La1-xSrxMnO3 Thin Films Research Articles

Magneto-Conductive and Magnetic Properties in La<sub>1-<i>x</i></sub>Sr<i><sub>x</sub></i>MnO<sub>3 </sub>Thin Films on a-SiO<sub>2</sub> Substrates Produced by Metal Organic Decomposition Method

We have studied magneto-conductive and magnetic properties of La1-xSrxMnO3 (LSMO) thin films on a-SiO2 substrates produced by the metal organic decomposition (MOD) method. LSMO thin films for x = 0, 0.15 and 0.3 have been produced in a pure O2 gas atmosphere. Although LaMnO3 (LMO) single crystal is an antiferromagnetic insulator (AFI), LMO thin films we have produced show ferromagnetic metal (FM) properties for suitable heat treatment conditions. We consider that the excess of O2- ions in LMO thin films produced in a pure O2 gas atmosphere induces the strong hole self-doping into those and the LMO thin films change from AFI to FM. Whereas, the ordinary hole doping is also occurred in LSMO thin films at x > 0. Thus, the carrier doping for LSMO thin films at x > 0 is caused by the hole self-doping by O2- ions and the ordinary hole doping by the replacement of La3+ ions by Sr2+ ones. To investigate the crystallographic and surface structures of the LSMO thin films, X-ray diffraction and SEM measurements have been performed, respectively. From the X-ray diffraction measurement, we have found that all LSMO thin films have perovskite structure and are polycrystalline. From the SEM measurement, we have seen that the LSMO thin films are formed of the aggregation of LSMO fine particles. Electrical resistivities (ERs) and magneto-resistivity (MR) ratios of the LSMO thin films have been measured on the temperature dependence (4K-300K). From MR ratio measurements, the coercive forces of them have been obtained as a function of temperature, and the Curie temperatures have been estimated from the temperature dependences of the coercive forces. We have discussed the origin of the magneto-conductive and magnetic properties of LSMO thin films.

Tailoring interface epitaxy and magnetism in La1−xSrxMnO3/SrTiO3 heterostructures via temperature-driven defect engineering

Defect engineering of La1−xSrxMnO3 (LSMO)—a strongly correlated oxide displaying half metallicity and ferromagnetism above room temperature—has been the focus of a long-standing quest aimed at the exploitation of this material as a functional building block for memory storage and spintronic applications. Here, we discuss the correlation between structural defects and magnetism in La0.74Sr0.26MnO3/SrTiO3 (LSMO/STO) epitaxial heterostructures as a function of growth temperature and post-deposition annealing. Upon increasing the growth temperature from 500 to 700 °C at a fixed oxygen partial pressure of 0.007 mbar, the sputter-deposited epitaxial LSMO films experience a progressive increase in Curie temperature Tc from 110 to 270 K and saturation magnetization Ms from 1.4 to 3.3 μB/u.c. owing to a reduction in oxygen deficiencies. Concurrently, however, growth temperatures above 600 °C trigger the formation of off-stoichiometric, dendritic-like SrMoOx islands at the film/substrate interface as a possible aftermath of temperature-driven diffusion of impurities from the STO substrate. Notably, although the interfacial spurious islands cause an increase in sample surface roughness, the heterostructure still preserves high-quality epitaxy. In general, the best compromise in terms of both structural and magnetic properties, comprising high-quality epitaxy, atomically flat surface, and robust ferromagnetism above room temperature, is obtained for LSMO films grown at a relatively low temperature of about 500–540 °C followed by a post-deposition annealing treatment at 900 °C for 1 h in air. Our study compares effective routes based on temperature-controlled defect engineering to finely tailor the complex interplay between microstructure and magnetism in LSMO thin films.

High room-temperature TCR and MR of La1-xSrxMnO3 thin films for advanced uncooled infrared bolometers and magnetic sensors

Perovskite manganese films had promising applications in the field of photoelectronic materials and devices. However, the utilization of manganese films remained severely challenging due to high fabrication cost and inferior electrical and magnetic properties. Herein, La1-xSrxMnO3 (LSMO) thin films were fabricated on La0.3Sr0.7Al0.65Ta0.35O3 (LSATO) substrates using a facile sol–gel spin-coating technique. All the LSMO films displayed (001)-orientation with orthorhombic structure. The LSMO films exhibited high temperature coefficient of resistivity (TCR) and magnetoresistance (MR) values at room-temperature. The maximum TCR value was 10.4 % K−1 (301.2 K, for x = 0.175). This result can be attributed to high carrier concentration and mobility of optimal × content LSMO films with low film-substrate lattice mismatch (-0.4%), leading to low resistivity, and thus, increased TCR value. The maximum MR value reached 42.85% (290.95 K, for x = 0.150). The LSMO films with high room-temperature TCR and MR values are potential materials for advanced uncooled infrared bolometers and magnetic sensors.

