Oriented SrTiO3 Substrates Research Articles

Thickness dependent uniaxial magnetic anisotropy due to step-edges in (1 1 1)-oriented La0.7Sr0.3MnO3 thin films

Abstract The magnetic anisotropy of films of La0.7Sr0.3MnO3 grown on vicinal (1 1 1)-oriented SrTiO3 substrates are investigated. For temperatures above the tetragonal – cubic structural phase transition temperature of the substrate, a step edge induced uniaxial magnetic anisotropy is found at remanence, with a thickness-driven change in easy axis direction, from perpendicular to the step edges to parallel to the step edges with increasing thickness. The anisotropy constant for the investigated (1 1 1)-oriented samples is of the same magnitude as for previously reported (0 0 1)-oriented samples. The data is discussed in the framework of in-plane rotations of the oxygen octahedra resulting in a uniaxial anisotropy. Furthermore, the magnetic anisotropy is sensitive to the structural phase transition at 105 K of the substrate, and the anisotropy constant increases drastically as the temperature is lowered below 105 K.

Interface structure and planar defects in the heterostructure of pyrochlore-type (Ca,Ti)2(Nb,Ti)2O7 film on SrTiO3 (0 0 1) substrate

Pyrochlore-type (Ca,Ti)2(Nb,Ti)2O7 thin film has been successfully prepared on (0 0 1)-oriented SrTiO3 substrate. The atomic-scale microstructure properties of the heterostructure have been investigated by advanced electron microscopy techniques. It is found that the (Ca,Ti)2(Nb,Ti)2O7 thin film is polycrystalline and an intermediate layer of perovskite-type Ca(Nb,Ti)O3 with a few unit cell thicknesses forms at the (Ca,Ti)2(Nb,Ti)2O7/SrTiO3 interface. In the (Ca,Ti)2(Nb,Ti)2O7 thin film, planar defects including {1 1 1}-type stacking faults, reflection twin and glide-reflection twin appear. The stacking faults have a projected displacement vector of (a/8)〈112〉. Cation segregation and boundary reconstruction occur at the twin boundaries and the stacking faults, which cause the change of the local stoichiometry and are believed to affect the electrical properties of the film.

Conduction mechanism and switchable photovoltaic effect in (1 1 1) oriented BiFe0.95Mn0.05O3 thin film

Epitaxial 200 nm BiFe0.95Mn0.05O3 (BFO) film was grown by pulsed laser deposition (PLD) on (1 1 1) oriented SrTiO3 substrate buffered with a 50 nm thick SrRuO3 electrode. The BFO thin film shows a rhombohedral structure and a large remnant polarization of Pr = 104 µC cm−2. By comparing I(V) characteristics with different conduction models we reveal the presence of both bulk limited Poole–Frenkel and Schottky interface mechanisms and each one dominates in a specific range of temperature. At room temperature (RT) and under 10 mW laser illumination, the as grown BFO film presents short-circuit current density (Jsc) and open circuit voltage (Voc) of 2.25 mA cm−2 and −0.55 V respectively. This PV effect can be switched by applying positive voltage pulses higher than the coercive field. For low temperatures a large Voc value of about −4.5 V (−225 kV cm−1) is observed which suggests a bulk non-centrosymmetric origin of the PV response.

Nonstoichiometry Driven Ferromagnetism in Double Perovskite La2Ni1–xMn1+xO6 Insulating Thin Films

In this work we report on the epitaxial growth of La2NiMnO6 double perovskite thin films on top of (001) oriented SrTiO3 substrates by RF magnetron sputtering. The influence of oxygen pressure (PO2) and growth temperature on the microstructure, stoichiometry of the films, and magnetic and transport properties is thoroughly investigated. It is found that high oxygen pressure promotes the growth of stoichiometric films, with a Ni/Mn ratio almost equal to 1. However, these films exhibit poor ferromagnetic properties with respect to the expected optimum values corresponding to ferromagnetic ordering mediated by superexchange interaction between Mn4+ and Ni2+ according to the Goodenough-Kanamori rules. Most interestingly, films grown at low PO2 exhibit Ni/Mn ratios below 1, but ferromagnetic properties close to the optimal ones. The valence balance between Ni and Mn ions in nonstoichiometric sample has been elucidated by X-ray absorption spectroscopy. The results indicate that Ni deficiency plays a crucial rol...

