Self-assembled antidots in La2/3Sr1/3MnO3 thin films

In this work, we report on the controlled fabrication of a self-assembled network of antidots in highly epitaxial La_2/3Sr_1/3MnO₃ thin films grown on top of SrTiO₃ (1 0 0) oriented substrates. To promote self-assembly process, it is fundamental to select a unique surface atomic termination of the STO substrates and the appropriate growth conditions. The evolution of the structural and magnetic properties as a function of deposition parameters is analyzed to clarify the growth mechanism leading to the formation of this self-assembled network of antidots. This morphology is of high interest envisaging a new route for the design and fabrication of oxide-based magnetoelectronic devices.     

Transport and magnetic properties of La1.48Nd0.4Sr0.12CuO4/La0.67Sr0.33MnO3 bilayer

We have grown La_1.48Nd_0.4Sr_0.12CuO₄/La_0.67Sr_0.33MnO₃ (LNSCO/LSMO) bilayer structure on SrTiO₃ (0 0 1) substrate. Both temperature dependences of resistivity and magnetization curves show anomalies between 60 < T < 80 K, where a low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) structural transition is observed in LNSCO bulk crystal. It is suggested that the formation of domains in LSMO layer can relax the strains caused by the LTO–LTT transition in LNSCO layer.     

Exchange interaction between ferromagnetic and antiferromagnetic phases in La1/3Nd1/3Ca1/3MnO3

Magnetization measurements of a La_1/3Nd_1/3Ca_1/3MnO₃ perovskite at magnetic field up to 6 T have revealed an anomalous behaviour – above 130 K the material exhibits a loop displacement about a field of 2 T. We assume that this is the result of an exchange interaction between ferromagnetic and antiferromagnetic phases in a magnetically inhomogeneous compound. At about 115 K a transition from a semiconducting to a metallic-like state has been observed.     

Maximum magnetic entropy change modulated toward room temperature in perovskite manganites La0.7−xNdx(Ca,Sr)0.3MnO3

With Nd³⁺ doping and Ca²⁺, Sr²⁺ modulating in the sol–gel technique, a series of polycrystalline perovskite samples La_0.7?xNd_x(Ca,Sr)_0.3MnO₃ (x = 0, 0.05, 0.1, 0.15, 0.20, 0.25) was prepared, their maximum magnetic entropy changes were tuned to room temperature (ΔS_H = ?1.47 J/kg K at 298 k for La_0.45Nd_0.25(Ca,Sr)_0.3MnO₃), an enhancement of the maximum magnetic entropy change (ΔS_H = ?1.89 J/kg K at 315 k) and its refrigerant capacity (about 45.3 J/kg) had also been obtained under 9 kOe magnetic field variation for La_0.55Nd_0.15(Ca,Sr)_0.3MnO₃ contrast to La_0.7(Ca,Sr)_0.3MnO₃.     

Chemical solution processing and properties of Sr2FeMoO6 thin films

Crack-free and oriented Sr₂FeMoO₆ (SFMO) thin film with double perovskite structure has been fabricated by the chemical solution deposition (CSD) method. A homogeneous and stable SFMO precursor solution was successfully prepared by controlling the reaction of starting metal-organic compounds in a mixture solvent of 1-propanol and 2-methoxyethanol. SFMO thin films with c-axis preferred orientation could successfully be synthesized on MgO (0 0 1) and SrTiO₃ (0 0 1) substrates by optimizing the several processing conditions. SFMO thin film prepared on SrTiO₃ (0 0 1) showed a magnetoresistance effect at a low magnetic field.     

Properties of thin manganite films grown on semiconducting substrates for spintronics applications

La_0.7Sr_0.3MnO₃ (LSMO) manganite thin films were grown by pulsed plasma deposition on silicon (Si) and gallium arsenide (GaAs) substrates covered by an amorphous oxide. Manganite films are characterized by polycrystalline structure. Ferromagnetic transition is above room temperature and for 50 nm thick film the Curie temperature was as high as 325 K and 305 K for LSMO/SiO_x/Si and LSMO/AlO_x/GaAs, respectively.     

Uniaxial pressure effect on magnetic susceptibility of perovskite manganite La0.66Ba0.34MnO3

Magnetization of La_0.66Ba_0.34MnO₃ and its temperature behavior under a uniaxial pressure of 0.1 kbar are measured between 5 and 270 K in magnetic fields 0<H<120 Oe. The magnetization represents nearly linear dependence on an external magnetic field. Temperature dependence of the magnetic susceptibility found represents a plateau, that is considered as an evidence of the formation of a long period magnetic structure (probably a sort of helix) below the Curie point. Pressure derivative of magnetization displays a sharp minimum at 200 K, pointing to an instability of electronic structure of the compound near this temperature.     

Room temperature ferroelectricity of tetragonally strained SrTiO3 thin films on single crystal Rh substrates

Epitaxial SrTiO₃ thin films were deposited on single crystalline Rh substrates by pulsed laser deposition. The tetragonally stained structure of the SrTiO₃ thin films with a c/a ratio of 1.04 was confirmed by x-ray diffraction experiments. The SrTiO₃ thin films exhibited good ferroelectric properties with a high remanent polarization (2P_r) of 8 μC/cm² and a canonical ferroelectric piezoresponse hysteresis loop at room temperature. We estimated a high activation electric field of about 6.4 MV/cm for domain wall creeping. This activation electric field is higher than that of typical ferroelectric materials such as PbTiO₃.     