Anisotropic spin-density distribution and magnetic anisotropy of strained La1\u2212xSrxMnO3 thin films: angle-dependent x-ray magnetic circular dichroism

Magnetic anisotropies of ferromagnetic thin films are induced by epitaxial strain from the substrate via strain-induced anisotropy in the orbital magnetic moment and that in the spatial distribution of spin-polarized electrons. However, the preferential orbital occupation in ferromagnetic metallic La1−xSrxMnO3 (LSMO) thin films studied by x-ray linear dichroism (XLD) has always been found out-of-plane for both tensile and compressive epitaxial strain and hence irrespective of the magnetic anisotropy. In order to resolve this mystery, we directly probed the preferential orbital occupation of spin-polarized electrons in LSMO thin films under strain by angle-dependent x-ray magnetic circular dichroism (XMCD). Anisotropy of the spin-density distribution was found to be in-plane for the tensile strain and out-of-plane for the compressive strain, consistent with the observed magnetic anisotropy. The ubiquitous out-of-plane preferential orbital occupation seen by XLD is attributed to the occupation of both spin-up and spin-down out-of-plane orbitals in the surface magnetic dead layer.

(Invited) Volume Resistive Switching in Metallic Perovskite Oxides Driven By the Metal-Insulator Transition

In recent years, huge efforts have been made in the research community to overcome the limitations in Flash and RAM memories, and consequently, new emerging technologies are competing in the race towards a new paradigm in information storage. In particular, nonvolatile memories based on the Resistive Switching (RS) effect, where two (or more) reversible resistance states can be induced upon application of an electric field, has emerged with excellent performance. This phenomenon has been observed in many oxide systems, in particular in perovskite oxides, which are materials showing outstanding properties giving rise to exotic physical phenomena due to the strong electronic correlation, such as metal-insulator transitions (MIT). This is the case of the metallic perovskite La1-xSrxMnO3 (LSMO), RENiO3 (RE=rare earth) and YBa2Cu3O7(YBCO) family compounds, which are able to display Volume RS effects induced by the MIT and therefore, small changes in carrier concentration can induce huge resistance changes. The mechanism underlying this phenomenon is still unclear although oxygen vacancies mobility plays an important role in the mechanism underneath this phenomenon. In this presentation we will discuss our studies on bipolar resistive switching of the mentioned perovskite oxides. Switching characteristics have been evaluated by C-SPM and I(V) curves with metal electrodes. Scanning tunneling microscopy and spectroscopy (STM/S), transport and resistive measurements were performed to gain insight into the local density of states of the material for different resistance states. Large resistance rations (102-104), over 100 cycles experiments, multilevel switching and switching dynamics have been evaluated. In addition, we have confirmed the strong influence of different atmospheres on the resistive switching properties of bare LSMO thin films. Remarkably, this influence disappears when the LSMO layer is capped with a CeO2thin layer, which acts as an oxygen reservoir, making this bilayer a proper material choice for encapsulation. A 3-terminal proof of principle device based on Ag/CeO2/LSMO/CeO2/Ag will be presented. We believe that these results contribute to a better understanding of the physical mechanism behind the robust RS effect of these metallic perovskite oxides films.

Control of Preferential Orientation of La<sub>1-x</sub>Sr<sub>x</sub>MnO<sub>3</sub> Thin Films by Pulsed Laser Deposition

La1-xSrxMnO3 (LSMO) thin films were grown on (100)- and (110)-oriented MgO substrates by pulsed laser deposition, so as to control the preferential orientation of the films. At the optimum deposition temperature (Td = 923K), LSMO thin films with a dense texture and a smooth surface were prepared. The XRD patterns and X-ray pole figures indicated that the LSMO thin films were epitaxially grown on MgO substrates, due to the small lattice mismatch between the films and substrates The possible orientation relationships were found to be LSMO (100) [100] // MgO (100) [100] and LSMO (110) [110] // MgO (110) [110], as for the (100) and (110) MgO substrates, respectively.

