NIRT: Active Multiferroic NanostructuresChristos 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 0609377 http://www.uic.edu/labs/AMReL/NIRT/index.html

Introduction

Recent Research Efforts

Recent Findings

Epitaxial LNMO & LCMO Thin Films

100 mTorr O2 1000 mTorr O2

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

100 mTorr O2

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

Double-perovskites: La2NiMnO6 (LNMO) and

La2CoMnO6 (LCMO ) thin films

Bi2NiMnO6 (BNMO) thin films (ongoing work)

SrTiO3 (perovskite)

La2NiMnO6 (double perovskite)

Ordering of Ni/Mn

A can be any rare earth or Bi cationB can be Ni or Co

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

A new mechanism for magnetoelectricityPi = ij Hj

Mi = ji Ej

is non-zero only in the absence of space-inversion and time-reversal symmetry, and is generally small

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

Response:

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

Pulsed Laser Deposition (PLD) of Multifunctional Double Perovskite Thin Films and Heterostructures

Nano-Materials for High Frequency Electronics

Objective:Synthesize nanocomposites of magnetic and dielectric oxides and study the magneto-electric interactions

Prepare nano bilayers, wires and tubes of ferroelectric materials, such as lead zirconium titanante or barium titanate, on ferromagnetic nickel- or cobalt ferrite.

Approach:

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. B 75, 224407 (2007)

Impact:New materials for Consumer Electronics and National DefenseTwo patents on the potential use of composites in sensors were

granted in 2007. The university intends to license the technology

Molecular Beam Epitaxy (MBE) Growth of BiFeO3 on SrTiO3 (001) Substrates

E

E

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.

Spaldin and Fiebig, Science 309 391 (2005)

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

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.

Calculated magnetoelectric response of interface

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

Mechanism:

Rondinelli, Stengel, and Spaldin, "Carrier-mediated magnetoelectricity in complex oxide heterostructures," Under Review at Nature Nanotechnology.

Guo et al, Appl. Phys. Lett. 89, 022509 (2006)

Model for Low-frequency ME effects in Bilayers

Low Pressure Chemical Vapor Deposition (CVD) of Fe2O3 & Bi2O3 Films for Multiferroic Applications

n-butylferroceneTriphenylbismuth

Hot wall quartz tube reactorCarrier gas (Argon) at 35 sccm, O2 at 40 sccmTotal pressure: 0.8 Torr; Reaction temperature: 400 – 600 °C

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

CVD Fe2O3 Thin Films

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

Epitaxial BiFeO3 Thin Films on STO

Kabelac et al, J. Vac. Sci. Technol. B 25(3) 1049-1052 (2007)

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.

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.

Education Outreach

Research Experience for High School Seniors