Epitaxial Pr0 Research Articles

Correlation of microstructure with magnetic properties in Pr0.67Sr0.33MnO3 thin films

Strain can be used to modulate magnetic properties of manganite thin film by affecting not only its crystal structure but also the microstructure, including the cluster size and fraction of ferromagnetic phase. In this work, epitaxial Pr0.67Sr0.33MnO3 thin films deposited on (001)-SrTiO3 single-crystal substrate are studied. The magnetization-temperature curves with zero-field cooling and field cooling show the typical feature of cluster-glass system. With the increase of film thickness, strain relaxation occurs in 80 nm Pr0.67Sr0.33MnO3 film with a wide temperature range of phase transition. By fitting the freezing temperature Tf dependence on magnetic field according to Almeida–Thouless equation, the 80 nm film has a larger local anisotropy and exchange bias compared to that of the 15 nm Pr0.67Sr0.33MnO3 film. Detailed analysis suggests that the strain-induced phase fraction and size distribution of ferromagnetic phase are responsible for the different magnetic properties of Pr0.67Sr0.33MnO3 film with different thickness.

Misfit Relaxation Mechanisms and Domain Ordering in Anisotropically Strained Manganite Thin Films.

The evolution of anisotropic strain in epitaxial Pr0.5Sr0.5MnO3 films grown on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7(110) substrates has been characterized by off-specular X-ray reciprocal space mappings on the (130), (310), (222), and (222̅) reflections in the scattering zone containing the [110] axis. We demonstrate that a multistage hierarchical structural evolution (single-domain-like structure, domain ordering, twin domains, and/or periodic structural modulations) occurs as the film thickness increases, and the structural modulation between the two transverse in-plane [11̅0] and [001] directions is quite different due to the monoclinic distortion of the film. We then show the relationship between the distribution of diffraction spots in reciprocal space and their corresponding domain configurations in real space under various thicknesses, which is closely correlated with thickness-dependent magnetic and magnetotransport properties. More importantly, the distribution and annihilation dynamics of the domain ordering are imaged utilizing home-built magnetic force microscope, revealing that the structural domains tilted toward either the [001] or [001̅] direction are arranged along the [11̅1] and [1̅11] crystal orientations. The direct visualization and dynamics of anisotropic-strain-related domain ordering will open a new path toward the control and manipulation of domain engineering in strongly correlated perovskite oxide films.

Digital- to Analog-Type Terahertz Modulation Controlled by Mosaicity of the Substrate Template in Rare-Earth Nickelate Thin Films.

The extreme sensitivity of the metal-insulator (M-I) transition in RNiO3 (R = rare-earth ion) nickelates to various extrinsic and intrinsic factors rely on mechanisms driving structure-property relations. Here, we demonstrate a unique way to control the M-I transition of epitaxial Pr0.5Sm0.5NiO3 thin films using a mosaic template of the LaAlO3(100) substrate; two sets of epitaxial films were deposited on highly oriented crystals and mosaic (with multiple crystallites) crystals. While the former films exhibit a robust and sharp M-I transition, the films on the mosaic substrate show distinctively much more subtle and broad transition, albeit same factors suggesting compositional purity. Terahertz (THz) dynamic conductivity too behaves very differently for the two types of films; Drude dynamics dominate the conductivity of highly crystalline films, whereas disorder-driven Drude-Smith conductivity prevails in mosaic films. Using this mosaic structure-controlled M-I transition and conductivity dynamics, we propose to implement these two templates of films for digital and analog THz transmission amplitude modulators.

Formation of Ruddlesden-Popper Faults and Their Effect on the Magnetic Properties in Pr0.5Sr0.5CoO3 Thin Films.

Epitaxial Pr0.5Sr0.5CoO3 thin films have been grown on single-crystalline (La0.289Sr0.712)(Al0.633Ta0.356)O3(001) substrates by the pulsed laser deposition technique. The magnetic properties and microstructure of these films are investigated. It is found that Ruddlesden-Popper faults (RP faults) can be introduced in the films by changing the laser repetition rate. The segregation of Pr at the RP faults is characterized by atomic-resolution chemical mapping. The formation of the RP faults not only contributes to the epitaxial strain relaxation but also significantly decreases the ferromagnetic long-range order of the films, resulting in lower magnetizations than those of the fault-free films. Our results provide a strategy for tuning the magnetic properties of cobalt-based perovskite films by modifying the microstructure through the film growth process.

