NdNiO3 Films Research Articles

Oxygen Vacancy Induced Room-Temperature Metal-Insulator Transition in Nickelate Films and Its Potential Application in Photovoltaics.

Oxygen vacancy is intrinsically coupled with magnetic, electronic, and transport properties of transition-metal oxide materials and directly determines their multifunctionality. Here, we demonstrate reversible control of oxygen content by postannealing at temperature lower than 300 °C and realize the reversible metal-insulator transition in epitaxial NdNiO₃ films. Importantly, over 6 orders of magnitude in the resistance modulation and a large change in optical bandgap are demonstrated at room temperature without destroying the parent framework and changing the p-type conductive mechanism. Further study revealed that oxygen vacancies stabilized the insulating phase at room temperature is universal for perovskite nickelate films. Acting as electron donors, oxygen vacancies not only stabilize the insulating phase at room temperature, but also induce a large magnetization of ∼50 emu/cm³ due to the formation of strongly correlated Ni²⁺ t(2g)⁶e(g)² states. The bandgap opening is an order of magnitude larger than that of the thermally driven metal-insulator transition and continuously tunable. Potential application of the newly found insulating phase in photovoltaics has been demonstrated in the nickelate-based heterojunctions. Our discovery opens up new possibilities for strongly correlated perovskite nickelates.

Key role of lattice symmetry in the metal-insulator transition of NdNiO3 films.

Bulk NdNiO3 exhibits a metal-to-insulator transition (MIT) as the temperature is lowered that is also seen in tensile strained films. In contrast, films that are under a large compressive strain typically remain metallic at all temperatures. To clarify the microscopic origins of this behavior, we use position averaged convergent beam electron diffraction in scanning transmission electron microscopy to characterize strained NdNiO3 films both above and below the MIT temperature. We show that a symmetry lowering structural change takes place in case of the tensile strained film, which undergoes an MIT, but is absent in the compressively strained film. Using space group symmetry arguments, we show that these results support the bond length disproportionation model of the MIT in the rare-earth nickelates. Furthermore, the results provide insights into the non-Fermi liquid phase that is observed in films for which the MIT is absent.

Competition between strain and dimensionality effects on the electronic phase transitions in NdNiO3 films.

Transition metal oxides host an array of exotic electronic phases, including superconductivity, ferroelectricity, quantum spin liquid and Mott insulators. Their extreme sensitivity to external stimuli enables various routes to manipulate the ground state, which greatly improves our understanding of the physics involved. Here, we report the competition between strain and dimensionality effects on the phase evolution in high quality NdNiO3 films down to several unit cells. While both compressive and tensile strains increase the Ni 3d band width and favor the metallic phase, reducing dimensionality, on the other hand, decreases the covalent band width and favors the insulating phase in NdNiO3. The experimental observations are well supported by ab initio calculations and improve our understanding of the electronic behavior in strongly correlated electron systems.

Large effects of epitaxial tensile strain on electrical transport of Mn-doped NdNiO3 thin films

Abstract Epitaxial thin films of NdNi1−xMnxO3 (x=0, 0.02, 0.10) were grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 (001) and NdGaO3 (001) single-crystals by pulsed laser deposition. The results were compared and analyzed for the effects of epitaxial tensile strain and Mn-doping on the temperature dependent electrical transport. With increased tensile strain of NdNiO3 films, the metal–insulator transition shifts to lower temperature and the resistivity clearly increases in both metallic and insulating states. Our results show that the effect of epitaxial tensile strain on the resistivity of NdNiO3 films is much larger on the metallic state than that on the insulating state. The effects of Mn-doping are distinguished from the effects of tensile strain. The resistivity data of the insulating films (Mn-doped) fit to variable range hopping model. Mn-doping scatters the charge carriers in the metallic state and it is unambiguously found to assist the hopping of charge carriers by increasing density of states in the insulating state of these films.

