thicknesses, and temperatures. In particular, spatially resolved MOKE hysteresis loops can be acquired in the FGT nanoflakes with a submicrometer resolution. By analyzing the evolution of the relative (saturated) MOKE intensity as a function of temperature, we further study the critical exponent and universality class and its dependence on the FGT nanoflake thickness. Combining the polar MOKE images with the calculated MOKE hysteresis loops, a detailed magnetic phase diagram summarizing an evolution of the stripe domain, single domain, and paramagnetic state is further validated. Our results suggest that the wide-field p-MIMM can be conveniently used for rapidly examining the magnetic properties of versatile vdW magnetic materials.

Kerr effect anomaly in magnetic topological insulator superlattices

We report the magneto-optical Kerr effect (MOKE) study of magnetic topological insulator superlattice films with alternating transition-metal and rare-earth doping. We observe an unexpected hump in the MOKE hysteresis loops upon magnetization reversal at low temperatures, reminiscent of the topological Hall effect (THE) reported in transport measurements. The THE is commonly associated with the existence of magnetic skyrmions, i.e. chiral spin textures originating from topological defects in real space. Here, the observation of the effect is tied to ferromagnetic ordering in the rare-earth-doped layers of the superlattice. Our study may provide a new approach for the non-invasive optical investigation of skyrmions in magnetic films, complementary to electrical transport measurements, where the topological Hall signal is often the only hint of non-trivial magnetization patterns.

Increase in Coercive Field by Magnetoelastic Coupling in Co/VO₂ Films

Herein, a ferromagnetic cobalt (Co) film was deposited on a VO₂ film exhibiting a structural phase transition from monoclinic to tetragonal at ~340 K to investigate the magnetoelastic effect caused by a phase-transition-induced strain. First, (100) and (101) VO₂ films with thicknesses of 11-43 nm were grown on Al₂O₃ using pulsed laser deposition, and 2.5-nm-thick Co films were deposited on top of them via sputtering. The magneto-optic Kerr effect hysteresis loops were measured with the temperature variation across the structural phase transition temperature of VO₂. Upon heating, an increase in the coercive field of the Co layer was observed at the structural phase transition temperature of VO₂, suggesting a magnetoelastic coupling between the Co and VO₂ layers. The coercive field increment diminished with decreasing VO₂ thickness, and it disappeared as the VO₂ became thinner than a critical value of ~11 nm. These results imply that a phase-transition-induced strain in the VO₂ layer can be used to change the coercive field of the neighboring magnetic layer.

Magneto-optical images of submicron-size Bi-substituted YIG patterns prepared by electron-beam irradiated metal-organic decomposition

We investigated the magneto-optical properties of submicron-size epitaxial Bi-substituted yttrium-iron garnet (Bi:YIG) patterns fabricated by an electron-beam irradiated metal-organic decomposition (EB-MOD) method. The advantage of this method is that no dry etching process is required. The Bi:YIG pattern showed the clear magneto-optical Kerr effect (MOKE) hysteresis loop, and it was in good agreement with the magnetization curve of the Bi:YIG patterns. The MOKE image measurements revealed that the Bi:YIG pattern with a width of ∼0.71 μm prepared by the EB-MOD showed proper contrast changing behavior depending on magnetic fields, and it can maintain its MO property even in submicron-size.

Multi-jump magnetization switching in Co2FeAl full Heusler alloy thin films: Experiments and simulations

Co2FeAl (CFA) thin films of 50 nm thickness have been grown on MgO (0 0 1) single crystal substrates at room temperature with and without post-annealing (PA) at 300 °C and 400 °C using dual ion-beam sputtering technique. The XRD pattern of the as-grown film revealed that CFA grows with preferred crystallographic orientation on the MgO (0 0 1) substrate. Temperature dependent anisotropy measurements on PA films revealed a dominating contribution from cubic anisotropy as confirmed by the analysis of azimuthal angle dependent longitudinal in-plane magneto-optical Kerr effect (MOKE) measurements. The contributions from the cubic and uniaxial anisotropies have also been quantified employing ferromagnetic resonance spectroscopy. Magnetization reversal is accompanied with a plateau in the MOKE hysteresis recorded at various azimuthal angles in the in-plane applied magnetic field configuration. The occurrence of the observed plateau is explained by the presence of a combination of domain walls such as 90°, 135° and 180° domain walls and/or complex domains which is supported by results from micromagnetic simulations. These results demonstrate the feasibility of manipulating the magnetization switching in one of the two ferromagnetic electrodes of the magnetic tunnel junction devices based on Heusler alloy ferromagnetic films.

