Magnetic properties of arrays of cobalt nanoparticles on the CaF<Subscript>2</Subscript>(110)/Si(001) surface

The Co/CaF₂/Si(001) heterostructures with the corrugated (110) surface of the CaF₂ buffer layer have been grown by molecular beam epitaxy. The structures are nanoparticle arrays of single-crystal Co, mostly of the cubic fcc modification. The behavior of the magnetic hysteresis loops as a function of the density of coverage of the substrate by cobalt islands, the island size, and the temperature is studied using the magnetooptical technique. At low coverage densities, where the effective cobalt film thickness d _eff is less than the critical value d _eff ^c , the magnetic structure of the films at T = 294 K can be visualized as an ensemble of superparamagnetic, weakly interacting nanoparticles and is characterized by small values of the coercive field H _c and the remanent magnetization M _rem. A decrease in the temperature leads to a strong increase in H _c and M _rem, which is associated with the transition of the islands to the blocked state. The blocking temperature of the structures is T _b ～ 280 K. The magnetic anisotropy parameter K and the saturation magnetization M _s of the islands depend on the growth temperature of cobalt T _Co. An increase in the coverage density above the critical thickness d _iff ^c at T = 294 K brings about a strong increase in H _c and M _rem and the appearance of a hysteresis loop anisotropy originating from the corrugated structure of the CaF₂ buffer layer. The experimental results are compared with the model of an ensemble of noninteracting superparamagnetic particles.     

Effect of an electron beam on CaF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript> epitaxial layers on Si

Atomically smooth CaF₂ and BaF₂ layers have been sequentially grown on Si(111) substrates by molecular beam epitaxy. Pore macrodefects have been revealed at the points of the action of an electron beam from a diffractometer when analyzing the crystal structure of the surface during the growth with the subsequent observation using atomic force microscopy. The formation of these macrodefects is associated with the decomposition of fluorides by high-energy electrons, which is accompanied by the desorption of fluorine and the drift current of positive ions from the electron charge drains.     

Memory element based on Si/CaF<Subscript>2</Subscript> periodic nanostructures

A memory element based on a Si/CaF₂ periodic nanostructure is proposed. In this element, information is recorded through charge capture by trap states in a CaF₂ dielectric. The high and low signal levels correspond to the current in the maximum and minimum of the negative differential resistance region, which forms as a result of the resonant tunnel distribution of charge carriers over trap levels in the dielectric. The speed of such logical elements depends on the rate of activation carrier trapping and the rate of tunnel carrier transfer from one state to another. It is shown that both Si/CaF₂-based logical elements and memory elements proposed operate at temperatures from 77 to 300 K, have a switching time of 10⁻¹²–10⁻¹⁰ s, and are compatible with silicon IC technology.     

Features of the surface morphology of CaF<Subscript>2</Subscript>/Si(100) films obtained by solid-phase epitaxy

The dielectric parameters of calcium fluoride films grown on the Si(100) surface by solid-phase epitaxy (group 2) and without it (group 1) are analyzed. It is established experimentally that the deposition mode of CaF₂ films immediately after growth of the Si buffer layer at a substrate temperature of 530°C is not suitable for producing high-quality dielectric layers. The use of solid-phase epitaxy at the initial stage of the nucleation of CaF₂ layers enables the production of single crystal uniformly thick films with high dielectric properties.     

Analysis of the oscillation intensity of RHEED specular reflection during the MBE growth of CaF<Subscript>2</Subscript>/Si/CaF<Subscript>2</Subscript> structures

The results of analysis of the oscillation intensity of RHEED specular reflection during the MBE growth of CaF₂/Si(111) structures in a wide temperature range from 100 to 600°С are presented. It is shown that the preliminary formation of a 2D Si buffer layer provides the two-dimensional growth of CaF₂ layers. Possible reasons which for the disruption of 2D growth at high substrate temperatures are discussed.     