Interfacial coupling-induced distinct magnetic structure in La1/2Sr1/2CoO2.5+δ/La2/3Sr1/3MnO3/ La1/2Sr1/2CoO2.5+δ heterostructure

The interfacial coupling between electron correlated complex oxides opens a promising avenue for achieving new forms of magnetism that arises as a consequence of charge transfer, orbital reconstruction and symmetry-breaking. Herein we reported a distinct magnetic structure in La2/3Sr1/3MnO3 (LSMO) film sandwiched between La1/2Sr1/2CoO2.5+δ (LSCO) bilayer grown on (001)-oriented SrTiO3 substrates. It is found that the LSMO layer of heterostructure contains two independent components that are respectively free layer (free-LSMO) and coupled layer with LSCO (coupled-LSMO). The easy axis of the free-LSMO is in-plane, while it is out-of-plane directed for the coupled-LSMO layer. The thickness of these two parts in the LSMO layer are 4uc and 12uc respectively. This is in sharp contrast to the single LSMO layer on STO, for which the easy axis is always in film plane. Different from LSMO, the easy axis of LSCO is in-plane and is ferromagnetically coupled with LSMO. This distinct magnetic structure could be ascribed to orbital reconstruction due to symmetry-mismatch at interfaces.

Antiferromagnetic interlayer coupling of (111)-oriented La0.67Sr0.33MnO3/SrRuO3 superlattices**Project supported by the National Basic Research Program of China (Grant Nos. 2016YFA0300701, 2017YFA0206300, and 2017YFA0303601) and the National Natural Science Foundation of China (Grant Nos. 11520101002, 51590880, and 11674378).

We report a strong antiferromagnetic (AFM) interlayer coupling in ferromagnetic La0.67Sr0.33MnO3/SrRuO3 (LSMO/SRO) superlattices grown on (111)-oriented SrTiO3 substrate. Unlike the (001) superlattices for which the spin alignment between LSMO and SRO is antiparallel in the in-plane direction and parallel in the out-of-plane direction, the antiparallel alignment is observed along both the in-plane and out-of-plane directions in the present sample. The low temperature hysteresis loop demonstrates two-step magnetic processes, indicating the coexistence of magnetically soft and hard components. Moreover, an inverted hysteresis loop was observed. Exchange bias tuned by the temperature and cooling field was also investigated, and positive as well as negative exchange bias was observed at the same temperature with the variation of the cooling field. A very large exchange field (HEB) was observed and both magnitude and sign of the HEB depend on the cooling field, which can be attributed to an interplay of Zeeman energy and AFM coupling energy at the interfaces. The present work shows the great potential of tuning a spin texture through interfacial engineering for the complex oxides whose spin state is jointly determined by strongly competing mechanisms.

Effect of induced strain by substrate orientation on magnetic properties in BaTiO3/La0.7Sr0.3MnO3 bilayers

Abstract Hybrid multiferroics composed of ferromagnetic (FM) / ferroelectric (FE) heterostructures offer a possible route to design artificial multiferroic material exhibiting, for example, magnetoelectric coupling. We have grown BaTiO3 (100 nm)/ La2/3Sr1/3MnO3 (25 nm) bilayers on (001)-, (110)-, and (111)-oriented SrTiO3 substrate to study the induced strain influence in their magnetic properties. From structural characterization by x-ray analysis we found that La2/3Sr1/3MnO3 (LSMO) layer grew epitaxially strained, whereas BaTiO3 (BTO) film grew relaxed with lattice parameter close to that for single film. The magnetic measurements show a ferromagnetic order for all bilayers at room temperature with a Curie temperature TC∼350 K exhibiting a slight decrease of the magnetic properties (MS, HC and TC) of the bilayers in comparison with those for the La2/3Sr1/3MnO3 film. These variations could be associated to strained/relaxed regions induced by substrate orientation, and dislocation formation close to BTO/LSMO interface, among others phenomena. The magnetization mechanism was studied using the Berger model, where a critical β exponent ∼ 0.25 was found for all LSMO films in different STO orientations, which agrees with the tricritical point model. However, we found a β=0.283 only for the bilayer deposited on (001)STO substrate that coincides with previous reports for YMnO3 multiferroic materials. This behavior is not observed n either in the bilayer grown on (110) STO or (111) STO and displays indirectly a possible magnetoelectrical coupling in this configuration.

