La2NiMnO6 Thin Films Research Articles

Top-Layer Engineering Reshapes Charge Transfer at Polar Oxide Interfaces.

Charge-transfer phenomena at heterointerfaces are a promising pathway to engineer functionalities absent in bulk materials but can also lead to degraded properties in ultrathin films. Mitigating such undesired effects with an interlayer reshapes the interface architecture, restricting its operability. Therefore, developing less-invasive methods to control charge transfer will be beneficial. Here, an appropriate top-interface design allows for remote manipulation of the charge configuration of the buried interface and concurrent restoration of the ferromagnetic trait of the whole film. Double-perovskite insulating ferromagnetic La2 NiMnO6 (LNMO) thin films grown on perovskite oxide substrates are investigated as a model system. An oxygen-vacancy-assisted electronic reconstruction takes place initially at the LNMO polar interfaces. As a result, the magnetic properties of 2-5 unit cell LNMO films are affected beyond dimensionality effects. The introduction of a top electron-acceptor layer redistributes the electron excess and restores the ferromagnetic properties of the ultrathin LNMO films. Such a strategy can be extended to other interfaces and provides an advanced approach to fine-tune the electronic features of complex multilayered heterostructures.

Oxygen defect engineered magnetism of La2NiMnO6 thin films

The double perovskite La2NiMnO6 (LNMO) exhibits complex magnetism due to the competition of magnetic interactions that are strongly affected by structural and magnetic inhomogeneities. In this work, we study the effect of oxygen annealing on the structure and magnetism of epitaxial thin films grown by pulsed laser deposition. The key observations are that a longer annealing time leads to a reduction of saturation magnetization and an enhancement in the ferromagnetic transition temperature. We explain these results based upon epitaxial strain and oxygen defect engineering. The oxygen enrichment by annealing caused a decrease in the volume of the perovskite lattice. This increased the epitaxial strain of the films that are in-plane locked to the SrTiO3 substrate. The enhanced strain caused a reduction in the saturation magnetization due to randomly distributed anti-site defects. The reduced oxygen defects concentration in the films due to the annealing in oxygen improved the ferromagnetic long-range interaction and caused an increase in the magnetic transition temperature.

Ferromagnetic insulating epitaxially strained La2NiMnO6 thin films grown by sputter deposition

The field of oxide spintronics can strongly benefit from the establishment of ferromagnetic insulators with near room-temperature Curie temperature. Here, we investigate the structural, electronic, and magnetic properties of epitaxially strained thin films of the double perovskite La2NiMnO6 (LNMO) grown by off-axis radio-frequency magnetron sputtering. We find that the films retain insulating behavior and a bulk-like Curie temperature in the order of 280 K independently of the epitaxial strain conditions. These results suggest a prospective implementation of LNMO films in multi-layer device architectures where a high-temperature ferromagnetic insulating state is a prerequisite.

Aqueous Chemical Solution Deposition of Functional Double Perovskite Epitaxial Thin Films.

Double perovskite structure (A2 BB'O6 ) oxides exhibit a breadth of multifunctional properties with a huge potential range of applications in fields as diverse as spintronics, magneto-optic devices, or catalysis, and most of these applications require the use of thin films and heterostructures. Chemical solution deposition techniques are appearing as a very promising methodology to achieve epitaxial oxide thin films combining high performance with high throughput and low cost. In addition, the physical properties of these materials are strongly dependent on the ordered arrangement of cations in the double perovskite structure. Thus, promoting spontaneous cationic ordering has become a relevant issue. In this work, our recent achievements by using polymer-assisted deposition (PAD) of environmentally friendly, water-based solutions for the growth of epitaxial ferromagnetic insulating double perovskite La2 CoMnO6 and La2 NiMnO6 thin films on SrTiO3 and LaAlO3 single-crystal substrates are presented. It is shown that the particular crystallization and growth process conditions of PAD (very slow rate, close to thermodynamic equilibrium conditions) promote high crystallinity and quality of the films, as well as favors spontaneous B-site cationic ordering.

Phase stability and B-site ordering in La2NiMnO6 thin films

Thin films of multiferroic double-perovskite La2NiMnO6 are prepared on (001)-oriented SrTiO3, (La0.289Sr0.712)(Al0.633Ta0.356)O3, and LaSrAlO4 substrates by pulsed laser deposition. Microstructure investigation by advanced electron microscopy shows that the La2NiMnO6 films have a monoclinic structure on the SrTiO3 substrates and a rhombohedral structure on the (La0.289Sr0.712)(Al0.633Ta0.356)O3 and LaSrAlO4 substrates. Atomic-scale elemental maps of the monoclinic and rhombohedral phases reveal a short-range and/or partial ordering of the B-sites. In addition, domains and columnar grains are found in the films. Our results demonstrate that the phase and microstructure of the La2NiMnO6 films can be tuned by epitaxial strains induced by different substrates.

