NIRT: Active Multiferroic Nanostructures

Christos G Takoudis

1

, Siddhartha Ghosh

1

, Arunava Gupta

2

, Nicola A Spaldin

3

, Gopalan Srinivasan

4

1

University of Illinois at Chicago,

2

University of Alabama,

3

University of California Santa Barbara,

4

Oakland University

Supported by the NSF, Grant # CMMI 0609377 http://www.uic.edu/labs/AMReL/NIRT/index.html

Introduction

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

Double-perovskites:

 La 2 NiMnO 6 (LNMO) and La 2 CoMnO 6 (LCMO ) thin films  Bi 2 NiMnO 6 (BNMO) thin films (ongoing work)

SrTiO

3

(perovskite) Recent Findings

Calculated magnetoelectric response of interface

Response:

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

α E,33

. The substrate pinning effects is negligible only when the wire radius is much greater than the sheath radius.

Spaldin and Fiebig, Science 309 391 (2005) 

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.

Recent Research Efforts La

2

NiMnO

6



(bulk):

Single phase  Mixture of rhombohedral and monoclinic at room temperature; monoclinic at low temperature   

T

C ~ 280 K; close to room temperature Semiconducting with ~ 10 2 Ω-cm at room temperature Magnetocapacitance effect observed above 220 K

La 2 NiMnO 6 (double perovskite) A

can be any rare earth or Bi cation

B

can be Ni or Co Ordering of Ni/Mn

Bi

2

NiMnO

6



(bulk):

Monoclinic structure; multiferroic   T T CE CM ~ 485 K ~ 140 K  Capacitance change is 0.4% at  T CM and 9 T Presence of 6s 2 lone pairs of Bi 3+ leads to noncentrosymmetric distorted structure

Nano-Materials for High Frequency Electronics A new mechanism for magnetoelectricity

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 P i =  ij H j M i =  ji E j  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

Objective:

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

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.

Molecular Beam Epitaxy (MBE) Growth of BiFeO 3 SrTiO 3 (001) Substrates on

    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

Low Pressure Chemical Vapor Deposition (CVD) of Fe 2 O 3 & Bi 2 O 3 Films for Multiferroic Applications

n-butylferrocene Triphenylbismuth Trial system: SrRuO 3 / SrTiO 3 heterostructures  Hot wall quartz tube reactor  Carrier gas (Argon) at 35 sccm, O 2 at 40 sccm  Total pressure: 0.8 Torr; Reaction temperature: 400 – 600 °C

E

Mechanism:    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

E

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

Nature Nanotechnology

.

Epitaxial LNMO & LCMO Thin Films

100 mTorr O

2

1000 mTorr O

2

100 mTorr O

2

  Curie temperature (

T

c ) 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 Petrov et al, Phys. Rev. B 75 , 224407 (2007)

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

Epitaxial BiFeO 3 Thin Films on STO

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

CVD Fe 2 O 3 Thin Films

 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    Low degree of crystallinity at 450 ºC Fe 2 O 3 preferred orientations: (113) at 40.85º, (024) at 49.48º, (116) at 54.09º, (018) at 57.59º Only Fe 2 O 3 peaks observed

Education Outreach

Guo et al,

Appl.

Phys. Lett. 89 , 022509 (2006)

Research Experience for High School Seniors

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.

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.