Temperature dependent photoluminescence down to 4.2 K in EuTFC

The temperature dependence of photoluminescence in Europium tris[3-(trifluoro-methylhydroxymethylene)-(+)-camphorate] (EuTFC) embedded in polymer films has been examined from 40 K down to 4.2 K with the goal of preparing sensor films for low-temperature thermal imaging. The behavior of EuTFC showed significant difference when based on polystyrene compared to poly(n-alkyl methacrylate)s. In poly(n-alkyl methacrylate)s prepared by standard methods for imaging applications, the photoluminescence is fully saturated below 30 K, whereas in polystyrene films there is a strong temperature dependence even down to 4.2 K. By optimizing the preparation procedure for films made of poly(butyl methacrylate) (PBMA) and poly(methyl methacrylate), also these polymers became very sensitive down to liquid helium temperature. The maximum temperature sensitivity of EuTFC in PBMA is found to be 1.0%/K at 4.2 K. The problem of delamination and cracking of the polymer film at cryogenic temperature is also avoided by the special preparation method.     

Magnetic properties of corrosion products investigated by CEMS at low temperatures near 4.2 K

Conversion electron Mössbauer measurements with a proportional counter at 6.3, 78 and 300 K have been done to investigate the magnetic properties of the corrosion products formed on the surface of iron foils, which consist of small particles of γ-Fe₂O₃. The growing period of the corroded layer formed in a solution of low oxygen concentration has insignificant influence on the size of particles composing the layer, however, when the corroded layer is grown in a solution of comparatively high oxygen concentration, the particle size becomes large with growing period of the layer.     

Local magnetic field distributions in superconducting niobium at 4.2 K by neutron diffraction

Neutron diffraction experiments on flux line lattices in superconducting Nb are reported. From the integral reflectivities of the (10), (11), (20), (21) and (31) reflections the absolute values of the formfactors were obtained as a function of the flux density. Considering certain properties of the flux line lattice it is also possible to determine the phases of the formfactors. From this the local field distribution in the mixed state could be calculated by Fourier-transformation. The dependence of the resulting maximum, minimum, and saddle point fields on the flux density and on the impurity parameter is discussed and compared with theory.     

Deformation of doped NaCl crystals during uniaxial compression at temperatures of 4.2–293 K

The impurity weakening of crystalline NaCl:Mg and NaCl:Mn associated with purification of the matrix in a melt is demonstrated. This effect is manifested in a consistent change in the yield strength and mobility of dislocations. Its unusual temperature dependence is determined: in a number of cases weakening becomes greater as the temperature is lowered within the interval 293–4.2 K. A deviation in the elastic region of the stress-strain curves is observed to increase as the temperature is lowered. Plasticizing of crystalline NaCl is established: when the deformation temperature is lowered from 293 to 77 and 4.2 K the rate of rise in the yield strength of Cd²⁺ and Sr²⁺ doped samples is lower than that of undoped samples. An explanation for this fact is proposed. Fiz. Tverd. Tela (St. Petersburg) 39, 313–317 (February 1997) Deceased.     

Metallic behavior of FeCl4-doped polyacetylene down to 1.9 K

A steep rise was observed in the temperature dependence of electrical conductivity of FeCl₄-doped polyacetylene by means of the voltage- shorted-compaction method. The temperature dependence was fitted to a T²-law.     

Frustrated magnetic structures: II. Antiferromagnetic structure of the ordered modified pyrochlore NH4FeIIFeIIIF6 at 4.2 K

The nuclear and magnetic structures of the ordered modified pyrochlore NH₄Fe²⁺Fe³⁺F₆ were solved at 4.2 K. NH⁺₄ tetrahedra are almost regular ($\text{〈}\text{N}\text{?}\text{H}\text{〉 =}\text{0.924}\text{A}\text{?}$) and the fluorinated skeleton at 4.2 K does not exhibit large deviations from the room-temperature positions (R_N = 0.042). Below T_N = 19 ± 1 K the magnetic and nuclear cells are identical. The strictly antiferromagnetic arrangement of Fe²⁺ spins (μ = 3.12(9)μB) along the [0 1 0] direction contrasts with the canted antiferromagnetic sublattice of Fe³⁺ ions (μ = 4.13 [8] μB) which is quasi-orthogonal (α = 76°) to the Fe²⁺ sublattice (R_mag = 0.078). The main component of Fe³⁺ moments lies along a. The antiferromagnetism of NH₄Fe³⁺Fe³⁺F₆ is compared to the spin-glass like behaviour which occurs when M²⁺ and M³⁺ ions are randomly distributed on the cationic sites.     

V nuclear magnetic relaxation in dilute Au-V alloys at temperatures 1–300 K

Nuclear magnetic relaxation times, T₁ and T₂ are measured in dilute Au-V alloys for 1–300 K. A value for T₁T = 17.0 ± 0.5 msK is obtained.     

Nonstochastic behavior of atomic surface diffusion on Cu(111) down to low temperatures

Atomic diffusion is usually understood as a succession of random, independent displacements of an adatom over the surface's potential energy landscape. Nevertheless, an analysis of molecular dynamics simulations of self-diffusion on Cu(111) demonstrates the existence of different types of correlations in the atomic jumps at all temperatures. Thus, the atomic displacements cannot be correctly described in terms of a random walk model. This fact has a profound impact on the determination and interpretation of diffusion coefficients and activation barriers.     

