Surface Magneto-optic Kerr Effect Research Articles

Revealing the Origin of Time-Reversal Symmetry Breaking in Fe-Chalcogenide Superconductor FeTe_{1-x}Se_{x}.

Recently, evidence has emerged in the topological superconductor Fe-chalcogenide FeTe_{1-x}Se_{x} for time-reversal symmetry breaking (TRSB), the nature of which has strong implications on the Majorana zero modes (MZM) discovered in this system. It remains unclear, however, whether the TRSB resides in the topological surface state (TSS) or in the bulk, and whether it is due to an unconventional TRSB superconducting order parameter or an intertwined order. Here, by performing in superconducting FeTe_{1-x}Se_{x} crystals both surface-magneto-optic-Kerr effect measurements using a Sagnac interferometer and bulk magnetic susceptibility measurements, we pinpoint the TRSB to the TSS, where we also detect a Dirac gap. Further, we observe surface TRSB in nonsuperconducting FeTe_{1-x}Se_{x} of nominally identical composition, indicating that TRSB arises from an intertwined surface ferromagnetic (FM) order. The observed surface FM bears striking similarities to the two-dimensional (2D) FM found in 2D van der Waals crystals, and is highly sensitive to the exact chemical composition, thereby providing a means for optimizing the conditions for Majorana particles that are useful for robust quantum computing.

Absence of spontaneous time-reversal symmetry breaking and ferromagnetism in superconducting NiBi3 single crystal

Recent experiments have pointed to chiral $p$-wave-like superconductivity in epitaxial Bi/Ni bilayers that are spontaneously time-reversal symmetry breaking (TRSB), making it a promising platform for exploring physics useful for topologically protected quantum computing. Quite intriguingly, evidence has emerged that, in nonepitaxial Bi/Ni bilayers, superconductivity arises due to the formation of $\mathrm{Ni}{\mathrm{Bi}}_{3}$, which has been reported to host coexisting ferromagnetic and superconducting orders at the surface. We perform high-resolution surface magneto-optic Kerr effect measurements using a Sagnac interferometer on single-crystal $\mathrm{Ni}{\mathrm{Bi}}_{3}$ and find no sign of any spontaneous Kerr signal except for contributions from trapped vortices. This strongly indicates the absence of TRSB in $\mathrm{Ni}{\mathrm{Bi}}_{3}$, whether due to TRSB in the superconducting state or any coexisting ferromagnetism, and we conclude that the superconductivity found in nonepitaxial Bi/Ni is distinctively different from that in epitaxial Bi/Ni.

Observing the magnetization reversal processes and anisotropic effective damping of epitaxial FeSi/MgO (001)

Epitaxial FeSi film on MgO (001) substrate was fabricated via a radio frequency magnetron sputtering technology. The epitaxial relationship of FeSi(001)/[110]//MgO(001)/[100] was characterized by crystal structure measurements and confirmed by in-plane biaxial magnetic anisotropy through vibrating sample magnetometer (VSM). By measuring Kerr magnetic hysteresis loops and recording the real-time magnetic domain images through surface magneto-optic Kerr effect (MOKE), two successive 90° domain wall displacement along easy axis and two discontinuous 90° domain wall displacement along hard axis were directly observed. Meanwhile, the difference of magnetic hysteresis loops obtained by VSM and MOKE devices were discussed. Furthermore, from the results of magnetic field sweeping ferromagnetic resonance measurements, it was found that FeSi film possesses an anisotropic effective damping constant of 0.0042 for easy axis direction and 0.0053 for hard axis direction, which was ascribed to the crystallographic defects induced two magnon scattering contributions.

Uniaxial magnetic anisotropy in three-bilayer Co/Cu and Co/Al superlattices

We present the magnetic properties of three-bilayer superlattices [Co(0.8 nm)/Cu(1.1 nm)]3 and Co(0.8 nm)/Al(2.2 nm)]3 grown by magnetron sputter deposition on SiO2 substrates. Using the surface magneto-optical Kerr effect we observed longitudinal magnetic hysteresis loops with low coercivity values of <17 × 10−4 T and <7 × 10−4 T from Co/Cu and Co/Al superlattices, respectively. From the hysteresis loops as a function of in-plane azimuthal angle of the Co/Cu and Co/Al superlattices, we observed an in-plane uniaxial anisotropy in coercivity and squareness. Using a four-point square configuration we observed positive and negative anisotropic magnetoresistances (AMR) with values +0.5% and −0.5% from Co/Cu superlattice depending on whether the applied current is perpendicular or parallel to the applied magnetic field direction, respectively. The surface morphology including vertical root-mean-square roughness and lateral correlation length were examined by atomic force microscopy. The number of bilayers and the thicknesses of Co, Cu and Al in the superlattices were examined by X-ray reflectivity. The observed magnetic properties were correlated to the interface roughness and layer thickness. The low coercivity, low saturation field, near-one squareness, and finite MR value of the three-bilayer Co/Cu superlattice has potential applications in low magnetic field detection and sensors.