Effect of Sr Doping on Structure and Emittance of La<sub>1<i>-x</i></sub>Sr<sub><i>x</i></sub>MnO<sub>3</sub> Smart Thermal Control Films

Sr-doped lanthanum manganite (La1-xSrxMnO3) is characteristic of thermochromic, which can act as a smart thermal control material used in the variable-emittance devices. In the present study, La1-xSrxMnO3 thin films were prepared on MgO(100) substrates by pulsed laser deposition, and the effect of Sr-doping (x = 0 ~ 0.4) on the structure and infrared emissivity was investigated. Single-phased La1-xSrxMnO3 films with (100)-orientation were obtained, which showed a dense texture with smooth surface. The ratio of Mn4+/Mn3+ in the films was increased with increasing Sr doping, leading to the enhancement in double-exchange interaction and electrical conductivity. As a result, the phase transition from metal to insulator was observed with the increasing of test temperature. For the La0.8Sr0.2MnO3 thin film, a large value of emittance (De = 0.28) was obtained, indicating good variable-emittance by appropriate Sr doping.

Pulsed laser deposition of La1−xSrxMnO3: thin-film properties and spintronic applications

Materials engineering on the nanoscale by precise control of growth parameters can lead to many unusual and fascinating physical properties. The development of pulsed laser deposition (PLD) 25 years ago has enabled atomistic control of thin films and interfaces and as such it has contributed significantly to advances in fundamental material science. One application area is the research field of spintronics, which requires optimized nanomaterials for the generation and transport of spin-polarized carriers. The mixed-valence manganite La1−xSrxMnO3 (LSMO) is an interesting material for spintronics due to its intrinsic magnetoresistance properties, electric-field tunable metal–insulator transitions, and half-metallic band structure. Studies on LSMO thin-film growth by PLD show that the deposition temperature, oxygen pressure, laser fluence, strain due to substrate–film lattice mismatch and post-deposition annealing conditions significantly influence the magnetic and electrical transport properties of LSMO. For spintronic structures, robust ferromagnetic exchange interactions and metallic conductivity are desirable properties. In this paper, we review the physics of LSMO thin films and the important role that PLD played in advancing the field of LSMO-based spintronics. Some specific application areas including magnetic tunnel junctions, multiferroic tunnel junctions and organic spintronic devices are highlighted, and the advantages, drawbacks and opportunities of PLD-grown LSMO for next-generation spintronic devices are discussed.

Role of doped Ru in coercivity-enhanced La0.6Sr0.4MnO3 thin film studied by x-ray magnetic circular dichroism

The coercivity of La1–xSrxMnO3 thin films can be enhanced by Ru substitution for Mn. In order to elucidate its mechanism, we performed soft x-ray absorption and magnetic circular dichroism measurements at the Ru M2,3 and Mn L2,3 edges. We found that the spin direction of Ru and Mn are opposite and that Ru has a finite orbital magnetic moment. Cluster-model analysis indicated that the finite orbital magnetic moment as well as the reduced spin moment of Ru result from local lattice distortion caused by epitaxial strain from the SrTiO3 substrate in the presence of spin-orbit interaction.

Relevancy between Thermochromic and Magnetic Property of La<sub>1-x</sub>Sr<sub>x</sub>MnO<sub>3</sub> (x=0.2 and 0.33) Smart Radiation Thin Film Materials Prepared by Magnetron Sputtering

La1-xSrxMnO3 thin films with x=0.2 and 0.33 were prepared by magnetron sputtering method for its potential application on thermal control of spacecraft. The materials show a phase transition from ferromagnetic metal phase with a low infrared emittance to paramagnetic insulator phase with a high emittance. Because of the thermochromic property, they can automatically change their infrared emittance greatly in response to environment temperature and thermal load and keep the spacecraft electronic components working normally. A superconduction quantum interference device magnetometer was used to study magnetization over the temperature range 100–315 K. Temperature dependence of total hemispherical emittance was carried out in a liquid nitrogen cooled vacuum chamber by a steady state calorimetric method. Thermal emittance results indicate an obvious tuneability of both La0.8Sr0.2MnO3 and La0.67Sr0.33MnO3 thin films. Compared to La0.8Sr0.2MnO3 thin film, La0.67Sr0.33MnO3 thin film has a higher transition temperature and bigger emittance tuneability. Based on phase separation concept, the thermal emittance was fitted by two-energy-level Boltzmann distribution of metal phase volume fraction fH. The measured magnetization curves were fitted in terms of mean-field approximation theory. The volume fraction of ferromagnetic phase fM was not coinciding with the fH very well. This result showed the fH and the fM plays a different role on thermal emittance property.

