Magnetic, electrical, and crystallographic properties of thin La1−x SrxMnO3 films

A study is reported of the magnetic, electrical, and crystallographic properties of La_1−x Sr_xMnO₃ (0.15⩽x⩽0.23) epitaxial films grown on single-crystal substrates of (001)ZrO₂(Y₂O₃) having the fluorite structure and (001)LaAlO₃ having the perovskite structure. It was found that films with close compositions for x=0.15 and 0.16, grown on different substrates, have different properties, namely, the film on a fluorite substrate is semiconducting and has a coercive strength 30 times that of the film on a perovskite substrate; the temperature dependence of electrical resistance of the latter film has a maximum around the Curie point T _C and follows metallic behavior for T<T _C. These differences are explained as due to different structures of the films. The x=0.23 film on the perovskite substrate has been found to exhibit a combination of giant magnetoresistance at room temperature with a resistance of ≈300 Ω which is useful for applications. The maxima in resistance and absolute value of negative magnetoresistance are accounted for by the existence of two-phase magnetic states in these films. Fiz. Tverd. Tela (St. Petersburg) 40, 290–294 (February 1998)     

Effect of the substrate crystalline structure on the magnetic, electrical, and crystallographic characteristics of La0.35Nd0.35Sr0.3MnO3 epitaxial films

A study has been made of the electrical resistivity ρ, magnetoresistance Δρ/ρ, and magnetization of La_0.35Nd_0.35Sr_0.3MnO₃ epitaxial films on ZrO₂(Y₂O₃), SrTiO₃, LaAlO₃, and MgO substrates. The first film can exist in four equivalent crystallographic orientations in the sample plane, while the other three have only one orientation. The maxima of ρ and Δρ/ρ of the first film are broadened considerably in the vicinity of the Curie point T _C compared to the three others, the magnitude of ρ itself being larger by 1.5 orders of magnitude, and a large negative magnetoresistance (|Δρ/ρ| ～ 10% in a field of 8.4 kOe) is observed at temperatures 80≤T≤200 K. In all films, the magnetic moment per molecule at 5 K is ～46% smaller than the pure spin value, due to the existence of magnetically disordered regions. The larger value of ρ of the film deposited on ZrO₂(Y₂O₃) is due to the electrical resistance of the boundaries separating regions with different crystallographic orientations, and the magnetoresistance is associated with polarized carriers tunneling through the boundaries coinciding with domain walls. The low-temperature magnetoresistance in fields above technical saturation is caused by the strong p-d exchange coupling within spin-ordered regions.     

Nature of the low-temperature colossal magnetoresistance of La0.35Nd0.35Sr0.3MnO3 epitaxial films

The colossal negative magnetoresistance (approximately 12%) in a field of 8.4 kOe over a wide range of temperatures below the Curie point T _C ≈240 K in a single-crystal La_0.35Nd_0.35Sr_0.3MnO₃ film on a single-crystal (001)ZrO₂(Y₂O₃) wafer substrate is discussed. Isotherms of the magnetoresistance of this film reveal that its absolute value increases with the field, abruptly in the technical magnetization range and almost linearly in stronger fields. For three single-crystal films of the same composition on (001)LaAlO₃, (001)SrTiO₃, and (001)MgO substrates, colossal magnetoresistance only occurred near T _C ≈240 K and at T<T _C it increased weakly, almost linearly, with the field. In the film on a ZrO₂(Y₂O₃) substrate the electrical resistivity was almost 1.5 orders of magnitude higher than that in the other three films. It is shown that this increase is attributable to the electrical resistance of the interfaces between microregions having four types of crystallographic orientations, while the magnetoresistance in the region before technical saturation of the magnetization is attributable to tunneling of polarized carriers across these interfaces which coincide with the domain walls (in the other three films there is one type of crystallographic orientation). The reduced magnetic moment observed for all four samples, being only 46% of the pure spin value, can be attributed to the existence of magnetically disordered microregions which originate from the large thickness of the domain walls which is greater than the size of the crystallographic microregions and is of the same order as the film thickness. The colossal magnetoresistance near T _C and the low-temperature magnetoresistance in fields exceeding the technical saturation level can be attributed to the existence of strong s-d exchange which is responsible for a steep drop in the carrier mobility (holes) and their partial localization at levels near the top of the valence band. Under the action of the magnetic field the carrier mobility increases and they become delocalized from these levels.     