Structural investigation and magnetic properties of oxygen adsorption on ultrathin Fe(110) film

We have investigated oxygen induced surface structures on Fe(110) thin film and their magnetic properties using low energy electron diffraction (LEED) and surface magneto-optic Kerr effect (SMOKE). We have determined two chemisorbed structures, p(2 × 2) at 0.25 monolayer (ML) and p(3 × 2) at 0.33 ML using a dynamical LEED analysis. We found that oxygen atoms adsorb in long bridge sites on p(2 × 2) and move to three fold hollow sites on p(3 × 2) due to a steric repulsion. During chemisorption, proceeding the formation of FeO, the topmost surface of Fe film is ferromagnetic and with further oxygen adsorption the surface loses ferromagnetism in accordance with the oxide formation.

Thickness dependent manipulation of uniaxial magnetic anisotropy in Fe-thin films by oblique deposition**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB921403 and 2016YFA0300701), the National Natural Science Foundation of China (Grant Nos. 51427801, 11374350, and 51671212), and the Chinese Government Scholarship (Grant No. 2015GXYG37).

The uniaxial magnetic anisotropy of obliquely deposited Fe(001)/Pd film on MgO(001) substrate is investigated as a function of deposition angle and film thickness. The values of incidence angle of Fe flux relative to surface normal of the substrate are 0°, 45°, 55°, and 70°, respectively. In-situ low energy electron diffraction is employed to investigate the surface structures of the samples. The Fe film thicknesses are determined to be 50 ML, 45 ML, 32 ML, and 24 ML (1 ML = 0.14 nm) by performing x-ray reflectivity on the grown samples, respectively. The normalized remanent magnetic saturation ratio and coercivity are obtained by the longitudinal surface magneto-optical Kerr effect. Here, the magnetic anisotropy constants are quantitatively determined by fitting the anisotropic magnetoresistance curves under different fields. These measurements show four-fold cubic anisotropy in a large Fe film thickness (50 ML) sample, but highly in-plane uniaxial magnetic anisotropies in thin films (24 ML and 32 ML) samples. In the obliquely deposited Fe films, the coercive fields and the uniaxial magnetic anisotropies (UMAs) increase as the deposition angle becomes more and more tilted. In addition, the UMA decreases with the increase of the Fe film thickness. Our work provides the possibility of manipulating uniaxial magnetic anisotropy, and paves the way to inducing UMA by oblique deposition with smaller film thickness.

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

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

MAGNETO-OPTIC PROPERTIES AND OXIDATION/REDUCTION OF ULTRATHIN MAGNETIC FILMS: FE FILMS ON Pt(111)

We have investigated magneto-optic properties of ultrathin Fe films grown on Pt(111) surfaces by using the in situ surface magneto-optic Kerr effect (SMOKE) and X-ray photoelectron spectroscopy (XPS). SMOKE measurements show that the Fe layers are not ferromagnetic when the film is thinner than approximately 4.5 MLs (monolayers), but the in-plane magnetization is present for a 4.1 ML Fe film on Pt(111) annealed at 550 K. Upon post-annealing at 770 K, a 9.2 ML Fe film does not show any Kerr signal, while a 6.3 ML Fe film has the in-plane Kerr signal with increased coercivity. The oxidation and reduction of ultrathin Fe films have also been studied by using XPS. Upon an oxygen exposure of 300 Langmuir at a film temperature of 873 K, the Fe layers were mostly oxidized as Fe3O4. When the Fe films were exposed to the same amount of oxygen at room temperature, a partial oxidation as Fe3O4 was observed for a 3 ML Fe film, while there was no oxidation for a 2 ML Fe film. On heating the 873 K oxidized films, Fe3O4O was reduced to FeO, and even the decomposition was observed. Underlying reasons for these chemical changes of Fe and iron-oxide films are discussed.

