Magnetotransport characteristics of strained La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> epitaxial manganite films

The electrical and magnetic characteristics of La_0.7Sr_0.3MnO₃ (LSMO) epitaxial manganite films are investigated by different methods under conditions when the crystal structure is strongly strained as a result of mismatch between the lattice parameters of the LSMO crystal and the substrate. Substrates with lattice parameters larger and smaller than the nominal lattice parameter of the LSMO crystal are used in experiments. It is shown that the behavior of the temperature dependence of the electrical resistance for the films in the low-temperature range does not depend on the strain of the film and agrees well with the results obtained from the calculations with allowance made for the interaction of electrons with magnetic excitations in the framework of the double-exchange model for systems with strongly correlated electronic states. Investigations of the magneto- optical Kerr effect have revealed that an insignificant (0.3%) orthorhombic distortion of the cubic lattice in the plane of the NdGaO₃(110) substrate leads to uniaxial anisotropy of the magnetization of the film, with the easy-magnetization axis lying in the substrate plane. However, LSMO films on substrates (((LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7)(001)) ensuring minimum strain of the films exhibit a biaxial anisotropy typical of cubic crystals. The study of the ferromagnetic resonance lines at a frequency of 9.76 GHz confirms the results of magnetooptical investigations and indicates that the ferromagnetic phase in the LSMO films is weakly inhomogeneous.     

Coercive field enhancement in microstructured (La<Subscript>0.4</Subscript>Pr<Subscript>0.6</Subscript>)<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> thin films

The perovskite material (La_0.4Pr_0.6)_0.67Ca_0.33MnO₃ (LPCMO) has complex electronic and magnetic behavior based on phase competition between ferromagnetic metallic (FMM) and insulating phases with similar free energies. Experimental evidence has indicated that in-plane stress anisotropy influences these phases and can affect electronic and magnetic properties. Here we investigate the roles that both stress and shape anisotropies may play in controlling the coercive field of the material. LPCMO thin films of various thicknesses (20, 25, and 30 nm) were deposited on (110) NdGaO₃ (NGO) substrates using pulsed laser deposition and the coercive fields were measured. Photolithography was then used to fabricate microstructured arrays of LPCMO on the NGO substrates for each of the films. The coercive fields of these arrays of LPCMO were compared to the behavior of the corresponding unpatterned LPCMO thin films across a range of temperatures. Microstructure arrays for the thicker (25 and 30 nm) films showed a substantial increase in the coercive field after forming the arrays, whereas a thinner film (20 nm) showed almost no change in the coercive field. Stress anisotropy continues to play a dominant role in the behavior of LPCMO thin films and dimensionality of the magnetic domains also influences the results. The films show 2D behavior when film thickness approaches the size of the critical radius for single-to-multidomain transitions. Making thicker films allows for 3D behavior and a role for shape anisotropy to influence the coercive fields.     

Modification of the “Transcritical” state in Ni<Subscript>75</Subscript>Fe<Subscript>16</Subscript>Cu<Subscript>5</Subscript>Mo<Subscript>4</Subscript> films produced by RF sputtering

The saturation magnetization, the perpendicular and rotational anisotropy constants, and the coercitivity of Ni₇₅Fe₁₆Cu₅Mo₄ thin magnetic films produced by rf sputtering are measured in the initial state and after annealing. A relation between the presence of perpendicular anisotropy and the “transcritical” state in the films is established. It is shown that, after additional thermal treatment, the magnetic softness of the films can be improved.     

