LaMnO3 Films Research Articles

Strain-dependent ultrafast carrier dynamics and spin–lattice interaction in LaMnO3 films

We investigate the ultrafast carrier dynamics and spin–lattice interaction in strained and unstrained LaMnO3 films via temperature-dependent femtosecond transient optical spectroscopy. The transient reflectivity measurements show two characteristic relaxation processes in both types of films, which are attributed to electron–phonon coupling and phonon-assisted spin–lattice interaction, respectively. The carrier dynamics and coupling between lattice and spin system are well described with the three-temperature model; the spin–lattice relaxation time constant is dominated by the temperature-dependent spin specific heat. Both the electron–phonon coupling and the spin–lattice interaction are enhanced in the strained film, as a result of the modified band structure and orbital ordering under biaxial compressive strain. Our results reveal the critical role of strain in the photo-induced dynamical interactions in LaMnO3.

Role of interdiffusion on the magnetism of ultrathin LaMnO3 films

We present a detailed study of the magnetic, spectroscopic and structural properties of ultrathin LaMnO3 films deposited on SrTiO3(001) substrates by oxide molecular beam epitaxy. We find that the as-grown LaMnO3 films are slightly reduced and present a significant magnetic moment, while annealing to 600 °C fully oxidizes and enhances its magnetic properties. From synchrotron x-ray photoemission spectromicroscopy, we find the presence of Sr, Ca and Si in the LaMnO3 film that diffuse from the SrTiO3 substrate; these impurities act as hole dopants, and can explain the presence of ferromagnetism in LaMnO3. This work highlights the importance of cation mobility at the elevated growth temperature in modifying the magnetic properties of ultrathin LaMnO3 films.

Epitaxial strain manipulation of the cluster glass state in LaMnO3 films.

As a new-type magnetic state, the cluster glass state in manganite is arousing considerable attention due to its important theoretical value and extensive application prospects in condensed matter physics and spintronics. Due to the complex magnetic interactions, the cluster glass state is difficult to form and regulate in single films. Studies report a new phenomenon that epitaxial strain can regulate the formation of the cluster glass state in LaMnO3 (LMO) films. Comparing LMO thin films with different thicknesses grown on a (001)-oriented LaAlO3 (LAO) single crystal substrate, we found that the 20-nm-thick LMO film is more likely to form the cluster glass state than the 60-nm-thick and 120-nm-thick films. This can be attributed to the uneven distribution of strain and Mn ions in the depth profile. Our work demonstrates that thickness is an important method for regulating the formation of the cluster glass state in LMO films.

Using a perovskite oxide buffer layer on Ca2Nb3O10 nanosheets for the epitaxial growth of Pb(Zr0.52Ti0.48)O3 for electrode-free thin films

Ca2Nb3O10 nanosheet layers were deposited on Gallium Nitride (GaN)-on-Si substrates by the Langmuir-Blodgett method to obtain (001)-oriented growth of perovskite oxide thin films using pulsed laser deposition. We investigated the influence of 10 nm thick SrTiO3, (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) and LaMnO3 films as buffer layers on the structural and ferroelectric properties of subsequentially grown Pb(Zr0.52Ti0.48)O3 (PZT) films, which may be used as gate capacitors in high power GaN devices. Scanning Electron Microscopy, Atomic Force Microscopy, X-Ray-Diffraction and in-plane polarization experiments were used to determine the influence of the oxide buffer layers on the PZT properties. Highly crystalline dense (001)-oriented PZT films with large in-plane polarizations were obtained using SrTiO3 and LSAT as buffer layers. For SrTiO3 and LSAT we observed an improvement of the structural and ferroelectric properties compared to PZT grown directly on the Ca2Nb3O10 nanosheets and or using a LaMnO3 buffer layer. The improved high-crystallinity PZT films grown on GaN using ultrathin buffer layers could enhance the performance of high-power semiconductor electronics or in other devices where there is a need for a ferroelectric layer without bottom electrode.