Magnetism in epitaxial PrCoO3 and Pr0.7Y0.3CoO3 thin films

Lattice distortions can stabilize magnetic ground states not observed in the bulk for thin films of undoped cobalt perovskites ACoO3. We have found that lattice distortions, in the form of tensile epitaxial strain, can stabilize a ferromagnetic ground state in PrCoO3 thin films below 65 K. However, in Pr0.7Y0.3CoO3, which has a smaller unit cell, similar levels of tensile strain are insufficient to induce ferromagnetism. These results indicate the importance of A-site cation choice for Co spin-state and Co-O-Co exchange interactions, offering another dimension to tune magnetic order in thin film cobalt perovskites.

The observation of valence band change on resistive switching of epitaxial Pr0.7Ca0.3MnO3 film using removable liquid electrode

The resistive switching (RS) phenomenon in transition metal oxides (TMOs) has received a great deal of attention for non-volatile memory applications. Various RS mechanisms have been suggested as to explain the observed RS characteristics. Many reports suggest that changes of interface and the role of oxygen vacancies originate in RS phenomena; therefore, in this study, we use a liquid drop of mercury as the top electrode (TE), epitaxial Pr0.7Ca0.3MnO3 (PCMO) (110) film of the perovskite manganite family for RS material, and an Nb-doped (0.7 at. %) SrTiO3 (100) single crystal as the substrate to observe changes in the interface between the TE and TMOs. The use of removable liquid electrode Hg drop as TE not only enables observation of the RS characteristic as a bipolar RS curve (counterclockwise) but also facilitates analysis of the valence band of the PCMO surface after resistive switching via photoelectron spectroscopy. The observed I-V behaviors of the low and high resistance states (HRS) are explained with an electrochemical migration model in PCMO film where accumulated oxygen vacancies at the interface between the Hg TE and PCMO (110) surface induce the HRS. The interpreted RS mechanism is directly confirmed via valence band spectrum analysis.

Effect of substrate and orientation on charge ordering behaviors in epitaxial Pr0.5Ca0.35Sr0.15MnO3 films

The charge ordering (CO) behaviors of Pr0.5Ca0.35Sr0.15MnO3 films grown on STO(100), STO(110) and LAO(100) are systematically investigated by transport measurements and transmission electron microscopy (TEM) examinations. From the transport measurements, the CO transition temperatures of all the three films are much higher than those of the bulk materials, showing that the film strain could enhance the CO transition. From TEM observations, many superlattice spots appear in the electron diffraction patterns taken from the films, indicating the appearance of the CO modulation structures at room temperature. The modulation vectors are determined to be (1/2, 0, 0) for STO (100), (1/2, 1/2, 1/2) for STO (110), and both (0, 1/2, 0) and (1/2, 1/2, 0) for LAO (100). It is shown that both the substrate orientation and the film strain have a great effect on the CO modulation structures. The CO state is much easier to appear in the compressive strain direction which is due to the Mn-O-Mn angle tilting.

Characterization on surface and interface structure of epitaxial Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3/Pr0.7Sr0.3MnO3 trilayer

Epitaxial trilayers consisting of ferromagnetic (FM) metallic Pr0.7Sr0.3MnO3 (PSMO) and antiferromagnetic (AFM) insulators La0.5Ca0.5MnO3 (LCMO) were fabricated on (001)-oriented single crystal MgO substrates using pulsed laser deposition technique. High resolution X-ray diffraction (HRXRD), grazing incidence X-ray reflectivity (GIXRR) and atomic force microscope were applied to characterize the surface and interface structure of PSMO/LCMO/PSMO trilayers. The HRXRD patterns of all trilayers implied high quality of epitaxial layer. A structural model including seven layers was presented to theoretically simulate GIXRR data. The perfect GIXRR data fitting of the trilayers indicated that the thickness of the top sublayer at air/PSMO interface was about 2.5nm for all the samples and the mass density of the top sublayer was about 77.66% of PSMO layer. In all trilayers, there existed two disordered layers with thickness of about 4nm resulting from interdiffusion at the PSMO/LCMO interfaces. The root-mean-square (rms) roughness of the surface and interface in the trilayers varied with respect to the thickness of LCMO layer. The surface roughness was consistent with observation from atomic force microscopy. A further analysis indicated that the rms roughness was related to the mismatch strain relaxation in film. Moreover, studies on magnetic properties of PSMO/LCMO/PSMO trilayers show that ferromagnetic clusters at the interface, large roughness at the surface and interface and interdiffusion between PSMO and LCMO layer probably lead to the disappearance of exchange bias.

