Magnetism in very thin films of permalloy measured by spin polarized cascade electrons

The spin polarizationP of the low energy cascade electrons excited with a primary unpolarized electron beam is measured with ultrathin films of permalloy (Ni₈₀Fe₂₀) as a function of film thickness, external magnetic field, and temperatureT. Surface adsorbates of small concentrations of less than 10% of a monolayer can change the Curie point and the saturation value ofP ₀(T0) by as much as 30%. The Ta-substrate induces a magnetically dead region in permalloy. Conventional spin wave theory cannot account for the observed smallT-dependence of the magnetizationM. Films on a nonmagnetic substrate are compared to similar films coupled to bulk permalloy over an interface of Ta. TheT-dependence ofM with the coupled films can be explained by spin wave theory. At lowT, the films coupled to the bulk exhibit a faster decrease ofM than the uncoupled films. We propose that this thermal stabilization of the magnetization in very thin ferromagnetic films is due to quenching of the long wavelength spin modes.     

Study of interaction between ZrO2 (YSZ) substrates and YBa2Cu3O7−δ superconducting films

The interactions between YBa₂Cu₃O_7– superconducting films and ZrO₂ substrates with different crystal orientations were investigated. Our results show that the crystal orientation of the substrate influences the orientations of the YBa₂Cu₃O_7– crystals, whose c-axis have preferential orientations parallel to the surface of 100 ZrO₂ substrates. The copper segregation in Y-Ba-Cu-O/ZrO₂ films results from substrate reactions. Y, Ba, and Cu atoms diffusing into substrates were detected by AES and RBS techniques. A thin silver film evaporated on ZrO₂ substrate as a buffer layer effectively slows down or eliminates the substrate interactions. The strong preferential c-axis orientation perpendicular to the substrate surface has been observed in the YBa₂Cu₃O_7– /Ag/ZrO₂ samples.This work has been supported by Science Foundation of Academia Sinica     

Preparation of single-crystal-like MgO films on Si and orientation control of platinum films on MgO/Si

Growth of a Pt/MgO bilayer on Si(100) was investigated by pulsed-laser deposition. The growth modes of both MgO and platinum films are layer-by-layer growth, which were revealed by in situ reflection high energy electron diffraction observations. Two kinds of orientations of platinum films, viz. epitaxially (100) and (111)-oriented platinum films, were obtained on the same MgO(100)/Si(100) substrate only by varying the laser fluence. The effect of laser fluence on the orientation of platinum films is briefly discussed. The platinum films prepared in our experiments are epitaxially grown and exhibit atomic-scale surface flatness. It is believed that the improvement in the quality of platinum films can be attributed to the perfectly single-crystalline quality of the MgO buffer layer, which was further confirmed by the excellent dielectric properties. For a 150 nm thick MgO film, the leakage current density was found to be 10^-7 Acm^-2 with an electric field of 8×10⁵ Vcm^-1 and the relative dielectric constant (_r) was 10.6. PACS 68.55.Jk; 81.15.Fg; 85.50.Gk     

Growth and melting behaviour of thin in films on Ge(100)

Growth and melting behaviour of thin indium films on Ge(100) have been investigated by Auger-electron spectroscopy (AES), atomic force microscopy (AFM) and perturbed angular correlation (PAC) spectroscopy, respectively. At room temperature inidium is found to grow in three-dimensional islands even at submonolayer coverages. A very rough film surface is observed for thicknesses up to 230 ML. The melting behaviour of such films has been studied by PAC. A reduction of the melting temperature T _m as well as a strong supercooling of the films is observed. The electric field gradient for ¹¹¹In(¹¹¹Cd) in the indium islands is determined as a function of temperature and is used to monitor the local crystalline order of the films up to temperatures just below the melting point.     

Mechanical instabilities of bubble clusters between parallel walls

We have carried out a systematic study of buckling-like mechanical instabilities in simple two- (2D) and three-dimensional (3D) symmetric foam clusters sandwiched between parallel planar walls. These instabilities occur when the wall separation w is reduced below a critical value, w^*, for which the foam surface energy E reaches its minimum, E^*. The clusters under investigation consist of either a single bubble, or of twin bubbles of fixed equal sizes (areas A in 2D or volumes V in 3D), which are either free to slide or pinned at the confining walls. We have numerically obtained w^* for both free and pinned 2D and 3D clusters. Furthermore, we have calculated the buckled configurations of 2D twin bubbles, either free or pinned, and of 3D free twin bubbles, whose energy is independent of w and equal to the minimum energy E^* of the unbuckled state. Finally, we have also predicted the critical w_t^* at which the terminal configurations under extension of 2D and 3D single and twin bubbles are realised. Experimental illustrations of these transitions under compression and extension are presented. Our results, together with others from the literature, suggest that a bubble cluster bounded by two parallel walls is stable only if the normal force it exerts on the walls is attractive, i.e., if dE/dw > 0; clusters that cause repulsion between the walls are unstable. We correlate this with the distribution of film orientations: films in a stable cluster cannot be too parallel to the confining walls; rather, their average tilt must be larger than for a random distribution of film orientations.     