Synthesis of LaXO3 (X = Fe, Mn, Cr, Ni) Thin Films Using a Simple Spin Coating Set-Up for Resistive Switching Memory Devices

This study focuses on the preparation of LaXO3(X=Fe,Mn,Cr,Ni) perovskite thin films using a simple set-up spin coating technique and the evaluation of their properties for application in switching memory devices. The properties of as-deposited films were thoroughly characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV-Vis spectroscopy, and a vibrating sample magnetometer (VSM). The results obtained revealed that the as-deposited films have a polycrystalline cubic structure. The film surfaces were uniform and densely packed without any voids, cracks, or pinholes. In addition, irregularly shaped grains were observed having an average size of 140, 120, 89, and 70 nm for LaFeO3, LaMnO3, LaCrO3, and LaNiO3 films, respectively. VSM analysis demonstrated that LaFeO3 film exhibited superior magnetic properties compared to the other films. Furthermore, memory devices with Au/LaXO3(X=Fe,Mn,Cr,Ni)/FTO structures were fabricated, and their I-V characteristics were measured. In order to assess their performance, an endurance test was conducted. The findings indicated that Au/LaFeO3/FTO device exhibited higher RHRSRLRS ratio (~7×105), low Set/Reset voltages, lower power consumption (2.7×10−4 W), and stable endurance with no significant degradation was observed in the LRS and HRS after 20 sweep cycles. These favorable parameters can be attributed to the reduced thickness, larger grain size, and excellent magnetic properties of the active-layer LaFeO3. Moreover, the conduction mechanism of the fabricated devices was investigated, revealing that the conduction in the LRS is primarily dominated by Ohmic behavior, while the HRS exhibited different conduction mechanisms.

Tuning charge transfer and magnetic properties in LaCoO3/LaMnO3/LaCoO3 trilayers by oxygen content

In this manuscript, perovskite oxide LaCoO3/LaMnO3/LaCoO3 (LLL) trilayers and the single-layer LaMnO3 (LMO) and LaCoO3 (LCO) films are epitaxially grown on (001)-oriented SrTiO3 substrates by polymer-assisted deposition combining with heat treating in Ar, air, and O2 atmospheres, respectively. The crystal structure and magnetic properties are investigated. It is found that the responses of LMO and LCO films to oxygen content are different, and the magnetic property of LMO is more sensitivity to oxygen content than that of LCO. The oxygen content can directly affect the structure and magnetic properties of LMO and LCO layers, which, in turn, modulate the interfacial magnetic properties of the LLL trilayers through interlayer coupling. By comparison with the magnetic properties of the single-layer films, an abnormal ferromagnetic (FM) transition is found in the LLL trilayers at around 216 K, besides two known FM transitions corresponding to those in the LCO and LMO layers, respectively. It is suggested that the abnormal FM transition is attributed to Mn4+–O–Co2+ FM super-exchange interaction, which is induced by the charge transfer from Mn3+ to Co3+ ions and the interlayer coupling between the LMO and LCO layers. The interlayer coupling of the trilayers, including the interfacial coupling and the diffusion-induced inlayer coupling, dominates the interface properties, which are tuned by oxygen content-dependent charge transfer. The cooperative function of the orbital/charge structure and the interlayer coupling dominates the magnetic properties of the LLL trilayer under the tensile strain. These findings provide new avenues to regulate the electronic and magnetic states of perovskite oxide multilayers and another way to the development of multifunctional electronic devices.

Microstructure-related enhancement of electrical properties in (La3BxMnO3) (LaB6)-based composite films

A series of (La1-xBxMnO3) (LaB6)-based composite films were prepared by a combined magnetron sputtering and ion implantation method, and their structure-function relationship have been investigated. After B3+ ion implantation, the as-sputtered LaMnO3 films still have perovskite structure and good surface flatness. However, the substitution and doping of B ions play the role of resistance regulation, and the formation of the second phase LaB6 reduces the resistivity of the film after ion implantation to a certain extent. The carrier concentration of the ion injected film will decrease from 1.33 × 1016 cm−3 to 1.80 × 1015 cm−3, while the resistivity value will a significant increase from 46 Ω cm to 381 Ω cm with slight variations in principle in thermal constant from 2229 to 2783 K. The secondary phase doping of (La1-xBxMnO3) (LaB6)-based perovskite materials bring an innovative perspective to the design and application of electronic materials.