Three-dimensional strain state and spacer thickness-dependent properties of epitaxial Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3/Pr0.7Sr0.3MnO3 trilayer structure

Epitaxial colossal magnetoresistive trilayer structures consisting of ferromagnetic metallic Pr0.7Sr0.3MnO3 (PSMO) and antiferromagnetic insulator La0.5Ca0.5MnO3 (LCMO) were fabricated on (001)-oriented single crystal MgO substrates using pulsed laser deposition technique. The evolution of three-dimensional strain states and electrical and magnetic transport properties of PSMO/LCMO/PSMO trilayers have been studied as a function of LCMO spacer thickness and lattice strain. When the thickness of LCMO spacer is 6 nm, lattice strain in the trilayer begins to be relaxed. Furthermore, trilayers with thickness of LCMO spacer up to 36 nm are not fully strain relaxation. The unit cell volume of the films is not conserved and exhibits the variation with LCMO layer thickness. Strain relaxation states are determined by bulk strain (εB) and Jahn–Teller (εJT) strain together. The electrical and magnetic transport properties, including metal-insulator transition temperatures TMI and saturation magnetization MS, also show systematic variations with respect to LCMO layer thickness.

Electric field tuning of phase separation in manganite thin films

In this paper, we investigate the electric field effect on epitaxial Pr0.65(Ca0.75Sr0.25)0.35MnO3 thin films in electric double-layer transistors. Different from the conventional transistors with semiconducting channels, the sub(micrometer)-scale phase separation in the manganite channels is expected to result in inhomogeneous distribution of mobile carriers and local enhancement of electric field. The field effect is much larger in the low-temperature phase separation region compared to that in the high-temperature polaron transport region. Further enhancement of electroresistance is achieved by applying a magnetic field, and a 250% modulation of resistance is observed at 80 K, equivalent to an increase of the ferromagnetic metallic phase fraction by 0.51%, as estimated by the general effective medium model. Our results illustrate the complementary nature of electric and magnetic field effects in phase-separated manganites, providing insights on such novel electronic devices based on complex oxides.

Giant Electroresistance Induced by Electric Currents in Epitaxial Thin Films of Pr<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub>

Electric-current induced electroresistance effect has been investigated in epitaxial Pr0.7Sr0.3MnO3thin film grown on the (100) SrTiO3substrate. A significant change ~38% in the ratio of the peak resistance at different currents with a current density up to ~3.3×103A/cm2was achieved. Such an ER effect is more remarkable in comparing with that reported in other manganite oxides with similar current densities. Compared with that of the as-grown films, the electroresistance of the post-annealed films is smaller, while the ER effect (~14%) is obtained. Although the nature behind such an electroresistance effect has not been well understood, the field tunability of the metal/insulator transition and the electroresistance effect induced by currents might be of potential for various applications such as filed effect devices.

Giant in-plane anisotropy in manganite thin films driven by strain-engineered double exchange interaction and electronic phase separation

We investigate epitaxial Pr0.65(Ca0.7Sr0.3)0.35MnO3 thin film grown on orthorhombic (110) NdGaO3 substrate which breaks the lattice symmetry and affects the phase separated ground state. As a result of the anisotropic substrate strain, giant in-plane magnetic and magnetotransport anisotropy are observed, which is related to the anisotropic coupling and competition between the double-exchange interaction and the Jahn-Teller distortion. Furthermore, the in-plane anisotropy shows a distinct enhancement near the metal-insulator transition, implying a significant contribution from the phase separation to the anisotropic transport behaviors.

Persistent photoconductivity induced by electric currents in epitaxial thin films of Pr0.7Sr0.3MnO3

The effects of electric currents on the photoconductivity of epitaxial Pr0.7Sr0.3MnO3 films were studied. The as-prepared films showed a transient photoconductivity in a wide temperature range. After the films were processed by a current of density ∼105 A/cm2 for certain duration at room temperature, a highly resistive and metastable state was excited at low temperature. This induced state exhibited significant electroresistance and persistent photoconductivity. The phenomena found in films grown on LaAlO3 were similar to those on SrTiO3. The observed effects might be related to the coexistence and instability of the multiphases in manganites.

Magnetic properties of pseudomorphic epitaxial films ofPr0.7Ca0.3MnO3under different biaxial tensile stresses

In order to analyse the effect of strain on the magnetic properties of narrow-band manganites, the temperature and field dependent susceptibilities of about 8.5 nm thick epitaxial Pr0.7Ca0.3MnO3 films, respectively grown on (001) and (110) SrTiO3 substrates, have been compared. For ultrathin samples grown on (001) SrTiO3, a bulk-like cluster-glass magnetic behaviour is found, indicative of the possible coexistence of antiferromagnetic and ferromagnetic phases. On the contrary, ultrathin films grown on (110) substrates show a robust ferromagnetism, with a strong spontaneous magnetization of about 3.4 mB /Mn atom along the easy axis. On the base of high resolution reciprocal space mapping analyses performed by x-ray diffraction, the different behaviours are discussed in terms of the crystallographic constraints imposed by the epitaxy of Pr0.7Ca0.3MnO3 on SrTiO3. We suggest that for growth on (110) SrTiO3, the tensile strain on the film c-axis, lying within the substrate plane, favours the ferromagnetic phase, possibly by allowing a mixed occupancy and hybridization of both in-plane and out-of-plane eg orbitals. Our data allow to shed some physics of inhomogeneous states in manganites and on the nature of their ferromagnetic insulating state.

