Interface effects and the evolution of ferromagnetism in La2/3Sr1/3MnO3 ultrathin films

Pulse laser deposited La_2/3Sr_1/3MnO₃ ultrathin films on SrTiO₃ substrates were characterized by polar and longitudinal Kerr magneto-optical spectroscopy. Experimental data were confronted with theoretical simulations based on the transfer matrix formalism. An excellent agreement was achieved for a 10.7 nm thick film, while a distinction in the Kerr effect amplitudes was obtained for a 5 nm thick film. This demonstrated the suppression of ferromagnetism due to the layer/substrate interface effects. A revised, depth-sensitive theoretical model with monolayer resolution described the experimental data well, and provided clear cross-section information about the evolution of ferromagnetism inside the film. It was found that the full restoration of the double-exchange mechanism, responsible for the ferromagnetic ordering in La_2/3Sr_1/3MnO₃, occurs within the first nine monolayers of the film. Moreover, all the studied films exhibited magneto-optical properties similar to bulk crystals and thick films. This confirmed a fully developed perovskite structure down to 5 nm.     

Interface and Bulk Charge Localization in Manganite Thin Films

Nuclear magnetic resonance (NMR) is utilized to map the hyperfine fields acting on ⁵⁵Mn nuclei of La_2/3Sr_1/3MnO₃ (LSMO) epitaxial thin films. In contrast to early thoughts, we show that phase separation (PS) in LSMO is not restricted to an interface‐related dead‐layer only a few nm thick. Instead, it propagates much deeper into the films, thus signalling different origins for the PS. These results indicate that both, surfaces and defects contribute to promote distinguishable PS effects. This view has been confirmed by detailed ⁵⁵Mn NMR analysis of LSMO films where engineered bulk‐like defects have been created by appropriate irradiation by energetic He⁺ ions. Moreover, from the analysis of the restoring fields sensed by the spins of the ⁵⁵Mn nuclei, we infer two sources of magnetic anisotropy. More specifically, a surface magnetic anisotropy –linked to the interfacial dead layer– is predominant for ultrathin samples (t < 10 nm), whereas structural film relaxation, provides a second mechanism for anisotropy. These findings provide guidelines for the growth of electric and magnetic homogeneous manganite films.     

Epitaxial PbZrxTi1−xO3 Ferroelectric Bilayers with Giant Electromechanical Properties

Giant electromechanical response viaferroelastic domain switching is achieved in epitaxial (001) ferroelectric tetragonal (T) PbZr_0.3Ti_0.7O₃/rhombohedral (R) PbZr_0.55Ti_0.45O₃ bilayers, grown on La_0.67Sr_0.33MnO₃ buffered SrTiO₃ substrates. X‐ray diffraction and transmission electron microscopy show that the domain structure of the T films is tuned as a function of its thickness, from a fully a₁/a₂‐domains (30 nm thick T layer) to a three domain stress‐free c/a₁/c/a₂ polytwin state (100 nm thick T layer). A large switchable polarization is found up to 65 μC cm⁻². Quantitative piezoelectric force microscopy reveals enhanced piezoelectric coefficients, with d₃₃ coefficients ranging from 250 to 350 pm V⁻¹, which is up to seven times higher than the nominal PbZr_xTi_1−xO₃ thin film values. These are attributed to the motion of nanoscale ferroelastic domains. Fatigue testing proves that these domains are reversible and repeatable up to 10⁷ cycles. In‐situ X‐ray synchrotron measurements reveal that the ferroelastic domain switching is promoted by a pulsating strain effect imposed by the R layer. The study reports a fundamental understanding of the origin of giant piezoelectric coefficients in epitaxial ferroelectric bilayers.     

Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary‐Condition Engineering

Tailoring and enhancing the functional properties of materials at reduced dimension is critical for continuous advancement of modern electronic devices. Here, the discovery of local surface induced giant spontaneous polarization in ultrathin BiFeO₃ ferroelectric films is reported. Using aberration‐corrected scanning transmission electron microscopy, it is found that the spontaneous polarization in a 2 nm‐thick ultrathin BiFeO₃ film is abnormally increased up to ≈90–100 µC cm^?2 in the out‐of‐plane direction and a peculiar rumpled nanodomain structure with very large variation in c /a ratios, which is analogous to morphotropic phase boundaries (MPBs), is formed. By a combination of density functional theory and phase‐field calculations, it is shown that it is the unique single atomic Bi₂O_3?x layer at the surface that leads to the enhanced polarization and appearance of the MPB‐like nanodomain structure. This finding clearly demonstrates a novel route to the enhanced functional properties in the material system with reduced dimension via engineering the surface boundary conditions.     

Voltage‐Controlled On/Off Switching of Ferromagnetism in Manganite Supercapacitors

The ever‐growing technological demand for more advanced microelectronic and spintronic devices keeps catalyzing the idea of controlling magnetism with an electric field. Although voltage‐driven on/off switching of magnetization is already established in some magnetoelectric (ME) systems, often the coupling between magnetic and electric order parameters lacks an adequate reversibility, energy efficiency, working temperature, or switching speed. Here, the ME performance of a manganite supercapacitor composed of a ferromagnetic, spin‐polarized ultrathin film of La_0.74Sr_0.26MnO₃ (LSMO) electrically charged with an ionic liquid electrolyte is investigated. Fully reversible, rapid, on/off switching of ferromagnetism in LSMO is demonstrated in combination with a shift in Curie temperature of up to 26 K and a giant ME coupling coefficient of ≈226 Oe V⁻¹. The application of voltages of only ≈2 V results in ultralow energy consumptions of about 90 µJ cm⁻². This work provides a step forward toward low‐power, high‐endurance electrical switching of magnetism for the development of high‐performance ME spintronics.     

Photoconductivity in BiFeO3/La0.7Sr0.3MnO3 heterostructure

In this letter, an oxide heterostructure has been fabricated by successively growing La_0.7Sr_0.3MnO₃ (LSMO) and BiFeO₃ (BFO) layers on LaAlO₃ (100) by pulsed laser deposition. Analysis of the leakage current at different temperature demonstrated that the Poole-Frenkel dominated the leakage current mechanism. Additionally, the BiFeO₃/La_0.7Sr_0.3MnO₃ heterostructure exhibits a positive colossal magnetoresistance (MR) effect over a temperature range of 50-320 K. The maximum MR values are determined to be about 45.32% at H = 0.5 T and 28.34% at H = 0.3 T. At last, we report photoconductivity in BiFeO₃/La_0.7Sr_0.3MnO₃ film under illumination from 160 mW/cm² and 200 mW/cm² green-light source, and photoconductivities increase with the intensity of light enhanced.     

Effect of high temperature post-annealing of La0.7Sr0.3MnO3 films deposited by radio frequency magnetron sputtering on SiO2/Si substrates heated at low temperature

La_0.7Sr_0.3MnO₃ thin films were deposited on SiO₂/Si substrates by RF magnetron sputtering under different oxygen gas flow rates with a sputtering power of 100 W. During deposition, the substrate was heated at 623 K. To investigate post-annealing effects, the as-deposited La_0.7Sr_0.3MnO₃ thin films were thermal-treated at 973 K for 1 h. The effects of oxygen gas flow rate and post-annealing treatment on the physical properties of the films were systematically studied. X-ray diffraction results show that the growth orientation and crystallinity of the films were greatly affected by the oxygen gas flow rate and substrate heating during deposition. The sheet resistance of the films gradually decreased with increasing oxygen gas flow rate, while the post-annealed films showed the opposite behavior. The temperature coefficient of resistance at 300 K of La_0.7Sr_0.3MnO₃ thin films deposited at an oxygen gas flow rate of 40 sccm decreased from − 2.40%/K to − 1.73%/K after post annealing. The crystalline state of the La_0.7Sr_0.3MnO₃ thin films also affected its electrical properties.     