Hexatic ordering in freely suspended liquid crystal films

In this paper, we report results from synchrotron X-ray scattering studies of thefluid/hexactic/solid phases and phase transitions in both very thick and very thin, freely suspended films of tilted hexatic liquid crystals. Contrary to the thick film case, the higher Fourier coefficients describing the bond orientational order are suppressed in very thin films. This suppression is consistent with a two-dimensional bond orientational order parameter, ₆, rather than the three-dimensional bond orientational order parameter found in very thick films. For a film containing twently-three (23) smectic layers we find that ₆ is two-dimensional whereas the positional order in the crystallineS _J phase is three-dimensional. We present an analysis of the thick film data in terms of the three-dimensionalXY-model and a new mean field theory model which incorporates explicitly the quasi two-dimensional nature of bulk smectic phases.     

Y1Ba2Cu3O7-δ thin films from KrF laser ablation with substrate heating and in situ patterning by CO2 laser radiation

Y₁Ba₂Cu₃O_7– thin films were deposited by KrF laser ablation while replacing conventional contact heating by cw CO₂ laser irradiation of the substrate front surface. The HTSC films obtained on (100)ZrO₂ showed T _c(R=0)=90 K, T(90–10%)=0.5 K, j _c=2.5 × 10⁶ A/cm², a sharp transition in the ac susceptibility X(T), and pure c-axis orientation. Micrographs of thin films (< 0.5 m) showed a smooth morphology while thick films (>1 m) contained many crystallites sticking in the bulk material. Furthermore, in situ patterning was achieved during deposition by local laser heating of a selected substrate surface area. The resulting planar films contained amorphous, semiconducting parts only 1 mm or less apart from crystalline material showing the above HTSC quality.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

The impact of twinning on the local texture of chalcopyrite‐type thin films

Twinning in a CuInS₂ layer in a completed thin‐film solar cell was analyzed by means of electron backscatter diffraction. This technique revealed the microstructure of the CuInS₂ thin films and local orientation relationships between the grains. At various locations within the layer it was possible to retrace how twinning occurred comparing the local orientations with the theoretically possible changes in orientation by twinning. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

     

Room‐temperature epitaxial growth of high‐quality m ‐plane InGaN films on ZnO substrates

The authors have grown high‐quality m ‐plane In_0.36Ga_0.64N (1 00) films on ZnO (1 00) substrates at room temperature (RT) by pulsed laser deposition (PLD) and have investigated their structural properties. m ‐plane InGaN films grown on ZnO substrates at RT possess atomically flat surfaces with stepped and terraced structures, indicating that the film growth proceeds in a two‐dimensional mode. X‐ray diffraction measurements have revealed that the m ‐plane InGaN films grow without phase separation reactions at RT. The full‐width at half‐maximum values of the 1 00 X‐ray rocking curves of films with X‐ray incident azimuths perpendicular to the c ‐ and a‐axis are 88 arcsec and 78 arcsec, respectively. Reciprocal space‐mapping has revealed that a 50 nm thick m ‐plane In_0.36Ga_0.64N film grows coherently on the ZnO substrate, which can probably explain the low defect density that is observed in the film. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of high-temperature annealing on magnetic ordering in thin films of Bi2O3-CaO-Fe2O3

Magnetic properties of rf sputtered (0.5–x) Bi₂O₃-x CaO-0.5 Fe₂O₃ (x=0–0.5) have been studied through magnetization and ferromagnetic resonance (FMR) measurements. Films prepared in a mixed oxygen-argon atmosphere are amorphous and paramagnetic. Samples annealed in air at temperatures of 700–1000 K show a ferrimagnetic behavior even though X-ray diffraction data for the films do not indicate the precipitation of any crystalline ferrimagnetic compounds. The room-temperature saturation magnetization 4M and the uniaxial anisotropy field H _u , decrease with increasing x. The Curie temperature and the gyromagnetic ratio increase with increase in the concentration of CaO. Studies on the effects of sputtering atmospheres on magnetic parameters show that films sputtered in oxygen-rich atmospheres have a large 4M and H _u , and a relatively small and FMR line-width. Ordered amorphous clusters are suggested to give rise to the observed ferrimagnetic character in the annealed films.     

