Epitaxial Constraints Research Articles

Structural, electronic, and magnetic properties ofSrRuO3under epitaxial strain

Using density functional theory within the local spin density approximation, structural, electronic and magnetic properties of SRO are investigated. We examine the magnitude of the orthorhombic distortion in the ground state and also the effects of applying epitaxial constraints, whereby the influence of large (in the range of $\pm 4%$) in-plane strain resulting from coherent epitaxy, for both [001] and [110] oriented films, have been isolated and investigated. The overall pattern of the structural relaxations reveal coherent distortions of the oxygen octahedra network, which determine stability of the magnetic moment on the Ru ion. The structural and magnetic parameters exhibit substantial changes allowing us to discuss the role of symmetry and possibilities of magneto-structural tuning of \SRO-based thin film structures.

Magnetic properties of epitaxial CrN films

The authors have investigated the structural and magnetic properties of CrN films grown on MgO(001) and sapphire(0001) by rf-plasma-assisted molecular beam epitaxy. CrN∕MgO(001) exhibits a better epitaxial quality than CrN/sapphire(0001). The CrN∕MgO film shows clear paramagnetic behavior at low temperatures, whereas CrN/sapphire exhibits a ferromagneticlike response with an order temperature above room temperature which resembles the magnetic behavior found in Cr-doped dilute magnetic semiconductors. Keeping in mind that bulk CrN exhibits antiferromagnetic behavior, the dramatically different magnetic behaviors found in epitaxial CrN films grown on MgO and sapphire demonstrate the importance of epitaxial constraints in determining their magnetic properties.

Structure and magnetism of Co a(1− x) Mn axGe b epitaxial films

The structural and magnetic phase diagrams of Co a(1− x) Mn ax Ge b epitaxial films near the composition of Heusler alloy Co 2MnGe have been studied using combinatorial molecular beam epitaxy techniques. Epitaxial growth on Ge (1 1 1) has been stabilized over nearly the entire composition range of the ternary system. Epitaxial constraints play an important role for the small number of observed structural phases. The variety of high-temperature magnetic properties, and the structural and chemical compatibilities with group IV elements make the ternary system promising for the science and applications of spintronics.

Pd bonded on Nb(001): Dependence of noble metal and ferromagnetic characteristics on film thickness

Experimental observations confirmed by density functional theory (DFT) calculations show that strong epitaxial bonds between Pd atoms and the substrate can induce two competing properties: nobleness and ferromagnetic order in the same material, i.e., Pd bonded on Nb(100). Angle-resolved ultraviolet photoelectron spectroscopy measurements, confirmed by first principles, self-consistent DFT calculations show that the strong, direct bonds between a Pd monolayer and Nb(001) push the $d$-band center of the monolayer toward lower binding energies, which results in the Pd reactivity comparable to that of the noble metal Ag. The strong epitaxial constraint of the Nb(001) substrate induces a $(11\overline{2}0)$-oriented hexagonal close-packed structure in thicker Pd films. First principles, self-consistent DFT calculations with spin-orbit coupling included performed at 0 K show that Pd in this structure is ferromagnetically ordered at the optimum lattice constant. Its bands at the Fermi level are flatter in comparison to those of Pd in its natural, nonmagnetic, face-centered-cubic structure, leading to the density of states (DOS) at the Fermi level which fulfills the Stoner criterion for ferromagnetism. We identify these bands in the bulk band structure and probe them with angle-resolved ultraviolet photoelectron spectroscopy in $(11\overline{2}0)$-oriented hexagonal close-packed Pd films.