Electron magnetic resonance (EMR) in sonochemically prepared doped nano-manganites

X-band electron magnetic resonance method was explored for study of sonochemically prepared nano-powders of La-based doped manganites: La_0.7Sr_0.3MnO₃ and La_0.9Ca_0.1MnO₃ at 115 ⩽ T ⩽ 600 K temperature interval, including main characteristic points of its magnetic transitions. The data obtained were compared with those for crushed bulk single crystals of the same compositions. It is shown that nano-powder of La_0.7Sr_0.3MnO₃ has the same ferromagnetic ordering as its bulk counterpart, being more homogeneous and less anisotropic. While, nano-powder of La_0.9Ca_0.1MnO₃ shows two distinct ferromagnetic phases in a marked contrast to the bulk sample, where complex magnetic structure comprising canted antiferromagnetic matrix and ferromagnetic clusters is observed. To explain the observed phenomena, the crucial role of nano-scale grain size and its interplay with spin and charge degrees of freedom in considered systems are discussed.     

Extraordinary fast oxide ion conductivity in La1.61GeO5−δ thin film consisting of nano-size grain

Thin films of La_1.61GeO_5−δ, a new oxide ionic conductor, were fabricated on dense polycrystalline Al₂O₃ substrates by a pulsed laser deposition (PLD) method and the effect of the film thickness on the oxide ionic conductivity was investigated on the nanoscale. The deposition parameters were optimized to obtain La_1.61GeO_5−δ thin films with stoichiometric composition. Annealing was found necessary to get crystalline La_1.61GeO_5−δ thin films. It was also found that the annealed La_1.61GeO_5−δ film exhibited extraordinarily high oxide ionic conductivity. Due to the nano-size effects, the oxide ion conductivity of La_1.61GeO_5−δ thin films increased with the decreasing thickness as compared to that in bulk La_1.61GeO_5−δ. In particular, the improvement in conductivity of the film at low temperature was significant .The electrical conductivity of the La_1.61GeO_5−δ film with a thickness of 373 nm is as high as 0.05 S cm^− 1 (log(σ/S cm^− 1) = − 1.3) at 573 K.     

La0.7Sr0.3MnO3 film prepared by dc sputtering on silicon substrate: Effect of working pressure

La_0.7Sr_0.3MnO₃ films were prepared by dc sputtering on Si (100) substrate at different working pressure. The possibility of controlling the magnetic and transport properties of colossal magnetoresistance film is investigated, which has attracted great research interest for practical application. The as-grown film shows different magnetic, transport and magnetoresistance change at different working pressure at room temperature, which is quite attractive from technological point of view. Maximum magnetoresistance (MR) of ?5.56%, Curie temperature (T_c) of 325 K and metal insulator transition temperature (T_MI) of 278 K was achieved at room temperature.     

Magnetic entropy change in perovskite manganites La0.65Nd0.05Ca0.3Mn0.9B0.1O3 (B=Mn,Cr, Fe)

We have investigated the effect of B-site dopant on magnetic entropy change in perovskite manganite samples of La_0.65Nd_0.05Ca_0.3MnO₃, La_0.65Nd_0.05Ca_0.3Mn_0.9Cr_0.1O₃, and La_0.65Nd_0.05Ca_0.3Mn_0.9Fe_0.1O₃ prepared by sol–gel technique. The maximum ΔS_H is in the order of −1.68 J/kg K and peaks at Curie temperature for La_0.65Nd_0.05Ca_0.3MnO₃ upon 10 kOe applied field change. For the sample with B-site dopant, a decrement of the maximum magnetic entropy change has been observed.     

Surface stoichiometry of La0.7Sr0.3MnO3 during in vacuo preparation; A synchrotron photoemission study

We present a study of the surface stoichiometry and contamination of La_0.7Sr_0.3MnO₃ thin films following exposure to air and subsequent in vacuo preparation. Samples were studied using both soft X-ray synchrotron photoemission (hν = 150 to 350 eV) and traditional Mg-Kα XPS (hν = 1253.6 eV) whilst annealing incrementally to ≈ 510°C in low pressures of O₂. In all cases, a Mn depleted and Sr rich surface oxide layer is observed, it is of reduced crystalline quality and is charge depleted. This surface layer is weakly affected by subsequent annealing, and is partially reversed by annealing in higher O₂ pressure. Surface carbon contamination is incrementally removed by annealing at increased temperatures, and at 270 °C, it is reduced to ≈ 0.4% of the topmost unit cell. The modification of the surface stoichiometry and electronic properties is consistent with the reported loss of magnetic properties in thin LSMO films.     

