Phonon and magnetoelastic coupling in Al0.5Ga0.5FeO3: Raman, magnetization and neutron diffraction studies

The intriguing coupling phenomena among spin, phonon, and charge degrees of freedom in materials having magnetic, ferroelectric and/or ferroelastic order have been of research interest for the fundamental understanding and technological relevance. We report a detailed study on structure and phonons of Al_0.5Ga_0.5FeO₃ (ALGF), a lead-free magnetoelectric material, carried out using variable temperature dependent powder neutron diffraction and Raman spectroscopy. Neutron diffraction studies suggest that Al³⁺ ions are distributed in one tetrahedrally (BO₄) and three octahedrally (BO₆) coordinated sites of the orthorhombic (Pc2₁n) structure and there is no structural transition in the temperature range of 7-800 K. Temperature dependent field-cooled and zero-field-cooled magnetization studies indicate ferrimagnetic ordering below 225 K (T_N), and that is reflected in the low temperature powder neutron diffraction data. An antiferromagnetic type arrangement of Fe³⁺ ions with net magnetic moment of 0.13 μ_B/Fe³⁺ was observed from powder neutron diffraction analysis and it corroborates the findings from magnetization studies. At the magnetic transition temperature, no drastic change in lattice strain was observed, while significant changes in phonons were observed in the Raman spectra. The deviation of several mode frequencies from the standard anharmonicity model in the ferrimagnetic phase (below 240 K) is attributed to coupling effect between spin and phonon. Spin-phonon coupling effect is discernable from Raman bands located at 270, 425, 582, 695, 738, and 841 cm^-1. Their coupling strengths (λ) have been estimated using our phonon spectra and magnetization results. BO_n (n = 4, 6) libration (restricted rotation) mode at 270 cm^-1 has the largest coupling constant (λ∼ 2.3), while the stretching vibrations located at 695 and 738 cm^-1 have the lowest coupling constant (λ∼ 0.5). In addition to the libration mode, several internal stretching and bending modes of polyhedral units are strongly affected by spin ordering.

Elastic and anelastic relaxation behaviour of perovskite multiferroics II: PbZr0.53Ti0.47O3 (PZT)-PbFe0.5Ta0.5O3 (PFT)

Elastic and anelastic properties of ceramic samples of multiferroic perovskites with nominal compositions across the binary join PbZr_0.53Ti_0.47O₃-PbFe_0.5Ta_0.5O₃ (PZT-PFT) have been assembled to create a binary phase diagram and to address the role of strain relaxation associated with their phase transitions. Structural relationships are similar to those observed previously for PbZr_0.53Ti_0.47O₃-PbFe_0.5Nb_0.5O₃ (PZT-PFN), but the magnitude of the tetragonal shear strain associated with the ferroelectric order parameter appears to be much smaller. This leads to relaxor character for the development of ferroelectric properties in the end member PbFe_0.5Ta_0.5O₃. As for PZT-PFN, there appear to be two discrete instabilities rather than simply a reorientation of the electric dipole in the transition sequence cubic-tetragonal-monoclinic, and the second transition has characteristics typical of an improper ferroelastic. At intermediate compositions, the ferroelastic microstructure has strain heterogeneities on a mesoscopic length scale and, probably, also on a microscopic scale. This results in a wide anelastic freezing interval for strain-related defects rather than the freezing of discrete twin walls that would occur in a conventional ferroelastic material. In PFT, however, the acoustic loss behaviour more nearly resembles that due to freezing of conventional ferroelastic twin walls. Precursor softening of the shear modulus in both PFT and PFN does not fit with a Vogel-Fulcher description, but in PFT there is a temperature interval where the softening conforms to a power law suggestive of the role of fluctuations of the order parameter with dispersion along one branch of the Brillouin zone. Magnetic ordering appears to be coupled only weakly with a volume strain and not with shear strain but, as with multiferroic PZT-PFN perovskites, takes place within crystals which have significant strain heterogeneities on different length scales.