Impact of granularity on transport properties of mechanically stressed La0.67Ca0.33MnO3 films

(La,Ca)MnO₃ is one of so called collosal magnetoresistive materials and it is of interest to correlate its transport properties to film growth in order to optimize its performance. Two-hundred nanometers thick (100)La_0.67Ca_0.33MnO₃ films were grown by laser ablation on (100)SrTiO₃, (100)LaAlO₃ and (100)(LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7 substrates. The films were granular in structure with low angle boundaries between the grains. The volume of the unit cell was considerably smaller for films grown on a SrTiO₃ substrate than on LaAlO₃. At temperatures higher than the one where the spins order, the strongest response of resistivity on temperature ρ(T) was measured for those (100)La_0.67Ca_0.33MnO₃ films that had the smallest volume of the unit cell. An increase of the lattice parameter for a film was accompanied by a decrease of the resistivity. The films grown on (LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7 substrates had the best crystallinity and the largest magnetoresistance.     

Colossal magnetoresistive and ferroelectric thin films deposited by excimer laser induced plasma

We prepared the colossal magnetoresistive La_0.8Sr_0.2MnO₃ thin films on single crystal MgO, SrTiO₃ and LaAlO₃ substrates by KrF excimer pulsed laser deposition technique. The thickness dependence of the structural, electric and magnetic properties of the LSMO films on different substrates is reported. The integration of the ferroelectrics with the colossal magnetoresistance thin film was achieved. The plasma plume ablated from target was investigated by intensified charge coupled device and optical emission spectroscopy.     

Colossal magnetoresistive and ferroelectric thin films deposited by excimer laser induced plasma

We prepared the colossal magnetoresistive La_0.8Sr_0.2MnO₃ thin films on single crystal MgO, SrTiO₃ and LaAlO₃ substrates by KrF excimer pulsed laser deposition technique. The thickness dependence of the structural, electric and magnetic properties of the LSMO films on different substrates is reported. The integration of the ferroelectrics with the colossal magnetoresistance thin film was achieved. The plasma plume ablated from target was investigated by intensified charge coupled device and optical emission spectroscopy.     

Preparation and properties of thick LaMnO3-based films

Thick (50 ±10 μm) La_1-xSr_xMnO₃ (x = 0.2, 0.4) and La_0.7Ca_0.3Mn0₃ films were prepared by screen printing, and their electrical properties were studied. The films were found to be close in magnetoresistance and resistivity to bulk materials     

Effect of internal strain on magnetic properties of La0.7Sr0.3MnO3 films

Oriented La_0.7Sr_0.3MnO₃ (LSMO) films were grown on (100) LaAlO₃ (LAO) and SrTiO₃ (STO) substrates by the method of radio frequency magnetron sputtering. X-ray diffraction patterns of LSMO films deposited on LAO and STO substrates showed a slight shift of diffraction peaks as compared to the corresponding peaks of the bulk LSMO. The shift of diffraction peaks was believed to result from the in-plane lattice mismatch between the film and substrate. The distinguished strain effects of substrates were found to be effective on the magnetic properties and the surface morphology of LSMO films on both substrates. The results indicate that STO substrate may be the promising candidate for room-temperature applications of LSMO film.     

Preparation and colossal magnetoresistance in a trilayer La0.67Sr0.33MnO3/La0.75MnO3/La0.67Sr0.33MnO3 device by dc magnetron sputtering

Epitaxial trilayer films of La_0.67Sr_0.33MnO₃ (LSMO)/La_0.75MnO₃ (L0.75MO)/La_0.67Sr_0.33MnO₃ (LSMO) have been prepared on (0 0 1) oriented LaAlO₃ substrates by dc magnetron sputtering. The structure and MR are studied. All as-deposited trilayer films exhibit a semiconductor to metal transition at temperature ranging from 116 to 185 K. The MR is also shown to be dependent on the thickness of the middle oxide layer. A maximum MR value of 32% (ΔR/R₀) has been obtained at 132 K under 0.4 T magnetic field for a LSMO (300 nm)/L0.75MO (70 nm)/LSMO (300 nm) trilayer film. The MR of trilayer film prefers to that of both LSMO and L0.75MO single layer films.     