Thickness-induced spin-reorientation originated from competing magnetic shape anisotropies

Engineering the surface morphology of magnetic film is one of the important methods to tune the magnetic anisotropy of ultrathin magnetic material. However, the influence of competing shape effects on magnetic anisotropy of ultrathin film is still not clearly demonstrated. Here, we investigated the magnetic anisotropy of obliquely deposited Fe films on vicinal Si(111) substrate by using in-situ and ex-situ surface magneto-optical Kerr effect (MOKE). Thickness-induced in-plane spin-reorientation transition, i.e. magnetization easy axis gradually rotates away from the step direction, was observed. MOKE measurements and micromagnetic simulation demonstrate this spin-reorientation transition process largely originated from the competition between step-induced magnetic shape anisotropy and oblique-deposition-induced magnetic shape anisotropy. Our study indicates the possibility of tuning magnetic spin order orientation by the competing magnetic shape anisotropies.

Effect of Ar+ beam sputtering on the magnetic anisotropy of Fe thin films deposited on the MgO(0 0 1) substrate

The Fe thin films with different initial thickness were prepared on MgO(001) substrate. Ar+ ion beam having energy of 3keV was used to sputter the film at incident angle of 80° with respect to the film normal. Low energy electron diffraction (LEED) was employed to study the morphology and crystal quality of the film. Moreover surface magneto-optical Kerr effect (SMOKE) and anisotropic magnetoresistance (AMR) setups were used to investigate the magnetic properties of the Fe film. The values of magnetocrystalline anisotropy K1 and induced in-plane uniaxial magnetic anisotropy (UMA) Ku were derived from torque curves on the base of AMR results and found that the value of Ku was increasing with increasing the sputtering time while the intrinsic magnetocrystalline anisotropy K1 was not affected by sputtering. Similarly it is observed that the value of K1 increases with the thickness. We noted that ion beam sputtering can be used as an effective technique of manipulating the magnetic anisotropy of ultrathin magnetic films.

Si/Fe flux ratio influence on growth and physical properties of polycrystalline β-FeSi2 thin films on Si(100) surface

This work investigates the Si/Fe flux ratio (2 and 0.34) influence on the growth of β-FeSi2 polycrystalline thin films on Si(100) substrate at 630°C. Lattice deformations for the films obtained are confirmed by X-ray diffraction analysis (XRD). The volume unit cell deviation from that of β-FeSi2 single crystal are 1.99% and 1.1% for Si/Fe =2 and Si/Fe =0.34, respectively. Absorption measurements show that the indirect transition (~ 0.704eV) of the Si/Fe =0.34 sample changes to the direct transition with a bandgap value of ~0.816eV for the sample prepared at Si/Fe =2. The absorption spectrum of the Si/Fe =0.34 sample exhibits an additional peak located below the bandgap energy value with the absorption maximum of ~0.36eV. Surface magneto-optic Kerr effect (SMOKE) measurements detect the ferromagnetic behavior of the β-FeSi2 polycrystalline films grown at Si/Fe =0.34 at T=10K, but no ferromagnetism was observed in the samples grown at Si/Fe =2. Theoretical calculations refute that the cell deformation can cause the emergence of magnetization and argue that the origin of the ferromagnetism, as well as the lower absorption peak, is β-FeSi2 stoichiometry deviations. Raman spectroscopy measurements evidence that the film obtained at Si/Fe flux ratio equal to 0.34 has the better crystallinity than the Si/Fe =2 sample.

Manipulating magnetic anisotropies of Co/MgO(001) ultrathin films via oblique deposition**Project supported by the Chinese Academy of Sciences–The World Academy of Sciences (CAS–TWAS) Fellowship Program, the National Basic Research Program of China (Grant Nos. 2015CB921403 and 2012CB933102), and the National Natural Science Foundation of China (Grant Nos. 51427801, 11374350, and 11274361).

We present a systematic investigation of magnetic anisotropy induced by oblique deposition of Co thin films on MgO (001) substrates by molecular beam epitaxy at different deposition angles, i.e., 0°, 30°, 45°, 60°, and 75° with respect to the surface normal. Low energy electron diffraction (LEED), surface magneto–optical Kerr effect (SMOKE), and anisotropic magnetoresistance (AMR) setups were employed to investigate the magnetic properties of cobalt films. The values of in-plane uniaxial magnetic anisotropy (UMA) constant Ku and four-fold magnetocrystalline anisotropy constant K1 were derived from magnetic torque curves on the base of AMR results. It was found that the value of Ku increases with increasing deposition angle with respect to the surface normal, while the value of K1 remains almost constant for all the samples. Furthermore, by using MOKE results, the Ku values of the films deposited obliquely were also derived from the magnetization curves along hard axis. The results of AMR method were then compared with that of hard axis fitting method (coherent rotation) and found that both methods have almost identical values of UMA constant for each sample.

