Size effect in the spin glass magnetization of thin AuFe films as studied by polarized neutron reflectometry

We used polarized neutron reflectometry to determine the temperature dependence of the magnetization of thin AuFe films with 3% Fe concentration. We performed the measurements in a large magnetic field of 6 T in a temperature range from 295 to 2 K. For the films in the thickness range from 500 to 20 nm we observed a Brillouin-type behavior from 295 K down to 50 K and a constant magnetization of about 0.9 micro(B) per Fe atom below 30 K. However, for the 10 nm thick film we observed a Brillouin-type behavior down to 20 K and a constant magnetization of about 1.3 micro(B) per Fe atom below 20 K. These experiments are the first to show a finite-size effect in the magnetization of single spin-glass films in large magnetic fields. Furthermore, the ability to measure the deviation from the paramagnetic behavior enables us to prove the existence of the spin-glass state where other methods relying on a cusp-type behavior fail.     

Retrieval of the complex reflection coefficient from nano scale thin films by using polarized neutrons and magnetic substrates

A method is proposed for overcoming the famous ‘phase problem’ in neutron specular reflectometry. It is shown that the complex reflection coefficient of any unknown non magnetic layer, with real scattering length density, can be determined by using a magnetic transmitted media and by measuring the polarization of the reflected beam relative to the incident beam. The method follows directly from a recent one which is limited to a one-dimensional neutron polarization. Here, the theory is generalized for a neutron polarization of arbitrary direction. We show that some combinations between the polarization of the incident and reflected beam must be used to determine the reflection coefficient. Also, it is shown that instead of full polarization or reflectivity analysis, some combinations between polarization and reflectivity can be used in the analysis process. The method is supplemented with a schematic example to test the method and its stability in the presence of experimental uncertainties and roughness of the interfaces. PACS 61.12.Ha; 28.20.-v     

Spinodal dewetting of thin liquid films at soft interfaces

We study theoretically the behavior of nanoscopic liquid films L (thickness e) intercalated between a solid S and a rubber R (elastic modulus μ). Thickness modulations involve a healing length , which results from a competition between elastic and disjoining pressure. With van der Waals interactions, , where a is a molecular size and h₀ the rubber capillary length ( , interfacial tension). If the Hamaker constant of the intercalated liquid is negative, the film dewets by amplification of peristaltic fluctuations (“spinodal dewetting”). The typical size of the contacts is predicted to scale like for films of thicknesses . The rise time of the fastest mode, predicted to scale like , should be very sensitive to the film thickness. Received 11 February 2000 and Received in final form 22 May 2000     

Retrieval of depth profile of nano-scale thin films by one directional polarization analysis in neutron specular reflectometry

Recently it has been shown that the modulus and phase of complex reflection coefficient can be determined using a magnetic substrate and polarized neutrons. Several other methods have also been worked out based on the measurement of polarizations of reflected neutrons from magnetic reference layers and magnetic substrate. However, due to the fact that available reflectometers are limited in the choice of polarization of reflected beam in the same direction as the polarization of the incident beam, neither of the methods, which are based on polarization analysis, have been proven to be experimentally practical. In this paper, we have proposed a new method for determining the phase of reflection coefficient that is based on two measurements of polarization, which correspond to two magnetic fields with the same magnitude and different orientations. The polarization analysis is performed in the same direction as the polarization of the incident beam and is well suited for available reflectometers.The problems envisaged in implementation of the method are also discussed.     

In‐situ neutron reflectometry probing competitive swelling and de‐swelling of thin polystyrene films

The competitive penetration of toluene with respect to oxygen, nitrogen and carbon dioxide into a thin and initially dry film of deuterated polystyrene (dPS) is compared to the reverse process of de‐swelling. In‐situ neutron reflectometry is used to follow the swelling and de‐swelling kinetics, allowing the determination of the total dPS thickness, roughness and solvent penetration. Swelling cannot be described in terms of case II diffusion due to the small film thickness and undersaturation results in a small swelling rate of 0.03 nm s^–1. De‐swelling is slower by more than two orders in magnitude, thereby underlining the difficulty to remove once incorporated solvent molecules. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

     

Application of polarized neutron reflectometry and X-ray resonant magnetic reflectometry for determining the inhomogeneous magnetic structure in Fe/Gd multilayers

The evolution of the magnetic structure of multilayer [Fe (35 Å)/Gd (50 Å)₅] with variation in temperature and an applied magnetic field was determined using a complementary approach combining polarized neutron and X-ray resonant magnetic reflectometry. Self-consistent simultaneous analysis of X-ray and neutron spectra allowed us to determine the elemental and depth profiles in the multilayer structure with unprecedented accuracy, including the identification of an inhomogeneous intralayer magnetic structure with near-atomic resolution.     