Crystal structure of nanostructured PbS films at temperatures of 293–423 K

The crystal structure of lead sulfide films prepared by the hydrochemical deposition has been studied by X-ray diffraction. The thickness of the films synthesized is ∼100 nm, the size of coherent scattering regions is ∼70 nm, and the value of microstrains is ∼0.20%. It is established, for the first time, that the as-synthesized PbS films and the same films annealed in the temperature range 293–423 K have a cubic crystal structure (space group Fm3m) different from the B1-type structure. In the crystal lattice of the structure revealed, sulfur atoms are located not only in the 4(b) positions but also in the 8(c) positions. The occupancies of the 4(b) and 8(c) positions by the S atoms are ∼0.84 and 0.08, respectively.     

Exotic behavior of hydrogen atoms in solid H2 at temperatures below 1 K

We present the first magnetic resonance study of atomic hydrogen embedded in solid H2 films for temperatures 150-900 mK. We found that at T approximately 150 mK average concentrations of H atoms of order 10(18 cm(-3) are very stable against recombination during two weeks of observations. The distribution of the population of the two lowest hyperfine states is found to be non-Boltzmann, with a very large occupation of the ground state. We consider the possibility of formation in solid H2 of regions with high local concentrations of H atoms, where collective quantum phenomena might occur.     

Crystallographic and magnetic properties of SrM thin films on Pt underlayer prepared at various substrate temperatures

Strontium ferrite (SrM) thin films deposited on thermally oxidized silicon wafer (SiO₂/Si) and single crystal sapphire with (0 0 l) orientation (Al₂O₃(0 0 l)) substrate using Pt underlayer were prepared by DC magnetron sputtering system. It was found that the intensity of (1 1 1) line for Pt and that of (0 0 l) diffraction line for SrM increases with increasing substrate temperature, T_u. The c-axis dispersion angle, Δθ₅₀, of SrM(0 0 8) depends on that of Pt underlayer. Both dispersion angle of Pt(1 1 1) and SrM(0 0 8) decrease with increasing temperature. It was observed that the saturation magnetization of SrM/Pt deposited on SiO₂/Si is higher than that of Al₂O₃ substrate. The coercivity and remanent squareness ratio in perpendicular direction are higher than that in in-plane direction. The maximum of coercivity in perpendicular direction of SrM/Pt films deposited on single crystal Al₂O₃ is about 4.2 kOe.     

Finite-size effect on magnetic ordering temperatures in long-period antiferromagnets: holmium thin films

The thickness dependence of the helical antiferromagnetic ordering temperature T(N) was studied for thin Ho metal films by resonant magnetic soft x-ray and neutron diffraction. In contrast with the Curie temperature of ferromagnets, T(N) was found to decrease with film thickness d according to [T(N)(infinity)-T(N)(d)]/T(N)(d) proportional variant (d-d(0))(-lambda(')), where lambda(') is a phenomenological exponent and d(0) is of the order of the bulk magnetic period L(b). These observations are reproduced by mean-field calculations that suggest a linear relationship between d(0) and L(b) in long-period antiferromagnets.     

Dewetting of thin polymer films at temperatures close to the glass transition

We present detailed studies on dewetting of thin polystyrene (PS) films which were deposited onto silicon wafers coated with a polydimethylsiloxane (PDMS) monolayer. Experiments were performed at temperatures close to the glass transition temperature of PS. Several significant deviations from the dewetting behaviour of Newtonian liquids were observed. The length of the PS molecules, and thus the viscosity, turned out to be of minor importance in determining the dewetting velocity, in particular for the later regimes. In stark contrast, the geometry of the drying spot had a striking influence on the dewetting velocity. Initially, dewetting from straight contact lines proceeded faster than the opening of circular holes. At later stages, the process slowed down significantly in both cases. Under the conditions at which our experiments were performed, PS cannot flow like a simple liquid. Thus, the observed dewetting has to be the consequence of plastic deformation induced by capillary forces. Our results indicate that under such conditions the energy dissipation process is strongly affected by geometry, which is not the case for viscous liquids.Received: 1 January 2003, Published online: 14 October 2003PACS: 68.60.-p Physical properties of thin films, nonelectronic - 61.41. + e Polymers, elastomers, and plastics - 68.55.-a Thin film structure and morphology - 83.50.-v Deformation and flow     

Study of nanoconductive and magnetic properties of nanostructured iron films prepared by sputtering at very low temperatures

A combined study of the surface nanostructure and electrical characteristics of iron thin films prepared on naturally passivated silicon wafers is presented. By means of conductive-scanning force microscopy, the influence of the substrate temperature during film preparation on both surface morphology and conductivity response is investigated. In addition, magnetic properties of these films are reported and correlated with the nanostructural properties. Films prepared at 200 K show granular core–shell magnetic behaviour exhibiting exchange bias. Both conductive and magnetic data indicate that samples prepared at low temperature behave as a percolated network of nanometric metallic iron clusters (with typical sizes of 20 nm) interconnected by oxidized chains (of 10 nm in diameter), showing an excellent system for nanotechnological exchange bias applications.