A-site-driven ferroelectricity in strained ferromagneticLa2NiMnO6thin films

We report on theoretical and experimental investigation of A-site driven ferroelectricity in ferromagnetic La2NiMnO6 thin films grown on SrTiO3 substrates. Structural analysis and density functional theory calculations show that epitaxial strain stretches the rhombohedral La2NiMnO6 crystal lattice along the [111]cubic direction, triggering a displacement of the A-site La ions in the double perovskite lattice. The lattice distortion and the A-site displacements stabilize a ferroelectric polar state in ferromagnetic La2NiMnO6 crystals. The ferroelectric state only appears in the rhombohedral La2NiMnO6 phase, where MnO6 and NiO6 octahedral tilting is inhibited by the 3-fold crystal symmetry. Electron localization mapping showed that covalent bonding with oxygen and 6s orbital lone pair formation are negligible in this material.

The magnetoelectric effect in thin films of ferromagnetic semiconductor La2 NiMnO6

Within a microscopic model and a Green's function technique we have calculated the temperature, size and magnetic field dependence of the phonon energy and damping of the mode, as well as the real and imaginary part of the dielectric function, and , respectively, in double perovskite La2 NiMnO6 thin films. All these quantities show anomalies at the magnetic phase transition temperature indicating a strong magnetoelectric coupling. They increase with decreasing film thickness. An external magnetic field enhances and and the kink disappears. Comparison to the bulk case and to experimental data is made.

Structural and magnetic properties of La2NiMnO6 thin films on LaAlO3 substrate with varying thickness

In the present work, the effect of film thickness on the structural and magnetic properties of La2NiMnO6 (LNMO) ferromagnetic double perovskite thin films has been systematically investigated. LNMO thin films of varying thickness from 54nm to 256nm were deposited using the pulsed laser deposition technique onto single-crystalline LaAlO3 substrate. The X-ray diffraction pattern and Raman scattering observations reveal that all films have c-axis epitaxial growth and an orthorhombic structure. The Raman spectra of the films are dominated by two broad peaks at around 534cm−1 and 676cm−1 assigned to the antisymmetric stretching (AS) and symmetric stretching (S) vibrations of the octahedra, respectively. The Raman peaks of the LNMO thin films are blue shifted in comparison to those of bulk LNMO and the shift increases with decreasing film thickness, indicating the increased influence of strain. The critical thickness for strain relaxation, as determined from the Raman spectra, is between 110nm and 156nm. However, the strain is observed to have large effect on the magnetic properties of films. Ferromagnetic to paramagnetic transition temperature (Tc) and the saturation magnetization were found to increase with increase in the film thickness. The film with thickness of 256nm exhibits a saturation magnetic moment of 4.5μB/f.u. and coercive field of 630Oe at 10K, with a Curie temperature close to 280K.

Synthesis and characterization of ordered and disordered polycrystalline La2NiMnO6 thin films by sol–gel

Polycrystalline La(2)NiMnO(6) thin films are prepared on Pt/Ti/SiO(2)/Si substrates by the sol-gel method. Through controlling the processing parameters, the cation ordering can be tuned. The disordered and ordered thin films exhibit distinct differences for crystal structures as well as properties. The crystal structure at room temperature characterized by X-ray diffraction and Raman spectra is suggested to be monoclinic (P2(1)/n) and orthorhombic (Pbnm) for the ordered and disordered thin films, respectively. The ferromagnetic-paramagnetic transition is 263 K and 60 K for the ordered and disordered samples respectively, whereas the saturation magnetic moment at 5 K is 4.9 μ(B) fu(-1) (fu = formula unit) and 0.9 μ(B) fu(-1). The dielectric constant as well as magnetodielectric effect is higher for the ordered La(2)NiMnO(6) thin films. The magnetodielectric effect for the ordered thin film is dominantly contributed to the intrinsic coupling of electric dipole ordering and fluctuations and magnetic ordering and fluctuations, while it is mainly contributed to Maxwell-Wagner (M-W) effects for the disordered thin film. The successful achievements of ordered and disordered polycrystalline La(2)NiMnO(6) thin films will provide an effective route to fabricate double-perovskite polycrystalline thin films by the sol-gel method.

Influence of substrates on epitaxial growth of B-site-ordered perovskite La2NiMnO6 thin films

Epitaxial La2NiMnO6 films were grown on (100)-oriented SrTiO3 and (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates under various growth conditions by pulsed laser deposition. Ferromagnetism and transport properties of B-site-ordered perovskite are greatly influenced by growth conditions. Optimum growth conditions of epitaxial La2NiMnO6 films, which exhibit the ferromagnetism similar to bulk properties, are different for SrTiO3 and LSAT substrates probably due to the difference in surface migration. Since the initial stage of epitaxial growth might vary with the type of substrates, the ordering of Ni and Mn ions is different. Therefore, the ferromagnetism of La2NiMnO6 depends on the epitaxial growth conditions and type of substrates.