Strain Phase Diagram and Domain Orientation inSrTiO3Thin Films

SrTiO3 thin films were used as a model system to study the effects of strain and epitaxial constraint on structural phase transitions of perovskite films. The basic phenomena revealed will apply to a variety of important structural transitions including the ferroelectric transition. Highly strained SrTiO3 films were grown on different substrates, providing both compressive and tensile strain. The measured strain-temperature phase diagram is qualitatively consistent with theory; however, the increase in the phase transition temperature is much larger than predicted. Because of the epitaxial strain and substrate clamping, the SrTiO3 lattice is tetragonal at all temperatures. The phase transitions involve only changes in internal symmetry. The low temperature phase under tensile strain has a unique structure with orthorhombic Cmcm space group but a tetragonal lattice, an interesting consequence of epitaxial constraint.

Destruction of spin cycloid in (111)c-oriented BiFeO3 thin films by epitiaxial constraint: Enhanced polarization and release of latent magnetization

In BiFeO3 films, it has been found that epitaxial constraint results in the destruction of a space modulated spin structure. For (111)c films, relative to corresponding bulk crystals, it is shown (i) that the induced magnetization is enhanced at low applied fields; (ii) that the polarization is dramatically enhanced; whereas, (iii) the lattice structure for (111)c films and crystals is nearly identical. Our results evidence that eptiaxial constraint induces a transition between cycloidal and homogeneous antiferromagnetic spin states, releasing a latent antiferromagnetic component locked within the cycloid.

Epitaxially induced high temperature (>900K) cubic-tetragonal structural phase transition in BaTiO3 thin films

For (001)c oriented BaTiO3 thin films, it has been found that epitaxial constraint can result in a dramatic increase in the temperature of a tetragonal (T) structural phase transition. For 2000-Å-thick films grown directly on SrTiO3 substrates, a T→cubic (C) phase transition was found on heating at >950K, where the lattice constant changed smoothly with temperature. It was also found for films of the same thickness that the T→C transition is nearly restored to that of bulk crystals by the use of a buffer layer, which relaxes the epitaxial constraint. These results provide evidence of an epitaxially induced high temperature structural phase transition in BaTiO3 thin films, where the ferroelectric (internal) and structural (external) aspects of the phase transition are decoupled.

Extending the magnetic order of MnAs films on GaAs to higher temperatures

Manganese arsenide is a promising candidate for new spintronics applications since it is ferromagnetic at room temperature and can be grown with high epitaxial quality on semiconductors. However, the transition temperature of ∼ 40 ∘ C is a limiting factor for device applications. Since the coupled magnetic and structural transition is of first order, it is in principle possible to shift the transition temperature by changing external parameters. Here we show that by either applying an external magnetic field or by growing the MnAs films on GaAs(1 1 1), i.e., by changing the epitaxial constraints which are equivalent to external pressure, ferromagnetic order can be stabilized well above the bulk-phase transition temperature.

Electric and Magnetic Properties of Hetero-Epitaxially Deposited BiFeO3 Thin Films

<TEX>$BiFeO_3$</TEX> films grown on (111) <TEX>$SrTiO_3$</TEX> substrate have a rhombohedral structure, identical to that of single crystals. On the other hand, films grown on (110) or (001) <TEX>$SrTiO_3$</TEX> substrate are monoclinically distorted from the rhombohedral structure due to the epitaxial constraint. The easy axis of spontaneous polarization is close to [111] for the variously oriented films. Dramatically enhanced polarization and magnetization have been found for hetero-epitaxially grown <TEX>$BiFeO_3$</TEX> thin films comparing to that of <TEX>$BiFeO_3$</TEX> crystals. The results are explained in terms of an epitaxially-induced transition between cycloidal and homogeneous spin states, via magneto-electric interactions.

Growth of single-phase c-axis aligned La 1.2Ca 1.8Mn 2O 7 films on SrTiO 3 (001)

We report the growth of single phase, c-axis aligned thin films of La 1.2Ca 1.8Mn 2O 7 on SrTiO 3 (001) substrates using a controlled pulsed laser deposition method. In this method, constraint of epitaxy is utilized to stabilize the Ruddlesdon–Popper (RP) phase of La 1.2Ca 1.8Mn 2O 7. Oxygen ambient pressure and the rate of deposition play a very important role in influencing the epitaxial growth as well as maintaining phase purity of the material. The oxygen pressure inside the deposition chamber was very precisely controlled and varied during the layer-by-layer growth of the film. Films, prepared by our method, show excellent electrical and magnetic characteristics with a sharp metal–insulator transition at T M–I=90 K, closely followed by a magnetic transition at T C=91 K.

