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Analysis of Ferromagnetic- Multiferroic interfaces in Epitaxial Multilayers of LSMO and BFO. Student: Peter Knapp Research Advisor: Professor Jeremiah Abiade. Overview.

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analysis of ferromagnetic multiferroic interfaces in epitaxial multilayers of lsmo and bfo

Analysis of Ferromagnetic-Multiferroic interfaces in Epitaxial Multilayers of LSMO and BFO

Student: Peter Knapp

Research Advisor: Professor Jeremiah Abiade

overview
Overview
  • Bilayers were fabricated from ferromagnetic (FM) LSMO (La0.7Sr0.3MnO3) and anti-ferromagnetic (AFM) BFO (BiFeO3) via Pulsed Laser Deposition (PLD)
  • Layers were analyzed using TEM (Transmission Electron Microscopy), XRD (X-ray Diffraction), and XPS (X-ray Photoelectron Spectroscopy) in order to confirm composition and observe structural detiails
motivation for project
Motivation For Project
  • Need to control the structure of oxide thin films and multilayers
  • Understand effects of structure & layering on magnetic interaction
  • Preliminary work for future experiments on properties of ferromagnetic/ferroelectric systems
introduction to multiferroic bilayers
Introduction to Multiferroic Bilayers
  • Materials where electric polarization influences ferromagnetic polarization, allowing manipulation of electric/magnetic order1
  • Contemporary research focuses on bilayers of FM and AFM materials
  • These structures demonstrate exchange bias (EB), exchange enhancement (EE), and exchange coupling (EC)
particular interest in lsmo and bfo
Particular Interest in LSMO and BFO
  • On their own LSMO and BFO possess useful characteristics
  • Combined they clearly exhibit exchange interactions that characterize multiferroic systems
  • Additional advantages include common perovskite structure and a close lattice parameter

(B)

(A)

All Perovskites have the same basic chemical formula: ABO3

interfacial effects
Interfacial Effects
  • Researchers know little about how interfacial effects impact magnetic effects
  • It is known that there is lattice mismatch and diffusion between LSMO and BFO layers.
  • It is necessary to understand how these phenomena can effect film properties

Lattice Mismatch

controlling structure
Controlling Structure
  • These experiments will focus on achieving structural control during deposition
  • Substrate will be varied between LaAlO3 or SrTiO3
  • The thickness of the layers will be varied
  • Layer order will be varied
potential applications of work
Potential Applications of Work
  • Could help demonstrate novel uses for materials like LMSO and BFO in memory devices and sensors, for instance Hard Drives and SQUIDs (superconducting quantum interference devices)
  • Development of novel heterostructures for unusual uses i.e. LMSO as electrode for ferroelectric films
  • Tailor structures to realize multicomponent multiferroic systems (e.g. electrical control of magnetism)
experimental procedures
Experimental Procedures
  • PLD for synthesis of the Bilayers.
  • TEM to observe local characteristics
  • XRD to observe interlayer interaction and structural characteristics
  • XPS to confirm composition
pulsed laser deposition
Pulsed Laser Deposition
  • Physical Vapor Deposition Technique
  • High Powered (Excimer Laser) focused on target (material to be deposited) in vacuum
  • Material is vaporized into plasma plume which extends from target
  • Proceeds to land on substrate forming a thin film
  • Highly Advantageous
transmission electron microscopy
Transmission Electron Microscopy
  • Beam of Electrons fired through specimen
  • Electrons interact with material in film
  • Image created on photographic film or a CCD camera
ii x ray reflectivity
II. X-Ray Reflectivity
  • Measurement: Specular reflection as a function of angle of incidence.
  • Result: electron density profile along substrate normal
  • Thickness and average electron density of the film.
  • Thickness and electron density can be used to infer roughness and structural defects like diffusion and lattice mismatch
  • X-ray techniques can also be used to analyze strain in the films

Thin Film or Multilayer

Thin Film or Multilayer

iii x ray photoelectron spectroscopy
III. X-ray Photoelectron Spectroscopy
  • XPS = X-Ray Photoelectron Spectroscopy
  • Kinetic Energy and Intensity of electrons emitted from material irradiated with X-Rays is measured
  • Yields elemental composition, empirical formula, chemical state, and electronic state

XPS Mechanism

pld results films deposited
PLD Results: Films Deposited
  • Target Substrate Distance=4.5 cm
  • Deposition Temp=6500 Celsius
  • O2 Background=0.02 Torr
  • Pulse Frequency=5 Hz
  • Laser Fluence =1.5 Jcm-2
  • Wavelength=248 nm
  • Used KrFExcimer Laser

Films deposited on both LaAlO3 and SrTiO3

tem results contd
TEM Results – Contd.

Unknown

LaAlO3

BFO

Glue

Clean Diffraction Pattern Indicates highly crystalline film

Growth rate of BFO twice what was expected

tem results
TEM - Results
  • PLD Allowed for deposition of films that are highly crystalline
  • At the interface there is a slight rotation (30o to 40o) between the crystalline plane of the substrate and film
  • Growth Rate of BFO is twice that of LSMO
xrd preliminary work
XRD Preliminary Work

Slit Collimation Geometry

S1 = 0.5 mm (h)  2 mm (v)

S2= 0.1 mm (h) 2 mm (v)

S3 and X Replaced with Soller Slitto lock out reflection from excess crystal planes/substrate

Sample : 5mmX5mmX0.5mm substrates

X

S3

S2

S1

Rigaku-ATXG diffractometer

crystallinity scans
Crystallinity Scans
  • Hold Omega at 0.5 degrees
  • Scan 2Theta from 20oto 600
  • If resulting graph has
    • Single Peak Single Crystal
    • Multiple Peaks Polycrystalline
    • No clear Peaks Amorphous

Amorphous

Polycrystalline

Nanocrystaline

sample scans
Sample Scans

Approaching Single Crystal

Amorphous

Polycrystalline

Nanocrystaline

crystallinity scan contd
Crystallinity Scan Contd.