Influence of substrate temperature in BiFeO3-CoFe2O4 nanocomposites deposited on SrTiO3 (0 0 1)

BiFeO₃-CoFe₂O₄ epitaxial nanocomposites have been deposited on SrTiO₃ (0 0 1) substrates by pulsed laser deposition. We present here a study of the influence of the deposition temperature (T_S), in the 550-800 °C range, on the film composition, morphology and microstructure. Electron-probe microanalysis shows strong reduction of the Bi content in the films when increasing T_S. Films prepared at T_S=750 °C and above are virtually Bi-free. X-ray diffraction (XRD) data show that, due to the volatility of Bi, there is a progressive reduction in the amount of BiFeO₃. The deposition temperature and the concomitant presence of Fe_xO_y spurious phases in the nanocomposites grown at high temperature promote radical changes in film morphology and magnetization. It thus follows that a temperature range suitable for controlled modification of nanocomposites morphology would be extremely narrow.     

Crystallographic,magnetic, and electrical properties of thin <Emphasis Type="Italic">Re</Emphasis><Subscript>0.6</Subscript>Ba<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> epitaxial films (<Emphasis Type="Italic">Re</Emphasis> = La,Pr, Nd,Gd)

Thin Re_0.6Ba_0.4MnO₃ epitaxial films (Re = La, Pr, Nd, Gd) grown on (001)SrTiO₃ and (001)ZrO₂(Y₂O₃) single crystal substrates have been prepared and studied. All the films were found to have a cubic perovskite structure, with the exception of the film with Re = La, which revealed rhombohedral distortion of the perovskite cell. The temperature dependences of the electrical resistivity and magnetoresistance pass through a maximum near the Curie point T_C, where the magnetoresistance reaches a colossal value. The magnetization isotherms M(H) are superpositions of a magnetization that is linear in field (like that of an antiferromagnet) and a weak spontaneous magnetization. The magnetic moment per formula unit is substantially smaller than that expected under complete ferro-or ferrimagnetic ordering. The magnetizations of samples cooled in a magnetic field (FC samples) and with no field applied (ZFC samples) differ by an amount that persists up to the highest measurement fields (50 kOe). The M(T) dependence obtained in strong magnetic fields is close to linear. Hysteresis loops of the FC samples are shifted along the field axis. The above magnetic and electric properties of thin films are explained in terms of two coexisting magnetic phases, which are due to strong s-d exchange coupling.     

Ferromagnetic resonance in epitaxial (Bi,Lu)3(Fe,Ga)5O12(210) films

Ferromagnetic resonance in epitaxial (Bi,Lu)₃(Fe,Ga)₅O₁₂ films grown on Gd₃Ga₅O₁₂(210) substrates is investigated. The spectrum contains a number of peaks, the most intense of which is related to the bulk of the film and the transition layer at the film-substrate interface. Most of the film volume is characterized by reduced magnetic anisotropy. The azimuthal and polar dependences of the resonance field exhibit 180° symmetry.     

Y2O3 stabilized ZrO2 thin films deposited by electron beam evaporation: Structural, morphological characterization and laser induced damage threshold

Four kinds of Y₂O₃ stabilized ZrO₂ (YSZ) thin films with different Y₂O₃ content have been prepared on BK7 substrates by electron-beam evaporation method. Structural properties and surface morphology of thin films were investigated by X-ray diffraction (XRD) spectra and scanning probe microscope. Laser induced damage threshold (LIDT) was determined. It was found that crystalline phase and microstructure of YSZ thin films was dependent on Y₂O₃ molar content. YSZ thin films changed from monoclinic phase to high temperature phase (tetragonal and cubic) with the increase of Y₂O₃ content. The LIDT of stabilized thin film is more than that of unstabilized thin films. The reason is that ZrO₂ material undergoes phase transition during the course of e-beam evaporation resulting in more numbers of defects compared to that of YSZ thin films. These defects act as absorptive center and the original breakdown points.     