Elastic and magnetoelastic relaxation behaviour of multiferroic (ferromagnetic + ferroelectric + ferroelastic) Pb(Fe0.5Nb0.5)O3 perovskite

Resonant Ultrasound Spectroscopy has been used to characterize elastic and anelastic anomalies in a polycrystalline sample of multiferroic Pb(Fe(0.5)Nb(0.5))O(3) (PFN). Elastic softening begins at ~550 K, which is close to the Burns temperature marking the development of dynamical polar nanoregions. A small increase in acoustic loss at ~425 K coincides with the value of T(*) reported for polar nanoregions starting to acquire a static or quasi-static component. Softening of the shear modulus by ~30-35% through ~395-320 K, together with a peak in acoustic loss, is due to classical strain/order parameter coupling through the cubic → tetragonal → monoclinic transition sequence of ferroelectric/ferroelastic transitions. A plateau of high acoustic loss below ~320 K is due to the mobility under stress of a ferroelastic microstructure but, instead of the typical effects of freezing of twin wall motion at some low temperature, there is a steady decrease in loss and increase in elastic stiffness below ~85 K. This is attributed to freezing of a succession of strain-coupled defects with a range of relaxation times and is consistent with a report in the literature that PFN develops a tweed microstructure over a wide temperature interval. No overt anomaly was observed near the expected Néel point, ~145 K, consistent with weak/absent spin/lattice coupling but heat capacity measurements showed that the antiferromagnetic transition is actually smeared out or suppressed. Instead, the sample is weakly ferromagnetic up to ~560 K, though it has not been possible to exclude definitively the possibility that this could be due to some magnetic impurity. Overall, evidence from the RUS data is of a permeating influence of static and dynamic strain relaxation effects which are attributed to local strain heterogeneity on a mesoscopic length scale. These, in turn, must have a role in determining the magnetic properties and multiferroic character of PFN.

Tunneling electroresistance in multiferroic heterostructures

We demonstrate the room temperature polar switching and tunneling in PbZr0.52Ti0.48O3 (PZT) ultra-thin films of thickness 3-7 nm, sandwiched between platinum metal and ferromagnetic La0.67Sr0.33MnO3 (LSMO) layers, which also shows magnetic field dependent tunnel current switching in Pt/PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 heterostructures. The epitaxial nature, surface quality and ferroelectric switching of heterostructured films were examined with the help of x-ray diffraction patterns, atomic force microscopy, and piezo force microscopy, respectively. The capacitance versus voltage graphs show butterfly loops above the coercive field (> ±3 V) of PZT for small probe area (∼16 μm(2)). The effect of ferroelectric switching was observed in current density versus voltage curves with a large variation in high-resistance/low-resistance (HRS/LRS) ratio (2:1 to 100:1), however, these effects were more prominent in the presence of in-plane external magnetic field. The conductance is fitted with Brinkman's model, and the parabolic conductance upon bias voltage implies electron tunneling governs the transport.

Studies of the Room-Temperature Multiferroic Pb(Fe0.5Ta0.5)0.4(Zr0.53Ti0.47)0.6O3: Resonant Ultrasound Spectroscopy, Dielectric, and Magnetic Phenomena

Recently, lead iron tantalate/lead zirconium titanate (PZTFT) was demonstrated to possess large, but unreliable, magnetoelectric coupling at room temperature. Such large coupling would be desirable for device applications but reproducibility would also be critical. To better understand the coupling, the properties of all 3 ferroic order parameters, elastic, electric, and magnetic, believed to be present in the material across a range of temperatures, are investigated. In high temperature elastic data, an anomaly is observed at the orthorhombic mm2 to tetragonal 4mm transition, T_ot = 475 K, and a softening trend is observed as the temperature is increased toward 1300 K, where the material is known to become cubic. Thermal degradation makes it impossible to measure elastic behavior up to this temperature, however. In the low temperature region, there are elastic anomalies near ≈40 K and in the range 160-245 K. The former is interpreted as being due to a magnetic ordering transition and the latter is interpreted as a hysteretic regime of mixed rhombohedral and orthorhombic structures. Electrical and magnetic data collected below room temperature show anomalies at remarkably similar temperature ranges to the elastic data. These observations are used to suggest that the three order parameters in PZTFT are strongly coupled.

Switching ferroelectric domain configurations using both electric and magnetic fields in Pb(Zr,Ti)O3-Pb(Fe,Ta)O3 single-crystal lamellae

Thin single-crystal lamellae cut from Pb(Zr,Ti)O3-Pb(Fe,Ta)O3 ceramic samples have been integrated into simple coplanar capacitor devices. The influence of applied electric and magnetic fields on ferroelectric domain configurations has been mapped, using piezoresponse force microscopy. The extent to which magnetic fields alter the ferroelectric domains was found to be strongly history dependent: after switching had been induced by applying electric fields, the susceptibility of the domains to change under a magnetic field (the effective magnetoelectric coupling parameter) was large. Such large, magnetic field-induced changes resulted in a remanent domain state very similar to the remanent state induced by an electric field. Subsequent magnetic field reversal induced more modest ferroelectric switching.