A Comparative Study of Angle Dependent Magnetoresistance in [001] and [110] La<Subscript>2/3</Subscript>Sr<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>

The angle dependent magnetoresistance study on [001] and [110] La_2/3Sr_1/3MnO₃ thin films shows that the anisotropy energy of [110] films is higher than that of a [001] oriented La_2/3Sr_1/3MnO₃ film of similar thickness. The data have been analyzed in the light of a multidomain model and it is seen that this model correctly explains the observed behavior.     

Characteristics of La0.7Ca0.3MnO3 Powders Prepared by the Solution Combustion and Solid State Reaction Methods for Colossal Magnetoresistance Applications

La_0.7Ca_0.3MnO₃ powders were prepared by both the solution combustion method and the solid state reaction method and were calcinated at various calcination temperatures and time intervals in air atmosphere. In the solid state reaction method, single-phase La_0.7Ca_0.3MnO₃ was obtained after heat treatment of the powder at 1000°C for 24 hr. In the solution combustion method, however, single-phase La_0.7Ca_0.3MnO₃ powder could be obtained easily when the powder was heat-treated at 650°C for only 30 min. Polycrystalline La_0.7Ca_0.3MnO₃ powder, using the solution combustion method, showed good powder characteristics, such as an average grain size of 50 nm and a specific surface area of 92 m²/g. The resistance as a function of temperature and the magnetoresistance ratio in La_0.7Ca_0.3MnO₃ thin films were attempted to examine the colossal magnetoresistance characteristics. These thin films also showed excellent colossal magnetoresistance properties in that 96% of the maximum magnetoresistance ratio was obtained at 97K.     

Comparative investigating the properties of Pb-based multilayer ferroelectric thin films for FeRAM

The PbZr_0.3Ti_0.7O₃(PZT) thin film and multilayer PbZr_0.3Ti_0.7O₃/PbTiO₃(PZT/PT), PbTiO₃/PbZr_0.3Ti_0.7O₃/ PbTiO₃(PT/PZT/PT) thin films were prepared by a Sol-Gel method on the Pt(111)/Ti/SiO₂/Si(100) substrate for FeRAM application. The microstructure, ferroelectric, fatigue, dielectric, and leakage current properties of these thin films were investigated. The results indicate that the multilayer PT/PZT/PT thin film have a better ferroelectric, fatigue, dielectric and leakage current density properties. Its remanent polarization P_r reaches a maximum value of 21.2 μC/cm² and the coercive field E_c gets to a minimum value of 64.2kV/cm. After 10¹⁰ cycles, it still has more than 80% remnant polarization. The PT/PZT/PT thin film exhibits lower dielectric constant and lower dielectric loss, which is beneficial for FeRAM application. It also has the lowest leakage current density. The leakage current mechanism of the PZT, PZT/PT and PT/PZT/PT thin films is correlated to the microstructure and can be modeled in terms of GBLC and SCLC theory. The microstructure and electric properties of these films are correlated. The double-sided PT seed layers enhance not only the microstructure but also the electric properties. It is evident that the PT/PZT/PT multilayer thin film is a promising candidate for FeRAM application.     

