Substrate-induced Strain Research Articles

Effect of strain on the stability and electronic properties of ferrimagnetic Fe2−xTixO3 heterostructures from correlated band theory

Based on density functional theory calculations including an on-site Hubbard U term, we investigate the effect of substrate-induced strain on the properties of ferrimagnetic hematite-ilmenite solid solutions and heterostructures. While the charge compensation mechanism through formation of a mixed Fe2+, Fe3+-contact layer is unaffected, strain can be used to tune the electronic properties of the system, e.g., by changing the position of impurity levels in the band gap. Straining Fe2O3–FeTiO3 films at the lateral lattice parameters of Al2O3(0001), commonly used as a substrate, is found to be energetically unfavorable as compared to films on Fe2O3(0001) or FeTiO3(0001)-substrates.

Effect of biaxial strain on the electrical and magnetic properties of (001) La0.7Sr0.3MnO3 thin films

We have studied the effect of biaxial strain on thin films of (001) La0.7Sr0.3MnO3. We deposited films by reactive molecular-beam epitaxy on different single crystalline substrates, varying the substrate-induced biaxial strain from −2.3% to +3.2%. Magnetization and electrical transport measurements reveal that the dependence of the Curie temperature on biaxial strain is in very good agreement with the theoretical predictions of Millis et al. [J. Appl. Phys. 83, 1588 (1998)].

Magnetic oxide nanowires with strain-controlled uniaxial magnetic anisotropy direction

While magnetic nanowires generally have a preferential magnetization direction along the wire axis to minimize magnetostatic energy, it is shown here for epitaxial magnetic oxide nanowires that substrate-induced strain can be used to tailor the magnetic easy axis in any direction. La0.67Sr0.33MnO3 (LSMO) nanowires were prepared by pulsed laser deposition of LSMO thin films on NdGaO3 (NGO) substrates of two different orientations [NGO(110)o and NGO(010)o], followed by patterning into arrays of nanowires by laser interference lithography. The uniaxial compressive strain from the substrate induces a strong uniaxial magnetic anisotropy in the LSMO that dominates the anisotropy. Hence, one obtains LSMO nanowires having a magnetic easy axis that can lie in any direction, including perpendicular to the wire axis. In marked contrast, similar nanowires on SrTiO3(001) substrates without significant uniaxial strain exhibit the usual preferential magnetization direction along the wire axis, as dictated by magnetostatic shape anisotropy. The tunable magnetic anisotropy direction is a useful feature for applications of magnetic nanowires in magnetic memory, sensor, and logic devices.

Ferromagnetism in epitaxial orthorhombic YMnO 3 thin films

Epitaxial orthorhombic YMnO 3 thin films, (0 0 1) oriented, have been grown by pulsed laser deposition on (0 0 1)SrTiO 3 substrates. Their crystal structure and magnetic response have been studied in detail. Although bulk o-YMnO 3 is antiferromagnetic, our magnetic measurements reveal intriguing thermal hysteresis between the zero-field-cooled and field-cooled curves below the onset of the antiferromagnetic ordering temperature, thus signaling a more complex magnetic structure with net ferromagnetic moments. We discuss on the possible origin of this net magnetization and we have found a correlation of the magnetic response with the strain state of the films. We propose that substrate-induced strain modifies the subtle competition of magnetic interactions and leads to a non-collinear magnetic state that can thus be tuned by strain engineering.

Enhanced magnetization of CuCr2O4 thin films by substrate-induced strain

The first synthesis of epitaxial spinel CuCr2O4 thin films is reported, which exhibit enhanced magnetization in excess of 200% of the accepted bulk value when grown on single-crystal (110) MgAl2O4 substrates. Bulk CuCr2O4 is a ferrimagnetic insulator with a low net magnetic moment of 0.5μB due to its tetragonal unit cell with a c∕a ratio of 1.29 and frustrated moment configuration. It is shown that through epitaxial growth and substrate-induced strain, it is possible to enhance the magnetization of these films by reducing the tetragonality of its unit cell which, in turn, effectively decreases magnetic moment frustration allowing for a greater net moment.