Robustness of topological states in Bi2Se3 thin film grown by Pulsed Laser Deposition on (0 0 1)-oriented SrTiO3 perovskite

We report on the reproducible surface topological electron states in Bi2Se3 topological insulator thin films when epitaxially grown by Pulsed Laser Deposition (PLD) on (001)-oriented SrTiO3 (STO) perovskite substrates. Bi2Se3 has been reproducibly grown with single (001)-orientation and low surface roughness as controlled by ex-situ X-ray diffraction and in situ scanning tunnel microscopy and low-energy electron diffraction. Finally, in situ synchrotron radiation angle-resolved photo-emission spectroscopy measurements show a single Dirac cone and Dirac point at EB∼0.38 eV located in the center of the Brillouin zone likewise found from exfoliated single-crystals. These results demonstrate that the topological surface electron properties of PLD-grown Bi2Se3 thin films grown on (001)-oriented STO substrates open new perspectives for applications of multi-layered materials based on oxide perovskites.

Tuning the crystallographic orientation and magnetic properties of multiferroic CuO epitaxial film on single crystalline SrTiO3 substrates

We report a comparative study between the crystal structure and magnetic properties of multiferroic epitaxial CuO thin films grown on (001) and (110) oriented SrTiO3 substrates. CuO thin films were successfully grown along the [010] direction, which is also the direction of their ferroelectric polarization, on (110) oriented SrTiO3 substrates. We correlate our structural findings with the magnetic properties. It was observed that the heterostructure with the CuO film grown along the [010] direction shows distinct anomalies in the temperature dependent magnetization plot at 213 K and 230 K, whereas no such anomaly was observed in the case of the heterostructure grown on the (001) SrTiO3 substrate with the [111] CuO film. Buffered MgO layers were used between the CuO layer and the substrate to improve the crystalline quality of the CuO film.

Evaluation of the AC conductivity and dielectric properties of YBaCo2O5±δ thin films at low temperatures

Textured YBaCo2O5±δ thin films were fabricated on (001)-oriented SrTiO3 substrates by dc magnetron sputtering. AC conductivity and dielectric properties of the films are studied in the frequency range 40 Hz-1 MHz and temperatures varying between 30 and 320 K. The frequency dependent AC conductivity is well described by the Jonscher’ dynamical law σac(ω) = σdc + Aωs. It is verified that the frequency exponent s has values ≤ 1. The value of s decreases with increasing temperature, suggesting hopping conduction mechanism. The activation energy for relaxation process ended up being 0.152 eV. It is also found that the AC conductivity is a thermally activated process with AC activation energy decreasing with increasing frequency. High values (∼104) are found for the real part of the dielectric permittivity on a wide temperature range. Moreover, the dependence of the permittivity on the frequency is weak over the entire experimental range and its value decreases with increasing temperature.

Interplay between current driven ferromagnetism in charge ordered antiferromagnetic Pr0.5Ca0.5MnO3 and superconducting YBa2Cu3O7−δ thin film multilayer

Thin film multilayer heterostructure of alternate YBa2Cu3O7-δ (YBCO) and Pr0.5Ca0.5MnO3 (PCMO) with thickness of each layer ∼60 nm has been deposited on (100) oriented SrTiO3 substrate by Pulsed Laser Deposition technique. A half portion of the base YBCO layer was masked in situ using mechanical shadow mask and in the remaining half portion, five alternate layers of PCMO and YBCO thin films were deposited. Magnetoresistance measurements were carried out under externally applied magnetic field and injection current. A noticeable damped oscillation of the superconducting transition temperature (TC) of this multilayer with respect to magnetic field is seen. Curiously, the field at which the first minimum in TC occurs, decreases as an injection current is driven perpendicular/parallel to the multilayers. Both these phenomena indicate that ferromagnetic correlation can be induced in antiferromagnetic PCMO thin films by (1) external magnetic field, or (2) injection current. While (1) is well researched, our study indicates that ferromagnetism can be induced by small amount of current in PCMO thin films. This unusual behavior points towards the strongly correlated nature of electrons in PCMO.

Structural investigation of (111) oriented (BiFeO3)(1-x)Λ/(LaFeO3)xΛ superlattices by X-ray diffraction and Raman spectroscopy

(BiFeO3)(1-x)Λ/(LaFeO3)xΛ superlattices (SLs) with varying x have been grown by pulsed laser deposition on (111) oriented SrTiO3 substrates. In order to obtain good epitaxy and flat samples, a conducting SrRuO3 buffer has been deposited prior to the superlattices to screen the polar mismatch for such (111) SrTiO3 orientation. X-ray diffraction reciprocal space mapping on a different family of planes was collected and evidenced a room temperature structural change at x = 0.5 from a rhombohedral/monoclinic structure for rich BiFeO3 to an orthorhombic symmetry for rich LaFeO3. This symmetry change has been confirmed by Raman spectroscopy and demonstrates the different phase stability compared to similar SLs grown on (100) SrTiO3. The strongly anisotropic strain and oxygen octahedral rotation/tilt system compatibility at the interfaces probably explain the orientation dependence of the phase stability in such superlattices.