Muon spin relaxation in ferromagneticPdMn

Positive-muon ( ⁺) spin relaxation experiments have been carried out in the dilute ferromagnetic alloy Pd+2 at.% Mn (T _c=5.8 K). In the paramagnetic state the inhomogeneous ⁺ linewidth is proportional to the bulk magnetization. BelowT _c the ⁺ linewidth and the width of the ⁺ local field distribution in zero applied field are both in qualitative accord with the Sherrington-Kirkpatrick theory of disordered magnets.This work was performed under the auspices of the U.S. Department of Energy, and was supported by the U.S. National Science foundation, grant nos. DMR-7909223 and DMR-8115543, and by the Netherlands Stichting voor Fundamenteel Onderzoek der Materie (FOM).     

Transport properties of Co<Subscript>2</Subscript>CrAl Heusler alloy films

The effect of atomic disorder on the electron transport and the magnetoresistance (MR) of Co₂CrAl Heusler alloy (HA) films has been investigated. We show that Co₂CrAl films with L2₁ order exhibit a negative value for the temperature coefficient of resistivity (TCR) in a temperature range of 10 < T < 290 K, and the temperature dependence of electric conductivity varies as T ^3/2 similarly to that of the zero-gap semiconductors. The atomic or the site disorder on the way of L2₁ → B2 → A2 → amorphous state in Co₂CrAl HA films causes the deviation from this dependence: reduction in the absolute value of TCR as well as decrease in the resistivity down to ?(T = 293 K) ～ 200 μΩ cm in comparison to ?(T = 293 K) ～ 230 μΩ cm typical for the Co₂CrAl films with L2₁ order. The magnetic-field dependence of MR of the Co₂CrAl films with L2₁ order is determined by two competing contributions: a positive Lorentz scattering and a negative s-d scattering. The atomic disorder in Co₂CrAl films drastically changes MR behavior due to its strong influence on the magnetic properties.     

Preparation of Pb2CrO5 thin films by an electron-beam evaporation technique

Pb₂CrO₅ thin films have been prepared by an electron-beam evaporation deposition technique on glass substrates using ceramic disks. The thin film fabrication conditions are studied by x-ray diffraction, replica electron micrography and scanning electron microscopy as parameters of substrate temperature, annealing temperature and annealing time. As-deposited Pb₂CrO₅ thin films in the amorphous state are crystallized by heat treatment. Annealed thin films are structurally classified into three types according to the preferred orientations which depend on substrate temperatures (T _s:

1. (i)

   (020) atT _s=room temperature

2. (ii)

   mainly (310) atT _s=100 ‡C, and

3. (iii)

   (200) atT _s=350 ‡C.

The substrate temperature contributes to film orientations; annealing temperature and time-enhance film crystallinity. The annealing temperature is fixed between 400 and 500 ‡C in making appropriate Pb₂CrO₅ thin films. These thin films, ranging between 0.3 and 2.0 Μm in thickness, are prepared at a deposition rate of 1500 å/m.

     

Growth of InN films on spinel substrates by pulsed laser deposition

We have grown InN films on MgAl₂O₄(111) substrates with atomically flat surfaces using pulsed laser deposition (PLD) and compared their structural properties with those grown on (Mn,Zn)Fe₂O₄(111) substrates. It has been revealed that InN(0001) films grow on MgAl₂O₄(111) with an in‐plane epitaxial relationship of InN[1 00] // MgAl₂O₄[1 0], achieving a lattice mismatch minimum. The InN films exhibited a clear sixfold rotational symmetry, without 30° rotational domains and with a full width at half maximum value of the InN 0002 rocking curve being 17.5 arcmin. Comparison between InN films grown on MgAl₂O₄ and those on (Mn,Zn)Fe₂O₄ led us to conclude that suppression of the interfacial reactions between the InN films and the substrate is inherently important to obtain high quality InN on substrates with a spinel structure. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetoconductivity of thin epitaxial silver films

The magnetoconductance (MC) of thin epitaxial Ag films on Si(111) surfaces is studied as a function of film thickness (1–125 monolayers (ML)) at 20 K under ultra high vacuum (UHV) conditions. Three different regimes of magnetoconductance are observed depending on the degree of disorder in the films which is controlled by film thickness and annealing procedures. Thick films (d>3 ML) with diffuse electron transport show in the case of large elastic scattering times ₀ a classical, negative MC B ² and in the case of small ₀ a positive MC due to weak localization effects. The MC of thin films (d<2 ML) which have a conductance smaller than e ²/h, i.e. localized electron states, is negative again.     