Magnetic Proximity Effect in LaMnO3-LaFeO3 Bilayer

Here, we report the magnetic properties of LaMnO3, LaFeO3 thin films and their heterostructures, fabricated using Pulse laser deposition technique (PLD) on SrTiO3(001) substrate. The crystal structure, surface morphology signifies the phase pure epitaxial growth of LaMnO3 and LaFeO3 films and their heterostructure. The magnetic characteristics of individual LaMnO3 and LaFeO3 films along with their heterostructures were studied to explore the proximity effect of the different antiferromagnets. We demonstrate that LaMnO3 thin film shows ferromagnetic signal with inconspicuous Tc. The ferromagnetic signal is stronger in case of LaMnO3 grown on buffered LaFeO3 and LaFeO3 on top of LaMnO3. The change of Tc is analyzed in heterostructures as compared to the bare LaMnO3. Additionally, our study illustrates the variation of anisotropic energy and spin flop coupling between the interface with alternate growth sequence of heterojunctions.

Enhanced stability and low operational voltage of resistive switching behavior in defect engineered LaMnO3 film

Enhanced stability and low operational voltage of resistive switching behavior in defect engineered LaMnO3 film

Improving the sensitivity of LaMnO3 NTC films by Al ion implantation

The Al ion implantation was employed to modify the NTC characteristics of LaMnO3 thin films sputtered on Si/SiO2 substrates under different annealing temperatures. The crystal structure, morphology and ion distribution of the as-sputtered LaMnO3 thin films and as-annealed Al-implanted films were carefully evaluated. Results indicated that (LaMnAlxO3)y(Al2O3)1-y composite phase existed over the annealing temperature range after Al3+ implanting into the LaMnO3 film of perovskite phase. The film was nucleated throughout and the size of the grain decreased with the increase in annealing temperature. The Al3+ implantation promoted the reduction of oxygen vacancy concentration while leading to the reduction in the equilibrium oxygen vacancy. As a result, the Mn3+/Mn4+ ratio declined and the number of carriers reduced, which helped to increase the carrier migration barrier and activation energy. The direct manifestation of the latter is the increase of the thermal constant B value from 3192 K to 2111 K based on the premise of resistivity is closed. The successful modification of a traditional perovskite-type NTC material will bring an innovative perspective to the design of advanced functional film materials and provide ideas for further exploration of the material in high B and low resistivity temperature sensor applications.

Epitaxial growth of LaMnO3 thin films on different single crystal substrates by polymer assisted deposition

Structure of epitaxial LaMnO3 thin films grown on different single crystal substrates by unconventional polymer assisted deposition (PAD) method was investigated. Epitaxial films were prepared from lanthanum manganite water based solutions deposited by spin coating on single crystal MgO (001), SrTiO3 (001) and SrTiO3 (110) substrates, and the influence of substrate type on the film structure was analysed. Better uniformity of the epitaxial LaMnO3 films on SrTiO3 substrates was obtained, but a non-stoichiometric La1-xMnO3 phase was formed after the heat treatment at 750 °C. In addition, the prepared thin films were multiple annealed at different temperatures up to 900 °C, in order to investigate importance of post-annealing treatment. Epitaxial nature of the prepared films was preserved after annealing at up to 900 °C and the structure rearrangement through formation of cell closer to bulk stochiometric LaMnO3 phase was observed.

Investigation into support effects for Pt and Pd on LaMnO3

The catalytic properties of Pt and Pd supported on thin-film LaMnO3 were investigated. The LaMnO3 film consisted of an ALD-grown 0.5-nm-thick film supported on a high-surface-area MgAl2O4. Pt and Pd were added to the LaMnO3 films by Atomic Layer Deposition (ALD) and then aged by repeated oxidation and reduction cycles at 1073 K. X-Ray Diffraction (XRD), together with Scanning Transmission Electron Microscopy (STEM)/Energy Dispersive Spectroscopy (EDS) showed that Pt underwent extensive sintering during these redox treatments and formed a Pt-Mn alloy following reduction at 1073 K. Consistent with this, the activity of the supported Pt catalysts for CO oxidation decreased progressively with the number of redox cycles. In contrast to the results for Pt, STEM/EDS results indicated that Pd remained well dispersed on LaMnO3, even after five redox cycles. The Pd catalyst showed high activity for CO oxidation at lower temperatures due to a low activation energy and positive reaction order in CO. XRD results with Rietveld analysis for the Pd catalyst indicated the presence of very large crystallites for PdO on the oxidized catalyst and metallic Pd on the reduced catalyst, inconsistent with the high dispersions measured by STEM and the high CO-oxidation rates. Possible explanations for these observations are discussed.

