NIRT: Active Multiferroic Nanostructures Christos G Takoudis 1 , Siddhartha Ghosh 1 , Arunava Gupta 2 , Nicola A Spaldin

1. NIRT: Active Multiferroic Nanostructures Christos G Takoudis1, Siddhartha Ghosh1, Arunava Gupta2, Nicola A Spaldin3, Gopalan Srinivasan4 1University of Illinois at Chicago, 2University of Alabama, 3University of California Santa Barbara, 4Oakland University Supported by the NSF, Grant # CMMI 0609377http://www.uic.edu/labs/AMReL/NIRT/index.html Introduction Pulsed Laser Deposition (PLD) of Multifunctional Double Perovskite Thin Films and Heterostructures Recent Findings Model for Low-frequency ME effects in Bilayers Nanowire structure of NFO-PZT grown on an MgO template. Estimated PZT volume fraction dependence of ME voltage coefficient αE,33. The substrate pinning effects is negligible only when the wire radius is much greater than the sheath radius. Petrov et al, Phys. Rev. B75, 224407 (2007) Double-perovskites: • La2NiMnO6 (LNMO) and La2CoMnO6 (LCMO ) thin films • Bi2NiMnO6 (BNMO) thin films (ongoing work) Calculated magnetoelectric response of interface SrTiO3 (perovskite) Response: La2NiMnO6 (bulk): • Single phase • Mixture of rhombohedral and monoclinic at room temperature; monoclinic at low temperature • TC ~ 280 K; close to room temperature • Semiconducting with ~ 102 Ω-cm at room temperature • Magnetocapacitance effect observed above 220 K La2NiMnO6 (double perovskite) E A can be any rare earth or Bi cation B can be Ni or Co Mechanism: Spaldin and Fiebig, Science309 391 (2005) Impact: • New materials for Consumer Electronics and National Defense • Two patents on the potential use of composites in sensors were granted in 2007. The university intends to license the technology • When an external field is applied, free carriers accumulate at the capacitor plates, which are partially screened by the dielectric polarization of the STO film • In the half-metallic limit all displaced electrons are spin-polarized in the same direction (up in the figure); in the present case there is a partial cancellation between spin-up and spin-down carriers • This process accumulates up-spin magnetization adjacent to the positively charged electrode, leaving behind an absence of up-spin magnetization (or equivalently down-spin polarized holes) at the negative plate • Materials in which ferromagnetism and ferroelectricity occur simultaneously in the same phase and allow coupling between the ferromagnetic and ferroelectric phase are known as MagnetoElectric (ME) Multiferroics • Multi-university interdisciplinary research program to • study magneto-electric property-structure-functionality relationships in thin films and nanostructures (nanowires and nanopillars) • carry out nano-epitaxy of magneto-electric multiferroic materials in thin film and nanostructure form, characterization of magneto-electric properties in various frequency ranges, detailed density functional theory based modeling of magneto-electric multilayered films and nanostructures • provide a science base for the development of miniature, passive (long-term deployable with batteries), ambient temperature operated (no need of cryostats), highly-sensitive (pico-Tesla), broad band (mHz) systems with the nanostructured magneto-electric composites. Such devices are expected to offer new capabilities in biomedical sensing, microwave circuits etc. Ordering of Ni/Mn Epitaxial BiFeO3 Thin Films on STO • Bi2NiMnO6 (bulk): • Monoclinic structure; multiferroic • TCE ~ 485 K • TCM ~ 140 K • Capacitance change is 0.4% at TCM and 9 T • Presence of 6s2 lone pairs of Bi3+ leads to noncentrosymmetric distorted structure E Rondinelli, Stengel, and Spaldin, "Carrier-mediated magnetoelectricity in complex oxide heterostructures," Under Review at Nature Nanotechnology. Nano-Materials for High Frequency Electronics Epitaxial LNMO & LCMO Thin Films Kabelac et al, J. Vac. Sci. Technol. B25(3) 1049-1052 (2007) Objective: CVD Fe2O3 Thin Films Synthesize nanocomposites of magnetic and dielectric oxides and study the magneto-electric interactions 100 mTorr O2 1000 mTorr O2 100 mTorr O2 Approach: Prepare nano bilayers, wires and tubes of ferroelectric materials, such as lead zirconium titanante or barium titanate, on ferromagnetic nickel- or cobalt ferrite. A model was developed for low-frequency ME effects in bilayers, pillars and nanowires of nickel ferrite (NFO) and lead zirconate titanate (PZT) on MgO templates. • Low degree of crystallinity at 450 ºC • Fe2O3 preferred orientations: (113) at 40.85º, (024) at 49.48º, (116) at 54.09º, (018) at 57.59º • Only Fe2O3 peaks observed Molecular Beam Epitaxy (MBE) Growth of BiFeO3 on SrTiO3 (001) Substrates • Low pressure: sharp and coherent interface; some defects develop away from interface that extend all the way to surface (red arrows) • High pressure: high quality interface; coherent and defect-free structure • Sharp and coherent interface • No major defects • Excellent crystalline quality • RF plasma atomic oxygen source with optical feedback control • Differentially pumped retractable QCM for accurate deposition rate measurement • BeO coated substrate heater – 950 °C @ 5x10-5 torr oxygen partial pressure • BeO crucibles for sources to withstand high oxygen partial pressures at high temperatures Recent Research Efforts Education Outreach Guo et al, Appl. Phys. Lett.89, 022509 (2006) A new mechanism for magnetoelectricity Research Experience for High School Seniors The linear magnetoelectric effect: • Arises from a novel carrier mediated mechanism • A universal feature of the interface between a dielectric and a spin-polarized metal Pi = ij Hj Mi = ji Ej Three high school interns were recruited from International Academy (Bloomfield, MI) for research training. Their summer projects on sensors and miniature antennas resulted in two reports for the Siemens-2006 competitions. One of the two teams was finalist for the Mid-West region and presented their findings at the University of Notre Dame.  is non-zero only in the absence of space-inversion and time-reversal symmetry, and is generally small Low Pressure Chemical Vapor Deposition (CVD) of Fe2O3 & Bi2O3 Films for Multiferroic Applications • Curie temperature (Tc) 270 - 275 K • Magnetic moment for films on STO and LAO is about twice that observed for film on MgO • Moment ~ 4.63 B/f.u. at 5 K; coercivity ~ 335 - 635 Oe Design new interfacial magnetoelectric from first principles: Tactic: use a ferromagnet (or material with magnetic ordering) to lift time-reversal symmetry use the interface to lift space inversion symmetry Trial system: SrRuO3 / SrTiO3 heterostructures n-butylferrocene The student interns Harini Srinivasan and Nimit Jain from International Academy worked at Oakland University on composites for mobile phone/radar antennas size reduction. The materials they developed will allow miniaturization. The team was a Regional Finalist in the 2006-Siemens Competition. Triphenylbismuth • Hot wall quartz tube reactor • Carrier gas (Argon) at 35 sccm, O2 at 40 sccm • Total pressure: 0.8 Torr; Reaction temperature: 400 – 600 °C