Study of the Au/SiO<Subscript>2</Subscript> + Co(65 at %)/GaAs heterostructure interfaces by the polarized neutron reflectometry method

Using the polarized neutron reflectometry method, we studied Au/SiO₂ + Co(60 at %)/GaAs granular films, which display a giant injection magnetoresistance effect in a narrow temperature range close to T = 300 K. We revealed the existence of a layer having particular magnetic properties at the boundary of a film with a semiconductor GaAs substrate. Experiments carried out at temperatures T = 300 K and T = 100 K showed an insignificant difference in the magnetic profile of the heterostructure.     

CoFe/TiZr多层膜材料界面结构与性能的极化中子反射研究

利用极化中子反射技术较系统地研究了CoFe/TiZr复合多层膜材料界面结构,结果表明.1)从多层膜超镜传输特性角度考虑,等厚对层不如非等厚对层膜结构好.2)等厚对层的最佳退火温度约250℃,非等厚对层退火温度低于250℃影响不明显,等厚和非等厚对层经350℃退火后膜层变化严重.3)从布拉格峰位变化看,随着退火温度的升高,等厚对层膜的厚度先变小后变大;非等厚对层与之相反. 关键词： CoFe/TiZr多层膜 退火影响 膜层结构 极化中子反射     

CoFe/TiZr多层膜材料界面结构与性能的极化中子反射研究

利用极化中子反射技术较系统地研究了CoFe/TiZr复合多层膜材料界面结构，结果表明.1)从多层膜超镜传输特性角度考虑，等厚对层不如非等厚对层膜结构好.2)等厚对层的最佳退火温度约250℃，非等厚对层退火温度低于250℃影响不明显，等厚和非等厚对层经350℃退火后膜层变化严重.3)从布拉格峰位变化看，随着退火温度的升高，等厚对层膜的厚度先变小后变大；非等厚对层与之相反.     

The total reflection of light at smooth and rough silver films and surface plasmons

Measurements of the reflection of thin silver films are reported which have been performed with the ATR method at constant wavelength λ as well as at constant angle of incidence θ. The backbending of the reflection minimum as a function of θ at constant λ is discussed. The influence of the interband absorption determines the R(θ,λ) dependence. Surface roughness displaces the turning point in the backbending curve.     

On the decay of plasmon surface polaritons at smooth and rough Ag-air interfaces: A reflectance and photo-acoustic study

Attenuated total reflection and photoacoustic measurements have been employed to study the decay channels of plasmon surface polaritons optically excited at the Ag-air interface. We show that it is possible to analyse the radiative and non-radiative decay quantitatively by comparing measurements taken with transparent and blackened cavities, respectively. In the first case only light that is thermalized by radiationless processes in the Ag film produces a photoacoustic signal. In the blackened cell, however, light that is either transmitted through the Ag film or reemitted by the radiative decay of excited polaritons also contributes to the photoacoustic intensity.     

Electrohydrodynamic controlled assembly and fracturing of thin colloidal particle films confined at drop interfaces

Particles can adsorb strongly at liquid interfaces due to capillary forces, which in practice can confine the particles to the interface. Here we investigate the electrohydrodynamic flow driven packing and deformation of colloidal particle layers confined at the surface of liquid drops. The electrohydrodynamic flow has a stagnation point at the drop equator, leading to assembly of particles in a ribbon shaped film. The flow is entirely controlled by the electric field, and we demonstrate that AC fields can be used to induce hydrodynamic “shaking” of the colloidal particle film. We find that the mechanical properties of the film is highly dependent on the particles: monodisperse polystyrene beads form packed granular monolayers which “liquefies” upon shaking, whereas clay mineral particles form cohesive films that fracture upon shaking. The results are expected to be relevant for understanding the mechanics and rheology of particle stabilized emulsions.     

X-ray and neutron reflectivity analysis of thin films and superlattices

X-ray and neutron reflectivity measurements provide a wealth of information on thickness and interfacial properties on the nanometer scale. This method is therefore well suited for the study of nano-structured thin films and superlattices. Neutrons provide a different contrast between the elements than X-rays and, in addition, are sensitive to the magnetization in the sample. Using polarized neutrons, magnetic as well as chemical profiles can be probed. In this review a basic introduction into the theory of X-ray and neutron reflectivity is provided along with some recent examples including the oxidation of Fe films and the structural and magnetic properties of Co/Cu superlattices.     