Tailoring the electronic transitions of NdNiO3 films through (111)pc oriented interfaces

Bulk NdNiO_3 and thin films grown along the pseudocubic (001)_pc axis display a 1st order metal to insulator transition (MIT) together with a N\'eel transition at T=200K. Here, we show that for NdNiO3 films deposited on (111)_pc NdGaO_3 the MIT occurs at T=335K and the N\'eel transition at T=230 K. By comparing transport and magnetic properties of layers grown on substrates with different symmetries and lattice parameters, we demonstrate a particularly large tuning when the epitaxy is realized on (111)_pc surfaces. We attribute this effect to the specific lattice matching conditions imposed along this direction when using orthorhombic substrates.

Correlation between stoichiometry, strain, and metal-insulator transitions of NdNiO3 films

The interplay of film stoichiometry and strain on the metal-insulator transition (MIT) and Hall coefficient of NdNiO3 films grown under different conditions is investigated. Unstrained lattice parameters and lattice mismatch strains are evaluated for films grown under a range of growth pressures and on different substrates. It is shown that both the temperature of the MIT and the Hall coefficient in the metallic phase are highly sensitive to film strain. In films grown with lower oxygen/total growth pressures, very large compressive in-plane strains can be obtained, which can act to suppress the MIT. Both the Hall coefficient and the temperature of the MIT are relatively insensitive to growth pressure, provided that films under the same strain are compared. The results support an itinerant picture of the transition that is controlled by the Ni eg bands, and that is relatively insensitive to changes in film stoichiometry.

Epitaxial strain effect in perovskite RENiO3 films (RE = La–Eu) prepared by metal organic decomposition

We report the synthesis of perovskite RENiO3 films (RE=La, Pr, Nd, Sm, and Eu) by metal organic decomposition (MOD). The RENiO3 family is an ideal system for studying the metal–insulator transition due to the simplicity of the materials. One of the drawbacks is that the bulk synthesis of the RENiO3 requires processing at high oxygen pressures to stabilize Ni3+. Fundamentally, MOD is similar to solid-state reaction, but it turned out that the MOD synthesis tends to stabilize RENiO3 without the need for high oxygen pressure. The films prepared by MOD show high crystallinity and low resistivity. Furthermore, we have investigated the epitaxial strain effect and observed a dramatic effect in PrNiO3 and NdNiO3 films on LaAlO3 substrates. The metal–insulator transition in the PrNiO3 films on LaAlO3 is fully suppressed, whereas the metal–insulator transition temperature is considerably lowered in the NdNiO3 films on LaAlO3.

Strain controlled metal-insulator transition in epitaxial NdNiO3 thin films

We have fabricated epitaxial thin films of NdNiO3 (NNO) on various single crystal substrates. The transport properties of NNO films are very sensitive to substrate-controlled epitaxial strain. As the strain varies from tensile to compressive, the Mott metal-insulator transition of NNO films shifts to low temperatures. Under a larger compressive strain, the film on LaSrAlO4 substrate exhibits a practically metallic transport characteristic. We have found that the conductivities of NNO films at low temperatures follow Mott's variable range hopping mechanism rather than thermal activation model and the epitaxial strain has a strong effect on Mott's parameters of NNO films. These findings demonstrate that the electronic transport of NNO thin films can be tuned by the epitaxial strain for next-generation perovskite-based microelectronic devices.

Heterointerface engineered electronic and magnetic phases of NdNiO3 thin films

Mott physics is characterized by an interaction-driven metal-to-insulator transition in a partially filled band. In the resulting insulating state, antiferromagnetic orders of the local moments typically develop, but in rare situations no long-range magnetic order appears, even at zero temperature, rendering the system a quantum spin liquid. A fundamental and technologically critical question is whether one can tune the underlying energetic landscape to control both metal-to-insulator and Néel transitions, and even stabilize latent metastable phases, ideally on a platform suitable for applications. Here we demonstrate how to achieve this in ultrathin films of NdNiO3 with various degrees of lattice mismatch, and report on the quantum critical behaviours not reported in the bulk by transport measurements and resonant X-ray spectroscopy/scattering. In particular, on the decay of the antiferromagnetic Mott insulating state into a non-Fermi liquid, we find evidence of a quantum metal-to-insulator transition that spans a non-magnetic insulating phase.