Preparation of Nd-doped BiFeO3 films and their electrical properties

The Nd-doped BiFeO₃ thin films were prepared on SnO₂(FTO) substrates spin-coated by the sol–gel method using Nd(NO₃)₃·6H₂O, Fe(NO₃)₃·9H₂O and Bi(NO₃)₃·5H₂O as raw materials. The microstructure and electric properties of the BiFeO₃ thin films were characterized and tested. The results indicate that the diffraction peak of the Nd-doped BiFeO₃ films is shifted towards right as the doping amounts are increased. The structure is transformed from the rhombohedral to pseudotetragonal phase. The crystal grain is changed from an elliptical to irregular polyhedron. Structure transition occurring in the Bi_0.85Nd_0.15FeO₃ films gives rise to the largest Pr of 64 μC/cm². The leakage conductance of the Nd doped thin films is reduced. The dielectric constant and dielectric loss of Bi_0.85Nd_0.15FeO₃ thin film at 10 kHz are 190 and 0.017 respectively.     

Polarization-enhanced bulk photovoltaic effect of BiFeO3 epitaxial film under standard solar illumination

In this Letter, we report the polarization-enhanced bulk photovoltaic effect (BPV) in pristine BiFeO₃ (BFO) epitaxial film under standard 1 sun AM 1.5 G illumination. High-quality epitaxial BFO films are grown on (001)-oriented niobium doped-SrTiO₃ substrates by pulsed laser deposition. The best BFO film based photovoltaic device achieves a power conversion efficiency of 0.0062% under standard illumination. Besides, it is found that the number of bipolar pulses plays a key role in improving the short-circuit current density and open-circuit voltage. These results are beneficial for further understanding of physical origin of the photovoltaic properties in ferroelectric oxides.     

Biaxial stress‐induced giant bandgap shift in BiFeO3 epitaxial films

Thickness‐dependent stress relaxation and its influence on the bandgap of BiFeO₃ have been investigated. BiFeO₃ films with 15 at% Bi excess allow the control of lattice mismatch‐induced biaxial stress up to 10.8 GPa on SrTiO₃(001) substrates. Heteroepitaxy of such films follows the Stranski–Krastanov growth mode, as suggested by island formation on the wetting layer accompanied with stress relaxation. The bandgap is linearly decreased with the in‐plane compressive stress, and a large stress coefficient of 67 meV/GPa was extracted. A 0.7 eV red‐shift of the bandgap was observed in the fully epitaxially strained film. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Morphology and orientation of iron oxide precipitates in epitaxial BiFeO3 thin films grown under two non-optimized oxygen pressures

Microstructures of multiferroic BiFeO₃ thin films epitaxially grown on SrRuO₃-buffered SrTiO₃ (001) substrates by laser molecular-beam epitaxy under two non-optimized oxygen pressures were characterized by means of transmission electron microscopy. The results showed that the films grown under oxygen pressures of 1 Pa and 0.3 Pa contain a secondary phase embedded in the BiFeO₃ matrix. High-angle annular dark-field imaging, elemental mapping and composition analysis in combination with selected area electron diffraction revealed that the parasitic phase is mainly antiferromagnetic α-Fe₂O₃. The α-Fe₂O₃ particles are semi-coherently embedded in the BiFeO₃ films, as confirmed by high-resolution transmission electron microscopy. In addition to the α-Fe₂O₃ phase, ferromagnetic Fe₃O₄ precipitates were found in the BiFeO₃ films grown under 0.3 Pa and shown to accumulate in areas near the film/substrate interfaces. In our heteroepitaxy systems, very low density misfit dislocations were observed at the interfaces between the BiFeO₃ and SrRuO₃ layers implying that their misfit strains may be relieved by the formation of the secondary phases. Using X-ray photoelectron spectroscopy it was found that Fe exists in the +3 oxidation state in these films. The possible formation mechanisms of the secondary phases are discussed in terms of film growth conditions.     