Investigation of substrate-induced strain effects in La0.7Ca0.15Sr0.15MnO3 thin films using ferroelectric polarization and the converse piezoelectric effect

We have fabricated manganite film/ferroelectric crystal heterostructures by growing La0.7Ca0.15Sr0.15MnO3 (LCSMO) films on ferroelectric 0.67Pb(Mg1/3Nb2/3)O3−0.33PbTiO3 (PMN-PT) single-crystal substrates. The efficient mechanical coupling at the interface, originated from ferroelectric polarization or the converse piezoelectric effect in the PMN-PT substrate, gives rise to large changes in the strain state, electrical resistance, magnetoresistance, and insulator-to-metal transition temperature (TP) of the film. We interpreted all these changes in terms of substrate-induced strain, which modifies the tetragonal distortion of MnO6 octahedra and the electron-lattice coupling strength in the film. Quantitative relationships between TP and induced strain in the LCSMO film have been established.

Magnetic properties of the insulating ferromagnetic phase in strained Pr0.6Ca0.4MnO3 thin films

Bulk magnetization in Pr0.6Ca0.4MnO3 thin films with tensile (SrTiO3) and compressive (LaAlO3) substrate-induced strain is compared to the magneto-optical Kerr effect (MOKE) measurements. In the absence of an external magnetic field, in both films, a stable ferromagnetic insulating majority phase coexists with an antiferromagnetic insulating phase below ∼120K. The MOKE measurements indicate that at 5K a metastable ferromagnetic metallic (FM) phase is formed at the surface of the stretched film in a magnetic field below 1.1T already; while in the bulk the FM phase starts to form in the field above ∼4T in both films.

Substrate-induced strain effect in La0.875Ba0.125MnO3 thin films grown on ferroelectric single-crystal substrates

The authors have studied the substrate-induced strain effect in La0.875Ba0.125MnO3 (LBMO) thin films grown on ferroelectric 0.67Pb(Mg1∕3Nb2∕3)O3–0.33PbTiO3 (PMN-PT) single-crystal substrates. Both the strain and resistance of the films can be in situ varied by applying an electric field across the PMN-PT substrates. X-ray diffraction analysis indicates that the variations of strain and resistance result from the induced strain in the PMN-PT substrate due to the ferroelectric polarization or the converse piezoelectric effect. The relationships between the resistance and the induced strain in the LBMO film and PMN-PT substrate have been quantitatively analyzed.

Microstructure and magnetic properties of strained Fe3O4 films

A comparable study of the microstructure and magnetic properties was performed for magnetite films deposited on (100)-oriented MgO and SrTiO3 (STO) substrates. The growth of strained high quality Fe3O4 films was confirmed by x-ray diffraction analysis and Raman spectroscopy measurements. The surface morphology and magnetic properties of the two films were found to be obviously different. Moreover, a stripelike magnetic domain structure was observed in the film on STO. Substrate-induced strain is believed to be responsible for these observations, which significantly affects the magnetic anisotropy and the magnetic coupling at the antiphase boundaries in the films.

Thickness dependence of substrate-induced strain in La 0.9Ca 0.1MnO 3 thin films

Thin films of La 0.9Ca 0.1MnO 3 (LCMO) with various thicknesses in the range of 40–200 nm were deposited on (1 0 0) SrTiO 3 (STO) substrates using the pulsed laser deposition technique. The as-grown LCMO films were characterized with respect to the structure as well as their magnetic and transport properties. The X-ray diffraction results show that high quality, single phase and ‘ b’ axis orientated epitaxial LCMO thin films have been produced successfully. Out-of-plane lattice parameters has been found to increase with the film thickness. Transport measurements, performed using standard four probe technique, and SQUID magnetometry revealed a ferromagnetic-insulating phase consistent with the bulk phase diagram.

Correlations between morphology, crystal structure, and magnetization of epitaxial cobalt-platinum films grown with pulsed laser ablation

The effects of growth rate (Gr), deposition temperature (Td), film thickness (tF), and substrate-induced strain (ϵ) on morphological, crystallographic, and magnetic characteristics of equiatomic CoPt epitaxial films synthesized with pulsed laser deposition are investigated. The (001)-oriented single-crystal substrates of MgO, SrTiO3, and LaAlO3 provide different degrees of epitaxial strain for growth of the disordered face-centered cubic (fcc) and ordered face-centered tetragonal (L10) phases of CoPt. The films deposited at Td≈600 °C on all three substrates are fcc with in-plane magnetization and a narrow hysteresis loop of width ≈200 Oe. The L10 phase, stabilized only at Td≥700 °C, becomes predominantly c-axis oriented as Td is increased to 800 °C. While the crystallographic structure of the films depends solely on the Td, their microstructure and magnetization characteristics are decided by the growth rate. At the higher Gr(≈1 Å/s) the L10 films have a maze-like structure which converts to a continuous film as the tF is increased from 20 to 50 nm. The magnetic coercivity of these films increases as the L10 phase fraction grows with Td and its orientation becomes out of the film plane. The evolution of microstructure with Td is remarkably different at a lower growth rate (≈0.4 Å/s). Here, the structure changes from a self-similar fractal pattern to a disordered assembly of nanodots as the Td is raised from 700 to 800 °C, and is understood in terms of the imbalance between strain and interfacial energies. Magnetic force microscopy of such films reveals no distinct domain walls within the nanoislands, while a clear contrast is seen between the islands of reversed magnetization. Magnetic relaxation measurements on these assemblies of single-domain islands show a negligible decay of magnetization unless a reverse field close to the coercive field (Hc≈30 kOe) is applied. The simple picture of coherent rotation of moment appears incompatible with the time dependence of the remanent magnetization in these films.