Temperature-dependence of the Hall coefficient of NdNiO3 thin films

The Hall coefficient of epitaxial NdNiO3 films is evaluated in a wide range of temperatures, from the metallic into the insulating phase. It is shown that for temperatures for which metallic and insulating regions co-exist, the Hall coefficient must be corrected for the time-dependence in the longitudinal resistance, which is due to a slow evolution of metallic and insulating domains. The positive Hall and negative Seebeck coefficients, respectively, in the metallic phase are characteristic for two bands participating in the transport. The change in the sign of the Hall coefficient to negative values in the insulating phase is consistent with the suppression of the contribution from the large hole-like Fermi surface, i.e., the formation of a (pseudo)gap due to charge ordering.

Anisotropic-strain-controlled metal-insulator transition in epitaxial NdNiO3 films grown on orthorhombic NdGaO3 substrates

NdNiO3 (NNO) films were grown by pulsed laser deposition on orthorhombic (110)-, (001)-, and (100)-oriented NdGaO3 substrates. It is found that all the films are tensile-strained but show dramatically different metal-insulator transition (MIT) temperatures (TMI) (160–280 K), as compared with the NNO bulk (∼200 K). A high resemblance in the sharpness of MIT and lattice variation across the MIT was observed. The TMI is highly dependent on the magnitude of the orthorhombic distortion induced by the different substrate surface plane and tends to recover the bulk value after annealing. Our results suggest that the anisotropic epitaxial strain can effectively tune the MIT of NNO films, and the NiO6 octahedra rotation and deformation involved in accommodating the tensile strain might cause the different TMI.

Orbital control of noncollinear magnetic order in nickel oxide heterostructures.

We have used resonant x-ray diffraction to develop a detailed description of antiferromagnetic ordering in epitaxial superlattices based on two-unit-cell thick layers of the strongly correlated metal LaNiO3. We also report reference experiments on thin films of PrNiO3 and NdNiO3. The resulting data indicate a spiral state whose polarization plane can be controlled by adjusting the Ni d-orbital occupation via two independent mechanisms: epitaxial strain and spatial confinement of the valence electrons. The data are discussed in light of recent theoretical predictions.

Publisher's Note: “Effect of structural and magnetic exchange coupling on the electronic transport of NdNiO3 films intercalated with La0.7Sr0.3MnO3 thin layers” [Appl. Phys. Lett. 103, 032403 (2013)

Publisher's Note: “Effect of structural and magnetic exchange coupling on the electronic transport of NdNiO3 films intercalated with La0.7Sr0.3MnO3 thin layers” [Appl. Phys. Lett. 103, 032403 (2013)

Effect of structural and magnetic exchange coupling on the electronic transport of NdNiO3 films intercalated with La0.7Sr0.3MnO3 thin layers

We have fabricated multilayer structure of half-metallic ferromagnet La0.7Sr0.3MnO3 and insulator-metal transition system NdNiO3 on SrTiO3 (100) substrate; thin layers of the former are intercalated in the major matrix of the later. The two structures exhibit pseudomorphic structural coupling with each other. We show that a large increase in the conductivity and an enhanced irreversibility across insulator-metal transition of NdNiO3 may be realized by varying the thickness of La0.7Sr0.3MnO3 and the ratio of thickness of the two layers. Corroborated by the magnetization data, it is shown that the combined effect of structure and magnetism by interfacial coupling is a much better technique than the chemical modification to control the electrical transport of NdNiO3.