Thin film solid oxide fuel cells

We have investigated the deposition of 91% ZrO₂ − 9% Y₂O₃ thin films by a variety of sputtering techniques for the application as electrolytes in thin film solid oxide fuel cells. The deposition by RF sputtering was accomplished by using an oxide target of the desired composition. The deposition rate in these initial tests was limited to 0.5 μm/hr and the morphology of the film was substantially modified by deposition rate and substrate temperature. Using DC magnetron sputtering we deposited metallic films from a metallic target with the desired chemical composition. We introduced oxygen into the sputtering chamber to reactively deposit the desired 91% ZrO₂ − 9% Y₂O₃ thin films; however, we encountered problems with target oxidation and growth rate reproducibility. We subsequently demonstrated that controlled oxidation of the metallic films could result in adhering, non porous yttria stabilized zirconia films. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Electrical properties of Sb-doped epitaxial SnO2 thin films prepared using excimer-laser-assisted metal–organic deposition

Excimer-laser-assisted metal–organic deposition (ELAMOD) was used to prepare Sb-doped epitaxial (001) SnO₂ thin films on (001) TiO₂ substrates at room temperature. The effects of laser fluence, the number of shots with the laser, and Sb content on the electrical properties such as resistivity, carrier concentration, and carrier mobility of the films were investigated. The resistivity of the Sb-doped epitaxial (001) SnO₂ thin film prepared using an ArF laser was lower than that of the film prepared using a KrF laser. The van der Pauw method was used to measure the resistivity, carrier concentration, and carrier mobility of the Sb-doped epitaxial (001) SnO₂ thin films in order to determine the effect of Sb content on the electrical resistivity of the films. The lowest resistivity obtained for the Sb-doped epitaxial (001) SnO₂ thin films prepared using ELAMOD with the ArF laser and 2 % Sb content was 2.5 × 10^?3 Ω cm. The difference between the optimal Sb concentrations and resistivities of the films produced using either ELAMOD or conventional thermal MOD was discussed.     

X-ray pole-figure study of the epitaxial growth of C60 thin films on mica (001)

The growth of epitaxial C₆₀ thin films on mica(001) by thermal evaporation has been studied in detail by X-ray pole-figure measurements. The influence of the deposition rate, the substrate temperature and the film thickness on the in-plane epitaxial arrangements and the formation of twins has been investigated. It has been demonstrated that the C₆₀ growth is determined by two independent and equivalent C₆₀-crystal grain alignments (type-A and type-B). The nearly six-fold symmetry of the mica(001)-substrate surface offers the three-fold fcc-(111)-oriented C₆₀-crystal grains two equivalent crystal alignments. A high deposition rate of 0.5 Å/s is responsible for the formation of twins at a substrate temperature of 150°C, which diminishes by a higher substrate temperature of 200°C. By a decrease of the deposition rate down to 0.08 Å/s the twins vanish at a film thickness of 200 nm and at the substrate temperature of 150°C. Under the same sublimation conditions, in addition to the type-A and type-B crystal orientations, the growth of the thin C₆₀ films starts with a slight fibre texture which does not appear at a larger film thickness.     

Experimental evidence of a two-phase magnetic state in thin epitaxial films of Re<Subscript>0.6</Subscript>Ba<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> (Re = La,Pr, Nd,Gd)

Thin epitaxial films of Re_0.6Ba_0.4MnO₃ (Re = La, Pr, Nd, Gd) on (001)-oriented single crystal SrTiO₃ and ZrO₂(Y₂O₃) substrates have been prepared and studied. All films possess a cubic perovskite structure, except for the film with Re = La, which exhibited a rhombohedral distortion of the perovskite lattice. The results show evidence for the presence of two magnetic phases, ferromagnetic (FM) and antiferromagnetic (AFM), in the films studied: (i) the magnetization isotherm M(H) appears as a superposition of a linear component (characteristic of antiferromagnets) and a small spontaneous magnetization component; (ii) the magnetic moment per formula unit is significantly reduced as compared to the value expected for the complete FM or ferrimagnetic ordering; (iii) there is a difference between magnetizations of the samples cooled with and without an applied magnetic field, which is preserved in the entire range of magnetic fields studied (50 kOe); (iv) the temperature dependence of the magnetization M(T) in strong magnetic fields is close to linear (for the composition with Re = Gd, M(T) is described by a Langevin function for superparamagnets with a cluster moment of 2μ_B); and (v) the magnetization hysteresis loops of the field-cooled samples are shifted along the field axis. The exchange integral (characterizing the Mn-O-Mn coupling via the FM-AFM phase boundary) estimated from the latter shift is | J|=10^?6 eV. This value is two orders of magnitude lower than the negative exchange integral between the FM layers in ReMnO₃, which makes the presence of a transition layer at the FM-AFM phase boundary unlikely. The temperature dependences of electrical resistance and magnetoresistance exhibit maxima at the Curie temperature (T_C), where the magnetoresistance reaches a colossal value. This behavior indicates that the two-phase magnetic state is caused by a strong s-d exchange.     

