Òxids ferroelèctrics sobre oblees de silici per a noves memòries

Algunes de les memòries actualment en ús en dispositius electrònics es basen en materials aïllants ferroelèctrics, que emmagatzemen la informació de manera permanent. No obstant això contenen òxids, com els compostos de plom, que són tòxics. Investigadors de l'Institut de Ciència de Materials de Barcelona han substituit aquests òxids de plom per altres que no són tòxics, aconseguint la integració epitaxial en oblees de silici, i han confirmat de manera experimental que presenten ferroelectricitat i, per tant, que poden utilitzar-se com a memòries.Algunas de las memorias actualmente usadas en dispositivos electrónico se basan en materiales aislantes ferroeléctricos que permiten almacenar la información de manera permanente. Sin embargo contienen óxidos, como compuestos de plomo, que son tóxicos. Investigadores del Instituto de Ciencia de Materiales de Barcelona han sustitui estos óxidos de plomo por otros que no son tóxicos, consiguiendo su integración epitaxial en obleas de silicio y han confirmado de manera experimental que presentan ferroelectricidad y,que por lo tanto, pueden utilizarse como memorias.Some of the memories currently used in electronic devices are based on ferroelectric insulating materials that permit non-volatile storing of information. However, they contain toxic oxides, such as lead-compounds. Researchers from the ICMAB-CSIC have replaced these lead-compounds by non-toxic oxides, achieving the epitaxial integration on silicon wafers and they have confirmed experimentally their ferroelectricity, so that they can be used as permanent memories

Magnetoelastic coupling in La2/3Sr1/3MnO3 thin films on SrTiO3

Clamping of epitaxial La2/3Sr1/3MnO3 (LSMO) magnetic thin films on SrTiO3 (STO) substrates is shown to promote a clear modification of their magnetic properties at the STO cubic-tetragonal transition. Two distinct mechanisms triggered by the STO transition, namely magnetic domain pattern reconstruction and creation of regions within the magnetically soft LSMO with enhanced magnetic anisotropy, are proposed to be behind the observed anomalous magnetic responses at low ac-magnetic field and at high dc-field, respectively. The persistence of these anomalies in LSMO films as thick as 220 nm shines new light into the magnetoelastic coupling mechanisms across interfaces

CoFe2O4/buffer layer ultrathin heterostructures on Si(001)

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Epitaxial films of ferromagnetic CoFe2O4 (CFO) were grown by pulsed laser deposition on Si(001) buffered with ultrathin yttria-stabilized zirconia (YSZ) layers in a single process. Reflection high-energy electron diffraction was used to monitor in real time the crystallization of YSZ, allowing the fabrication of epitaxial YSZ buffers with thickness of about 2 nm. CFO films, with thicknesses in the 2-50 nm range were subsequently deposited. The magnetization of the CFO films is close to the bulk value. The ultrathin CFO/YSZ heterostructures have very flat morphology (0.1 nm roughness) and thin interfacial SiOx layer (about 2 nm thick) making them suitable for integration in tunnel (e.g., spin injection) devices

High ferroelectric polarization in c-oriented BaTiO3 epitaxial thin films on SrTiO3/Si(001)

Scigaj, M. et al.The integration of epitaxial BaTiO3 films on silicon, combining c-orientation, surface flatness, and high ferroelectric polarization is of main interest towards its use in memory devices. This combination of properties has been only achieved so far by using yttria-stabilized zirconia buffer layers. Here, the all-perovskite BaTiO3/LaNiO3/SrTiO3 heterostructure is grown monolithically on Si(001). The BaTiO3 films are epitaxial and c-oriented and present low surface roughness and high remnant ferroelectric polarization around 6 μC/cm2. This result paves the way towards the fabrication of lead-free BaTiO3 ferroelectric memories on silicon platforms.ICMAB-CSIC authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496) and the MAT2014-56063-C2-1-R project, and from Generalitat de Catalunya (2014 SGR 734). Work at Oak Ridge National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. C. H. Chao acknowledges the NSC-CSIC 2014 Summer Program in Spain for Taiwanese PhD students. I. Fina acknowledges Juan de la Cierva – Incorporación postdoctoral fellowship (IJCI-2014-19102) from the Spanish Ministry of Economy and Competitiveness of Spanish Government. INL gratefully acknowledges the European commission and the national French research agency (ANR) for funding, through the projects SITOGA (FP7-ICT-2013-11-619456), TIPS (H2020‐ICT‐02-2014‐1-644453), ANR HIRIS and ANR DIAMWAFEL. INL also acknowledges P. Regreny, C. Botella and J.-B. Goure for MBE technical assistance.Peer reviewe

Multiple strain-induced phase transitions in LaNiO3 thin films

Weber, M.C. et al.Strain effects on epitaxial thin films of LaNiO3 grown on different single crystalline substrates are studied by Raman scattering and first-principles simulation. New Raman modes, not present in bulk or fully-relaxed films, appear under both compressive and tensile strains, indicating symmetry reductions. Interestingly, the Raman spectra and the underlying crystal symmetry for tensile and compressively strained films are different. Extensive mapping of LaNiO3 phase stability is addressed by simulations, showing that a variety of crystalline phases are indeed stabilized under strain which may impact the electronic orbital hierarchy. The calculated Raman frequencies reproduce the principal features of the experimental spectra, supporting the validity of the multiple strain-driven structural transitions predicted by the simulations.J.K., M.W., M.G., and J.I. acknowledge support from the National Research Fund, Luxembourg through a Pearl grant (Grant No. FNR/P12/4853155). ICMAB-CSIC authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015- 0496) and the MAT2014-56063-C2-1-R and MAT2013- 40581-P projects, and from Generalitat de Catalunya (2014 SGR 734). UB authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, project MAT2013-41506-P and from Generalitat de Catalunya (2014 SGR 672).Peer reviewe