Effect of double layer thickness on magnetoelectric coupling in multiferroic BaTiO3-Bi0.95Gd0.05FeO3 multilayers

The effect of double-layer thickness and partial substitution of Bi3+ by Gd3+ is demonstrated for multiferroic BaTiO3–BiFeO3 2–2 heterostructures. Multilayers of 15 double layers of BaTiO3 and Bi0.95Gd0.05FeO3 were deposited onto (0 0 1) oriented SrTiO3 substrates by pulsed laser deposition with various double layer thicknesses. X-ray diffraction and high resolution transmission electron microscopy investigations revealed a systematic strain tuning with layer thickness via coherently strained interfaces. The multilayers show increasingly enhanced magnetoelectric coupling with reduced double layer thickness. The maximum magnetoelectric coupling coefficient was measured to be as high as 50.8 V cm−1 Oe−1 in 0 T DC bias magnetic field at room temperature, and 54.9 V cm−1 Oe−1 above 3 T for the sample with the thinnest double layer thickness of 22.5 nm. This enhancement is accompanied by progressively increasing perpendicular magnetic anisotropy and compressive out-of-plane strain. To understand the origin of the enhanced magnetoelectric coupling in such multilayers, the temperature and magnetic field dependency of is discussed. The magnetoelectric performance of the Gd3+ substituted samples is found to be slightly enhanced when compared to unsubstituted BaTiO3–BiFeO3 multilayers of comparable double-layer thickness.

Unexpected metal-insulator transition in thick Ca1-xSrxVO3 film on SrTiO3 (100) single crystal

Epitaxial Ca1-xSrxVO3 (0 ≦ x ≦ 1) thin films were grown on (100)-oriented SrTiO3 substrates by using the pulsed laser deposition technique. In contrast to the previous report that metal-insulator transition (MIT) in Ca1-xSrxVO3 (CSVO) was achieved only for extremely thin films (several nm thick), MIT was observed at 39, 72, and 113 K for films with a thickness of 50 nm. The electronic structure was investigated by hard and soft X-ray photoemission spectroscopy (HX-PES and SX-PES). The difference between these PES results was significant due to the variation in an escape depth of photoelectrons of PES. While HX-PES showed that the V 2p3/2 spectra consisted of four peaks (V5+, V4+, V3+, and V2+/1+), SX-PES showed only three peaks (V5+, V4+, and V3+). This difference can be caused by a strain from the substrate, which leads to the chemical disorder (V5+, V4+, V3+, and V2+/1+). The thin film near the substrate is affected by the strain. The positive magnetoresistance is attributed to the effect of electron-electron interactions in the disorder system. Therefore, the emergence of MIT can be explained by the electron-electron interactions from the chemical disorder due to the strain.

Microstructure and oxidation resistance of relaxed epitaxial nickel thin films grown on (100)- and (110)-SrTiO3 substrates by pulsed laser deposition

The relaxed epitaxial growth of Ni thin films has been successfully performed on both (100)- and (110)-oriented SrTiO3 substrates by pulsed laser deposition.

The Exchange Bias of LaMnO3/LaNiO3 Superlattices Grown along Different Orientations

With the goal of observing and explaining the unexpected exchange bias effect in paramagnetic LaNiO3-based superlattices, a wide range of theoretical and experimental research has been published. Within the scope of this work, we have grown high-quality epitaxial LaMnO3(n)-LaNiO3(n) (LMO/LNO) superlattices (SLs) along (001)-, (110)-, and (111)-oriented SrTiO3 substrates. The exchange bias effect is observed in all cases, regardless of growth orientation of the LMO/LNO SLs. As a result of a combination of a number of synchrotron based x-ray spectroscopy measurements, this effect is attributed to the interfacial charge transfer from Mn to Ni ions that induces localized magnetic moments to pin the ferromagnetic LMO layer. The interaction per area between interfacial Mn and Ni ions is nearly consistent and has no effect on charge transfer for different orientations. The discrepant charge transfer and orbital occupancy can be responsible for the different magnetic properties in LMO/LNO superlattices. Our experimental results present a promising advancement in understanding the origin of magnetic properties along different directions in these materials.