Engineering Magnetic Anisotropy and Emergent Multidirectional Soft Ferromagnetism in Ultrathin Freestanding LaMnO3 Films

The combination of small coercive fields and weak magnetic anisotropy makes soft ferromagnetic films extremely useful for nanoscale devices that need to easily switch spin directions. However, soft ferromagnets are relatively rare, particularly in ultrathin films with thicknesses of a few nanometers or less. We have synthesized large-area, high-quality, ultrathin freestanding LaMnO3 films on Si and found unexpected soft ferromagnetism along both the in-plane and out-of-plane directions when the film thickness was reduced to 4 nm. We argue that the vanishing magnetic anisotropy between the two directions is a consequence of two coexisting magnetic easy axes in different atomic layers of the LaMnO3 film. Spectroscopy measurements reveal a change in Mn valence from 3+ in the film interior to approximately 2+ at the surfaces where considerable hydrogen infiltration occurs due to the water dissolving process. First-principles calculations show that protonation of LaMnO3 decreases the Mn valence and switches the magnetic easy axis from in-plane to out-of-plane as the Mn valence approaches 2+ from its 3+ bulk value. Our work demonstrates that ultrathin freestanding films can exhibit functional properties that are absent in homogeneous materials, concomitant with their convenient compatibility with Si-based devices.

Evidence for redispersion of Ni on LaMnO3 films following high-temperature oxidation

LaMnO3 films, 0.5-nm thick, were deposited by atomic layer deposition (ALD) onto γ-Al2O3 that had been modified with 15- wt% CaO. The CaO was shown to be effective in preventing formation of LaAlO3 that formed when La2O3 was deposited directly onto γ-Al2O3. Lattice fringes on the resulting CaAl2O4/γ-Al2O3 substrate were weakly resolved, allowing a detailed characterization of the LaMnO3 films. High-resolution transmission electron microscopy (HR-TEM) images showed that the LaMnO3 formed two-dimensional crystallites, ∼10–15 nm wide, that covered most of the surface. Crystallites with (001) and (111) orientation were clearly identified. High-temperature oxidation caused Ni to spread over the LaMnO3 film, suggesting there is a reaction of the Ni2+ cations with the perovskite lattice. Ni formed by high-temperature reduction on these films remained well dispersed and significantly more active for CO2 reforming of CH4 compared to Ni on MgAl2O4, even after repeated oxidation and reduction cycles at 1073 K. The implications of these results for understanding metal-support interactions between Ni and LaMnO3 are discussed.

Raman spectroscopy of the Jahn–Teller phonons in a magnetic LaMnO3 thin film grown on KTaO3

Bulk LaMnO3 is an A-type antiferromagnetic material in which the Jahn–Teller (JT) planes each have a ferromagnetic arrangement, but are stacked in antiparallel. Here, we report the growth of epitaxial LaMnO3 films on (001)-oriented cubic KTaO3 substrates in such a way that the JT plane is parallel to the nonmagnetic substrate. We verify the presence of the JT distortion and confirm the orientation of the plane by symmetry analysis with angle-resolved polarized Raman spectroscopy. The temperature evolution of the Raman spectra indicates that the B1g JT phonon softens below the magnetic transition temperature of ∼145 K, where ferro- or ferri-magnetism with magnetic easy axis along the out-of-plane direction is exhibited.

Modifying Jahn–Teller distortion by epitaxial stress in LaMnO3 films for tunning electron localization

The effect of epitaxial stress on Jahn–Teller (JT) distortion in epitaxial LaMnO3 (LMO) films has been investigated. Both 2θ–ω scans and reciprocal space maps (RSMs) indicate that LMO samples are subjected to compressive stress. Obvious Laue oscillations can be detected in 2θ–ω scans, indicating the high quality of samples. RSMs of symmetry peak (001) and asymmetry peak (−103) imply different epitaxial stress for LMO films deposited on different substrates. Raman spectra measurements reveal that the degree of JT distortion can be well tuned via the epitaxial stress which may further influence on the electron localization in the films. This study might benefit to understanding the correlation between crystalline structure and electrical transport properties of LMO films and related LMO-based superlattices.