Elastic and orbital effects on thickness-dependent properties of manganite thin films

We report on the structural and magnetic characterization of (110) and (001) ${\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) epitaxial thin films simultaneously grown on (110) and $(001)\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ substrates, with thicknesses $t$ varying between 8 and $150\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. It is found that while the in-plane interplanar distances of the (001) films are strongly clamped to those of the substrate and the films remain strained up to well above $t\ensuremath{\approx}100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the (110) films relax much earlier. Accurate determination of the in-plane and out-of-plane interplanar distances has allowed concluding that for $tg20$ nm, unit cell volume expansion does not change substantially for (001) films whereas it relaxes towards bulk value for (110) ones. However, in all cases, an abnormal unit cell expansion is observed for $tl20$ nm. It is observed that the magnetic properties (Curie temperature and saturation magnetization) of the (110) films are significantly improved compared to those of (001) films. These observations, combined with $^{55}\mathrm{Mn}$-nuclear magnetic resonance data and x-ray photoemission spectroscopy, signal that the depression of the magnetic properties of the more strained (001)LCMO films is not caused by an elastic deformation of the perovskite lattice but rather due to the electronic and chemical phase separation caused by the substrate-induced strain. On the contrary, the thickness dependence of the magnetic properties of the less strained (110)LCMO films are simply described by the elastic deformation of the manganite lattice. We will argue that the different behavior of (001) and (110)LCMO films is a consequence of the dissimilar electronic structure of these interfaces.

Photoinduced phase separation inBi0.4Ca0.6MnO3thin films

In this paper, we report a study of photoinduced and current-induced resistivity changes in ${\mathrm{Bi}}_{0.4}{\mathrm{Ca}}_{0.6}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films. The photoinduced effects were investigated using both far-field and near-field optical techniques. It was found that substrate-induced strain affects the stability of the charge ordering. The magnitude of the energy barrier separating the metastable conducting phase and the charge-ordered insulating phase estimated from the temperature dependence of the lifetime of the photoinduced conducting phase was found to be large, of the order of the charge-ordering temperature.

Decrease of the Curie temperature in La 0.67Sr 0.33MnO 3 thin films induced by Au capping

Ferromagnetism of La 0.67Sr 0.33MnO 3 is extremely sensitive to external perturbations like substrate-induced strain, charge injection and chemical interactions with neighbour layers. In this paper we discuss the perturbation induced by the presence of a metallic overlayer, typically deposited for electric contacts, in the prototypical case of the Au/La 0.67Sr 0.33MnO 3 interface. In particular we found a sizable decrease of the Curie temperature in thin films of La 0.67Sr 0.33MnO 3 after gold capping: 65 K for 5 nm thickness of the manganite. Apart from chemical reactions at the interface, charge injection–depletion induced by the difference in the work function between Au and La 0.67Sr 0.33MnO 3 could partially explain this phenomenon.

Microstructure and electrical properties of (120)O-oriented and of (001)O-oriented epitaxial antiferroelectric PbZrO3 thin films on (100) SrTiO3 substrates covered with different oxide bottom electrodes