Self-assembling of Ge quantum dots in the CaF<Subscript>2</Subscript>/Ge/CaF<Subscript>2</Subscript>/Si heteroepitaxial system and the development of tunnel-resonance diode on its basis

A CaF₂/Ge/CaF₂/Si(111) heteroepitaxial structure with Ge quantum dots was grown by molecular-beam epitaxy. A negative differential conductivity and conductivity oscillations caused by resonant hole tunneling were observed at room temperature. The energy spacing between the levels in quantum dots, as determined from the oscillation period, is 40–50 meV depending on the Ge dot size.     

Resonant-Optical Detection of Nuclear Magnetization in the Si/CaF<Subscript>2</Subscript> Nanostructure

We have considered theoretically the characteristic features of optical detection of nuclear magnetization in the Si/CaF₂ structure under the conditions of EPR and NMR by measuring luminescence polarization. We show that application of EPR makes it possible to detect weak nuclear fields, but the time of spin relaxation of electrons imposes its constraint. The application of NMR is limited by nuclear fields of no less than 15–20 Gs. The possibility of using optical NMR for direct measurement of the nuclear field from changes in the luminescence polarization spectrum is shown. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 5, pp. 644–649, September–October, 2005.     

Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> nanosquaresheets grown on Si substrate

A novel morphology of Bi₂O₃ nanomaterial (nanosquaresheets) has been successfully synthesized in large area by thermal evaporation of commercial Bi₂O₃ powder at high temperatures. The Bi₂O₃ nanosquaresheets (NSSs) are perfect regular squares and have sharp, uniform edges. The typical length of the sides is in the range of 200–600 nm. The thickness varies from 30 to 100 nm. Electron microscopy observations show that the Bi₂O₃ NSSs are single crystalline. The growth of Bi₂O₃ NSSs is probably controlled by a vapor–solid mechanism. The dominate growth directions are [2̄10] and [1̄2̄2] within the (245) planes. PACS 81.05.Hd; 81.10.Bk; 81.16.Be     

Magnetoresistance of La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films grown on a substrate with low lattice mismatch

The structure, electrical resistivity, and magnetoresistance of La_0.67Sr_0.33MnO₃ heteroepitaxial films (120-nm thick) practically unstrained by lattice mismatch with the substrate were studied. A strong maximum of negative magnetoresistance of ≈27% (for μ₀H = 4 T) was observed at T ≈360 K. While the magnetoresistance decreased monotonically in magnitude with decreasing temperature, it was still in excess of 2% at 150 K. For T < 250 K, the temperature dependence of the electrical resistivity ρ of La_0.67Sr_0.33MnO₃ films is fitted well by the relation ρ = ρ₀ + ρ ₁(H)T^2.3, where ρ₀ = 1.1×10^?4 Ω cm, ρ₁(H = 0) = 1.8×10^?9 Ω cm/K^2.3, and ρ₁(μ₀H = 4 T)/ρ₁(H = 0) ≈0.96. The temperature dependence of a parameter γ characterizing the extent to which the electrical resistivity of the ferromagnetic phase of La_0.67Sr_0.33MnO₃ films is suppressed by a magnetic field (μ ₀H = 5 T) was determined.     

Magnetotransport in thin epitaxial Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> films

The magnetoconductivity of thin Bi₂Se₃ films covered by a protective Se layer and grown at (111) BaF₂ substrates is studied. It is shown that the negative magnetoconductivity observed at low magnetic fields and caused by the effect of weak antilocalization, as well as the Shubnikov?de Haas oscillations at higher fields, is determined only by the magnetic field component perpendicular to the film plane. The obtained experimental results can be reasonably interpreted under the assumption that the studied films exhibit two-dimensional topologically protected electron states. Moreover, the contribution of these states to the total conductivity turns out to be the dominant one.     