Evidence of Kinetically Stable Glassy Phase Formation in Ultrathin NdNiO3 Films

We are reporting the time and temperature dependence of the electrical resistivity exhibited by ultrathin NdNiO3 films. Films with thicknesses ranging from 0.6 to 120 nm were grown on single crystal LaAlO3 substrate (001) using a pulsed laser deposition technique. Temperature dependence of the electrical resistivity indicated dissipation of strain effects for thicknesses ranging from 0.6 to 6 nm, below the critical thickness for defect formation, approximately 12 nm. Strain dissipation prior to defect formation occurred due to a decrease in the Ni–O–Ni bond angle with increasing film thickness resulting from an increased freedom of movement in the NiO6 octahedra. Subsequent defect formation and clustering in the 60 and 120 nm samples resulted in a domination of the material properties by defects rather than strain effects. Our study showed that below the metal–insulator transition temperature, electrical resistivity of the films was very strongly time dependent. A plausible mechanism of kinetically stable glassy phase formation has been proposed which not only explains the time dependence of the electrical resistivity data but also accounts for its anomalous temperature dependence at low temperatures.

Strain dependent stabilization of metallic paramagnetic state in epitaxial NdNiO3 thin films

We report here the strain dependent study of epitaxial NdNiO3 films deposited on LaAlO3 and SrTiO3 substrates using pulsed laser deposition. Electrical transport and magnetic properties of films are found to be controlled by the substrate induced strain. NdNiO3 film on SrTiO3 substrate exhibits behaviour similar to that of bulk NdNiO3, while stabilization of low temperature metallic paramagnetic phase has been observed for film deposited on LaAlO3 substrate. Invariance of Raman spectra, with temperature, of the film on LaAlO3 reveals that the melting of charge ordering under compressive strain is responsible for the stabilization of metallic phase at lower temperature.

Electrical Transport in Ultrathin NdNiO3 Films

ABSTRACTElectrical transport properties in ultrathin NdNiO3 films grown on single crystal LaAlO3(001) substrate were characterized. Films with thicknesses ranging from 0.6 nm to 12 nm were grown using a pulsed laser technique. Four probe resistivity as a function of temperature measurements indicated a strong dissipation of strain effects from 0.6 nm to 6 nm as well as the presence of defects in the 12 nm sample. A proposed mechanism of kinetically stable glassy phase formation explains the time dependence of the resistivity in both cooling and heating cycles.

Probing the metal-insulator transition of NdNiO3 by electrostatic doping

Modulation of the charge carrier density in a Mott material by remote doping from a highly doped conventional band insulator is proposed to test theoretical predictions of band filling control of the Mott metal-insulator transition without introducing lattice distortions or disorder, as is the case for chemical doping. The approach is experimentally tested using ultrathin (2.5 nm) NdNiO3 films that are epitaxially grown on La-doped SrTiO3 films. We show that remote doping systematically changes the charge carrier density in the NdNiO3 film and causes a moderate shift in the metal-insulator transition temperature. These results are discussed in the context of theoretical models of this class of materials exhibiting a metal-insulator transition.

Heterogeneous nucleation and metal-insulator transition in epitaxial films of NdNiO3

We have investigated the temperature driven first order metal-insulator (M-I) transition in thin films of NdNiO3 and have compared it with the bulk behavior. The M-I transition of thin films is sensitive to epitaxial strain and its partial relaxation creates an inhomogeneous strain field in the films which broadens the M-I transition. Both the thin film and the bulk samples exhibit nonequilibrium features in the transition regime which are attributed to the presence of high temperature metallic phases in their supercooled state. The degree of supercooling in the thin films is found to be much smaller than in the bulk which suggests that the M-I transition in the thin film occurs through heterogeneous nucleation.

Strain-mediated metal-insulator transition in epitaxial ultrathin films of NdNiO3

We have synthesized epitaxial NdNiO3 ultrathin films in a layer-by-layer growth mode under tensile and compressive strain on SrTiO3 (001) and LaAlO3 (001), respectively. A combination of x-ray diffraction, temperature dependent resistivity, and soft x-ray absorption spectroscopy has been applied to elucidate electronic and structural properties of the samples. In contrast to the bulk NdNiO3, the metal-insulator transition under compressive strain is found to be completely quenched, while the transition remains under the tensile strain albeit modified from the bulk behavior.