Large-distance rf- and dc-sputtering of epitaxial La1 − xSrxMnO3 thin films

Studies on large-distance sputtering as an effective alternative to molecular beam epitaxy, pulsed laser deposition or off-axis sputtering for the deposition of epitaxial La1−xSrxMnO3 (LSMO) thin films, are reported. The focus of this study is on the quality of the samples and their structural and magnetic properties. The dependence of the characteristics of the LSMO films on the sputtering mode (rf, dc) and the sputtering parameters, in particular on the oxygen partial pressure is established and discussed. It is shown that large-distance sputtering can provide high quality LSMO thin films without the need for post-annealing.

Computer Controlled Thermostat for the Resistivity measurements of the La1-xSrxMnO3 thin films

Sistema susideda is termostato celės kuri yra patalpinama tarp elektromagneto polių. Celė susideda is Peltier elemento kuris gali kaitinti arba saldyti bandymų objektą. Pratekantis vanduo palaiko pastovią Peltier elemento kitos pusės temperatūrą. Grįžtamam rysiui naudojamas platininis temperatūros jutiklis prijungtas prie universalaus matuoklio „Tektronix DMM 4050“. Peltier elementas maitinamas is universalaus laboratorinio reguliuojamo srovės saltinio „TTI QL 564P“. Abu prietaisai sujungti su kompiuteriu per GPIB sąsają ir per GPIB-USB sąsajų konverterį. Naudojant LabView programavimo aplinką buvo sukurta programa naudojanti PID algoritmą bei valdanti prijungtus prietaisus. Programa užtikrina pasirinktos temperatūros stabilizavimą. Matavimų rezultatai parodė, kad norima temperatūra pasiekiama per kelias minutes, priklausomai nuo temperatūrų skirtumo. Pasiekus norimą temperatūrą, ji yra palaikoma su 0.02 °C paklaida. Toks tikslumas yra pakankamas manganitų sluoksnų varžos matavimams magnetiniame lauke. Il. 8, bibl. 8 (anglų kalba; santraukos anglų ir lietuvių k.). DOI: http://dx.doi.org/10.5755/j01.eee.119.3.1363

Hard X-ray Photoemission Spectroscopy for Probing Bulk Electronic Structures of Solids

Recently, hard X-ray photoemission spectroscopy using high-brilliance third-generation synchrotron radiation at SPring-8 is realized. Specially designed apparatus including X-ray optics achieves both high energy-resolution and high efficiency comparable with the soft X-ray photoemission spectroscopy. Large probing depth of about 10 nm enables us to probe intrinsic bulk electronic structure free from surface condition and electronic structure of deeply-buried layers. The importance and capability of this technique are demonstrated by the drastic difference in the Mn 2p core-level photoemission spectra of La1-xSrxMnO3 thin films that is obtained by using soft and hard X-rays.

Thickness Dependence of Magnetic and Electronic Properties in La1-xSrxMnO3 Thin Films

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Pulsed laser deposition of atomically flat La1-xSrxMnO3 thin films using a novel target geometry

A new ablation target geometry is presented that was used to produce thin films of La1-xSrxMnO3 grown heteroepitaxially on SrTiO3 by pulsed reactive crossed-beam laser ablation. The films were grown in order to perform angle-resolved photoelectron spectroscopy, which demands that the surface be atomically flat. In situ and ex situ analysis shows that this condition was met, even after depositing to a thickness of over 100 nm.