Study of ultrathin iron silicide films grown by solid phase epitaxy on the Si(001) surface

Ultrathin films of iron silicide have been grown by high-temperature annealing of 0.14-to O.5O-nm-thick Fe films deposited on the Si(001) surface at room temperature. It has been found that annealing leads to the formation of nanoislands of iron silicide on the surface, so that their type depends on the thickness of the Fe film. High-energy electron diffraction and atomic force microscopy measurements have revealed that the deposition of Fe films less than 0.32 nm thick on the Si(001) surface stimulates epitaxial growth of both three-dimensional β-FeSi₂ and two-dimensional γ-FeSi₂ islands. It has been found that, for Fe coverages of more than 0.32 nm thick, a complete transition to solide phase epitaxy is observed only for two-dimensional β-FeSi₂ islands. The effect of prolonged annealing at 850°C on the morphology of the surface of the iron silicide film has been investigated.     

Conversion electron Mössbauer spectroscopic study of YIG substituted with Bi,Ti, Ga and La

YIG films, substituted with Bi, Ti, Ga and La, were grown on a (111) plane of Gd₃Ga₅O₁₂ or (Gd, Ca)₃(Ga, Mg, Zr)₅O₁₂ by a liquid phase epitaxial method. With the increase of the concentration of Bi atoms, which are substituted at 24c sites of YIG, the magnetic hyperfine fields at 16a and 24d Fe sites increased and the direction of magnetic moments changed from parallel to 45° to the (111) plane. With the increase of Ga concentration, the hyperfine fields at 16a and 24d Fe sites decreased. The hyperfine interaction and the substitution effect of YIG films are discussed.     

Magnetooptical visualization of stray magnetic fields of sample rocks using magnetouniaxial garnet ferrite film

The stray magnetic fields of rock samples with inclusions of the magnetic grains and magnetic crystals were observed using epitaxial (Bi,Lu)₃(Fe,Ga)₅O₁₂ films with uniaxial magnetic anisotropy. Basalt rocks with inclusions of titano-magnetite and native elsen but also crystals of titano-magnetite and pyrrhotin were investigated. Stray magnetic fields were visualized with the Faraday Effect.     

Epitaxial growth of C<Subscript>60</Subscript> thin films on the Bi(0001)/Si(111) surface

The morphology and atomic structure of C₆₀ fullerene films on the Bi(0001)/Si(111)?7 × 7 surface with different coverages have been studied by scanning tunneling microscopy and spectroscopy and low-energy electron microscopy in high vacuum. It is shown that the most favorable sites for nucleation of C₆₀ islands are double steps and domain boundaries on the surface of epitaxial Bi film.     

Impurity substitution effects in BiFeO3 thin films—From a viewpoint of FeRAM applications

Impurity substitution effects in BiFeO₃ thin films are reviewed from a viewpoint of FeRAM (ferroelectric random access memory) applications, in which such characteristics as a large remanent polarization, a low coercive voltage, and excellent fatigue endurance are most important. First, it is described that substitution experiments for Bi and Fe atoms in the films have already been conducted using almost all rare earth and transition metal elements. A list of the published paper is given in a form of the periodic table of elements. Then, two typical cases, La substitution for the Bi site and Mn substitution for the Fe site, are reviewed in detail. Particular attention is paid to the role of these impurity atoms by which the ferroelectric and insulating properties of BiFeO₃ films are significantly improved. Finally, impurity effects due to substitution or co-substitution of other elements are reviewed.