Superconductivity in Ti-doped iron-arsenide compound Sr<Subscript>4</Subscript>Cr<Subscript>0.8</Subscript>Ti<Subscript>1.2</Subscript>O<Subscript>6</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

Superconductivity was achieved in Ti-doped iron-arsenide compound Sr₄Cr_0.8Ti_1.2O₆Fe₂As₂ (abbreviated as Cr-FeAs-42622). The X-ray diffraction measurement shows that this material has a layered structure with the space group of P4/nmm, and with the lattice constants a = b = 3.9003 Å and c = 15.8376 Å. Clear diamagnetic signals in ac susceptibility data and zero-resistance in resistivity data were detected at about 6 K, confirming the occurrence of bulk superconductivity. Meanwhile we observed a superconducting transition in the resistive data with the onset transition temperature at 29.2 K, which may be induced by the nonuniform distribution of the Cr/Ti content in the FeAs-42622 phase.     

Investigation of the magnetic properties and magnetic structure parameters of nanocrystalline Fe<Subscript>79</Subscript>Zr<Subscript>10</Subscript>N<Subscript>11</Subscript> films

The magnetic properties (magnetization curve, ferromagnetic resonance spectrum) of nanocrystalline Fe₇₉Zr₁₀N₁₁ films obtained by RF magnetron sputtering with subsequent annealing were studied experimentally, along with the fundamental magnetic constants of these films (saturation magnetization M _S, local magnetic anisotropy energy K, and the exchange coupling constant A). The magnetic properties are discussed within the random magnetic model, which determines the correlation of the magnetic properties with the fundamental magnetic constants and nanostructure parameters (grain size, magnetic anisotropy, and correlation radius R _C). The exchange correlation length 2R _L for the film magnetic microstructure was determined by correlation magnetometry.     

Effects of BaTiO<Subscript>3</Subscript> and SrTiO<Subscript>3</Subscript> as the buffer layers of epitaxial BiFeO<Subscript>3</Subscript> thin films

BiFeO₃ (BFO) thin films with BaTiO₃ (BTO) or SrTiO₃ (STO) as buffer layer were epitaxially grown on SrRuO₃-covered SrTiO₃ substrates. X-ray diffraction measurements show that the BTO buffer causes tensile strain in the BFO films, whereas the STO buffer causes compressive strain. Different ferroelectric domain structures caused by these two strain statuses are revealed by piezoelectric force microscopy. Electrical and magnetical measurements show that the tensile-strained BFO/BTO samples have reduced leakage current and large ferroelectric polarization and magnetization, compared with compressively strained BFO/STO. These results demonstrate that the electrical and magnetical properties of BFO thin films can be artificially modified by using a buffer layer.     

Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

Fast electrochromic response of porous-structured cobalt oxide (Co<Subscript>3</Subscript>O<Subscript>4</Subscript>) thin films by novel nebulizer spray pyrolysis technique

Electrochromic effect of cobalt oxide thin films was studied as a function of substrate temperature (573–673 K). Tricobalt tetraoxide (Co₃O₄) thin films were deposited on glass and fluorine-doped tin oxide (FTO) substrates by nebulized spray technique using cobalt nitrate as precursor material. The XRD patterns indicated (311) plane was dominant for all the films irrespective of the deposition temperature. Williamson-Hall (W-H) analysis was made to understand the strain variation in the prepared Co₃O₄ films under different deposition temperature by employing uniform deformation model (UDM). Scanning electron microscopy images revealed porous morphology for the film prepared at 623 K. The optical parameters such as refractive index (n), extinction coefficient (k), and band gap were derived from UV-visible spectra of Co₃O₄ films. The electrochromic performance of the deposited Co₃O₄ films was analyzed through cyclic voltammetry, chronocoulometry, chronoamperometry, and iono-optical studies.     

Thermal sensors based on Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> and (Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>)<Subscript>70</Subscript>(Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>)<Subscript>30</Subscript> thin films

Thin films of Sb₂Te₃ and (Sb₂Te₃)₇₀(Bi₂Te₃)₃₀ alloy and have been deposited on precleaned glass substrate by thermal evaporation technique in a vacuum of 2?×?10^?6 Torr. The structural study was carried out by X-ray diffractometer, which shows that the films are polycrystalline in nature. The grain size, microstrain and dislocation density were determined. The Seebeck coefficient was determined as the ratio of the potential difference across the films to the temperature difference. The power factor for the (Sb₂Te₃)₇₀ (Bi₂Te₃)₃₀ and (Sb₂Te₃) is found to be 19.602 and 1.066 of the film of thickness 1,500 Å, respectively. The Van der-Pauw technique was used to measure the Hall coefficient at room temperature. The carrier concentration was calculated and the results were discussed.     