An Optical Magnetometry Mechanism Above the Surface of Seawater

This paper analyzes a mechanism for the remote optical measurements of magnetic field variations above the surface of seawater. This magnetometry mechanism is based on the polarization rotation of reflected polarized laser light, in the presence of the earth’s magnetic field. Here the laser light is reflected off the surface of the water and off an underwater object. Two mechanisms responsible for the polarization rotation are the surface magneto-optical kerr effect (SMOKE) and the Faraday effect. In both the mechanisms, the degree of polarization rotation is proportional to the earth’s local magnetic field. Variations in the earth’s magnetic field due to an underwater object will result in variations in the polarization rotation of the laser light reflected off the water’s surface (SMOKE) and off the underwater object (Faraday effect). An analytical expression is obtained for the polarization-rotated field when the incident plane wave is at arbitrary angle and polarization with respect to the water’s surface. We find that the polarization rotated field due to SMOKE is small compared with that due to the Faraday effect.

Variation of blocking temperatures for exchange biased CoO/Co/Ge(100) films

Variations of the blocking temperature and related structures for CoO/Co/Ge(100) films are investigated by employing reflection high energy electron diffraction, Auger electron spectroscopy, and surface magneto-optic Kerr effect measurements. By increasing the CoO thickness, the blocking temperature is smaller than the Neel temperature of CoO. The monotonous increase of the blocking temperature is mainly attributed to the increasing thermal stability of the antiferromagnetic grains by way of increasing the antiferromagnetic thickness. The deviation of the blocking temperature from the linear relation and the full widths at half maximum of the diffraction spots show a similar trend. The minimums appear around 25 monolayer of CoO and are related to the formation of larger grains.

Shape-dependent magnetic properties of Co nanostructure arrays synthesized by pulsed laser melting

One dimensional (1D) magnetic nanowires show unique magnetic behaviors, such as large coercivity and high remanence, in comparison to the bulk and thin film materials. Here, planar arrays of Co nanowires, nanorods and nanoparticles were fabricated from thin Co films by a nanosecond pulsed laser interference irradiation technique. Magnetic force microscopy (MFM) and surface magneto-optic Kerr effect (SMOKE) techniques were used to study the individual and average magnetic properties of the nanostructures. Magnetic domain orientation was found to depend on the in-plane aspect ratio of the nanostructure. The magnetic orientation was out-of-plane for in-plane aspect ratio ranging from 1 to 1.4 and transitioned to an in-plane orientation for aspect ratios greater than 1.4 (such as in nanorods and nanowires). Our results also showed that polycrystalline Co nanowires showed much higher coercivity and remanence as compared to bulk and thin film materials, as well as shapes with smaller aspect ratio. This result was attributed mainly to the shape anisotropy. This study demonstrated that nanosecond pulsed laser synthesis is capable of fabricating various nanostructures in a simple, robust and rapid manner and SMOKE is a reliable technique to rapidly characterize such magnetic nanostructures.

Surface Magneto-Optical Kerr Effect Study of Magnetization Reversal in Epitaxial Fe(100) Thin Films

Surface Magneto-optic Kerr Effect (SMOKE) technique is used to study the magnetization rotation in single crystalline Fe thin films, deposited onto MgO (100) substrates by magnetron sputtering, with thicknesses, tFe, varied within the range from 70 Å to 250 Å. The crystalline structure and epitaxial relation are determined employing X-ray diffraction (XRD). The SMOKE loops are interpreted in the framework of a phenomenological model for coherent rotation of the magnetization. The switching behavior of these films is characterized by two different reversal regimes. In films with thicknesses less than a critical value (tFe<tc), there is a superposition of the magneto crystalline anisotropy to an uniaxial in-plane anisotropy, and as a consequence the magnetization rotation is non-coherent with motion of Néel and Bloch domain walls. As the thickness is increased from the critical thickness, and the uniaxial anisotropy is overwhelmed by the magneto crystalline anisotropy, the magnetization reversal turns coherent with Bloch domain walls motion. The thickness dependence of all anisotropy constants follow a function of 1/tFe as expected for interface effects.