Electro-optic properties of epitaxial Sr<Subscript>0.6</Subscript>Ba<Subscript>0.4</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> films grown on MgO substrates using Li<Subscript>x</Subscript>Ni<Subscript>2-x</Subscript>O buffer layer

Textured Li_xNi_2-xO (LNO) thin films have been fabricated on (001)MgO substrates by pulsed laser deposition technique. The as-deposited LNO films shows a conductivity of 2.5×10^-3 Ω m and possess a transmittance of about 35% in the visible region. Subsequent deposition of Sr_0.6Ba_0.4Nb₂O₆ (SBN60) thin film on these LNO-coated MgO substrates resulted in a textured SBN layer with a 〈001〉 orientation perpendicular to the substrate plane. Phi scans on the (221) plane of the SBN layer indicated that the films have two in-plane orientations with respect to the substrate. The SBN unit cells were rotated in the plane of the film by ± 8.2° as well as ± 45° with respect to the LNO/MgO substrate. Besides the highly (00l)-orientation, the SBN films also exhibited a dense microstructure as shown by scanning electron microscopy. The electro-optic coefficient (r₃₃) of the SBN film was measured to be 186 pm/V. On the basis of our results, we have demonstrated that the LNO film can be used as a buffer layer as well as a transparent bottom electrode for waveguide applications. The SBN/LNO heterostructure is also a suitable candidate for integrated electro-optics devices. PACS  42.79.Gn; 42.82.Et; 78.20.Ci     

Growth and magneto-optical properties of LiTb(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

Lithium terbium molybdate (LiTb(MoO₄)₂) single crystal was grown by the Czochralski method. The lattice parameters of the crystal were determined by X-ray diffraction analysis. The absorption coefficient and the Faraday rotation spectrum (B=1.07 T) were investigated at wavelengths of 400–1500 nm at room temperature. Verdet constants of LiTb(MoO₄)₂ crystal at 532-, 633- and 1064-nm wavelengths were measured by the extinction method. The results show that LiTb(MoO₄)₂ crystal has a larger magneto-optical figure of merit than that of terbium gallium garnet at wavelengths of 600–1500 nm.     

Phosphorescence of CaF<Subscript>2</Subscript>-Dy crystals

The decay kinetics of phosphorescence excited by x-rays in a CaF₂-Dy crystal is investigated. It is found that localized charge carriers recombine through tunneling. The conclusion is drawn that, in the initial stages of x-ray irradiation, the accumulation of charge carriers occurs in pairs. This process is caused by the formation of excimer-like molecular states during excitation of the crystal.     

Structural and electrical properties of 0.7Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>–0.3PbTiO<Subscript>3</Subscript> thin films grown on Ir/MgO buffered Si(100) substrates

Thin films of 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN-PT) have been grown on Ir/MgO buffered Si(100) substrates at different substrate temperatures by pulsed laser deposition. Crystalline phases as well as preferred orientations in the PMN-PT films were investigated by X-ray diffraction analysis (XRD). The microstructure, dielectric and ferroelectric properties of PMN-PT film prepared at 650 °C were studied. The results show that the film prepared at 650 °C exhibits pure perovskite phase and single c-axis orientation. The dielectric constant and dissipation factor of the single c-axis oriented film are 1000 and 0.04 at a frequency of 1 kHz, while the remnant polarization and coercive field are about 13.0 μC/cm² and 100 kV/cm under an electric field of 480 kV/cm, respectively. PACS 81.15.Fg; 77.80.-e; 77. 22.Ej; 77.55.+f; 85.50.Gk     

Exciton luminescence in orthorhombic-structure MnF<Subscript>2</Subscript> epitaxial films

The low-temperature photoluminescence (PL) of 100 to 300-nm thick MnF₂ epitaxial films of the α-PbO₂-type orthorhombic structure was studied. The PL spectrum consists mainly of a broad band peaking around 575 nm and a slowly decaying long-wavelength wing. The short-wavelength part of the main band revealed relatively weak spectral features, which are due to magnon replicas of the Mn²⁺ excitonic line perturbed by Mg and Ca impurities. These features were found to shift toward shorter wavelengths by 12 nm relative to their position in bulk MnF₂ crystals. The shift can be accounted for by a change in the crystal field acting on the Mn²⁺ ions in the orthorhombic phase.     