Structure,the lattice dynamic,and the dielectric characteristics of Sr<Subscript>0.5</Subscript>Ba<Subscript>0.5</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> films

Solid solution Sr_0.5Ba_0.5Nb₂O₆ films have been synthesized on a (111)Pt/(001)Si substrate by rf deposition in an oxygen atmosphere. The depolarized Raman spectra, the structure, and the dielectric characteristics of the films have been studied over a wide temperature range. It is found that the films were singlephase, had the tetragonal tungsten bronze structure, and had a pronounced axial texture with axis 001 directed perpendicular to the substrate surface. It is shown that the film material undergoes a diffuse phase transition to the state of a relaxor ferroelectric in the temperature range 300–425 K. Possible reasons of the regularities observed are discussed.     

Strain effect on transport properties of <Emphasis Type="Bold">SrRuO</Emphasis><Subscript><Emphasis Type="Bold">3</Emphasis></Subscript> films grown by laser MBE

Transport properties of SrRuO₃ thin films were studied as a function of the epitaxial strain. SrRuO₃ films were grown on (100) SrTiO₃ substrates by the Pulsed Laser Deposition technique equipped with Reflection High Energy Electrons Diffraction (RHEED). Samples thickness has been varied from a few unit cells to above 1000 ? while monitoring RHEED intensity oscillations. In thicker films epitaxial strain was found to be progressively relaxed. SrRuO₃ relaxed films (thickness 1000 ?) show metallic behavior for the whole temperature range with a ferromagnetic ordering at about 150 K. For thinner films, ferromagnetic ordering occurs at progressively lower temperatures, until in films thinner than 400 ? it disappears. Films thinner than 80 ? show a semiconducting behavior at low temperatures. Our results provide direct evidence of the crucial role of the strain effect for conducting and magnetic properties of SrRuO₃. Received 16 July 2001 and Received in final form 22 October 2001     

Gamma irradiation effects on structural and optical properties of amorphous and crystalline Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> thin films

The structural and optical properties of RF sputtered Nb₂O₅ thin films are studied before and after gamma irradiation. The films are subjected to structural and surface morphological analyses by using X-ray (XRD) and field emission scanning electron microscope techniques. In the wavelength range of 300–2000 nm, the optical parameters for amorphous and crystalline Nb₂O₅ thin films are estimated at differently exposed γ-irradiation doses (0, 50, 100 and 200 kGy). The optical constants, such as optical energy band gap, absorption coefficient, refractive index and oscillators parameters of amorphous and crystalline Nb₂O₅ thin films are calculated. The optical band gaps of γ-irradiated amorphous and crystalline Nb₂O₅ thin films are determined. In the non-absorbing region, the real part of the refractive index of amorphous and crystalline Nb₂O₅ thin films slightly increases with the increase in the exposed γ-irradiation dose.     

Crystallographic orientation dependence of the dielectric constant in polymorphic BaNb<Subscript>2</Subscript>O<Subscript>6</Subscript> thin films deposited by laser ablation