Dynamic nanocrystal response and high temperature growth of carbon nanotube-ferroelectric hybrid nanostructure

A long standing problem related to the capping of carbon nanotubes (CNT) by inorganic materials at high temperature has been solved. In situ dynamic response of Pb(Zr0.52Ti0.48)O3 (PZT) nanocrystals attached to the wings of the outer surface of PZT/CNT hybrid-nanostructure has been demonstrated under a constant-energy high-resolution transmission electron microscopy (HRTEM) e-beam. PZT nanocrystals revealed that the crystal orientations, positions, faces, and hopping states change with time. HRTEM study has been performed to investigate the microstructure of hybrid nanostructures and nanosize polycrystal trapped across the wings. Raman spectroscopy was utilized to investigate the local structures, defects, crystal qualities and temperature dependent growth and degradation of hybrid nanostructures. Raman spectra indicate that MWCNT and PZT/MWCNT/n-Si possess good quality of CNT before and after PZT deposition until 650 °C. The monoclinic Cc/Cm phase of PZT which is optimum in piezoelectric properties was prominent in the hybrid structure and should be useful for device applications. An unusual hexagonal faceting oscillation of the nano-crystal perimeter on a 10-30 s period is also observed.

N-doped ZnO thin film for development of magnetic field sensor based on surface plasmon resonance

Magnetic-field-dependent optical properties of nitrogen-doped ZnO (ZnO:N) thin films were investigated using surface plasmon resonance (SPR) and a highly sensitive (4.65/Tesla) magnetic field sensor has been realized. The refractive index (RI) of ZnO:N film increases from 1.949 to 2.025 with increase in N doping from 0% to 10% demonstrating tunable RI. In contrast to pure ZnO, SPR curves for ZnO:N films exhibit a shift toward lower angles with increasing applied magnetic field from 0 to 35 mT due to change in reflectance of light upon reflection from ferromagnetic surface. Results indicate promising application of ferromagnetic ZnO:N film as a magnetic field sensor.

Magnetic switching of ferroelectric domains at room temperature in multiferroic PZTFT

Single-phase magnetoelectric multiferroics are ferroelectric materials that display some form of magnetism. In addition, magnetic and ferroelectric order parameters are not independent of one another. Thus, the application of either an electric or magnetic field simultaneously alters both the electrical dipole configuration and the magnetic state of the material. The technological possibilities that could arise from magnetoelectric multiferroics are considerable and a range of functional devices has already been envisioned. Realising these devices, however, requires coupling effects to be significant and to occur at room temperature. Although such characteristics can be created in piezoelectric-magnetostrictive composites, to date they have only been weakly evident in single-phase multiferroics. Here in a newly discovered room temperature multiferroic, we demonstrate significant room temperature coupling by monitoring changes in ferroelectric domain patterns induced by magnetic fields. An order of magnitude estimate of the effective coupling coefficient suggests a value of ~1 × 10⁻⁷ sm⁻¹.

Multiferroic materials that exhibit coupled ferromagnetic and ferroelectric characteristics could be useful in the development of non-volatile digital storage. Evans et al. report a single-phase multiferroic material whose room-temperature magnetoelectric coupling appears to be unusually strong.

Temperature tuned defect induced magnetism in reduced graphene oxide

The existence of ferromagnetism in the wonder material graphene has opened up the path for many future spintronics and memory applications. But simultaneously it is very important to understand the variation of these properties with temperature in regards to the device applications. Here we observed defect induced ferromagnetism in chemically reduced graphene and the effect of temperature on it. Several theoretical studies have proved that the main cause of ferromagnetism in graphene is due to various defects. The observed results established that these defects can be mended by treating the samples at elevated temperatures but sacrificing the ferromagnetism simultaneously. Hence, temperature plays a crucial role in controlling the magnetism as well as the defects in graphene. In this study we revealed that at 600 °C the self-repair mechanism helps the defects to mend but resulting in the decrement of magnetization and providing a good quality graphene with less defects.

