Epitaxial La0 Research Articles

Self-assembled La0.67Sr0.33MnO3 epitaxial nanocolumn films towards future flexible spintronic devices

Flexible epitaxial magnetic oxide nanostructures with different electric and magnetic characteristics have attracted intensive explorations because of the potential applications in compact flexible devices. However, either expensive equipment or complicated processing is often indispensable in the traditional fabrication technology. Besides, there still lacks the report on high-quality metallic or half-metallic oxide nanocolumn arrays integrated on flexible substrates, which is one of the most important part in the future all-oxide nanostructure-based flexible spintronics. Herein, we demonstrate a spontaneous formation method to fabricate the epitaxial La0.67Sr0.33MnO3 nanocolumn films on flexible mica substrates with an SrTiO3 buffer layer. The morphology and density of the nanoscale columns can be easily tuned by changing the film thickness, which in turn influences the magnetic properties of the film. Under different levels of bending, the films exhibit morphology-dependent performance during the ferromagnetic resonance measurements. This study provides a viable route of fabricating tunable nanostructures for flexible spintronic devices with optimized performances.

Modification of magnetocrystalline anisotropy via ion-implantation

The ability to systematically modify the magnetic properties of epitaxial La0.7Sr0.3MnO3 thin films is demonstrated through the use of Ar+ ion implantation. With increasing implant dose, a uniaxial expansion of the c-axis of the unit cell leads to a transition from in-plane toward perpendicular magnetic anisotropy. Above a critical dose of 3 × 1013 Ar+/cm2, significant crystalline disorder exists leading to a decrease in the average Mn valence state and near complete suppression of magnetization. Combined with lithographic techniques, ion implantation enables the fabrication of magnetic spin textures consisting of adjacent regions with tunable magnetic anisotropy in complex oxide thin films.

Magnetic properties of [formula omitted][formula omitted][formula omitted] epitaxial thin films grown on [formula omitted](0 0 1)

Epitaxial La0.67Sr0.33MnO3 thin films play important roles in several applications due to their half-metallic conductivity and high spin polarization. Since these properties are very sensitive to minor structural changes arising from slightly different growing conditions it is important to understand their effect on the electronic and magnetic properties of the resulting samples. Here, we study two sets of La0.67Sr0.33MnO3 thin films, thicker (90 nm) and thinner (7 nm) ones, focusing on the influence of laser fluence and post-annealing oxygen partial pressure on their electronic structure and magnetism. Regardless of the preparation conditions, the thicker films present a tensile strain that leads to MnO6 octahedral deformation as detected by Raman active modes. The thinner films exhibit sizable in-plane magnetic anisotropy which depends on the sample preparation that leads to distinct surface morphology and electronic properties (Mn4+/Mn3+ ratio).

Effects of Oxygen Modification on the Structural and Magnetic Properties of Highly Epitaxial La0.7Sr0.3MnO3 (LSMO) thin films

La0.7Sr0.3MnO3, a strong semi-metallic ferromagnet having robust spin polarization and magnetic transition temperature (TC) well above 300 K, has attracted significant attention as a possible candidate for a wide range of memory, spintronic, and multifunctional devices. Since varying the oxygen partial pressure during growth is likely to change the structural and other physical functionalities of La0.7Sr0.3MnO3 (LSMO) films, here we report detailed investigations on structure, along with magnetic behavior of LSMO films with same thickness (~30 nm) but synthesized at various oxygen partial pressures: 10, 30, 50, 100, 150, 200 and 250 mTorr. The observation of only (00 l) reflections without any secondary peaks in the XRD patterns confirms the high-quality synthesis of the above-mentioned films. Surface morphology of the films reveals that these films are very smooth with low roughness, the thin films synthesized at 150 mTorr having the lowest average roughness. The increasing of magnetic TC and sharpness of the magnetic phase transitions with increasing oxygen growth pressure suggests that by decreasing the oxygen growth pressure leads to oxygen deficiencies in grown films which induce oxygen inhomogeneity. Thin films grown at 150 mTorr exhibits the highest magnetization with TC = 340 K as these thin films possess the lowest roughness and might exhibit lowest oxygen vacancies and defects. Interpretation and significance of these results in the 30 nm LSMO thin films prepared at different oxygen growth pressures are also presented, along with the existence and growth pressure dependence of negative remanent magnetization (NRM) of the above-mentioned thin films.

