We have investigated the evolution of the electronic properties of La1-xSrxCrO3 (for the full range of x) epitaxial films deposited by molecular beam epitaxy (MBE) using x-ray diffraction, x-ray photoemission spectroscopy, Rutherford backscattering spectrometry, x-ray absorption spectroscopy, electrical transport, and ab initio modeling. LaCrO3 is an antiferromagnetic insulator whereas SrCrO3 is a metal. Substituting Sr2+ for La3+ in LaCrO3 effectively dopes holes into the top of valence band, leading to Cr4+ (3d2) local electron configurations. Core-level and valence-band features monotonically shift to lower binding energy with increasing x, indicating downward movement of the Fermi level toward the valence band maximum. The material becomes a p-type semiconductor at lower doping levels and an insulator-to-metal transition is observed at x greater than or equal to 0.65, but only when the films are deposited with in-plane compression via lattice-mismatched heteroepitaxy. Valence band x-ray photoemission spectroscopy reveals diminution of electronic state density at the Cr 3d t2g-derived top of the valence band while O K-edge x-ray absorption spectroscopy shows the development of a new unoccupied state above the Fermi level as holes are doped into LaCrO3. The evolution of these bands with Sr concentration is accurately captured using density functional theory with a Hubbard U correction of 3.0 eV (DFT + U). Resistivity data in the semiconducting regime (x less than or equal to 0.50) do not fit perfectly well to either a polaron hopping or band conduction model, but are best interpreted in terms of a hybrid model. The activation energies extracted from these fits are well reproduced by DFT + U.
We report herein fabrication and characterization of a thin-film transistor (TFT) using single-crystalline, epitaxial SrTiO3 film, which was grown by a pulsed laser deposition technique followed by the thermal annealing treatment in an oxygen atmosphere. Although TFTs on the polycrystalline epitaxial SrTiO3 films (as-deposited) exhibited poor transistor characteristics, the annealed single-crystalline SrTiO3 TFT exhibits transistor characteristics comparable with those of bulk single-crystal SrTiO3 FET: an on/off current ratio >10^5, sub-threshold swing ~2.1 V/decade, and field-effect mobility…
The interrelation between the epitaxial strain and oxygen deficiency in La0.7Ca0.3MnO3-δ thin films was studied in terms of structural and functional properties. The films with a thickness of 1000Å were prepared using a PLD system equipped with a RHEED facility and a pyrometric film temperature control. The epitaxial strain and the oxygen deficiency in the samples were systematically modified using three different substrates: SrTiO3, (LaAlO3)0.3-(Sr2AlTaO6)0.7 and LaSrAlO4, and four different oxygen pressures during film growth ranging from 0.27mbar to 0.1mbar. It could be demonstrated that th…
Ferromagnetic insulators play a crucial role in the development of low-dissipation quantum magnetic devices for spintronics. Epitaxial LaCoO3 thin film is a prominent ferromagnetic insulator, in which the robust ferromagnetic ordering emerges owing to epitaxial strain. Whereas it is evident that strong spin-lattice coupling induces ferromagnetism, the reported ferromagnetic properties of epitaxially strained LaCoO3 thin films were highly consistent. For example, even under largely modulated degree of strain, the reported Curie temperatures of epitaxially strained LaCoO3 thin films lie within 8…
The capability to control the type and amount of charge carriers in a material and, in the extreme case, the transition from metal to insulator is one of the key challenges of modern electronics. By employing angle resolved photoemission spectroscopy (ARPES) we find that a reversible metal to insulator transition and a fine tuning of the charge carriers from electrons to holes can be achieved in epitaxial bilayer and single layer graphene by molecular doping. The effects of electron screening and disorder are also discussed. These results demonstrate that epitaxial graphene is suitable for ele…
We report x-ray synchrotron experiments on epitaxial films of uranium, deposited on niobium and tungsten seed layers. Despite similar lattice parameters for these refractory metals, the uranium epitaxial arrangements are different and the strains propagated along the a-axis of the uranium layers are of opposite sign. At low temperatures these changes in epitaxy result in dramatic modifications to the behavior of the charge-density wave in uranium. The differences are explained with the current theory for the electron-phonon coupling in the uranium lattice. Our results emphasize the intriguing …
We report on the preparation and characterization of epitaxial thin films of the double-perovskite Sr$_2$CrWO$_6$ by Pulsed Laser Deposition (PLD). On substrates with low lattice mismatch like SrTiO$_3$, epitaxial Sr$_2$CrWO$_6$ films with high crystalline quality can be grown in a molecular layer-by-layer growth mode. Due to the similar ionic radii of Cr and W, these elements show no sublattice order. Nevertheless, the measured Curie temperature is well above 400 K. Due to the reducing growth atmosphere required for double perovskites, the SrTiO$_3$ substrate surface undergoes an insulator-me…
A reversible structural transition of an epitaxial La2/3Sr1/3MnO3 film deposited on the LaAlO3 substrate has been investigated by means of in situ high-resolution transmission electron microscopy (HREM) and electron diffraction, combined with image and diffraction calculations. We observe that the crystallographic symmetry of the film can be lowered via electron beam irradiation, leading to a rhombohedral-monoclinic transition. This transition can be attributed to the cooperating effect of the mismatch stress and the irradiation-induced thermal-stress.
