Polarized neutron reflectivity and X-ray scattering measurements as tools to study properties of Pt/Co/Pt ultrathin layers irradiated by femtosecond laser pulses

ABSTRACT

We have used polarized neutron reflectivity, X-ray diffraction, X-ray reflectivity and magneto-optical Kerr effect in polar configuration to study the properties of ultrathin Pt/Co/Pt films. Structures consisting of a 5-nm thick Pt buffer, 3-nm thick Co layer and 5-nm thick Pt cover layer were deposited onto (0001)-oriented Al₂O₃ substrate by the molecular beam epitaxy (MBE) method. Irreversible modifications of film properties, resulting from its illumination by single femtosecond laser pulses, of duration of 40 fs and wavelength of 800 nm, were observed and analyzed. As prepared films exhibited magnetization in-plane, but after laser irradiation, the direction of magnetization was rotated to out-of-plane state. Formation of Co–Pt alloy phase caused by quasi-uniform film irradiation was demonstrated by the results of X-ray and neutron scattering measurements. Moreover, polarized neutron and X-ray reflectivity data showed that after illumination Co was distributed mostly in the area of nominal Co layer and Pt cover layer and its diffusion into the Pt buffer was less significant.     

Abnormal reduction of coercivity on magnetic cobalt–platinum alloy films

Ultrathin Co–Pt alloy films as substrate were studied by the surface magneto-optical Kerr effect. As the growth of Ni, the films show uniquely high polar Kerr responses without any in-plane signals. The coercivity decreased until the thickness of Ni film was higher than 5 ML. A new surface structure was discovered at 7–10 ML Ni/Co–Pt films by the low-energy electron diffraction. Interestingly, polar Kerr signal and coercivity of the 10 ML Ni/Co–Pt(1 1 1) template film reduced rapidly as Co films were further deposited onto only about 1–2 ML. Then the films show a canted magnetization with a rollback hysteresis in the polar configuration during the growth of Co. Coercivity of the 7 ML Co/Ni/Co–Pt film was found unusually down to almost 100 Oe.The corresponding magic number at around 7 ML of Co in the abnormal reduction of coercivity may be attributed to the cluster formations of Co.     

Preisach modeling of aftereffect in a magneto-optical medium with perpendicular magnetization

The aftereffect of Co/Pt multilayer films with perpendicular magnetization has been measured with a magneto-optical Kerr effect (MOKE) magnetometer and calculated with a newly developed Preisach model. Compared to materials such as traditional magnetic recording media, Co/Pt multilayer films show a more complete picture of the progression of aftereffect because the magnetization of this material decays from saturation almost all the way to a ground state in a reasonable length of time. The magnetization measurements for times equal to negative and positive infinity are asymptotically horizontal, with a transition region that is linear on a logarithmic time scale. In contrast, typical published aftereffect analyses exhibit only a very small percentage of the total aftereffect that could be observed if time were not a factor in making measurements. A Preisach–Arrhenius model is used to calculate the magnetic aftereffect in the Co/Pt multilayer. Comparisons of the model to experimental results show not only the validity of the model, but also its value in predicting very short-time and long-time aftereffect behavior, and low levels of aftereffect occurring in noisy data, all of which are difficult to observe experimentally.     

Parameters of the transition layer in the exchange-biased NiFe/DyCo film structure

The parameters of the transition layer in exchange-biased film structures are necessary agents to understand the mechanism of formation of unidirectional anisotropy. The layer thickness in NiFe/DyCo films has been determined by comparison of signals of the polar magneto-optical Kerr effect from a reference DyCo film and a hard magnetic layer of the exchange-biased structure. The layer thickness obtained is one order of magnitude larger than that characteristic of ferromagnet-antiferromagnet bilayer films. The mechanism of magnetization reversal of the structure under study has been explained within the model suggesting the formation of 180° boundaries in the interface.     