Composition and Growth-Temperature Effect on the Microstructure of Epitaxial La1−x Srx MnO3 (x = 0, 0.2) Thin Films on (001) LaAlO3

La0.8Sr0.2MnO3 (LSMO) and LaMnO3 (LMO) thin films epitaxially grown on (001) LaAlO3 substrate at 700 and 800°C were studied using cross-section and plan-view transmission electron microscopy. In both LSMO and LMO films deposited at 700°C, a two-layered structure was observed: a continuous cubic perovskite layer epitaxially grown on the substrate followed by epitaxially grown orthorhombic column nanostructures. The orthogonal nano columns in LSMO were very well defined with a narrow size distribution. The LMO films exhibited a somewhat distorted orthogonal shape for the nanostructured columns and a wider size distribution. LSMO and LMO epitaxial films deposited at 800°C consisted of only a continuous single layer of cubic perovskite. These results reveal that formation of epitaxial column nanostructures in the La1-xSrxMnO3 thin films strongly depends on the growth temperature. Sr substitution on La sites in La1-xSrxMnO3 can improve uniformity of column size distribution and perfection of the orthogonal shape of nano columns.

Low-frequency noise in patterned La0.7Sr0.3MnO3 thin films

The low frequency noise in patterned La0.7Sr0.3MnO3 thin film bridges with different geometries was measured at 300 K in ambient magnetic field. Films were deposited by pulsed laser ablation on (100) SrTiO3 substrates. The measured 1/f noise was well described by the semiempirical Hooge relation, which indicates that the low frequency noise could be originated by resistance fluctuations. The measured noise magnitude was compared with bibliographic data for epitaxial La1−xSrxMnO3 (LSMO) and La1−xCaxMnO3 thin films. The normalized Hooge parameter α/n for our films was in the range of 2–4×10−28 m3, which is the order of magnitude of the best results reported in literature for oxygen annealed films. Finally the noise equivalent temperature of a thermometer fabricated with such LSMO films was estimated to be 3.4×10−6 K/Hz at 300 K, 10 Hz, and I=100 μA. These results demonstrate that the intrinsic low frequency noise of LSMO is not a limitation and that LSMO thin films can advantageously be used as performant thermometers for the fabrication of room temperature microbolometers.

YBa2Cu3O7-x and La1-xSrxMnO3 thin films grown by pulsed injection MOCVD

YBa 2 Cu 3 O 7-x (YBCO) and La 1-x Sr x MnO 3 (LSMO) layers were grown by single liquid source pulsed injection MOCVD technique in hot wall vertical MOCVD reactors. Y, Ba, Cu, Sr, La, Mn thd complexes (thd -2,2,6,6-tetramethylheptane-3,5-dionate) were used as precursor materials and monoglyme (1,2-dimethoxyethane) as a solvent. The influence of deposition temperature and thickness on the properties of films on LaAlO 3 and MgO substrates was studied. Thin YBCO layers deposited at 825 °C on LaAlO 3 substrates exhibited epitaxial quality, sharp superconducting transition (∼0.2 K) and a high critical current density (= 4MA/cm 2 at 77 K). Films on MgO substrates were of lower quality, Epitaxial LSMO films were deposited on various substrates, and the tensile or compressive strains and their influence on the film properties was obvous in thinner films on LaAlO 3 and SrTiO 3 substrates.

Enhanced Magnetoresistance of La1-xSrxMnO3 Thin Films by Reaction with Substrates.

Magnetic and magnetotransport properties of polycrystalline La1-xSrxMnO3 (LSMO) thin films annealed at high temperatures were investigated. The reaction of the LSMO thin films with YSZ substrates effectively induces enhancement of low-field MR up to 13.8 % at 78 K for the film annealed at 1350°C. The results are consistently explained by the formation of potential barriers at grain boundaries by the annealing.

Superconductivity and scanning tunneling microscopy (STM) / spectroscopy (STS) of transition metal oxide artificial lattices

High Tc superconducting artificial lattices and superlattices have been constructed by a laser molecular beam epitaxy. Layer-by-layer growth method controlled by RHEED and STM is a promising method for the construction of these artificial lattices. Formation and properties of superconducting artificial lattices and super-conducting / ferromagnetic superlattices are presented as the typical examples. In order to investigate the electronic state of carrier doped strongly correlated electronic system, STS measurements have been performed on La2−xSrxCuO4, La2−xSrxNiO4, and La1-xSrxMnO3 thin films. Based on the similarities and the differences between the STS spectra of the three types of the films, electronic states of the strongly correlated system are discussed.