We report a giant resistance drop induced by dc electrical currents in La0.67Ca0.33MnO3 epitaxial thin films. Resistance of the patterned thin films decreases exponentially with increasing current and a maximum drop shows at the temperature of resistance peak Tp. Variation of resistance with current densities can be scaled below and above Tp, respectively. This work can be useful for the future applications of electroresistance.
We show that a ferromagnetic (FM) order in the orthorhombic CaRuO3, which is a non-magnetic and iso-structural analog of FM system SrRuO3, can be established and stabilized by the means of tensile epitaxial strain. Investigations on the structural and magnetic property correlations in the CaRuO3 films with different degrees of strain reveal that the FM moment increases with increasing the tensile strain. This is an experimental verification to the theoretical predictions of scaling of the tensile epitaxial strain and the magnetic order in this system. Our studies further establish that the ten…
We have synthesized epitaxial NdNiO$_{3}$ ultra-thin films in a layer-by-layer growth mode under tensile and compressive strain on SrTiO$_{3}$ (001) and LaAlO$_3$ (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 NdNiO$_{3}$, 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.
Electrical transport properties of RNiO3 (R= Pr, Nd, Sm) thin films grown by pulse laser deposition have been studied. RNiO3 films grow in (100) direction on LaAlO3 (100) substrate. Unlike in polycrystalline solid, PrNiO3 film showed metallic behavior. The first order metal to insulator transition observed in polycrystalline solids is suppressed in RNiO3 films. The effect of lattice strain in the films influensing the transport properties has been studied by varying the thickness of PrNiO3 film on LaAlO3 and also by growing them on SrTiO3 and sapphire substrates. Deviation in the transport pro…
We have measured the transport spin-polarization of epitaxial thin films of the conductive ferromagnetic oxide, SrRuO3, using Point Contact Andreev Reflection Spectroscopy (PCAR). In spite of the fact that spin-up and spin-down electronic densities of states at the Fermi level for SrRuO3 calculated from band structure theory are practically the same, the experimental transport spin polarization for these films was found to be about 50%. This result is a direct consequence of the Fermi velocity disparity between the majority and minority bands and is in good agreement with our theoretical estim…
We report on the functionalization of multiferroic BiFeO3 epitaxial films for spintronics. A first example is provided by the use of ultrathin layers of BiFeO3 as tunnel barriers in magnetic tunnel junctions with La2/3Sr1/3MnO3 and Co electrodes. In such structures, a positive tunnel magnetoresistance up to 30% is obtained at low temperature. A second example is the exploitation of the antiferromagnetic spin structure of a BiFeO3 film to induce a sizeable (~60 Oe) exchange bias on a ferromagnetic film of CoFeB, at room temperature. Remarkably, the exchange bias effect is robust upon magnetic f…
The double perovskite Sr2CrReO6 is an interesting material for spintronics, showing ferrimagnetism up to 635 K with a predicted high spin polarization of about 86%. We fabricated Sr2CrReO6 epitaxial films by pulsed laser deposition on (001)-oriented SrTiO3 substrates. Phase-pure films with optimum crystallographic and magnetic properties were obtained by growing at a substrate temperature of 700 degree C in pure O2 of 6.6x10-4 mbar. The films are c-axis oriented, coherently strained, and show less than 20% anti-site defects. The magnetization curves reveal high saturation magnetization of 0.8 …