Exchange bias effect in epitaxial La0.35Sr0.65MnO3/La0.7Sr0.3MnO3 bilayers: Impact of antiferromagnet growth conditions

Antiferromagnets have shown great potential for replacing ferromagnets in spintronics applications in recent years. In this work, antiferromagnetic La0.35Sr0.65MnO3 (AFM-LSMO) thin films were grown on ultrathin ferromagnetic La0.7Sr0.3MnO3 (FM-LSMO) layer over a wide temperature range. The different AFM-LSMO growth temperatures introduced variation of strain states in the AFM-LSMO layer, changing the AFM-LSMO magnetic properties. Considering the thermally activated switching model of antiferromagnetic grains, the behaviour of AFM-LSMO growth temperature-dependent exchange bias effect are explained. This work demonstrates the modulation of exchange bias effect through the control of AFM-LSMO growth conditions, suggesting possibility to control and probe oxide antiferromagnets via exchange bias coupling.

Polarization Screening Mechanisms at La0.7Sr0.3MnO3-PbTiO3 Interfaces.

The structural, electronic, and magnetic properties of interfaces between epitaxial La0.7Sr0.3MnO3 and PbTiO3 have been explored via atomic resolution transmission electron microscopy of a functional multiferroic tunnel junction. Measurements of the polar displacements and octahedral tilting show the competition between the two distortions at the interface and demonstrate strong dependence on the polarization orientation. The density functional theory provides information on the electronic and magnetic properties, where the interface termination plays a crucial role in the screening mechanisms.

Vertically aligned nanostructure control and tunable low-field magnetoresistance in La0.5Ca0.5MnO3 single-phase thin films manipulated by a high magnetic field

Vertically aligned nanostructured (VAN) epitaxial La0.5Ca0.5MnO3 (LCMO) single-phase thin films have been achieved on (LaAlO3)0.3(Sr2AlTaO6)0.7 (001) [LSAT (001)] substrates under high magnetic fields applied in pulsed laser deposition processing. Low-field magnetoresistance (LFMR) in the LCMO VAN films can be effectively manipulated through varying the high magnetic field strength. The tunability of VAN on the electrical transport properties is dependent on control of the high magnetic field on the microstructures, including the geometrical arrangement, vertical interfaces, and vertical grain boundaries (GBs). An LFMR value as high as 45% at 150 K and 1 T has been achieved in an LCMO VAN film grown at 10 T, and its LFMR values are larger than 25% at 127–200 K and 1 T. The tunable and enhanced LFMR in the LCMO VAN films over a wide temperature range can be attributed to the increase in vertical interfaces and GB density with the increasing high magnetic field, which are highly related to the spin-polarized tunneling effect. Applying a high magnetic field in film deposition to control the microstructures of VAN single-phase films is a feasible route to achieve tunable and desirable physical properties.

Strain-induced exchange bias transition at La0.7Sr0.3MnO3/NiO interface

The exchange bias effect with anisotropy perpendicular to the device plane is promising for exploring the next generation of spintronics devices. However, the effective modulating of exchange bias remains challenging because the understanding of the mechanism on tuning the exchange bias anisotropy is lacking and worth investigation. In this study, epitaxial La0.7Sr0.3MnO3/NiO (LSMO/NiO) bilayers were deposited on the (001)-orientated SrTiO3 and MgO substrates by pulse laser deposition to further explore the mechanism behind the anisotropy of EB effect at the LSMO/NiO interface. The results demonstrate the exchange bias transition from in-plane to out-of-plane by changing the substrate from SrTiO3 to MgO. According to the strain analyses, the substrate strain-induced spin reorientation transition of NiO plays an important role on the exchange coupling at the LSMO/NiO interface, leading to the EB effect transition. The significance of the interaction between strain and spin at the heterointerface is revealed, which provides a possible way for the regulation of interfacial magnetism.

Spin current and magnetic measurements in heterostructure SrIrO3/La0.7Sr0.3MnO3

The dc voltage on the film SrIrO3 (SIO) generated under ferromagnetic resonance condition has been studied in the heterostructure based on manganite thin epitaxial films La0.7Sr0.3MnO3 (LSMO). The SIO film is a paramagnetic metal with strong spin-orbit interaction in the temperature range 20-300K. The effect is shown to be caused by two different phenomena: (1) the resonance dc electromotive force related to anisotropic magnetoresistance (AMR) in the manganite film and (2) pure spin current (spin pumping) registered by means of the inverse spin Hall effect in upper layer. We also give the results of the magnetic parameters measuring for the heterostructure SIO/LSMO using ferromagnetic resonance in a wide temperature range.