Well-crystallized barium metaniobate (BaNb₂O₆) thin films were fabricated on fused quartz substrates by pulsed laser deposition. The influence of substrate temperature and oxygen pressure on the crystal structure and preferred orientation were studied to understand the growth mechanism of BaNb₂O₆ thin films. The films formed at 600 °C at an oxygen pressure of 100 mTorr exhibited predominantly the orthorhombic (040) orientation, and turned to the orthorhombic (230) orientation at 800 °C. It was found that (220)-oriented hexagonal thin films were formed at 600 °C at an oxygen pressure less than 50 mTorr. The dielectric constant of the BaNb₂O₆ thin films was measured by scanning microwave microscopy (SMM). Preferentially (230)-oriented orthorhombic and (220)-oriented hexagonal BaNb₂O₆ thin films were shown to have significantly enhanced dielectric constants of 47.8 and 56.7, respectively. This could be attributed to the dependence of the dielectric constant on crystallographic orientation. PACS 77.55.+f; 77.84.Dy     

Phase transition behavior of highly (100) textured sol-gel-derived Ba<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> thin films

We have investigated the relations between microstructure and dielectric properties in order to fabricate sol-gel-derived highly (100) oriented Ba_0.5Sr_0.5TiO₃ (BST 50/50) thin films with properties comparable to those of the bulk material. For the first time, we were able to fabricate BST thin films which exhibited the orthorhombic-to-tetragonal transition in addition to the commonly observed tetragonal-to-cubic transition. We were successful in explaining the commonly observed degradation of the dielectric behavior of BST thin films, when compared to that of the bulk material, in terms of grain size, compositional inhomogeneity (measured in terms of Sr/Ba ratio) between the grain bulk and grain boundary, and mechanical stresses. PACS 77.55.+f; 77.22Gm; 77.80.Bh; 77.80.Dj     

Ferroelectric and dielectric properties of SrBi<Subscript>4</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript> thin films grown on Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> film layer

Ferroelectric and dielectric properties of bilayered ferroelectric thin films, SrBi₄Ti₄O₁₅ grown on Bi₄Ti₃O₁₂, were investigated. The thin films were annealed at 700°C under oxygen atmosphere. The bilayered thin films were prepared on a Pt(111)/Ti/SiO₂/Si substrate by a chemical solution deposition method. The dielectric constant and dielectric loss of the bilayered thin films were 645 and 0.09, respectively, at 100 kHz. The value of remnant polarization (2P _r) measured from the ferroelectric thin film capacitors was 60.5 μC/cm² at electric field of 200 kV/cm. The remnant polarization was reduced by 22% of the initial value after 10¹⁰ switching cycles. The results showed that the ferroelectric and dielectric properties of the SrBi₄Ti₄O₁₅ on Bi₄Ti₃O₁₂ ferroelectric thin films were better than those of the SrBi₄Ti₄O₁₅ grown on a Pt-coated Si substrate suggesting that the improved properties may be due to the different nucleation and growth kinetics of SrBi₄Ti₄O₁₅ on the c-axis-oriented Bi₄Ti₃O₁₂ layer or on the Pt-coated Si substrate.     

Optical characteristics of nickel and boron carbonitride films in Si(100)/Ni/BC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> structures

The optical characteristics of nickel films deposited on Si(100) substrates by vacuum thermal evaporation have been studied. The thickness and optical constants of the films are determined using monochromatic zero ellipsometry, while the inverse problems are solved within the three-layer optical model of the samples. It is shown that thermal annealing leads to a change in the optical constants of nickel films in the heating-temperature range of 500–900°C. Boron carbonitride layers deposited on silicon substrates with a nickel sublayer are analyzed within multilayer optical models, which make it possible to determine the refractive index and absorption coefficient distributions along the thickness of the synthesized Si(100)/Ni/BC _x N _y structure.     