Multiferroic BiFeO3 epitaxial films with thickness ranging from 40 nm to 960 nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes. X-ray characterization shows that the structure evolves from angularly-distorted tetragonal with c/a ~ 1.04 to more bulk-like distorted rhombohedral (c/a ~ 1.01) as the strain relaxes with increasing thickness. Despite this significant structural evolution, the ferroelectric polarization along the body diagonal of the distorted pseudo-cubic unit cells, as calculated from measurements along the normal direction, barely ch…
Thin films of perovskite Ruthenates of the general formula ARuO3 (A = Ca and Sr) are versatile electrical conductors for viable oxide electronics. They are also scientifically intriguing, as they exhibit non-trivial electromagnetic ground states depending on the A-site element. Among them, realization of the cubic perovskite (3C) BaRuO3 thin film has been a challenge so far, because the 3C phase is metastable with the largest formation energy among the various polymorph phases of BaRuO3. In this study, we successfully prepared 3C BaRuO3 thin films employing epitaxial stabilization. The 3C BaRu…
Temperature dependent impedance spectroscopy enables the many contributions to the dielectric and resistive properties of condensed matter to be deconvoluted and characterized separately. We have achieved this for multiferroic epitaxial thin films of BiFeO3 (BFO) and BiMnO3 (BMO), key examples of materials with strong magneto-electric coupling. We demonstrate that the true film capacitance of the epitaxial layers is similar to that of the electrode interface, making analysis of capacitance as a function of film thickness necessary to achieve deconvolution. We modeled non-Debye impedance respon…
Using pulsed laser deposition and a unique fast quenching method, we have prepared SrCoOx epitaxial films on SiTiO3 substrates. As electrochemical oxidation increases the oxygen content from x = 2.75 to 3.0, the films tend to favor the discrete magnetic phases seen in bulk samples for the homologous series SrCoO(3-n/8) (n = 0, 1, 2). Unlike bulk samples, 200nm thick films remain single phase throughout the oxidation cycle. 300 nm films can show two simultaneous phases during deoxidation. These results are attributed to finite thickness effects and imply the formation of ordered regions larger …
We have successfully grown epitaxial La$_{1.67}$Sr$_{0.33}$NiO$_4$ films with a small crystalline mosaic using pulsed laser deposition. With synchrotron radiation, the x-ray diffraction peaks associated with charge stripes have been successfully observed for relatively thick films. Anomalies due to the charge-ordering transition have been examined using four-point probe resistivity measurements. X-ray scattering provides direct evidence for suppression of the stripe phase in thin samples; the phase disappears for film thicknesses $\leqslant$ 2600 ~Å. The suppression appears to be a result of s…
We have grown epitaxial thin-films of the orthorhombic phase of YMnO3 oxide on Nb:SrTiO3(001) substrates and their structure, magnetic and dielectric response have been measured. We have found that a substrate-induced strain produces an in-plane compression of the YMnO3 unit cell. The temperature-dependent magnetization curves display a significant ZFC-FC hysteresis at temperatures below the Neel temperature (TN around 40K). The dielectric constant increases gradually (up to 26%) below TN and mimics the ZFC magnetization curve. We argue that these effects are a manifestation of magnetoelectric…
Magneto-transport properties have been investigated for epitaxial thin films of B20-type MnSi grown on Si(111) substrates. Both Lorentz transmission electron microscopy (TEM) images and topological Hall effect (THE) clearly point to the robust formation of skyrmions over a wide temperature-magnetic field region. New features distinct from those of bulk MnSi are observed for epitaxial MnSi films: a shorter (nearly half) period of the spin helix and skyrmions, and an opposite sign of THE. These observations suggest versatile features of skyrmion-induced THE beyond the current understanding.