Relaxor-ferroelectric superlattices: high energy density capacitors

We report the breakdown electric field and energy density of laser ablated BaTiO(3)/Ba((1-x))Sr(x)TiO(3) (x = 0.7) (BT/BST) relaxor-ferroelectric superlattices (SLs) grown on (100) MgO single crystal substrates. The dielectric constant shows a frequency dispersion below the dielectric maximum temperature (T(m)) with a merger above T(m) behaving similarly to relaxors. It also follows the basic criteria of relaxor ferroelectrics such as low dielectric loss over wide temperature and frequency, and 50 K shift in T(m) with change in probe frequency; the loss peaks follow a similar trend to the dielectric constant except that they increase with increase in frequency (~40 kHz), and satisfy the nonlinear Vogel-Fulcher relation. Well-saturated ferroelectric hysteresis and 50-80% dielectric saturation are observed under high electric field (~1.65 MV cm(-1)). The superlattices demonstrate an 'in-built' field in as grown samples at low probe frequency (<1 kHz), whereas it becomes more symmetric and centered with increase in the probe frequency system (>1 kHz) which rules out the effect of any space charge and interfacial polarization. The P-E loops show around 12.24 J cm(-3) energy density within the experimental limit, but extrapolation of this data suggests that the potential energy density could reach 46 J cm(-3). The current density versus applied electric field indicates an exceptionally high breakdown field (5.8-6.0 MV cm(-1)) and low current density (~10-25 mA cm(-2)) near the breakdown voltage. The current-voltage characteristics reveal that the space charge limited conduction mechanism prevails at very high voltage.

Emerging memories: resistive switching mechanisms and current status

The resistance switching behaviour of several materials has recently attracted considerable attention for its application in non-volatile memory (NVM) devices, popularly described as resistive random access memories (RRAMs). RRAM is a type of NVM that uses a material(s) that changes the resistance when a voltage is applied. Resistive switching phenomena have been observed in many oxides: (i) binary transition metal oxides (TMOs), e.g. TiO(2), Cr(2)O(3), FeO(x) and NiO; (ii) perovskite-type complex TMOs that are variously functional, paraelectric, ferroelectric, multiferroic and magnetic, e.g. (Ba,Sr)TiO(3), Pb(Zr(x) Ti(1-x))O(3), BiFeO(3) and Pr(x)Ca(1-x)MnO(3); (iii) large band gap high-k dielectrics, e.g. Al(2)O(3) and Gd(2)O(3); (iv) graphene oxides. In the non-oxide category, higher chalcogenides are front runners, e.g. In(2)Se(3) and In(2)Te(3). Hence, the number of materials showing this technologically interesting behaviour for information storage is enormous. Resistive switching in these materials can form the basis for the next generation of NVM, i.e. RRAM, when current semiconductor memory technology reaches its limit in terms of density. RRAMs may be the high-density and low-cost NVMs of the future. A review on this topic is of importance to focus concentration on the most promising materials to accelerate application into the semiconductor industry. This review is a small effort to realize the ambitious goal of RRAMs. Its basic focus is on resistive switching in various materials with particular emphasis on binary TMOs. It also addresses the current understanding of resistive switching behaviour. Moreover, a brief comparison between RRAMs and memristors is included. The review ends with the current status of RRAMs in terms of stability, scalability and switching speed, which are three important aspects of integration onto semiconductors.

Electric control of magnon frequencies and magnetic moment of bismuth ferrite thin films at room temperature

Here, we report the tuning of room-temperature magnon frequencies from 473 GHz to 402 GHz (14%) and magnetic moment from 4 to 18 emu∕cm(3) at 100 Oe under the application of external electric fields (E) across interdigital electrodes in BiFeO(3) (BFO) thin films. A decrease in magnon frequencies and increase in phonon frequencies were observed with Magnon and phonon Raman intensities are asymmetric with polarity, decreasing with positive E (+E) and increasing with negative E (-E) where polarity is with respect to in-plane polarization P. The magnetoelectric coupling (α) is proved to be linear and a rather isotropic α = 8.5 × 10(-12) sm(-1).