Interaction transfer of silicon atoms forming Co silicide for Co/3×3R30°-Ag/Si(111) and related magnetic properties

Combined scanning tunneling microscopy, Auger electron spectroscopy, and surface magneto-optic Kerr effect studies were employed to study the microscopic structures and magnetic properties for ultrathin Co/3×3R30°-Ag/Si(111). As the annealing temperature increases, the upward diffusion of Si atoms and formation of Co silicides occurs at temperature above 400 K. Below 600 K, the 3×3R30°-Ag/Si(111) surface structure persists. We propose an interaction transferring mechanism of Si atoms across the 3×3R30°-Ag layer. The upward transferred Si atoms react with Co atoms to form Co silicide. The step height across the edge of the island, a separation of 0.75 nm from the analysis of the 2 × 2 structure, and the calculations of the normalized Auger signal serve as strong evidences for the formation of CoSi2 at the interface. The interaction transferring mechanism for Si atoms enhances the possibility of interactions between Co and Si atoms. The smoothness of the surface is advantage for that the easy axis of magnetization for Co/3×3R30°-Ag/Si(111) is in the surface plane. This provides a possible way of growing flat magnetic layers on silicon substrate with controllable silicide formation and shows potential applications in spintronics devices.

Effect of CoSi2 buffer layer on structure and magnetic properties of Co films grown on Si (001) substrate**Project supported by the National Basic Research Program of China (Grant Nos. 2011CB921801 and 2012CB933102), the National Natural Science Foundation of China (Grant Nos. 11374350, 11034004, 11274361, and 11274033), and the Research Fund for the Doctoral Program of Higher Education of China (Grant

Buffer layer provides an opportunity to enhance the quality of ultrathin magnetic films. In this paper, Co films with different thickness of CoSi2 buffer layers were grown on Si (001) substrates. In order to investigate morphology, structure, and magnetic properties of films, scanning tunneling microscope (STM), low energy electron diffraction (LEED), high resolution transmission electron microscopy (HRTEM), and surface magneto-optical Kerr effect (SMOKE) were used. The results show that the crystal quality and magnetic anisotropies of the Co films are strongly affected by the thickness of CoSi2 buffer layers. Few CoSi2 monolayers can prevent the interdiffusion of Si substrate and Co film and enhance the Co film quality. Furthermore, the in-plane magnetic anisotropy of Co film with optimal buffer layer shows four-fold symmetry and exhibits the two-jumps of magnetization reversal process, which is the typical phenomenon in cubic (001) films.

In situ surface magneto-optical Kerr effect (s-MOKE) study of ultrathin soft magnetic FeCuNbSiB alloy films

Herein we report on an in situ surface magneto-optical Kerr effect (s-MOKE) study of ion-beam-sputtered ultra-thin films of an amorphous Fe73.9Cu0.9Nb3.1Si13.2B8.9 (FINEMET) alloy with film growth that ranges from a fraction of a nm to a few tens of nms. Extrapolating the linear variation of the Kerr signal with film thickness suggests the absence of a magnetic dead layer at the substrate/FINEMET film interface, and hence the absence of any intermixing. The presence of a thin SiO2 film at the surface of the Si substrate may be responsible for preventing possible intermixing of Fe with Si to form nonmagnetic silicide. Close to the onset of ferromagnetic ordering, a steep increase in the coercive field with film thickness can be explained in terms of the Volmer–Weber growth of the film. Furthermore, the temperature dependence of the hysteresis loops of a 41 nm-thick FINEMET film has been studied. The Curie temperature of the amorphous film is found to be lower than that of a ribbon of the same composition. The origin of a uniaxial magnetic anisotropy in the as-prepared stage is attributed to the generation of some long-range stresses in the film, which are relieved close to the onset temperature for nanocrystallization.

Signal enhancement of magneto-optical Kerr effect measurements by weak value amplification

Magneto-optic effects are widely applied in the analysis of magnetic thin films. Specifically, due to its local probing characteristic, sensitivity, and experimental simplicity, the surface magneto-optic Kerr effect has been shown to be very useful in the investigation of the magnetization structure of materials. In this work, we propose a weak measurement-based protocol to enhance the pointer deflection, namely the polarization rotation of an incident light beam upon reflection from a magnetic surface, in some orders of magnitude, without loss in precision. It could provide a new insight into magnetic research.