Magneto-optical recording on the multilayers systems with plasmon excitation

The maximization of the sensitivity of magneto-optical recording media in the short wavelength region is discussed. We show that a combination of the optimized thickness of dielectric and Ag (or CoAg) films can increase the magneto-optic Kerr effect in FeTbCo media by about five times of magnitude in the UV and blue spectral range. It has been shown that the local plasmon excited in the Ag-based films strongly enhances the magneto-optical properties in amorphous FeTbCo magnetic film. The Kerr rotation spectra get the maxima values for the resonance conditions when the local plasmon can be excited in the Ag or CoAg films.     

PtMn/Co多层膜的磁光特性研究

采用磁控溅射法制备了一系列的Mn掺杂的PtMn/Co多层膜.通过测量得到的磁光常数和光学常 数,计算介电张量的对角元和非对角元,得到在掺杂较大时,克尔角的变化是由于介电张量 的非对角元引起的;在掺杂量较小时,克尔角的变化是由介电张量的对角元和非对角元以及 光学常数的变化引起的.     

Magnetooptical,optical, and magnetotransport properties of Co/Cu superlattices with ultrathin cobalt layers

We investigated the field dependences of the magnetization and magnetoresistance of superlattices [Co(t _x, Å)/Cu(9.6 Å)]30 prepared by magnetron sputtering, differing in the thickness of cobalt layers (0.3 Å ≤ t _Co ≤ 15 Å). The optical and magnetooptical properties of these objects were studied by ellipsometry in the spectral region of hω= 0.09–6.2 eV and with the help of the transverse Kerr effect (hω= 0.5–6.2 eV). In the curves of an off-diagonal component of the tensor of the optical conductivity of superlattices with t _Co = 3–15 Å, a structure of oscillatory type (“loop”) was detected in the ultraviolet region, resulting from the exchange splitting of the 3d band in the energy spectrum of the face-centered cubic structure of cobalt (fcc Co). Based on magnetic experiments and measurements of the transverse Kerr effect, we found the presence of a superparamagnetic phase in Co/Cu superlattices with a thickness of the cobalt layers of 3 and 2 Å. The transition from superlattices with solid ferromagnetic layers to superparamagnetic cluster-layered nanostructures and further to the structures based on Co and Cu (t _Co = 0.3–1 Å) with a Kondo-like characteristics of the electrical resistivity at low temperatures is analyzed.     

Growth-induced perpendicular magnetic anisotropy and clustering in NixPt1−x alloys

Polycrystalline and epitaxial (1 0 0), (1 1 0), and (1 1 1)-oriented Ni₃Pt, NiPt, and NiPt₃ films were deposited over a range of growth temperatures from 80°C to 700°C. Films grown at moderate temperatures (200–400°C) exhibit growth-induced properties similar to Co–Pt alloys: enhanced and broadened Curie temperature, perpendicular magnetic anisotropy and large coercivity. As in Co–Pt, the magnetic properties suggest a clustering of Ni into platelets on the growth surface, as the films are being grown. Unlike Co–Pt, however, NiPt films exhibit a strong orientational dependence of anisotropy and enhanced Curie temperature, possibly resulting from different types of surface reconstructions which affect the growth surface.     

Influence of annealing temperature on optical properties of zinc oxide thin films analyzed by spectroscopic ellipsometry method

Zinc oxide (ZnO) thin films were sol–gel spin coated on glass substrates, annealed at various temperatures 300 °C, 400 °C and 500 °C and characterized by spectroscopic ellipsometry method. The optical properties of the films such as transmittance, refractive index, extinction coefficient, dielectric constant and optical band gap energy were determined from ellipsometric data recorded over the spectral range of 300–800 nm. The effect of annealing temperature in air on optical properties of the sol–gel derived ZnO thin films was studied. The transmission values of the annealed films were about 65% within the visible range. The optical band gap of the ZnO thin films were measured between 3.25 eV and 3.45 eV. Also the dispersion parameters such as single oscillator energy and dispersive energy were determined from the transmittance graph using the Wemple and DiDomenico model.     