Untangling Electrostatic and Strain Effects on the Polarization of Ferroelectric Superlattices

Khestanova, Ekaterina et al.The polarization of ferroelectric superlattices is determined by both electrical boundary conditions at the ferroelectric/paraelectric interfaces and lattice strain. The combined infl uence of both factors offers new opportunities to tune ferroelectricity. However, the experimental investigation of their individual impact has been elusive because of their complex interplay. Here, a simple growth strategy has permitted to disentangle both contributions by an independent control of strain in symmetric superlattices. It is found that fully strained short-period superlattices display a large polarization whereas a pronounced reduction is observed for longer multilayer periods. This observation indicates that the electrostatic boundary mainly governs the ferroelectric properties of the multilayers whereas the effects of strain are relatively minor.Financial support by the Spanish Government [Projects MAT2014- 56063-C2-1-R and MAT2013-41506 ] and Generalitat de Catalunya ( 2014-SGR-734 and 2014-SGR-672 ) is acknowledged. ICMAB-CSIC authors acknowledge fi nancial support from the Spanish Ministry of Economy and Competitiveness , through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV- 2015-0496 ). I.F. acknowledges Juan de la Cierva – Incorporación postdoctoral fellowship (IJCI-2014- 19102) from the Spanish Ministry of Economy and Competitiveness. The transmission electron microscopy works were conducted in the Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragón (Universidad de Zaragoza). The authors acknowledge the LMA-INA for offering access to their instruments and expertise. The authors thank Massimiliano Stengel for useful discussions.Peer reviewe

Strain tuned magnetoelectric coupling in orthorhombic YMnO3 thin films

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Orthorhombic YMnO3epitaxialthin films were grown on Nb(0.5%)-doped SrTiO3(001) substrates. Film's thickness was varied to tune the epitaxial strain. Structural and magnetic properties are well correlated, presenting a more pronounced ferromagnetic behavior as the unit cell becomes more distorted. Dielectric properties were investigated as a function of the temperature and magnetic field. The dielectric peak occurring at temperatures below the antiferromagnetic ordering is proved to be magnetoelectric and its amplitude is dependent on the unit cell distortion. These findings allow tailoring ferromagnetic and magnetoelectric properties via epitaxial strain

Distinct magnetism in ultrathin epitaxial NiFe2O4 films on MgAl2O4 and SrTiO3single crystalline substrates

Spinel ferrites are being considered for advanced spintronic applications. Here, we report on the magnetic properties of ultrathin (3-37 nm) epitaxial films of NiFe2O4 (NFO) on MgAl2O4 (MAO) and SrTiO3 (STO) single crystalline substrates. It is found that NFO films on STO display superparamagnetic response down to 50 K, whereas films grown on MAO display ferrimagnetic response up to room temperature. Microstructural information indicates that this distinct response can be attributed to the different growth mechanisms of the spinel ferrite on the isostructural MAO substrate (two-dimensional growth) and the perovskite STO (Volmer-Weber three-dimensional growth). We discuss the reasons for this distinct behavior and its relevance for the integration of ferrites in epitaxial heterostructures for tunnel devices

Electric field control of exchange bias in multiferroic epitaxial heterostructures

The magnetic exchange bias between epitaxial thin films of the multiferroic (antiferromagnetic and ferroelectric) hexagonal YMnO3 oxide and a soft ferromagnetic (FM) layer is used to couple the magnetic response of the ferromagnetic layer to the magnetic state of the antiferromagnetic one. We will show that biasing the ferroelectric YMnO3 layer by an appropriate electric field allows modifying and controlling the magnetic exchange bias and subsequently the magnetotransport properties of the FM layer. This finding may contribute to pave the way towards a new generation of electric-field controlled spintronics devices.Comment: 15 pages, 5 figures, submitte

Structural and functional characterization of (110)-oriented epitaxial La2/3Ca1/3MnO3 electrodes and SrTiO3 tunnel barriers

La2/3Ca1/3MnO3 (LCMO) films have been deposited on (110)-oriented SrTiO3 (STO) substrates. X-ray diffraction and high-resolution electron microscopy reveal that the (110) LCMO films are epitaxial and anisotropically in-plane strained, with higher relaxation along the [1¿10] direction than along the [001] direction; x-ray absorption spectroscopy data signaled the existence of a single intermediate Mn3+/4+ 3d-state at the film surface. Their magnetic properties are compared to those of (001) LCMO films grown simultaneously on (001) STO substrates It is found that (110) LCMO films present a higher Curie temperature (TC) and a weaker decay of magnetization when approaching TC than their (001) LCMO counterparts. These improved films have been subsequently covered by nanometric STO layers. Conducting atomic-force experiments have shown that STO layers, as thin as 0.8 nm, grown on top of the (110) LCMO electrode, display good insulating properties. We will show that the electric conductance across (110) STO layers, exponentially depending on the barrier thickness, is tunnel-like. The barrier height in STO (110) is found to be similar to that of STO (001). These results show that the (110) LCMO electrodes can be better electrodes than (001) LCMO for magnetic tunnel junctions, and that (110) STO are suitable insulating barriers