Epitaxial thin films of Dirac semimetal antiperovskite Cu3PdN

The growth and study of materials showing novel topological states of matter is one of the frontiers in condensed matter physics. Among this class of materials, the nitride antiperovskite Cu3PdN has been proposed as a new three-dimensional Dirac semimetal. However, the experimental realization of Cu3PdN and the consequent study of its electronic properties have been hindered due to the difficulty of synthesizing this material. In this study, we report fabrication and both structural and transport characterization of epitaxial Cu3PdN thin films grown on (001)-oriented SrTiO3 substrates by reactive magnetron sputtering and post-annealed in NH3 atmosphere. The structural properties of the films, investigated by x-ray diffraction and scanning transmission electron microscopy, establish single phase Cu3PdN exhibiting cube-on-cube epitaxy (001)[100]Cu3PdN||(001)[100]SrTiO3. Electrical transport measurements of as-grown samples show metallic conduction with a small temperature coefficient of the resistivity of 1.5 × 10−4 K−1 and a positive Hall coefficient. Post-annealing in NH3 results in the reduction of the electrical resistivity accompanied by the Hall coefficient sign reversal. Using a combination of chemical composition analyses and ab initio band structure calculations, we discuss the interplay between nitrogen stoichiometry and magneto-transport results in the framework of the electronic band structure of Cu3PdN. Our successful growth of thin films of antiperovskite Cu3PdN opens the path to further investigate its physical properties and their dependence on dimensionality, strain engineering, and doping.

Novel visible-light-driven photoconductive properties of LaCoO3 epitaxial nano-thin films

LaCoO3 nano-thin films were grown on (001), (110) and (111) oriented SrTiO3 substrates by the polymer assisted deposition method, respectively. All the LaCoO3 films are in epitaxial growth in accordance with the orientation of STO, with biaxial tensile strain. The films, especially for (111) oriented film, exhibit excellent visible-light photoconductive properties. The strain-induced increase of Co-O bond length may contribute to reduce the t2g-eg gap of Co3+ and further increase the number of photoexcited eg itinerant electrons. The different photoconductive activity of LaCoO3 epitaxial films may be attributed to the different surface energies of crystal planes. It suggests that the LaCoO3 epitaxial film can be considered as a potential candidate for visible-light photodetector.

Signature of exchange bias and magneto-electric coupling in BiFeO3/SrRuO3 heterostructure

The magnetic interaction between BiFeO3 and SrRuO3 layers in a heterostructure grown on (001) oriented SrTiO3 substrate is investigated. A two-step magnetization reversal was observed in M−H hysteresis loop measurement of the heterostructure at 10K. The first step in the hysteresis loop is associated with the switching of the free SrRuO3 moments whereas the second step arises from the switching of the pinned moments. The total amount of the pinned SrRuO3 moment was observed to decrease with increasing thickness of the BiFeO3 layer. The presence of exchange bias effect in the heterostructure was confirmed by the field cooled M−H measurements where only the second step of hysteresis loop was observed to shift along the field axis. The coercivity of the second step of hysteresis loop decreases with increasing temperature and merges with that of the first step above 100K which infers the desertion of the pinned magnetic moments. Temperature dependent capacitance measurement shows a kink at the ferromagnetic transition temperature of SrRuO3 and a pronounced dip was observed in the second derivative of capacitance with respect to temperature, indicating a strong magneto-electric coupling between the two layers.

Charge transfer-induced magnetic exchange bias and electron localization in (111)- and (001)-oriented LaNiO3/LaMnO3 superlattices

High-quality lattice-matched LaNiO3/LaMnO3 superlattices with monolayer terrace structure have been grown on both (111)- and (001)-oriented SrTiO3 substrates by pulsed laser deposition. In contrast to the previously reported experiments, a magnetic exchange bias is observed that reproducibly occurs in both (111)- and (001)-oriented superlattices with the thin single layers of 5 and 7 unit cells, respectively. The exchange bias is theoretically explained by charge transfer-induced magnetic moments at Ni atoms. Furthermore, magnetization data at low temperature suggest two magnetic phases in the superlattices, with Néel temperature around 10 K. Electrical transport measurements reveal a metal-insulator transition with strong localization of electrons in the superlattices with the thin LaNiO3 layers of 4 unit cells, in which the electrical transport is dominated by two-dimensional variable range hopping.