Ferromagnetic resonance in double perovskite epitaxial thin films of La2NiMnO6 on SrTiO3 and NdGaO3 substrates

Ferromagnetic resonance (FMR) studies of epitaxial La2NiMnO6 (LNMO) thin films on (100) oriented SrTiO3 and (110) oriented NdGaO3 substrates at room temperature are presented. Observation of FMR spectra above the Curie temperature of this compound confirms the presence of magnetic ordering in LNMO thin films at room temperature. Best fitting of FMR spectra has been made on the assumption of the coexistence of two magnetic phases with different easy and hard axis periodicities in the film plane of LNMO. The spectra of the films on various substrates are characterized by different in-plane and out-of-plane symmetries, which indicates the existence of different epitaxial growth on the substrates with different crystal symmetries and orientations.

Preparation of La2NiMnO6 thin films on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition

Double perovskite La2NiMnO6 thin films were prepared on Pt/TiO2/SiO2/Si substrates by the pulsed laser deposition process, and the crystal structure and microstructures were investigated. The temperature and the oxygen pressure played the primary roles dominating the crystallization behavior and the morphology of La2NiMnO6 thin films. The well crystallized La2NiMnO6 thin films could be obtained at 873 and 923 K under all oxygen pressures investigated here, and the fine morphology was obtained under the oxygen pressures equal to or higher than 50 and 100 Pa, respectively, while phase constitution was significantly affected by the oxygen pressure for La2NiMnO6 thin films prepared at 1,023 K where the higher oxygen pressure led to the appearance of some secondary phase.

Fabrication of polycrystalline La2NiMnO6 thin films on Si (1 0 0) substrates by chemical solution deposition

Polycrystalline double perovskite La2NiMnO6 thin films are successfully deposited on Si (1 0 0) substrates via chemical solution deposition. The X-ray diffraction and Raman scattering spectroscopy are used to characterize all the films, it is found that all films are single phase and polycrystalline. The field-emission scanning electron microscopy shows that the films are relatively smooth and dense. The magnetic measurements indicate that one sample exhibits a Curie temperature of about 271 K, which is close to that of the bulk material. Moreover, the low-temperature magnetization of the films is lower than that of the films deposited on LaAlO3 (1 0 0) substrates, which can be attributed to the large mismatch between the films and the Si substrates.

Long-range Ni/Mn structural order in epitaxial double perovskiteLa2NiMnO6thin films

We report and compare the structural, magnetic, and optical properties of ordered La2NiMnO6 thin films and its disordered LaNi0.5Mn0.5O3 counterpart. An x-ray diffraction study reveals that the B-site Ni/Mn ordering induces additional XRD reflections as the crystal symmetry is transformed from a pseudocubic perovskite unit cell in the disordered phase to a monoclinic form with larger lattice parameters for the ordered phase. Polarized Raman spectroscopy studies reveal that the ordered samples are characterized by additional phonon excitations that are absent in the disordered phase. The appearance of these additional phonon excitations is interpreted as the clearest signature of Brillouin zone folding as a result of the long-range Ni/Mn ordering in La2NiMnO6. Both ordered and disordered materials display a single ferromagnetic-to-paramagnetic transition. The ordered films display also a saturation magnetization close to 4.8 mB/f.u. and a transition temperature (FM-TC) around 270 K, while the disordered ones have only a 3.7 mB/f.u. saturation magnetization and a FM-TC around 138 K. The differences in their magnetic behaviours are understood based on the distinct local electronic configurations of their Ni/Mn cations.

Ferromagnetic properties of epitaxial La2NiMnO6 thin films grown by pulsed laser deposition

La 2 NiMnO 6 (LNMO) thin films were fabricated on (100) oriented SrTiO3 single crystal substrates by pulsed laser deposition method. The film growth conditions were found to significantly affect the magnetic moment and ferromagnetic transition temperature of LNMO films. Epitaxial LNMO films exhibiting the ferromagnetism equivalent to that of bulk polycrystal were grown only in such a narrow region of growth temperature and oxygen pressure that stabilized Ni2+–O–Mn4+ superexchange ferromagnetic interaction. Out of this narrow region, the ferromagnetism is significantly suppressed by the appearance of secondary phases, locally disordered Ni and Mn ions, and oxygen vacancies.

Absence of long-range Ni∕Mn ordering in ferromagnetic La2NiMnO6 thin films

Epitaxial La2NiMnO6 thin films have been grown on (001)-orientated SrTiO3 using the pulsed laser deposition technique. The thin film samples are semiconducting and ferromagnetic with a Curie temperature close to 270K, a coercive field of 920Oe, and a saturation magnetization of 4.85μB/formula unit. Transmission electron microscopy, conducted at room temperature, reveals a majority phase having an “I-centered” structure with a≈c≈asub2 and b≈2asub along with minority phase domains having a “P-type” structure (asub being the lattice parameter of the cubic perovskite structure). A discussion on the absence of Ni∕Mn long-range ordering, in light of recent literature on the ordered double-perovskite La2NiMnO6 is presented.