Study of exchange bias effect in a patterned Fe/Pt multilayer with the thermal annealing

The present work reports the preparation of 5 μm wide and 10 μm grating periodicity patterned [Fe(2.0nm)/Pt(2.5nm)]X10 multilayer and its thermal annealing behavior. Intermixing across the Fe and Pt layers is observed with vacuum annealing at 650 K resulting in the formation of disordered face centered cubic (FCC) and ordered L10 face centered tetragonal (FCT) FePt phases. The longitudinal magneto-optical Kerr effect (L-MOKE) hysteresis loop, measured from the single 5 μm FePt stripe shows double loop behavior indicating the presence of two magnetic phases, one with the low coercivity (HC) and the second one with the higher HC values. This could be due to the presence of disordered FCC and ordered L10 FCT phases of FePt, known to be soft and hard magnetic phases. Conversion electron Mössbauer spectroscopy of the annealed sample shows the presence of FCC and FCT FePt phases corroborating the x-ray diffraction and MOKE measurements. The hysteresis loop of the soft phase shows the exchange bias effect with the remnant state of the hard magnetic phase as initial condition akin to the exchange bias as observed in conventional ferromagnetic - antiferromagnetic bilayers.

Magneto-optical properties of BaTiO3/La0.76Sr0.24MnO3/BaTiO3 heterostructures

The magnetic properties of epitaxial BaTiO3/La0.76Sr0.24MnO3/BaTiO3 (BTO/LSMO/BTO) heterostructures have been studied using magneto-optic Kerr effect (MOKE) technique. Both longitudinal and polar MOKE were probed as a function of magnetic field and temperature (in the range between 80 and 320 K) for epitaxial films of BTO/LSMO/BTO and LSMO grown on TiO2-terminated SrTiO3 (001) substrates by pulsed laser deposition technique. The LSMO film without the BTO layers exhibits nearly square field-dependent MOKE hysteresis loops with low saturation fields below a bulk-like Curie temperature (TC) of ∼ 350K. In contrast, the film with the BTO layers exhibits a significantly suppressed TC of 155 K, accompanied by significantly enhanced coercive fields and perpendicular magnetic anisotropy.

Naturally Produced Co/CoO Nanocrystalline Magnetic Multilayers: Structure and Inverted Hysteresis.

Cobalt-based multilayers with excellent sequencing are grown via radiofrequency magnetron sputtering with the use of one Co target and natural oxidation. The Co layers are continuous, fully textured {111} and have the face centered cubic structure. At the end of deposition of each Co layer air is let to flow into the vacuum chamber via a fine (leak) valve. The top of Co is oxidized. The oxidized layers consist of cubic CoO crystallites. Near the film surface hexagonal Co(OH)2 is also detected. Magneto-optical Kerr effect hysteresis loops show in-plane magnetized films. The magnetic saturation field in the out-of-plane measurements is large exceeding 12 kOe. This observation supports indirectly the fact that Co is face centered cubic; if it was c-axis textured hexagonal the magnetocrystalline anisotropy would be large resulting in smaller values of the saturation field. As the Co-layer thickness decreases the in-plane loops show reduced remanence, slow approach to magnetic saturation and the out-of-plane loops show inverted hysteresis and/or crossing loop features with sizeable remanence. The effects are discussed with respect to the enhanced orbital magnetic moment of Co and the antiferromagnetic coupling between Co spins at the Co/CoO interface.

Hydrogen absorption-induced reversible change in magnetic properties of Co–Pd alloy films

The effects of reversible hydrogen absorption on the magnetism of Co–Pd alloy thin films were investigated. Evident enhancement of magnetic coercivity Hc and squareness, measured by magneto optical Kerr effect (MOKE), was observed in the magnetic hysteresis loops of films with Pd content of 61%, 76%, and 86%, whereas no observable changes were noted in the MOKE hysteresis loops of Co-rich alloy films. For Co39 Pd61, perpendicular magnetic anisotropy increased as film thickness increased from 10 to 20 nm. Hydrogenation induced three times enhancement of perpendicular Hc for 20 nm Co39 Pd61, whereas the Hc change in 10 nm Co39 Pd61 was within 10%. A Pd-rich alloy composition and appropriate film thickness for perpendicular magnetization are both crucial for hydrogen absorption-induced effect on magnetic properties, i.e. the enhancement of Hc and squareness as well as the reduction of Kerr rotation angle.