The absence of charge-density-wave sliding in epitaxial charge-orderedPr0.48Ca0.52MnO3 films

For an epitaxial Pr0.48Ca0.52MnO3 film on NdGaO3, we use transmission electron microscopy to observe a ‘charge-ordered’ superlattice along the in-planedirection a. The same film shows no electrical signatures of charge order. The in-plane electrical anisotropyρa/ρc = 28 is constant, and there is no evidence of sliding charge density waves up to the large field of ∼ 103 V cm − 1.

Resistance Switching Characteristic and Charge Carrier Self-Trapping in Epitaxial Pr0.7(Ca1–xSrx)0.3MnO3 Thin Films

Carrier injection performed in Pr0.7Ca0.3MnO3 junctions demonstrate resistance switching (RS) characteristic with dramatic changes in both resistances and interface barriers, which suggests a charge carrier self-trapping model in strongly correlated electronic framework. Un-stable RS behaviour without electric fields in epitaxial Pr0.7(Ca1–xSrx)0.3MnO3 (PCSMO) films shows dependences on insulator–metal transition temperature, which indicates that RS process is really related to the intrinsic property of carriers. The switched resistance of epitaxial PCSMO films also depends on the amount of current pulses, which should be another evidence of the carrier self-trapping model, similarly to the dependence on the amount of self-trapped charge carriers.

Trap-controlled space-charge-limited current mechanism in resistance switching at Al∕Pr0.7Ca0.3MnO3 interface

Well-defined metal-insulator-metal trilayered structures composed of epitaxial Pr0.7Ca0.3MnO3 insulator layers, epitaxial LaNiO3 bottom electrodes, and Al metal top electrodes were fabricated on LaAlO3 (100) substrates. The I-V characteristics of the trilayer structures show electric-field-induced resistance switching. The resistance switching ratio of the heterostructures was up to 100 when positive and negative pulsed voltages were applied. Detailed I-V analysis indicates the importance of both trap-controlled space-charge-limited current and Poole–Frenkel effect in resistance switching at Al∕Pr0.7Ca0.3MnO3 interfaces.

Defect-induced charge-order melting in thin films of Pr0.5Ca0.5MnO3

We have investigated the relation between defect structure and charge order melting in thin films of epitaxial Pr0.5Ca0.5MnO3 (PCMO), grown under strain on SrTiO3. We compared the behavior of an 80nm film grown in one deposition step at 840°C with the behavior of a film grown in two steps. In the two-step case, a thin PCMO layer of 10nm was deposited at 120°C, followed by 70nm deposited at 840°C. The increase of the growth temperature leads to complete crystallization of the first layer and the lattice constants of the two-step grown film indicate that tensile strain is still present. On the other hand, a magnetic field of only 5T is required to melt the charge-order state in the two-step grown film, which is a much lower than the value for the normally grown film. This appears to be connected to a larger amount of threading dislocations present in the first (recrystallized) layer.

Highly rectifying Pr0.7Ca0.3MnO3∕SrTi0.9998Nb0.0002O3 p-n junction

We have fabricated epitaxial Pr0.7Ca0.3MnO3∕SrTi0.9998Nb0.0002O3 (PCMO∕Nb:STO) junctions and characterized the interface electronic properties. The PCMO∕Nb:STO junctions show highly rectifying current density–voltage (J–V) characteristics without an apparent breakdown in the reverse bias up to 100V at room temperature. The J–V characteristics of the diodes agree well with the conventional diffusion theory for a p-n diode. The forward bias J–V and reverse bias capacitance-voltage characteristics result in an identical built-in potential of ∼0.7eV. Based on the experimental results, a plausible band diagram of the PCMO∕Nb:STO p-n diode is proposed.

Hopping conductivity of magnetic polarons in epitaxial Pr0.5Sr0.5MnO3 films

We present data on the electrical transport and negative magnetoresistivity effect in epitaxial films of Pr0.5Sr0.5MnO3, which undergoes two magnetic phase transitions, both accompanied by apparent changes in the conductive mechanism. It is shown that conductivity arises from hopping processes, while the giant negative magnetoresistivity seems to be of a similar origin in the ferro-and in the antiferromagnetic phase.