Ferromagnetic Enhancement in LaMnO3 Films with Release and Flexure (Adv. Mater. Interfaces 24/2021)

Ferromagnetic Enhancement in LaMnO3 Films with Release and Flexure In article number 2101499, Hongbao Yao, Kuijuan Jin, and co-workers report the achievement of doubly-enhanced magnetization and raised Curie temperature in the freestanding manganite thin films by releasing the misfit strain and introducing the flexure, demonstrating that flexible mechanical modulations provide an extraordinary opportunity to design and manipulate correlated electronic states of functional oxides.

Photocatalytic reduction of CO2 over LaMO3 (M: Fe, Co, Mn) /CuxO films

LaMO3/CuxO (M: Co, Mn, Fe) thin films were prepared in two steps. First, lanthanum perovskites were deposited by spin coating, and then a CuxO layer by Chemical Vapor Deposition. The X-ray diffraction characterization confirmed the formation of the crystalline perovskite phases. The topography of the films was observed by AFM analysis, where fine needles grains appeared in LaMnO3/CuxO and LaCoO3/CuxO perovskite films, being attributed to the CuxO layer. XPS characterization confirms the presence of a larger concentration of oxygenated species over the surface of the films; besides, it was possible to elucidate a mixed-valence state of copper nanoparticles. DR UV–vis analyses show a redshift displacement for LaCoO3 and LaMnO3 films with the addition of copper nanoparticles, related to improved light absorption. The evaluation of the CO2 photoreduction using the pure perovskite films was performed. The reaction showed that adsorbed oxygenated species (CO32−, OH−, among others) over the surface could hinder active sites causing low performance. Once films were decorated with CuxO nanoparticles, their performance was improved significantly in the formic acid evolution. Copper nanoparticles acted like active sites for CO2 reduction, promoting the separation of the charges.LaMnO3/CuxO films presented the higher production of formic acid associated with low presence of oxygenated species in the surface and the formation of scheme Z between semiconductors, allowing a better charge transfer.

Ferromagnetic Enhancement in LaMnO3 Films with Release and Flexure

AbstractA variety of novel phenomena and functionalities emerge from lowering the dimensionality of materials and enriching the degrees of freedom in modulation. In this work, it is found that the saturation magnetization of LaMnO3 (LMO) films is largely enhanced by 56% after releasing from a brand‐new phase of tetragonal strontium aluminate buffer layer, and is significantly increased by 92% with bending films to a curvature of 1 mm−1 using a water‐assisted direct‐transferring method. Meanwhile, the Curie temperature of LMO films has been improved by 13 K. High‐resolution spherical aberration‐corrected scanning transmission electron microscopy and first‐principles calculations unambiguously demonstrate that the enhanced ferromagnetism is attributed to the strengthened Mn–O–Mn super‐exchange interactions from the augmented characteristics of the unconventional P21/n structure caused by the out‐of‐plane lattice shrinking after strain releasing and increased flexure degree of freestanding LMO films. This work paves a way to achieve large‐scale and crack‐and‐wrinkle‐free freestanding films of oxides with largely improved functionalities.

Dry reforming of methane over Ni supported on LaMnO3 thin films

Thin films of LaMnO3 were deposited onto MgAl2O4 by Atomic Layer Deposition (ALD) and studied as supports for Ni in the Dry Reforming of Methane (DRM). Scanning Transmission Electron Microscopy (STEM) with Energy-Dispersive X-ray Spectroscopy (EDS) demonstrated that LaMnO3 covered the support uniformly, and X-ray diffraction (XRD) showed that the LaMnO3 films maintained their perovskite structure after 5 redox cycles at 1073 K. The Ni also remained well dispersed after 5 redox cycles at 1073 K. The LaMnO3-supported catalyst was more active than Ni on the unmodified MgAl2O4 and showed superior resistance against coke formation. Finally, the results for Ni on the LaMnO3 films are compared to previous results for Ni on LaFeO3, CaTiO3, SrTiO3, and BaTiO3 films.