Ferromagnetic Resonance of Single-Crystalline La0.67Sr0.33MnO3 Thin Film Integrated on Silicon

Integration of high-quality multifunctional oxide thin films in silicon technology promises a wide range of potential applications in low loss spintronic devices such as sensors, detectors, data storage media, and so on. However, the heteroepitaxial growth of functional complex oxides on silicon substrates has been proved challenging, which limits the development of semiconductor-based spintronic devices. In this work, epitaxial single-crystalline La0.67Sr0.33MnO3 (LSMO) thin films have been integrated on silicon substrates by epitaxy and transfer carried out at room temperature. The microwave magnetisms of the LSMO thin films transferred on silicon substrates have been investigated under multiple directions of magnetic field by ferromagnetic resonance. The transferred LSMO thin films on silicon substrates preserve the microwave magnetic characteristics of the primary as-grown LSMO thin films. Our results demonstrate that the epitaxy and transfer method has enormous potential in future spintronic applications of functional oxide devices compatible with semiconductor technology without thermal damage.

Defects induced huge magnetoresistance in epitaxial La1–xSrxMnO3 thin films deposited by magnetic sputtering

In this work, epitaxial La1–xSrxMnO3 (LSMO) films were fabricated on SrTiO3 substrates at temperatures (Ts) ranging from 550 to 750 °C by RF magnetron sputtering. Significant Ts-dependent structural, magnetic, and magnetotransport properties were observed. The LSMO (Ts = 750 °C) film exhibits the colossal magnetoresistance (CMR) of −47% under the magnetic field (H) of 5 T. In contrast, the LSMO (Ts = 650 °C) film demonstrates a huge magnetoresistance (MR) of −98% (H = 5 T) around the metal-insulator transition temperature and –59% at 5 K. The spin-glass-like behaviors indicate that the defects, particularly the oxygen vacancies, in the epitaxial LSMO (Ts = 650 °C) films destroy the double exchange. The huge MR is related to the defect modulated magnetic structures and spin-dependent magnetotransport properties. Our work helps to understand the physical mechanism of the CMR and provides a way for tuning the magnetotransport properties of the perovskite films.

Magnetic Interactions on Oxide Ferromagnet/Ferromagnetic Intermetallide Interface

The magnetic properties of heterostructures consisting of two films are studied. The upper layer involves rare-earth intermetallic nanostructured superlattices consisting of exchange-coupled layers (TbCo2/FeCo)n (TCFC), and the lower layer includes either epitaxial manganite La0.7Sr0.3MnO3 (LSMO) with optimum strontium doping or an epitaxial film of an yttrium–iron garnet Y3Fe5O12 (YIG) with a Bi additive. TCFC is a ferromagnet having high Curie temperature and provides controllable induced magnetic anisotropy. Experimental studies showed that the interlayer interaction of the TCFC/LSMO heterostructure is antiferromagnetic. There was an increase in the FMR line width in the structures due to the flow of a spin current through the interface between two films. There was electric voltage in the TCFC/YIG heterostructure induced in the TCFC intermetallide film, due to an inverse spin Hall effect under ferromagnetic resonance conditions.

Isotropic magnetoresistance and enhancement of ferromagnetism through repetitious bending moments in flexible perovskite manganite thin film

For achieving more functionalities of perovskite manganese oxides film, an epitaxial La0.8Sr0.2MnO3 (LSMO) film was deposited on a flexible substrate (mica) by pulsed laser deposition. The film shows a paramagnetic-ferromagnetic (PM-FM) phase transition at 134 K together with metal-insulator transition (MIT) at 149 K. The measurement of hysteresis loop indicates that the easy-magnetization axis lied in film plane. The transport behavior can be fitted well with the small polaron model, giving out the activation energy Ehop = 176.9 meV. Different from the common anisotropic magnetoresistance (AMR) in manganite film, an isotropic magnetoresistance (IMR) effect was observed in LSMO film and the relatively small thickness of LSMO film (∼50 nm) is a major reason for it. After the cyclic flexural bending for 1.0×104 times, we found the magnetic properties of LSMO film were improved. Micromagnetic detection shows that the activation energy for polaron hopping (Ea = 144.7 meV) remarkably decreases by the cyclic flexural bending process. We suggested that the inhomogeneous/incompact part located at the zone of neighbouring domains become uniform and tight after circulating bending, which facilitates the rotation of domains and the polarons hopping so that the magnetic and electronic performance are enhanced.