Effects of heating on the Raman spectra of YBa2Cu3O7−δ

We report effects of heating to the intensities of Raman modes in the spectra of single-crystals embedded in ceramic YBa₂Cu₃O_7– pellets, and compare the results with those from YBa₂Cu₃O_7– thin-films. Heating was done by either increasing the laser-beam power or resistively heating in a heating cell. From the measurements on a single-crystal in z(xx) polarization, we find that the relative intensity of the B_1g mode of the plane oxygens to that of the A_1g mode of apical oxygens decreases strongly as the samples are heated. For a crystal in near x(zz) polarization, the relative intensity of the two modes increases up to certain value. In thin-films, however, no such change was observed even when heated to higher temperatures than the crystals were. Therefore, we interpret that the change in the relative intensity of the two modes of single-crystals is not due to electronic or structural changes nor thermal decomposition in the heated samples. We propose that a single-crystal, when heated, is fractured into many micro-crystals and these micro-crystals thermally fluctuate in orientations. For thin-films, such fluctuation in orientations might be hindered because there are only a few layers of micro-crystals on top of the substrate. Scanning electron micrographs (SEM) of the heated part of the samples seem to provide the evidence of the micro-crystals.     

Galvano- and Thermomagnetic phenomena in thin films with nonidentical surfaces

A theory of galvano- and thermomagnetic phenomena in thin films with nonidentical surfaces is presented. The kinetic equation is solved with consideration of boundary conditions in films with nonidentical surfaces in the presence of a magnetic field perpendicular to the film plane, and an electric field and temperature gradient within the film plane. General expressions are found for the components of the galvano- and thermomagnetic tensors _ik, _ik, _ik which are oscillating functions of film thickness and magnetic field. A detailed analysis is performed of the dependence of magnetic resistance(d, H), the Nernst — Ettingshausen(N-E) coefficient Q(d, H), the electron component of thermal conductivity(d, H), and the thermo-emf(d, H) on film thickness and magnetic field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 86–91, October, 1977.In conclusion, the authors express their gratitude to B. M. Askerov for his constant interest in the study and fruitful evaluation of the results.     

Control of a- and c-plane preferential orientations of ZnO thin films

We report orientation-controllable growth of ZnO thin films and their orientation-dependent electrical characteristics. ZnO thin films were deposited on single-crystalline (1 0 0) LaAlO₃ and (1 0 0) SrTiO₃ substrates using pulsed laser deposition (PLD) at different substrate temperatures (400-800 °C). It was found that the orientation of ZnO films could be controlled by using different substrates of single-crystalline (1 0 0) LaAlO₃ and (1 0 0) SrTiO₃. The a-plane () and c-plane (0 0 0 2) oriented ZnO films are formed on LaAlO₃ and SrTiO₃, respectively. In both cases, the degree orientation increased with increasing deposition temperature T_s. Both the surface free energy and the degree of lattice mismatch are ascribed to play an important role for the orientation-controllable growth. Further characterization show that the grain size of the films with both orientations increases for a substrate temperature increase (i.e. from T_s = 400 °C to T_s = 800 °C), whereas the electrical properties of ZnO thin films depend upon their crystalline orientation, showing lower electrical resistivity values for a-plane oriented ZnO films.     

Correlation between extrinsic magnetoresistance and electroresistance in La0.6Pb0.4MnO3 thin films as revealed from current-voltage and ferromagnetic resonance studies

We have investigated magnetoresistance (MR) and electroresistance (ER) of well characterized La_0.6Pb_0.4MnO₃ (LPMO) films having two different crystallinity (i) (00l) oriented single-crystalline, SC, i.e. without any grain boundaries, and (ii) nanocrystalline, NC, with an average grain size of 17 nm. Both MR and ER were remarkably different for the two films, that is, (i) NC films exhibited a highest MR of ∼100% near the metal-insulator transition temperature; while for SC films the MR was in the range of 40-60%, (ii) NC films exhibited a finite ER, which increased monotonically with decreasing temperature; while in SC films ER was completely absent. Using current-voltage characteristic and ferromagnetic resonance studies we demonstrate that both enhanced MR and ER in NC films are extrinsic in nature and originate due to the presence of spin glassy grain boundaries.     

Eliashberg theory for disordered superconductors. II. Tunneling into disordered two-dimensional Pb films

A series ofc-axis oriented YBa₂Cu₃O _x -films with different oxygen content were prepared by laser deposition. The oxygen contentx was determined by X-ray diffraction and by resonant Rutherford-back-scattering (RRBS) measurements. Thec-axis length in these films of YBa₂Cu₃O _x is about 0.5% larger compared to bulk values. We describe transport measurements in magnetic fields up to 7 Tesla between room temperature andT _c in samples with an oxygen content between the orthorhombic-to-tetragonal transition (x6.4) and full oxygenation (x7). The ratio /R _H was investigated with respect to the two-dimensional Luttinger liquid theory and the model of the two-dimensional ionic metal. We report on deviations from the expected quadratic temperature behaviour of /R _H , especially in films with high oxygen content.