Magnetic control of ferroelectric interfaces

We report the strong magnetic field dependence of ferroelectric PbZr(0.52)Ti(0.48)O(3) (PZT) films on half-metallic oxide La(0.67)Sr(0.33)MnO(3) (LSMO) electrodes. As the field H is increased, the hysteresis loop first broadens (becomes lossy) and then disappears at approximately H = 0.34 T and ambient temperatures. The data are compared with the theories of Pirč et al (2009 Phys. Rev. B 79 214114), Parish and Littlewood (2008 Phys. Rev. Lett. 101 166602) and Catalan (2006 Appl. Phys. Lett. 88 102902). The results are interpreted as due not to magnetocapacitance but to the sharp negative magnetoresistance in LSMO at low magnetic fields (Hwang et al 1996 Phys. Rev. Lett. 77 2041), which causes a dramatic increase in leakage current through the PZT.

Characterization of Conductive RuO2 Thin Film as Bottom electrodes for Ferroelectric Thin Films

Ruthenium Oxide (RuO2) thin films were prepared on silicon substrates by solution chemistry technique. X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), micro-Raman, X-ray photoelectron spectroscopy (XPS), and four probe Van-der-paw technique were used for the film characterization. X-ray analysis shows a rutile structure in these films. The films annealed at 700°C showed lowest resistivity of 29 × 10−5 ohm-cm. The presence of Eg, A1g, and B2g modes is consistent with the Raman spectrum of rutile phase. These modes as well as additional unidentified band at about 477 cm−1 were investigated by temperature dependent Raman studies. Based on the result, band at 477 cm−1 that disappears above 370 K is attributed to hydrated RuO2 present in the films. XPS analysis show stoichiometric rutile RuO2 present in the films. Small concentrations of RuCl3, RuO3 and hydrated RuO2 were also detected. Pb0.9La0.15TiO3 (PLT15) thin films were deposited on RuO2/Si substrates and characterized for its ferroelectric properties to demonstrate that solution deposition technique offers an alternative approach for preparing high quality RuO2 bottom electrodes.

Effect of Precursor Sol Ageing on Sol-Gel Derived Ruthenium Oxide Thin Films

In the present work we have optimized the process parameters to yield homogeneous, smooth ruthenium oxide (RuO2) thin films on silicon substrates by a solution deposition technique using RuCl3.×.H2O as the precursor material. Films were annealed in a temperature range of 300°C to 700°C, and it was found that RuO2 crystallizes at a temperature as low as 400°C. The crystallinity of the films improves with increased annealing temperature and the resistivity decreases from 4.86µΩ-m (films annealed at 400°C) to 2.94pµΩ (films annealed at 700°C). Ageing of the precursor solution has a pronounced effect on the measured resistivities of RuO2 thin films. It was found that the measured room temperature resistivities increases from 2.94µΩ-m to 45.7µΩ-m when the precursor sol is aged for aged 60 days. AFM analysis on the aged films shows that the grain size and the surface roughness of the annealed films increase with the ageing of the precursor solution. From XPS analysis we have detected the presence of non-transformed RuCl3 in case of films prepared from aged solution. We propose, that solution ageing inhibits the transformation of RuCl3 to RuO2 during the annealing of the films. The deterioration of the conductivity with solution ageing is thought to be related with the chloride contamination in the annealed films.

New cryogenic phase transitions in SrSnO3

Strontium stannate is under study as an ultra-stable dielectric material for microelectronic applications at low temperatures. It is known to have a remarkably temperature-independent dielectric constant from 27 K to room temperature. However, we report anomalies in the Raman spectra, dielectric response, and differential thermal analysis of strontium stannate compatible with a structural phase transition at 160 K. Further anomalies are seen in calorimetric and Raman data (but not dielectric data) that suggest another phase transition at 270 K. A preliminary x-ray powder diffraction study confirms a small change in the pseudo-cubic lattice constant a(T) at the lower transition.