Giant birefringence in zinc-blende-based artificial semiconductors

We use extended-basis empirical tight-binding calculations and examine the anisotropy of the refractive index in ultrashort-period superlattices of materials sharing no common atom. We find that a strong birefringence can be engineered in these articial semiconductors, allowing phase matching for frequency difference generation. The prominent role of epitaxial constraint and bond-length alternation is evidenced.

Magnetism of ultrathin nanowires: ab initio study

The results of ab initio calculations within density-functional theory are presented for ultrathin wires composed of fifth and sixth row elements. At first free-standing wires are examined and then, similar to experimental conditions, wires are placed along the step ledges of vicinal silver surfaces. Free-standing wires show mostly a weak tendency towards magnetism at the calculated equilibrium bond lengths. The largest magnetic moments are found for wires with an antiferromagnetic ground state. Unexpectedly, a faint magnetism is predicted also for Pd and the simple metals In and Tl. The epitaxial constraint imposed by the silver substrate induces an increased intrawire bond length favoring magnetic ordering whereas hybridization effects work against magnetism. We find that on Ag substrate most elements tend to antiferromagnetism. The possible growth modes of wires, a row-by-row growth and island formation are discussed.

Dramatically enhanced polarization in (001), (101), and (111) BiFeO3 thin films due to epitiaxial-induced transitions

Dramatically enhanced polarization has been found for (001), (101), and (111) films, relative to that of BiFeO3 crystals. The easy axis of spontaneous polarization lies close to (111), for the various oriented films. BiFeO3 films grown on (111) have a rhombohedral structure, identical to that of single crystals; whereas films grown on (101) or (001) are monoclinically distorted from the rhombohedral structure, due to the epitaxial constraint.

Ab initiostudy of the phase diagram of epitaxialBaTiO3

Using a combination of first-principles and effective-Hamiltonian approaches, we map out the structure of BaTiO3 under epitaxial constraints applicable to growth on perovskite substrates. We obtain a phase diagram in temperature and misfit strain that is qualitatively different from that reported by Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)], who based their results on an empirical thermodynamic potential with parameters fitted at temperatures in the vicinity of the bulk phase transitions. In particular, we find a region of `r phase' at low temperature where Pertsev et al. have reported an `ac phase'. We expect our results to be relevant to thin epitaxial films of BaTiO3 at low temperatures and experimentally-achievable strains.

Interplay of elastic and magnetic properties in MnAs/GaAs heterostructures

Ferromagnetic α-MnAs is a good candidate for electrical spin injection into semiconductors. Due to the epitaxial constraints in MnAs/GaAs(0 0 1) heterostructures, where the in-plane lattice-spacings a and c cannot change during the first-order phase transition, the α- and β-phases coexist over a wide temperature range (10–40°C) and exhibit a unique periodic stripe pattern along MnAs[0 0 0 1]. This coexistence of elastic domains of two phases minimizes the elastic energy. The MnAs-on-GaAs(0 0 1) films exhibit an additional out-of-plane component of the magnetization which led us to a new concept for programmable magneto-logic devices.

Spontaneous transition from epitaxially constrained to equilibrium Ge nanocrystals on silicon-on-insulator ( [formula omitted

Epitaxial constraints define the stress-driven self-assembly of faceted nanocrystals and in particular one of their key characteristics––their shape. Here we identify a technologically relevant system, Ge islands grown on ultrathin silicon-on-insulator (SOI) substrate in which nanocrystals, whose shape is initially defined by epitaxial constraints, spontaneously overcome those constraints and transform to their equilibrium shape. Ge nanocrystals on ultrathin SOI form initially as huts and then transform into domes, similar to the sequence of epitaxially constrained shapes they assume on bulk Si(1 0 0). While the sequence on bulk Si ends here, we observe further dramatic morphological changes on ultrathin SOI: a spontaneous transformation to equilibrium-shaped Ge nanocrystals.