Synthesis of Ultrafine-Particle La0.7Sr0.3MnO3 via Chelate Homogenization and Microwave Processing

Ultrafine La_0.7Sr_0.3MnO₃ powders were prepared via homogenization in chelate solutions, followed by microwave dehydration, using polynuclear heterometallic diethylenetriaminepentaacetates as precursors. To assess the effect of the dehydration procedure on the phase composition and grain size of La_0.7Sr_0.3MnO₃ ceramics, three routes were tested: concentration of chelate solutions by evaporation until the formation of a glassy precursor, microwave dehydration of chelate solutions, and a combination of gelation and microwave dehydration. Phase-pure La_0.7Sr_0.3MnO₃ with a crystallite size of 30–40 nm (as determined by transmission electron microscopy) could be obtained via microwave dehydration of heterobimetallic precursor solutions, followed by calcination at a temperature as low as 800°C.     

Griffiths Phase and Disorder in Perovskite Manganite Oxides La1?xCaxMnO3 and La0.7Sr0.3MnO3

Polycrystalline samples La_1−x Ca _x MnO₃ (x=0.17, 0.15, 0.10) and La_0.7Sr_0.3MnO₃ were prepared in order to investigate the Griffiths-like features induced by disorder compared with their counterpart single crystals. The magnetization data exhibit the traditional transition from ferromagnetic phase to paramagnetic phase. From the temperature dependence of inverse susceptibility, it can be testified that the Griffiths-like features still exist in as-prepared Ca doped samples, while non-Griffiths-like features exist in La_0.7Sr_0.3MnO₃. All these samples, however, exhibit the large effective spins resulting from formation of the short-order ferromagnetic clusters. The O K-edge X-ray absorption spectra indicate the Jahn–Teller (J-T) distortions are definitely present due to the J-T ion Mn³⁺, which indicate that static J-T distortion is not a sufficient condition for the existence of Griffiths phase in Sr-doped system. And, the size of J-T distortion is a little larger in polycrystalline La_0.7Sr_0.3MnO₃ than that in polycrystalline samples La_1−x Ca _x MnO₃ (x=0.17, 0.15, 0.10), revealed by X-ray diffraction parameters and extended X-ray fine structure absorption data of Mn K-edge. It also testifies that the disorder in La_0.7Sr_0.3MnO₃ caused by both chemical doping and J-T distortions is lower than that in polycrystalline samples La_1−x Ca _x MnO₃, which may be the reason of non-Griffiths-like phase existing in La_0.7Sr_0.3MnO₃ samples.     

Improvement of the remnant polarization of 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 thin films in a multilayer composite

It has been previously reported that the remnant polarization of 0.65Pb(Mg_1/3Nb_2/3)O₃-0.35PbTiO₃ (PMNT) thin films on Si-based substrates is much lower than that of bulk ceramics with the same composition, which is a problem for its integration in microdevices. The preparation of multilayer composites of PMNT and PbTiO₃ layers, which maintain large remnant polarization values even for the thinnest films, is explored in this work as an alternative for obtaining PMNT-based films with enhanced remanence. The multilayer composite thin films were fabricated onto Pt/TiO₂/SiO₂/Si (100) substrates by Chemical Solution Deposition. The deposited layers are ultrathin and the results show that there is no significant interdiffusion among them. Although the typical constraints related to the small grain size found in ultrathin layers and the associated lack of ferroelastic domains are present, the results show an improvement of the remanence of the PMNT layer in the multilayer composite film and that these composites can be good candidates for the integration of PMNT in devices.     

Probing electronic defect states in manganite/SrTiO3 heterostructures by surface photovoltage spectroscopy

The energetic distribution of electronic defect states in oxide heterostructures made of ultrathin lanthanum manganite (La_0.7Ca_0.3MnO₃, La_0.7Ce_0.3MnO₃) films on SrTiO₃ substrates is investigated by surface photovoltage (SPV) spectroscopy. Within a comparative evaluation of the SPV spectra of both the film/substrate structures and pure substrates at different temperatures we were able to elaborate a map of defect states across the SrTiO₃-bandgap and we find that the defect state distribution is mainly affected by the substrate, i.e., no specific film-induced defect states were detected. Possible origins of the defect states are discussed within the framework of a semiconductor band scheme, taking into account complementary photoconductivity and SPV transient data.