The discovery of high temperature superconductivity in FeSe films on SrTiO3 substrate has inspired great experimental and theoretical interests. First-principles density functional theory calculations, which have played an important role in the study of bulk iron-based superconductors, also participate in the investigation of interfacial superconductivity. In this article, we review the calculation results on the electronic and magnetic structures of FeSe epitaxial films, emphasizing on the interplay between different degrees of freedom, such as charge, spin, and lattice vibrations. Furthermor…
We have synthesized thermodynamically metastable Ca2IrO4 thin-films on YAlO3 (110) substrates by pulsed laser deposition. The epitaxial Ca2IrO4 thin-films are of K2NiF4-type tetragonal structure. Transport and optical spectroscopy measurements indicate that the electronic structure of the Ca2IrO4 thin-films is similar to that of Jeff = 1/2 spin-orbit-coupled Mott insulator Sr2IrO4 and Ba2IrO4, with the exception of an increased gap energy. The gap increase is to be expected in Ca2IrO4 due to its increased octahedral rotation and tilting, which results in enhanced electron-correlation, U/W. Our…
We have investigated the transport and optical properties along the c-axis of a-axis-oriented Sr2IrO4 epitaxial thin-films grown on LaSrGaO4 (100) substrates. The c-axis resistivity is approximately one order of magnitude larger than that of the ab-plane. Optical absorption spectra with E//b polarization show both Ir 5d intersite transitions and charge-transfer transitions (O 2p to Ir 5d), while E//c spectra show only the latter. The structural anisotropy created by biaxial strain in a-axis-oriented thin-films also changes the electronic structure and gap energy. These a-axis-oriented, epitaxi…
We have fabricated Na_xCoO_2 thin films via lateral diffusion of sodium into Co_3O_4 (111) epitaxial films (reactive solid-phase epitaxy: Ref. 4). The environment of thermal diffusion is key to the control of the sodium content in thin films. From the results of x-ray diffraction and in-plane resistivity, the epitaxial growth and the sodium contents of these films were identified. The thermoelectric measurements show a large thermoelectric power similar to that observed in single crystals. The quasiparticle scattering rate is found to approach zero at low temperatures, consistent with the smal…
We have investigated structural, transport, and optical properties of tensile strained (Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$ (x = 0, 0.025, 0.05) epitaxial thin-films. While High-T$_c$ superconductivity is predicted theoretically in the system, we have observed that all of the samples remain insulating with finite optical gap energies and Mott variable-range hopping characteristics in transport. Cross-sectional scanning transmission electron microscopy indicates that structural defects such as stacking faults appear in this system. The insulating behavior of the La-doped Sr$_3$Ir$_2$O$_7$ thin-fil…
High–quality epitaxial YNixMn1-xO3 thin films have been successfully grown on SrTiO3 (100) (STO) by pulsed laser deposition. X-ray diffraction studies showed that the films deposited on STO are fully c-axis oriented and exhibit in-plane alignment. The magnetic transition temperatures (Tc) of the films (both x=0.33 and 0.5) are equivalent to the values of the corresponding bulk samples. However, when x=0.5, the films show magnetic properties quite different from those of bulk samples. This difference may be caused by the structure distortion in these films.
For an epitaxial Pr0.48Ca0.52MnO3 film on NdGaO3, we use transmission electron microscopy to observe a "charge-ordered" superlattice along the in-plane direction a. The same film shows no electrical signatures of charge order. The in-plane electrical anisotropy (rho)a/(rho)c = 28 is constant, and there is no evidence of sliding charge density waves up to the large field of ~10^3 V/cm.
We have synthesized epitaxial Ba2IrO4 (BIO) thin-films on SrTiO3 (001) substrates by pulsed laser deposition and studied their electronic structure by dc-transport and optical spectroscopic experiments. We have observed that BIO thin-films are insulating but close to the metalinsulator transition boundary with significantly smaller transport and optical gap energies than its sister compound, Sr2IrO4. Moreover, BIO thin-films have both an enhanced electronic bandwidth and electronic-correlation energy. Our results suggest that BIO thin-films have great potential for realizing the interesting ph…
Effect of growth orientation on charge- and orbital-ordering (CO-OO) phenomena has been studied for Pr0.5Ca0.5MnO3 epitaxial thin films fabricated on (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates by means of resistivity, synchrotron x-ray diffraction, and polarized optical microscopy measurements. CO-OO transition is observed around 220 K for a film grown on an LSAT (011) substrate ((011)-film), similarly to a bulk sample, while a film grown on a (001) plane of LSAT ((001)-film) shows much higher transition temperature around 300 K. The domain size of OO is approximately 3 times as large i…
We have studied structural and transport properties of epitaxial NaxCoO2 thin films on (0001) sapphire substrate prepared by topotaxially converting an epitaxial Co3O4 film to NaxCoO2 with annealing in Na vapor. The films are c axis oriented and in-plane aligned with [10 1 0] NaxCoO2 rotated by 30 degrees from [10 1 0] sapphire. Different Na vapor pressures during the annealing resulted in films with different Na concentrations, which showed distinct transport properties.