Magneto-optical properties of nanogranulated Co-Ti-O films

The results from studies of the magneto-optical properties of Co-Ti-O nanogranulated films obtained under conditions of solid-state reaction with oxygen exchange in CoO/Ti layered structures are presented. The magneto-optical spectra of the polar Kerr effect in films with a relative volume of the magnetic fraction of 0.38 to 0.64 has a resonance character in the wavelength range of 450–650 nm. The Kerr rotation angle of the films increases by a factor of 3–5, relative to conventional Co films.     

Trigonally-compressed octahedral geometry of hexaamminecobalt(III) complex cation in aqueous solution investigated from the electronic spectra

Electronic spectra of hexaamminecobalt(III) complex cation in aqueous solution were analyzed to obtain spectral components. Subsequently, based on the spectral components, the coordination geometry around the cobalt(III) ion was investigated, using the reverse angular overlap model method. The result indicates that the geometry is a trigonally compressed octahedron with the polar angle of 57.9?±?1.0° under D_3d symmetry, where the polar angle is the angle between the trigonal axis and the Co–N bond. From this angle, the top and side N–Co–N bond angles are calculated as 94.4° and 85.6°, respectively. The density functional theory computation supported this trigonally compressed structure in aqueous solution.     

The role of nucleation field of media in heat-assisted magnetic probe recording

We characterize a method of heat-assisted magnetic probe recording on perpendicular media. Heating source is field emission current from a scanning tunneling microscope (STM) tip. Recording media are three kinds of magnetic films, Co/Pt, CoNi/Pt, and Co/Pd multilayers with different nucleation fields. Pulses with amplitude of 5 V were applied between the STM tip and the recording medium. Experiments show that magnetic marks with an average size of 180 nm were formed on both Co/Pt and CoNi/Pt films whose nucleation fields are greater than their saturation magnetization. No marks were observed on the Co/Pd film whose nucleation field is smaller than its saturation magnetization. A model is built to simulate the dynamic process of domain formation in probe-based magnetic recording system. Simulation results agree with experiments and it explains the effect of the nucleation field of medium in perpendicular recording.     

PtMn/Co多层薄膜结构及磁光克尔效应的研究

用磁控溅射法制备了不同Mn含量的PtMn/Co多层膜,通过大,小角X射线衍射谱对该多层膜进行结构分析,研究了该多层膜的层状结构同磁光克尔效应的关系,通过测定该多层膜在不同杂质浓度下的克尔谱,椭偏率谱及克尔回线,发现克尔角随Mn含量增加的变化规律,并分析了它的产生机制。     

Scaling properties of magnetic domain walls in Pt/Co/Pt trilayers on MgO (1 1 1)

We present a scaling analysis of the time evolution of domain walls in ultrathin magnetic films that are subject to different forms of uncorrelated and correlated disorder caused by the microstructure of the underlying template. The study is performed on ultrafine modulated Pt/Co/Pt trilayers grown on as-supplied and structured MgO (1 1 1) substrates employing polar Kerr microscopy for the imaging of the magnetic domains.     

Synthesis and optical properties of basic fuchsin dye-doped PMMA polymeric films for laser applications: wide scale absorption band

Basic fuchsin dye-doped poly(methyl methacrylate) polymeric films were sensitized with various dye concentrations ranging from 0.0833 to 1.667 wt% of basic fuchsin. Their structure, linear absorption, and optical limiting properties were examined. The films were prepared using a simple and fast casting technique dissolved in chloroform for both the dye and the polymer. Structural characterizations were achieved by XRD, and the films showed an amorphous hump supporting the noncrystalline structure of studied polymeric composites. Spectrophotometer measurements were used to estimate the spectral absorption measurements of the films such as transmittance, absorbance with the calculations of absorption index (k), and optical energy band gap (E _g ) in the wavelength region from 190 to 2500 nm. Results show that the optical constants change with increasing the dye doping concentrations. It has been found that optical energy gap (E _g ) appearing that, both direct and indirect optical transitions are conceivable for these films. Optical limiting properties of the films with various dye concentrations were studied using a continuous wave He–Ne laser operating at 632.8 nm. The results appeared that the sample has an obvious optical limiting effect. The designed BF/PMMA composites can be applicable in wide-scale applications.     