Geometry and field dependence of the formation of magnetic antivortices in pound-key-like structures

In this work, we assess the effects of field history and structure shape on the formation of magnetic antivortices. The magnetic reversal process was investigated for a series of patterned micron-sized permalloy pound-key structures with varying degrees of asymmetry using magneto-optical Kerr effect hysteresis measurements combined with magnetic force microscopy. The largest number of antivortices was observed in the structures with the highest level of structure asymmetry, which also show an intermediate state in the hysteresis loop. A significant enhancement of the antivortex formation rate—from 5% to almost 80%—was achieved by adjusting the structure dimensions. Images of the magnetic states obtained at various points in the hysteresis loop show that the highest rate of antivortex formation occurs near the coercive field, also the nucleation field, and that the antivortex formation is also sensitive to the angle of the applied field, where the highest antivortex formation rate is observed when the field is aligned along the structure diagonal. A comparison of the experimental results with micromagnetic simulations shows that the areas with lower shape anisotropy lead the reversal in the formation step and the upper field limit for the antivortex stability is related to the reversal of the regions with higher shape anisotropy, although the simulations suggest that the annihilation mechanism will change to one that involves domain wall propagation when the smallest structure dimensions are below ∼60 nm. These results demonstrate how shape anisotropy can be used to promote the formation of isolated magnetic antivortices, which will facilitate future investigations of this topological magnetic state.

Texture and Magnetism of Nanocrystalline Ni Films and Multilayers

Nickel films are grown by radio frequency magnetron sputtering on Corning glass, polyimide foils and on the native oxide of Si (100) wafers at low (1x10-3 mbar) and relatively high (2.5x10-2 mbar) Argon pressure at 100 °C. The base pressure of the high vacuum chamber is 1x10-7 mbar. X-ray diffraction experiments are performed to reveal the different texture of various Ni films. Magneto-optical Kerr effect hysteresis loops and magnetic force microscopy images show correlation between Argon pressure, texture and magnetic anisotropy of the films. The results are discussed with respect to relevant measurements of Ni/NiO magnetic multilayers prepared under similar experimental conditions.

Optical spectroscopy of sputtered nanometer-thick yttrium iron garnet films

Nanometer (nm)-thick yttrium iron garnet (Y3Fe5O12, YIG) films present interest for spintronics. This work employs spectral ellipsometry and magneto-optic Kerr effect (MOKE) spectra to characterize nm-thick YIG films grown on single-crystal Gd3Ga5O12 substrates by magnetron sputtering. The thickness (t) of the films ranges between 10 nm and 40 nm. Independent on t, the polar MOKE hysteresis loops saturate in the field of about 1.8 kOe, consistent with the saturation magnetization in bulk YIG (4πMs ≈ 1.75 kG). The MOKE spectrum measured at photon energies between 1.3 eV and 4.5 eV on the 38-nm-thick film agrees with that measured on single-crystal YIG bulk materials. The MOKE spectrum of the 12-nm-thick film still preserves the structure of the bulk YIG but its amplitude at lower photon energies is modified due to the fact that the radiation penetration depth exceeds 20 nm. The t dependence of the MOKE amplitude is consistent with MOKE calculations. The results indicate that the films are stoichiometric, strain free, without Fe2+, and preserve bulk YIG properties down to t ≈ 10 nm.

Magnetic and structural properties of Co2FeAl thin films grown on Si substrate

The correlation between magnetic and structural properties of Co_{2} FeAl (CFA) thin films of different thickness (10 nm<d< 100 nm) grown at room temperature on MgO-buffered Si/SiO2 substrates and annealed at 600\lyxmathsym{\textdegree}C has been studied. XRD measurements revealed an (011) out-of-plane texture growth of the films. The deduced lattice parameter increases with the film thickness. Moreover, pole figures showed no in-plane preferential growth orientation. The magneto-optical Kerr effect hysteresis loops showed the presence of a weak in-plane uniaxial anisotropy with a random easy axis direction. The coercive field measured with an applied field along the easy axis direction and the uniaxial anisotropy field increase linearly with the inverse of the CFA thickness. The microstrip line ferromagnetic resonance measurements for in-plane and perpendicular applied magnetic fields revealed that the effective magnetization and the uniaxial in-palne anisotropy field follow a linear variation versus the inverse CFA thickness. This allows deriving a perpendicular surface anisotropy coefficient of -1.86 erg/cm2

Probing the magnetization reversal process of permalloy nano-rings with high wall height-to-thickness ratios

We report a study of magnetization reversal process on a tall magnetic nano-ring with a wall height-to-thickness ratio of up to 21. Samples in a hexagonal lattice pattern of ring array with an outer/inner diameter of 300/260 nm and a ring height of 420 nm are fabricated using electron beam lithography in conjunction with an ion beam etching technique. A longitudinal magneto-optical Kerr effect (MOKE) measurement reveals that meta-stable states exist during the magnetization reversal process. In particular, the exact magnetization configurations at the top end of the nano-ring are illustrated using magnetic force microscopy (MFM) that is facilitated with a photoresist etch-back technique. Most interestingly, MFM images of two sets of head-to-head (H2H)/tail-to-tail (T2T) domain walls are captured during the magnetization reversal process. It is believed that the domain walls formation and evolution processes at the top end of the nano-ring are mediated by the anti-symmetrical distribution of the vortex domain wall on the tubular sidewalls. Simulation results using Object Oriented Micromagnetic Framework confirm the MOKE hysteresis loop and MFM imaging.