We report on the synthesis of ultrathin films of highly distorted EuNiO3 (ENO) grown by interrupted pulse laser epitaxy on YAlO3 (YAO) substrates. Through mapping the phase space of nickelate thin film epitaxy, the optimal growth temperatures were found to scale linearly with the Goldschmidt tolerance factor. Considering the gibbs energy of the expanding film, this empirical trend is discussed in terms of epitaxial stabilization and the escalation of the lattice energy due to lattice distortions and decreasing symmetry. These findings are fundamental to other complex oxide perovskites, and pro…
We investigate skyrmion formation in both a single crystalline bulk and epitaxial thin films of MnSi by measurements of planar Hall effect. A prominent stepwise field profile of planar Hall effect is observed in the well-established skyrmion phase region in the bulk sample, which is assigned to anisotropic magnetoresistance effect with respect to the magnetic modulation direction. We also detect the characteristic planar Hall anomalies in the thin films under the in-plane magnetic field at low temperatures, which indicates the formation of skyrmion strings lying in the film plane. Uniaxial mag…
A systematic study of the temperature and magnetic field dependence of the longitudinal and transverse resistivities of epitaxial thin films of magnetite (Fe3O4) is reported. The anisotropic magnetoresistance (AMR) and the planar Hall effect (PHE) are sensitive to the in-plane orientation of current and magnetization with respect to crystal axes in a way consistent with the cubic symmetry of the system. We also show that the AMR exhibit sign reversal as a function of temperature, and that it shows significant field dependence without saturation up to 9 T. Our results provide a unified descript…
A quantitative mathematical model for the critical thickness of strained epitaxial metal films is presented, at which the magnetic moment experiences a reorientation from in-plane to perpendicular magnetic anisotropy. The model is based on the minimum of the magnetic anisotropy energy with respect to the orientation of the magnetic moment of the film. Magnetic anisotropy energies are taken as the sum of shape anisotropy, magnetocrystalline anisotropy and magnetoelastic anisotropy, the two latter ones being present as constant surface and variable volume contributions. Other than anisotropy mat…
We report on a comprehensive investigation of the effects of strain and film thickness on the structural and magnetic properties of epitaxial thin films of the prototypal $J_\mathrm{eff}=1/2$ compound Sr$_2$IrO$_4$ by advanced X-ray scattering. We find that the Sr$_2$IrO$_4$ thin films can be grown fully strained up to a thickness of 108 nm. By using X-ray resonant scattering, we show that the out-of-plane magnetic correlation length is strongly dependent on the thin film thickness, but independent of the strain state of the thin films. This can be used as a finely tuned dial to adjust the out…
Strong interplay of fundamental order parameters in complex oxides are known to give rise to exotic physical phenomena. The 4d transition metal oxide SrRhO3 has generated much interest, but advances have been hindered by difficulties in preparing single crystalline phases. Here, we have epitaxially stabilized high quality single crystalline SrRhO3 films and investigated their structural, electronic, and magnetic properties. We determine that their properties significantly differ from the paramagnetic metallic ground state that governs bulk samples and are strongly related to rotations of the R…
Quantum wells made of simple polyvalent metals represent a novel family of doped 2D Mott-Hubbard insulators. As scanning tunneling microscopy experiments show, these systems exhibit an anisotropic form of metal-insulator transition. Their elementary excitations possess coherent wave-like properties along the normal axis, and show an incoherent behavior in-plane. The development of such an anisotropic coherence is most likely related to Coulomb interaction between localized and delocalized thin film electronic states - 2D Kondo screening.
A detailed investigation of the magnetization processes in epitaxial MnSi thin film reveals the existence of elliptically distorted skyrmion strings that lie in the plane of the film. We provide proof that the uniaxial anisotropy stabilizes this state over extended regions of the magnetic phase diagram. Theoretical analysis of an observed cascade of first-order phase transitions is based on rig- orous numerical calculations of competing chiral modulations, which shows the existence of helicoids, elliptic skyrmions, and cone phases.
Transport measurements along the node and anti-node directions in the a-b plane of optimally doped and epitaxial thin films of YBCO are reported. Low bias magnetoresistance measurements near and below T_c show that the flux flow resistivity along the node and anti-node directions versus magnetic field are indistinguishable. This result suggests that within the experimental error of our measurements, no correspondence is found between the flux pinning properties in YBCO and the d-wave nature of the order parameter.