Preparation and characterization of nano and microcrystalline ZnO thin films by PLD

Nano and microcrystalline ZnO thin films were prepared on glass substrates using pulsed laser deposition technique under a vacuum of 3 × 10⁻⁷ Torr at different laser power density. Composition analyses show that the films deposited at low laser power density have more structural defects than the film deposited at high laser power density. It confirms that the content of Zn in free-state decreased greatly at high laser power density. Atomic force microscopy analysis shows that the surface roughness of the deposited films increases with an increase in laser power density. X-ray diffraction analyses show that all the films are oriented along (0 0 2) direction independently on the laser power density applied. The structural quality increases with an increase in laser power density. It is due to the fact that the increase of laser power density leads to the enhancement of peak intensity. The increase of laser power density reduces the film transmission in the visible range of the spectra. The optical band gap value is found to be in the range from 3.42 to 3.39 eV. It shows that the optical band gap value decreases with an increase in laser power density. FTIR analysis shows that the hydrated oxide content in the deposited films decreases with an increase in laser power density.     

Polarized neutron reflectometry of the influence of Fe/Si,Fe/FeSi2 and Au/Fe interfaces on the magnetic properties of epitaxial Fe films

The magnetic and structural properties of epitaxial Fe films grown on Si(1 1 1) are investigated by polarized neutron reflectometry (PNR) at room temperature. The influence of different types of interfaces, Fe/Si, Fe/FeSi₂ and Au/Fe on the magnetic properties of Fe films deposited by molecular beam epitaxy onto Si(1 1 1) are characterized. We observe a drastic reduction of the magnetic moment in the entire Fe film deposited directly on the silicon substrate essentially due to strong Si interdiffusion throughout the whole Fe layer thickness. The use of a silicide FeSi₂ template layer stops the interdiffusion and the value of the magnetic moment of the deposited Fe layer is close to its bulk value. We also evidence the asymmetric nature of the interfaces, Si/Fe and Fe/Si interfaces are magnetically very different. Finally, we show that the use of Au leads to an enhancement of the magnetization at the interface.     

Calibration and applications of polarized neutron thermometry at milli- and microkelvin temperatures

We have utilized the polarized neutron scattering technique for the determination of the nuclear temperature in copper from 20 mK down to 100 K. Using a cooling facility where two adiabatic demagnetization stages work in series we have calibrated the flipping ratio of neutrons scattered from the (200) Bragg-peak against the nuclear polarization over the whole polarization range. The observed calibration curve deviates considerably from the curve expected for an extinction-free sample. The polarized neutron thermometer is discussed and applied in measurements of the spin-lattice relaxation in copper.     

Co:BaTiO3纳米复合薄膜的制备及其结构

用脉冲激光沉积技术(PLD)在MgO(100)基底上生长了嵌埋Co纳米晶的BaTiO3复合薄膜. 分别利用x射线衍射(XRD)、原子力显微镜(AFM)以及拉曼光谱(Raman)对薄膜的微观结构、表面 形貌进行了表征. 结果表明该薄膜为c轴取向的四方晶体结构，薄膜表面均匀、致密、 具有原子尺度的光滑性，其均方根表面粗糙度(RMS)达到015nmCo以纳米晶形式嵌埋BaTi O3基体中，呈单分散性均匀分布，其粒径随激光脉冲数的增加而增大. Co：BaTiO3纳米 复合薄膜拉曼峰的强度随钴纳米晶粒径的增加明显减弱，但是峰的宽度逐渐增加. 关键词： Co：BaTiO3 纳米复合薄膜 脉冲激光沉积     

Dynamics of rough interfaces in chemical vapor deposition: experiments and a model for silica films

We study the surface dynamics of silica films grown by low pressure chemical vapor deposition. Atomic force microscopy measurements show that the surface reaches a scale invariant stationary state compatible with the Kardar-Parisi-Zhang (KPZ) equation in three dimensions. At intermediate times the surface undergoes an unstable transient due to shadowing effects. By varying growth conditions and using spectroscopic techniques, we determine the physical origin of KPZ scaling to be a low value of the surface sticking probability, related to the surface concentration of reactive groups. We propose a stochastic equation that describes the qualitative behavior of our experimental system.