Epitaxial antiferroelectric PbZrO3 (PZO) thin films of two different crystallographic orientations were grown by pulsed laser deposition on (100)-oriented SrTiO3 single crystal substrates. The latter were covered either with SrRuO3 epitaxial bottom electrodes, or with an epitaxial BaZrO3 buffer layer and an epitaxial BaPbO3 bottom electrode, respectively. Their crystal orientation and microstructure were characterized by x-ray diffraction, transmission electron microscopy, and electron diffraction. The orthorhombic (index O) PZO films on SrRuO3∕SrTiO3 were predominantly (120)O oriented and consisted of four azimuthal domains forming 90° and 60° boundaries, whereas those grown on BaPbO3∕BaZrO3∕SrTiO3 were (001)O oriented. All films showed well-defined double P-E hysteresis loops, four distinct switching peaks in the current-voltage characteristics, and piezoelectric double loops recorded by piezoresponse scanning force microscopy. The values of the saturation polarization PS and the critical field EC of the (120)O-oriented PZO films (PS=41μC∕cm2; EC=445kV∕cm) are different from those of the (001)O-oriented films (PS=24μC∕cm2; EC=500kV∕cm). A transition temperature to the paraelectric phase of 260°C has been found, which is 30K higher than the bulk value, probably indicating a stabilization of the antiferroelectric phase by substrate-induced strain.

Dielectric anomaly and magnetic response of epitaxial orthorhombic YMnO3 thin films

The structure, magnetic response, and dielectric response of the grown epitaxial thin films of the orthorhombic phase of YMnO3 oxide on Nb:SrTiO3 (001) substrates have been measured. We have found that a substrate-induced strain produces an in-plane compression of the YMnO3 unit cell. The magnetization versus temperature curves display a significant zero-field cooling (ZFC)-field cooling hysteresis below the Néel temperature (TN ≈ 45 K). The dielectric constant increases gradually (up to 26%) below the TN and mimics the ZFC magnetization curve. We argue that these effects could be a manifestation of magnetoelectric coupling in YMnO3 thin films and that the magnetic structure of YMnO3 can be controlled by substrate selection and/or growth conditions.

Effect of substrate-induced strain on transport and magnetic properties of epitaxial La0.66Sr0.33MnO3 thin films

Electrical transport and magnetic properties of epitaxial 500Å La0.67Sr0.33MnO3 (LSMO) thin films, grown on different substrates having different lattice strains, are found to exhibit strong correlation with Jahn-Teller (JT) strain. The authors’ study reveals that a sufficiently large JT strain gives rise to distinct insulating state in LSMO films even below the respective para-ferromagnetic Curie temperature, which is a contradiction with the established phase diagram of Sr doped manganites. The authors have presented a microscopic model for the analysis of their data instead of the usual expansion around the undistorted state. The model incorporates two relevant eg orbitals and the effect of both JT and bulk strains on the transition temperature via double exchange.

Effect of substrate-induced strains on the spontaneous polarization of epitaxial BiFeO3 thin films

A single-domain thermodynamic theory is employed to predict the spontaneous polarizations of (001)c, (101)c, and (111)c oriented epitaxial BiFeO3 thin films grown on dissimilar substrates. The effects of various substrate-induced strains on the spontaneous polarization were studied. The dependences of the spontaneous polarization on film orientations and the types of substrate-induced strains were analyzed.

Effect of strain and electric field on the electronic soft matter in manganite thin films

We have studied the effect of substrate-induced strain on the properties of the hole-doped manganite (La$_{1-y}$Pr$_{y}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ ($y$ = 0.4, 0.5 and 0.6) in order to distinguish between the roles played by long-range strain interactions and quenched atomic disorder in forming the micrometer-scale phase separated state. We show that a fluid phase separated (FPS) state is formed at intermediate temperatures similar to the strain-liquid state in bulk compounds, which can be converted to a metallic state by applying an external electric field. In contrast to bulk compounds, at low temperatures a strain stabilized ferromagnetic metallic (FMM) state is formed in the $y$ = 0.4 and 0.5 samples. However, in the $y$ = 0.6 sample a static phase separated (SPS) state is formed similar to the strain-glass phase in bulk compounds. Hence, we show that long-range strain interaction plays a dominant role in forming the micrometer-scale phase separated state in manganite thin films.

Nanoscale Current Modulations in Pr0.7Ca0.3MnO3 Thin Films

Many theoretical and experimental efforts have been focused on the origin of the electric-pulse-induced resistance change effect. However, there are still various reports of the origin supporting either the bulk nature or the interface nature. To resolve the controversies, nanoscale electronic measurements may provide essential clues. In this work, we report microscopic electrical properties of Pr(0.7)Ca(0.3)MnO(3) thin films. The resistance of a single-crystalline grain is not homogeneous in nanometer scale. We deduce that nanoscale inhomogeneity is related to the periodic relaxation of substrate-induced strain, which is caused by the lattice mismatch between the substrate and the thin film.