Electron-electron interactions in nano-patterned La0.3Sr0.7MnO3 thin films

Understanding the transport in ultrathin epitaxial La0.3Sr0.7MnO3 (LSMO) is a topic widespread current interest. Here, the authors explore electron-electron interactions in low temperature magneto-transport in straight and zigzag nanowires fabricated from ultrathin epitaxial LSMO films grown to different thicknesses on SrTiO3 (100) substrates. They find that three-dimensional electron-electron interactions can explain the resistivity upturn, including many changes observed with film thickness, nano-patterning, and magnetic field.

Observing a previously hidden structural-phase transition onset through heteroepitaxial cap response

Characterization of the onset of a phase transition is often challenging due to the fluctuations of the correlation length scales of the order parameters. This is especially true for second-order structural-phase transition due to minute changes involved in the relevant lattice constants. A classic example is the cubic-to-tetragonal second-order phase transition in SrTiO3 (STO), which is so subtle that it is still unresolved. Here, we demonstrate an approach to resolve this issue by epitaxially grown rhombohedral La0.7Sr0.3MnO3 (LSMO) thin films on the cubic STO (100) substrate. The shear strain induced nanotwinning waves in the LSMO film are extremely sensitive to the cubic-to-tetragonal structural-phase transitions of the STO substrate. Upon cooling from room temperature, the development of the nanotwinning waves is spatially inhomogeneous. Untwinned, atomically flat domains, ranging in size from 100 to 300 nm, start to appear randomly in the twinned phase between 265 and 175 K. At ∼139 K, the untwinned, atomically flat domains start to grow rapidly into micrometer scale and finally become dominant at ∼108 K. These results indicate that the low-temperature tetragonal precursor phase of STO has already nucleated at 265 K, significantly higher than the critical temperature of STO (∼105 K). Our work paves a pathway to visualize the onset stages of structural-phase transitions that are too subtle to be observed using direct-imaging methods.

Strain induced extrinsic magnetocaloric effects in La0.67Sr0.33MnO3 thin films, controlled by magnetic field

The magnetic control of strain-induced giant caloric properties of epitaxial La0.67Sr0.33MnO3 (LSMO) thin films grown on (0 0 1) BaTiO3 (BTO) single crystal substrates is investigated. Arising from the crystallographic structural transformation of the BTO substrate during its 3-phase transitions, there is a drastic change of 3D film-lattice strain, which affects the local magnetic anisotropy of the LSMO at the interface and changes the entropy of the film. The thermal hysteresis observed in temperature dependent resistivity measurement can be manipulated by applying a magnetic field. Detailed transport and magnetic studies in different crystal orientations reveal the origin of such field-dependent magnetocaloric properties induced by the different phase transitions of the BTO substrate. Finally, a large entropy change of 1.03–1.95 J · Kg−1 · K−1 · T−1 was obtained at the monoclinic–tetragonal (M–T) transition at 283 K which compares to <1 J · Kg−1 · K−1 · T−1 for previously reported films.

Resistivity of Manganite Thin Film Under Strain

A complex study of the electron transport and magnetic characteristics of epitaxial manganite films La0.7Ba0.3MnO3 (LBMO) was carried out under conditions of the crystal structure tension caused by a mismatch between the parameters of the LBMO crystal and the substrate. The epitaxial thin films with the thickness 40–100 nm were grown by pulsed laser deposition at T = 700–800 °C in pure oxygen pressure 0.3–1 mbar. The substrates (110) NGO, (001) STO, (001) LAO, and (001) LSAT were used. By comparison of the lattice parameter of LBMO targets with substrate’s one, the lattice mismatches were derived. We used substrates in which the lattice parameter was less than for the LBMO crystal one. It is shown that the temperature dependence of the film resistance in the low-temperature region does not depend on the film stress and is in good agreement with the calculation that takes into account the interaction of carriers with magnetic excitations in the presence of strongly correlated electron states. A nonmonotonic temperature dependence of the resistance of an LBMO film deposited on ferroelectric crystals PMN-PT that was observed. This feature is typical for manganites, and indicating the presence of ferromagnetism in the system was observed.