Photoluminescence probed minority processes in La2/3Sr1/3MnO3 thin films

La_2/3Sr_1/3MnO₃ thin films are studied with temperature variable photoluminescence (PL) spectroscopy. Two emission peaks are assigned to the minority carriers related transition processes. The temperature independent 2.526 eV peak is attributed to the charge transfer type inter-band transition, while the redshifted doublet peak around 1.686 eV to the spin flip process. Band structures are obtained within the density functional theory, which show the consistent band gaps with the PL data. The temperature dependence of the intensity of PL emission suggests that these minority carrier processes are relevant to polaron formation.     

Strain induced phase separation on La0.5Ca0.5MnO3 thin films

The effect of epitaxial strain on La_0.5Ca_0.5MnO₃ films of various thicknesses grown on SrTiO₃, SrLaAlO₄, and SrLaGaO₄ substrates is studied by Raman spectroscopy, magnetic, and resistivity measurements. The transport and magnetic properties as well as Raman spectra of the films are affected by epitaxial strains. The energy of the A_g(2) mode and the tilting angle of the MnO₆ octahedra is affected by the strain imposed by the substrate. In the spectra of the films deposited on the (1 0 0) SrTiO₃ substrate strong Jahn-Teller (JT) modes appear, which couple with charge-ordering. In all other films these modes are suppressed and no additional Raman lines are present at low temperatures contrary to the bulk compound. The low frequency continuum scattering decreases at low temperatures indicating a coupling with both the charge and orbital transitions. Comparison of the Raman spectra with the magneto-transport properties suggests an interpretation in terms of a strain induced phase separation between ferromagnetic metallic and antiferromagnetic insulating states.     

Oxygen assisted deposition of Sr2FeMoO6 thin films on SrTiO3(1 0 0)

Epitaxial thin films of Sr₂FeMoO₆ (SFMO) were prepared by pulsed laser deposition on SrTiO₃(1 0 0) substrate. Thin films have been grown under different gas environments and they were structurally characterised by XRD. In contrast to previous reports, deposition carried out in the presence of a small amount of O₂ with Ar yields high-quality SFMO films with a saturation magnetic moment of 3.8 μ_B. These SFMO films were strained in such a way that they were elongated along the c-axis and compressed in the ab-axes directions. The large low-field magnetoresistance seen in these films has been attributed to the tunneling across the antisite boundaries.     

Measuring magnetic profiles at manganite surfaces with monolayer resolution

The performance of manganite-based magnetic tunnel junctions (MTJs) has suffered from reduced magnetization present at the junction interfaces that is ultimately responsible for the spin polarization of injected currents; this behavior has been attributed to a magnetic “dead layer” that typically extends a few unit cells into the manganite. X-ray magnetic scattering in resonant conditions (XRMS) is one of the most innovative and effective techniques to extract surface or interfacial magnetization profiles with subnanometer resolution, and has only recently been applied to oxide heterostructures. Here we present our approach to characterizing the surface and interfacial magnetization of such heterostructures using the XRMS technique, conducted at the BEAR beamline (Elettra synchrotron, Trieste). Measurements were carried out in specular reflectivity geometry, switching the left/right elliptical polarization of light as well as the magnetization direction in the scattering plane. Spectra were collected across the Mn L_2,3 edge for at least four different grazing angles to better analyse the interference phenomena. The resulting reflectivity spectra have been carefully fit to obtain the magnetization profiles, minimizing the number of free parameters as much as possible. Optical constants of the samples (real and imaginary part of the refractive index) in the interested frequency range are obtained through absorption measurements in two magnetization states and subsequent Kramers–Krönig transformation, allowing quantitative fits of the magnetization profile at different temperatures. We apply this method to the study of air-exposed surfaces of epitaxial La_2/3Sr_1/3MnO₃ (0 0 1) films grown on SrTiO₃ (0 0 1) substrates.     

Magnetic properties of high quality superconducting laser ablated thin films

We present experimental results obtained forRBa₂Cu₃O_7 − x(R = Y,Er) expitaxial thin films obtained through pulsed laser deposition (PLD) and grown on yttria stabilized zirconia (YSZ) and SrTiO₃(STO) substrates. The films have been deposited by using low deposition rates (f = 4 Hz) and with control of the film surface temperature rather than that of the sample holder leading to a high quality of the epitaxy.     

The growth and field evaporation of Er and deuterated Er films

The morphological structure of clean and deuterated Er films deposited on W substrates and their removal by field evaporation have been investigated as part of a program directed toward the development of deuterium ion sources for neutron generators. Annealed Er films up to ~ 20 monolayers in thickness deposited on W < 110 > substrates appear pseudomorphic. Thicker annealed films form a hexagonal close-packed < 0001 > orientated over-layer with the Pitsch–Schrader orientation relation. The pseudomorphic and hexagonal close-packed character of the films is retained up to the last atomic layer that forms the film-substrate interface. Deuterated Er films appear polycrystalline. At 77 K in Ar, annealed Er films field evaporate at 2.5 V/Å primarily as Er^2 + and deuterated Er films evaporate at ~ 2.4 V/Å primarily as ErD_x^2 +. Field evaporation of both clean and deuterated Er films shows signs of space charge induced field lowering when film thicknesses exceeding ~ 10 layers were field evaporated using 20 ns duration voltage pulses.