Microstructural evolution and electrical properties of La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> thin films grown on SrTiO<Subscript>3</Subscript> substrates by excimer-laser assisted metal-organic deposition

Thin films of La_0.7Ca_0.3MnO₃ were successfully grown epitaxially on (100) single-crystal SrTiO₃ substrates by excimer-laser assisted metal-organic deposition. Initial amorphous LCMO thin films were obtained by metal-organic deposition at 500 °C. Crystallization and epitaxial growth of the films was achieved using a KrF pulsed laser irradiation while the film/substrate samples were kept at 500 °C. High resolution transmission electron microscopy observations on cross-sections demonstrate the formation mechanism of the epitaxial films. The crystallization process starts at the LCMO/STO interface and grows by increasing the number of laser shots. A fully crystallized film was obtained after 5 min of irradiation. The film/substrate interface was found to be sharp and abrupt. The temperature dependence of the resistance R(T) shows various behaviors, starting from insulating to semiconducting and metal–insulator transition material during the formation of the manganite film. The oxygen content was also improved by increasing the irradiation time. Promising values of the temperature coefficient of resistance were obtained from these manganite films for prospect integration in silicon based microbolometric devices. PACS 81.15.-z; 81.15.Np; 73.61.-r; 71.30.+h     

Investigation on optical properties of Bi<Subscript>2.85</Subscript>La<Subscript>0.15</Subscript>TiNbO<Subscript>9</Subscript> thin films by prism coupling technique

Layered-perovskite ferroelectric Bi_2.85La_0.15TiNbO₉ (LBTN) optical waveguiding thin films were grown on fused silica substrates by pulsed laser deposition (PLD). X-ray diffraction (XRD) revealed that the film is highly (00l) textured. We observed sharp and distinct transverse electric (TE) and transverse magnetic (TM) multimodes and measured the refractive indices of LBTN thin films at 632.8 nm. The ordinary and extraordinary refractive indices were calculated to be n _TE=2.358 and n _TM=2.464, respectively. The film homogeneity and the film-substrate interface were analyzed using an improved version of the inverse Wentzel–Kramer–Brillouin (iWKB) method. The refractive index of the film remains constant at n ₀ within the waveguiding layer. The average transmittance of the film is 70% in the wavelength range of 400–1400 nm and the optical waveguiding properties were evaluated by the optical prism coupling method. Our results showed that the LBTN films are very good electro-optical active material.     

Electrical properties of Sr<Subscript>3</Subscript>Bi<Subscript>4</Subscript>Ti<Subscript>6</Subscript>O<Subscript>21</Subscript> thin films

Highly c-axis-oriented Sr₃Bi₄Ti₆O₂₁ (SBTi) thin films were fabricated on Pt-coated Si substrates by pulsed laser deposition (PLD). The structures were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). No peaks of SrTiO₃ (STO) could be detected in the XRD pattern, indicating the existence of the SBTi single phase. Good ferroelectric hysteresis loops of the films with Pt electrodes were obtained. With an applied field of 400 kV/cm, the measured remanent polarization (P_r) and coercive field (E_c) values were 4.1 C/cm² and 75 kV/cm respectively. The films showed little fatigue after 2.22×10⁹ switching cycles: the nonvolatile polarizations decreased by less than 5% of the initial values. The dielectric constant and the loss tangent of the films were measured to be 363 and 0.04 at 100 kHz. These results might be advantageous for nonvolatile ferroelectric random access memory (NVFRAM) and dynamic random access memory (DRAM). PACS 77.84.Dy; 77.22.-d; 68.55.Jk     

Modulation of ultrafast laser-induced magnetization precession in BiFeO<Subscript>3</Subscript>-coated La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> thin films

The ultrafast laser-excited magnetization dynamics of ferromagnetic (FM) La_0.67Sr_0.33MnO₃ (LSMO) thin films with BiFeO₃ (BFO) coating layers grown by laser molecular beam epitaxy are investigated using the optical pump-probe technique. Uniform magnetization precessions are observed in the films under an applied external magnetic field by measuring the time-resolved magneto-optical Kerr effect. The magnetization precession frequencies of the LSMO thin films with the BFO coating layers are lower than those of uncoated LSMO films, which is attributed to the suppression of the anisotropy field induced by the exchange interaction at the interface between the antiferromagnetic order of BFO and the FM order of LSMO.