The significant and temperature-insensitive magnetoresistance observed in Co-doped (La0.7Sr0.3)MnO3 thin films

Magnetic compounds with significant and temperature-insensitive magnetoresistance are quite promising for device applications. Here, we intentionally doped different amounts (0.025-0.1) of Co into epitaxial (La0.7Sr0.3)MnO3 thin films. The evolution of electrical, magnetic, magneto-transport, and valence state of this series of Co-doped (La0.7Sr0.3)MnO3 have been systematically characterized. Remarkably, the magnetoresistance in 0.1Co-doped (La0.7Sr0.3)MnO3 can reach significantly 35% within an extremely large temperature window (from 40 to 200 K). Other Co-doped epitaxial (La0.7Sr0.3)MnO3 thin films show similar sharp MR-peaks around Curie temperature as that in (La0.7Sr0.3)MnO3. The evolution of magnetic and magneto-transport properties of Co-(La0.7Sr0.3)MnO3 films can be explained by antiferromagnetic coupling induced by paramagnetic/low-spin Co3+(Co2+) ions in (La0.7Sr0.3)MnO3. Our work demonstrates that doping paramagnetic/low-spin Co into manganites is an effective way to optimize their magneto-transport properties.

Reversible Control of Physical Properties via an Oxygen-Vacancy-Driven Topotactic Transition in Epitaxial La0.7 Sr0.3 MnO3- δ Thin Films.

The vacancy distribution of oxygen and its dynamics directly affect the functional response of complex oxides and their potential applications. Dynamic control of the oxygen composition may provide the possibility to deterministically tune the physical properties and establish a comprehensive understanding of the structure-property relationship in such systems. Here, an oxygen-vacancy-induced topotactic transition from perovskite to brownmillerite and vice versa in epitaxial La0.7 Sr0.3 MnO3- δ thin films is identified by real-time X-ray diffraction. A novel intermediate phase with a noncentered crystal structure is observed for the first time during the topotactic phase conversion which indicates a distinctive transition route. Polarized neutron reflectometry confirms an oxygen-deficient interfacial layer with drastically reduced nuclear scattering length density, further enabling a quantitative determination of the oxygen stoichiometry (La0.7 Sr0.3 MnO2.65 ) for the intermediate state. Associated physical properties of distinct topotactic phases (i.e., ferromagnetic metal and antiferromagnetic insulator) can be reversibly switched by an oxygen desorption/absorption cycling process. Importantly, a significant lowering of necessary conditions (temperatures below 100 °C and conversion time less than 30 min) for the oxygen reloading process is found. These results demonstrate the potential applications of defect engineering in the design of perovskite-based functional materials.

Speciation and Electronic Structure of La1−xSrxCoO3−δ During Oxygen Electrolysis

Cobalt-containing perovskite oxides are promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolyzers. However, a lack of fundamental understanding of oxide surfaces impedes rational catalyst design for improved activity and stability. We couple electrochemical studies of epitaxial La1−xSrxCoO3−δ films with in situ and operando ambient pressure X-ray photoelectron spectroscopy to investigate the surface stoichiometry, adsorbates, and electronic structure. In situ investigations spanning electrode compositions in a humid environment indicate that hydroxyl and carbonate affinity increase with Sr content, leading to an increase in binding energy of metal core levels and the valence band edge from the formation of a surface dipole. The maximum in hydroxylation at 40% Sr is commensurate with the highest OER activity, where activity scales with greater hole carrier concentration and mobility. Operando measurements of the 20% Sr-doped oxide in alkaline electrolyte indicate that the surface stoichiometry remains constant during OER, supporting the idea that the oxide electrocatalyst is stable and behaves as a metal, with the voltage drop confined to the electrolyte. Furthermore, hydroxyl and carbonate species are present on the electrode surface even under oxidizing conditions, and may impact the availability of active sites or the binding strength of adsorbed intermediates via adsorbate–adsorbate interactions. For covalent oxides with facile charge transfer kinetics, the accumulation of hydroxyl species with oxidative potentials suggests the rate of reaction could be limited by proton transfer kinetics. This operando insight will help guide modeling of self-consistent oxide electrocatalysts, and highlights the potential importance of carbonates in oxygen electrocatalysis.