Effect of the substrate ferroelastic transition on epitaxial La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> films grown on LaAlO<Subscript>3</Subscript>

Thin films of La_0.7Sr_0.3MnO₃ were grown by molecular beam epitaxy on (001)LaAlO₃ crystals. High resolution X-ray diffraction analysis proves the presence of twins in the films at room temperature, showing that the twin structure of the substrate which forms at the ferroelastic transition at T_F = 813 K served as a template for the film microstructure. Magnetic measurements indicate a thermomagnetic irreversibility which is ascribed to the quenched disorder related to twinning and discussed in terms of coexisting ferromagnetic and spin disordered regions connected with the undeformed domain cores and strained domain walls respectively.     

Magnetic properties of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> surface

The magnetic properties of the magnetite Fe₃O₄(110) surface have been studied by spin resolved Auger electron spectroscopy (SRAES). Experimental spin resolved Auger spectra are presented. The results of calculation of Auger lines polarization carried out on the basis of electronic state density are presented. Problems related to magnetic moments of bivalent (Fe²⁺) and trivalent (Fe³⁺) ions on the Fe₃O₄(110) surface are discussed. It is established that the deposition of a thin bismuth film on the surface results in significant growth of polarization of iron Auger peaks, which is due to additional spin-orbit scattering of electrons by bismuth atoms.     

FMR study of the anisotropic properties of an epitaxial Fe<Subscript>3</Subscript>Si film on a Si(111) vicinal surface

The anisotropic characteristics of an iron silicide (Fe₃Si) epitaxial thin magnetic film grown on a Si(111) silicon vicinal surface with a misorientation angle of 0.14° have been measured by the ferromagnetic resonance method. It has been shown that the polar and azimuth misorientation angles of the crystallographic plane of the substrate can be determined simultaneously from the angular dependences of the ferromagnetic resonance field of the epitaxial film. The effective saturation magnetization of the film M _eff = 1105 G and the constant of the cubic magnetocrystalline anisotropy K ₄ = 1.15 × 10⁵ erg/cm³ have been determined. The misorientation of the substrate plane leads to the formation of steps on the film surface and, as a result, to the appearance of uniaxial magnetic anisotropy of the magnetic dipole nature with the constant K ₂ = 796 erg/cm³. Small unidirectional magnetic anisotropy (K ₁ = 163 erg/cm³), which may be associated with symmetry breaking on the steps of the film and is due to the Dzyaloshinskii–Moriya interaction, has been detected.     

Spin Polarization of Fluorine Nuclei in the Si/CaF<Subscript>2</Subscript> Nanostructure Under Optical Excitation

The laws of fluorine nuclear spin polarization in the Si/CaF₂ nanostructure under optical excitation of the charge carriers in it has been considered theoretically. It has been shown that maximum values (up to 3% of the concentration of nuclei in the lattice) are attained under a high rate of optical excitation (>10⁻⁹ sec⁻¹) when the nuclear spin diffusion process and the Auger recombination prevail. In this case, the nuclear relaxation time in an individual layer reaches 100–300 sec and the spin diffusion radius decreases to 0.3 nm. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 397–403, May–June, 2005.     

Additive coloring of CaF<Subscript>2</Subscript> optical ceramic

The specificity of additive coloring of CaF₂ optical ceramic (formation of color centers in it and photothermochemical transformation of these centers in colored ceramic samples) has been considered. Under the same coloring conditions, this process occurs more slowly in ceramics rather than in crystals; at the same time, the limiting concentration of color centers that can be introduced into ceramics is much higher. The photothermochemical transformations of color centers in crystals and ceramics, which occur under illumination at different wavelengths and upon heating, have been studied. The specific features of introduction of color centers into ceramic and their transformation under illumination and heating are likely to be related to the mass twinning of ceramic grains.