Surface-oxidation studies of cube-textured, pure nickel to form NiO as a potential YBa2Cu3O7−x-coated conductor buffer layer

The growth of nickel oxide (NiO) on {100}〈001〉 cube textured, rolling-assisted biaxially textured substrates (RABiTS) of pure nickel was studied. Single-phase (100) NiO formed only in a narrow temperature range at 1250 ± 5 °C. At lower or higher temperatures, other orientations, namely (111), (311), and (220), also formed. At 1250 °C, practically single phase (100) NiO was observed for short oxidation times t of 0.2–10 min (oxide thickness &lt; 10 μm). For 10 &lt; t &lt; 120 min, small quantities of (111) NiO formed in addition to (100), but near-single-phase (100) NiO formed once again after oxidation for &gt;150 min (thickness &gt; 35 μm). The ratio of (100) to (111) textures with oxidation time is explained in terms of epitaxial constraints, growth rates, and oxygen absorption on the (100) and (111) grains. The optimum oxidation conditions are oxidation for approximately 0.5 min at 1250 °C in flowing oxygen, yielding (100) NiO, a few microns in thickness, and root-mean-square roughness of approximately 40 nm on the length-scale of the grain size.

Strain-mediated phase coexistence in MnAs heteroepitaxial films on GaAs: An x-ray diffraction study

The temperature-dependent phase coexistence between crystalline phases in heteroepitaxial films of MnAs on GaAs is studied. The epitaxial constraints on the film expansion are analyzed. The x-ray-diffraction data are fitted to a model of periodic elastic domains. The temperature dependencies of phase fractions, the domain sizes, and the misfits are simultaneously obtained. The domain sizes correspond to the minimum of elastic energy, which proves the equilibrium state of the heteroepitaxial system at each temperature.

Strain-induced tuning of metal–insulator transition in NdNiO3

We have used the lattice-mismatch epitaxial strain, induced by the constraint of epitaxy, to tune the metal–insulator (M–I) transition temperature of NdNiO3 films grown on Si(100) substrate. Films were integrated with the Si(100) substrate using several combinations of thin buffer layers. A systematic variation in the electrical transport properties has been observed with the change in the lattice mismatch between NdNiO3 and the buffer layer just below it. It was shown that by the proper selection of the substrate and thickness of film, it is possible to control and precisely tune the M–I transition temperature of NdNiO3 to any desired value between 12 and 300 K (temperature range of this study). Fine control over the M–I transition temperature of these films is likely to boost the potential of these films for their applications in bolometers, actuators, and thermal/optical switches in next-generation perovskite-based microelectronic devices.

Band gap in epitaxial NaCl-structure CrN(001) layers

B1–NaCl-structure CrN(001) layers were grown on MgO(001) at 600 °C by ultrahigh vacuum reactive magnetron sputter deposition in pure N2 discharges. X-ray diffraction analyses establish the epitaxial relationship as cube-on-cube, (001)CrN∥(001)MgO with [100]CrN∥[100]MgO, while temperature-dependent measurements show that the previously reported phase transition to the orthorhombic Pnma structure is, due to epitaxial constraints, absent in our layers. The resistivity increases with decreasing temperature, from 0.028 Ω cm at 400 K to 271 Ω cm at 20 K, indicating semiconducting behavior with hopping conduction. Optical absorption is low (α&amp;lt;2×104 cm−1) for photon energies below 0.7 eV and increases steeply at higher energies. In situ ultraviolet photoelectron spectra indicate that the density of states vanishes at the Fermi level. The overall results provide evidence for CrN exhibiting a Mott–Hubbard type band gap.