Fabrication of 1 Teradot/in.2 CoCrPt bit patterned media and recording performance with a conventional read/write head

1 Teradot/in.2 (Td/in.2) CoCrPt alloy bit patterned media (BPM) disks were patterned by direct write e-beam lithography, and the recording performance was measured with a commercial recording head. Recording analysis showed a minimum error rate of 2 × 10−3, which was limited by the fraction of BPM patterning defects Continuous magnetic media disks were coated with a 20 nm thick carbon hard mask film by PECVD followed by a 8.5 nm thick hydrogen silsesquioxane (HSQ) resist by spin coating. A series of 1 Td/in.2 dot patterns were e-beam written in the HSQ, and the patterns were etched into the carbon hard mask by reactive ion etching. The underlying magnetic media was physically etched with 200 eV Ar. The carbon hard mask maximum thickness was limited by erosion of the HSQ dots during the carbon hardmask etch and shadowing of the mask during the magnetic media etch. The minimum carbon thickness and the maximum CoCrPt thickness were determined by erosion of the hardmask pillars during etching of the CoCrPt magnetic media. The optimal carbon hard mask thickness was determined to be ∼20 nm (for our PECVD carbon). The optimal CoCrPt magnetic media thickness was 6 nm, as determined by etch selectivity and magnetic properties. A Silvaco Monte Carlo 3D model simulation was used to describe the magnetic media etching process. Additional patterning steps formed physical support, surrounding the patterns, for the recording head that scanned in contact with the patterned magnetic media. Analysis of top down SEM micrographs of BPM patterns showed defect rates as low as 3 × 10−4 and a 1-sigma dot placement tolerance of 0.9 nm. Magnetic coercivity and switching field distribution width were measured from polar magneto-optic micro Kerr effect hysteresis loops (with a spot size of 20–50 μm). Patterning process conditions that produced a higher fraction of eroded or merged magnetic islands reduced the BPM coercivity and increased the relative width of the switching field distribution. Magnetic recording was performed with a commercial recording head of magnetic write width 90 nm in a shingled writing method. The recording error rate minimum varied with the fraction of defects in a similar manner as the magnetic switching field distribution width. A higher fraction of defects resulted in increased recording error probability due to data erasure by stray magnetic fields from the head.

Iron-based bimagnetic core/shell nanostructures in SiO2: a TEM, MEIS, and energy-resolved XPS analysis

Structural, compositional, and chemical changes during the growth of ion beam-synthesised nanoparticles under electron beam annealing (EBA) at 1,000 °C in high vacuum conditions were investigated using medium energy ion scattering, energy-resolved X-ray photoelectron spectroscopy, and transmission electron microscopy. The nanoparticles evolved from <5 to 30 nm diameter Fe-metal (core)/amorphous FexSi33−xOy (shell) structures to single 5–30 nm Fe crystalline nanoparticles sitting on the SiO2 as the annealing time was increased. The stoichiometry of the shell after 60 s EBA at 1,000 °C was found to be (O2)x:Fe14Si19O52 with x between 1 and 7.5 where x increases with increasing depth. This amorphous-shell phase has not been previously reported and a reanalysis of the magnetization data shows that it is ferromagnetic with a magnetic moment of about 1.7 μB per Fe atom even at 300 K. The low-temperature magnetic hysteresis and magneto-optical Kerr-effect hysteresis at room temperature are enhanced when the core/shell nanoparticles are present. A maximum coercive field of about 740 Oe was found with a corresponding Kerr rotation of about 0.04°.

Magneto-electric tuning of the phase of propagating spin waves

The utilization of a magnetoelectric film composite to control, by an electric field, the phase of magnetostatic surface spin waves propagating along thin films is reported. Laminates of ferromagnetic films of Ni and NiFe are deposited on a ferroelectric substrate, lead magnesium niobate-lead titanate. The phase of propagating spinwaves is shown to be modulated by an electric field while traveling a finite distance along the surface. The observed phase change in the spinwaves is in agreement with the anisotropy field changes measured with magneto optical Kerr effect hysteresis loops. A quantitative agreement is demonstrated.