Magnetic control of large room-temperature polarization

Numerous authors have referred to room-temperature magnetic switching of large electric polarizations as 'the Holy Grail' of magnetoelectricity. We report this long-sought effect, obtained using a new physical process of coupling between magnetic and ferroelectric nanoregions. Solid state solutions of PFW [Pb(Fe(2/3)W(1/3))O(3)] and PZT [Pb(Zr(0.53)Ti(0.47))O(3)] exhibit some bi-relaxor qualities, with both ferroelectric relaxor characteristics and magnetic relaxor phenomena. Near 20% PFW the ferroelectric relaxor state is nearly unstable at room temperature against long-range ferroelectricity. Here we report magnetic switching between the normal ferroelectric state and a magnetically quenched ferroelectric state that resembles relaxors. This gives both a new room-temperature, single-phase, multiferroic magnetoelectric, (PbFe(0.67)W(0.33)O(3))(0.2)(PbZr(0.53)Ti(0.47)O(3))(0.8) ('0.2PFW/0.8PZT'), with polarization, loss (<1%), and resistivity (typically 10(8)-10(9) Ω cm) equal to or superior to those of BiFeO(3), and also a new and very large magnetoelectric effect: switching not from +P(r) to -P(r) with applied H, but from P(r) to zero with applied H of less than a tesla. This switching of the polarization occurs not because of a conventional magnetically induced phase transition, but because of dynamic effects: increasing H lengthens the relaxation time by 500 × from<200 ns to>100 µs, and it strongly couples the polarization relaxation and spin relaxations. The diverging polarization relaxation time accurately fits a modified Vogel-Fulcher equation in which the freezing temperature T(f) is replaced by a critical freezing field H(f) that is 0.92 ± 0.07 T. This field dependence and the critical field H(c) are derived analytically from the spherical random bond random field model with no adjustable parameters and an E(2)H(2) coupling. This device permits three-state logic (+P(r),0,-P(r)) and a condenser with >5000% magnetic field change in its capacitance; for H = 0 the coercive voltage is 1.4 V across 300 nm for +P(r) to -P(r) switching, and the coercive magnetic field is 0.5 T for +P(r) to zero switching.

Effect of Growth Temperature on Bamboo-shaped Carbon-Nitrogen (C-N) Nanotubes Synthesized Using Ferrocene Acetonitrile Precursor

This investigation deals with the effect of growth temperature on the microstructure, nitrogen content, and crystallinity of C-N nanotubes. The X-ray photoelectron spectroscopic (XPS) study reveals that the atomic percentage of nitrogen content in nanotubes decreases with an increase in growth temperature. Transmission electron microscopic investigations indicate that the bamboo compartment distance increases with an increase in growth temperature. The diameter of the nanotubes also increases with increasing growth temperature. Raman modes sharpen while the normalized intensity of the defect mode decreases almost linearly with increasing growth temperature. These changes are attributed to the reduction of defect concentration due to an increase in crystal planar domain sizes in graphite sheets with increasing temperature. Both XPS and Raman spectral observations indicate that the C-N nanotubes grown at lower temperatures possess higher degree of disorder and higher N incorporation.

Ion relaxation dynamics and nearly constant loss behavior in polymer electrolyte

The broadband conductive spectroscopy covering the range 10(-2) to 10(6) Hz has been employed to examine both the ionic and the segmental motions in polymer salt complexes (PSCs), consisting of polyethylene oxide (PEO), LiClO4, or LiCF3SO3. The temperature dependence of dc conductivity has been analyzed using Vogel-Tamman-Fulcher (VTF) equation and the results suggest that the temperature dependence of the dc conductivity can be well described in terms of the free volume changes with temperature. The ac conductivity at low temperatures (below 223 K) remains unchanged with the temperature, exhibiting nearly linear frequency dependence. This observation, not previously reported in polymers, indicates clearly that nearly constant loss (NCL) phenomenon, usually observed in glassy and ceramic ion conductors, is also operative in polymer electrolytes. Furthermore, a crossover from a NCL to cooperative ion hopping at higher temperatures in the frequency dependence of the conductivity has been observed. The analysis of the temperature dependence of both dc and ac conductivities demonstrates that there exists a direct correlation between the ionic conductivity and the segmental relaxation processes.

Probing Nanoscale Ferroelectricity by Ultraviolet Raman Spectroscopy

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature (Tc) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (with Tc as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. Tc was tuned by approximately 500 kelvin by varying the thicknesses of the BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

Low frequency Raman scattering from acoustic phonons confined in ZnO nanoparticles

We report here the first observation of the low frequency Raman scattering from acoustic phonons in semiconducting zinc oxide (ZnO) nanoparticles without embedding in any solid matrix. ZnO nanoparticles (size 5-10 nm) with nearly spherical shape have been synthesized using a chemical route. A shift in the phonon peaks toward higher frequencies along with broadening was observed with a decrease in particle size. The size dependence of the acoustic phonons in ZnO nanoparticles is explained using Lamb's theory that predicts the vibrational frequencies of a homogeneous elastic body of spherical shape. Our results show that the observed low frequency Raman scattering originates from the spherical (l = 0) and quadrupolar vibrations (l = 2) of the spheroidal mode due to the confinement of acoustic vibrations in ZnO nanoparticles.