Modes of periodic domain wall motion in ultrathin ferromagnetic layers

Magnetization reversal in a periodic magnetic field is studied on an ultrathin, ultrasoft ferromagnetic Pt/Co(0.5 nm)/Pt trilayer exhibiting weak random domain wall (DW) pinning. The DW motion is imaged by polar magneto-optic Kerr effect microscopy and monitored by superconducting quantum interference device susceptometry. In close agreement with model predictions, the complex linear ac susceptibility corroborates the dynamic DW modes segmental relaxation, creep, slide, and switching.     

Magneto-optical Static Recording Performances of Sputtered CoPt Alloy Films

CoPt alloy films with good magnetic and mageto-optical properties were prepared on Pt buffer layer or directly on glass subst rate by sputtering a composite target. Their static recording properties were investigated. The results showed that static signals could be easily written into the CoPt alloy films without Pt buffer layers using moderate recording power. The introduction of Pt buffer layer in CoPt alloy films not only led to a smaller Kerr rotation but also greatly increased the recording power.     

Numerical calculation of Kerr spectra for Co/Pt magnetic multilayered films

The Kerr spectra as function of wavelength, incident angle and layer number are calculated with 4×4 matrix method. It is found that the calculated results are in good agreement with experimental ones for [Co (0.3 nm)/Pt (0.4 nm)]×43/glass (1.21 mm) and [Co (0.4 nm)/Pt (1.1 nm)]×53/glass (1 mm). In addition, for [Co (0.3 nm)/Pt (0.4 nm)]×43/glass (1.21 mm), it shows a maximum Kerr rotation at N = 10. For [Co (0.4 nm)/Pt (1.1 nm)]×53/glass (1 mm), the calculated Kerr rotation as a function of incident angle reveals maximum when the incident angle is 89°.     

Third-order nonlinear optical response of Au-core CdS-shell composite nanoparticles embedded in BaTiO<Subscript>3</Subscript> thin films

Au-core CdS-shell composite nanoparticles were synthesized by a direct self-assembly process and integrated into BaTiO₃ thin films. Characterization by transmission electron microscopy showed that the average diameter of these composite nanoparticles was about 8 nm. Using the femtosecond time-resolved optical Kerr effect method, we investigated the third-order nonlinear optical response of the Au@CdS nanoparticles embedded in the BaTiO₃ thin films at a wavelength of 800 nm. An ultrafast nonlinear response and a large effective third-order nonlinear susceptibility of χ⁽³⁾=7.7×10^-11 esu were observed. We attributed the enhancement of the third-order optical nonlinearity to a localized electric field effect originating from the core-shell structure under off-surface-plasmon resonance conditions. Received: 13 May 2002 / Revised version: 23 October 2002 / Published online: 3 April 2003 RID="*" ID="*"Corresponding author. Fax: +86-21/6510-4949, E-mail: sxqian@fudan.ac.cn     

Tensile strain induced changes in the optical spectra of SrTiO3 epitaxial thin films

Effect of biaxial tensile strains on optical function and band edge transitions of ultra thin epitaxial films was studied using as an example a 13 nm thick SrTiO₃ films deposited on KTaO₃ (100) single-crystal substrates. Optical functions in the 200–1200 nm spectral range were determined by spectroscopic ellipsometry technique. It was found that tensile strains result in a shift of the low energy band gap optical transitions to higher energies and decrease the refractive index in the visible region. Comparison of the optical spectra for strained SrTiO₃ films and for homoepitaxial strain-free SrTiO₃: Cr (0.01 at %) films deposited on SrTiO₃ (100) single crystalline substrates showed that this “blue” shift of the band gap could not be related to technological imperfections or to reduced thickness. The observed effect is connected with changes in the lowest conduction and in the top valence bands that are due to increase of the in-plane lattice constant and/or onset of the polar phase in the tensile strain-induced ultra-thin epitaxial SrTiO₃ films.