Amorphous

Nanocrystalline or Amorphous

Nanoctystalline or Amorphous

Polycrystaline

crystallinity scans contd
Crystallinity Scans Contd.

Amorphous

Amorphous

Amorphous

Amorphous or Nanocrystalline

crystallinity scans contd1
Crystallinity Scans Contd.

Amorphous

Amorphous

Amorphous

Amorphous

crystallinity scans contd2
Crystallinity Scans Contd.

Amorphous

Amorphous or Nanocrystalline

  • Results
  • Majority of Films are amorphous
  • Several Films appear to be Polycrystalline or Nanocrystalline
  • New BFO film created with alternate deposition parameters
nanocrystaline samples
Nanocrystaline Samples

Possible to determine the size of crystallites using the Scherrer Eqn.

B(2) = Peak Width (radians)

λ = .1542 nm

L = Crystallite Width (nm)

 = d-spacing (radians)

K = Scherrer Constant (Assumed to be 1)

new 150 nm bfo film on srtio3
New 150 nm BFO Film on SrTiO3
  • Used standard Laser Fluence and Pulse Frequency
  • Modified Annealing Process
  • Deposition at 670o C at .02 Torr
  • Cool to 390o C, anneal for 1 hour
  • Cool to room temperature at 5o C/min

Data indicates Amorphous film. XPS analysis used to confirm composition allowing us to draw a more accurate conclusion.

crystallinity scans results
Crystallinity Scans - Results
  • Majority of films are amorphous with some polycrystalline and nanocrystalline samples
  • Likely due to diffusion of oxygen during annealing
  • Indicated deposition process still requires optimization
x ray reflectivity 150nm bfo 150nm lsmo srtio3
X-Ray Reflectivity 150nm_BFO_150nm_LSMO_SrTiO3

GE111 Compressor Crystal

S1 = 0.5 mm (h)  2 mm (v)

S2 = 0.1 mm (h)  2 mm (v)

S3 = 0.2 mm (h)  5 mm (v)

X = 0.2 mm (h)

Flux: ~ 2.1*106photons/s

Sample : 5mmX5mmX0.5mm substrates

conclusion xrr
Conclusion - XRR
  • Thickness and SLD data seems reasonable but contrasts with data on growth rate from TEM
  • Unfitted drop results from having a high roughness film and low X-ray intensity during scanning
  • Top residue Layer is Likely a Combination of organics and silver particles from adhesive
xps analysis
XPS Analysis

XPS Results for original 150nm_BFO_SrTiO3: Proper Stoichiometry Observed

XPS Results for New 150nm_BFO_SrTiO3: Proper Stoichiometry not Observed

xps results
XPS - Results
  • Stoichiometry of films very similar to target material
  • Currently no explanation for iron deficiency in the new BFO film
summary conclusion
Summary/Conclusion
  • While the constructed films were not epitaxial many were highly crystalline
  • The Stoichiometry of films examined by XPS was consistent with the target material
  • XRR indicated the films have a large roughness
  • The deposition process for LSMO and BFO still requires optimization.
acknowledgements
Acknowledgements

The financial support from the National Science Foundation, EEC-NSF Grant # 1062943 is gratefully acknowledged. I would like to thank Professors Jursich and Takoudis for organizing the REU Program. I would like to thank the LORE lab in general and Professor Jeremiah Abiade specifically for providing me with the opportunity to work in their lab.

sources
Sources

1P.S. Sankara Rama Krishnan, M. Arredondo, M. Saunders, Q. M. Ramase, M. Valanoor: ‘Microstructural analysis of interfaces in a ferromagnetic-multiferroic epitaxial heterostructure’, J. Appl. Phys., 2011, 109 034103 (2011), 1-7.

2L. W. Martin, Y-H. Chu, M. b. Holcomb, M. Huijben. P. Yu, S-J. Han, D. Lee, S. X. Wang, R. Ramesh: ‘Nanoscale Control of Exchange Bias with BiFeO3 Thin Films’, Nano Letters, 2008, Vol. 8, No. 7, 2050-2055.

3X. Ke, L. J. Belenkey, C. B. Eom, M. S. Rzchowski: ‘Antiferromagnetic exchange-bias in epitaxial ferromagnetic La0.67Sr0.33MnO3 /SrRuO3 bilayers’, J. Appl. Phys., 2005, 97 10k115 (2005), 1-3.

4M. Kharrasov, I. Kyzyrgulov, F. Iskahkov: ‘Exchange enhancement of the magnetoelastic interaction in a LaMnO<sub>3</sub> crystal’, Doklady Physics, 2003, Vol. 48, No. 9, 499-500.

5S. Habouti, R. K. Shiva, C-H. Solterbeck, M. Es-Souni, V. Zaporojtcheko: ‘La0.8Sr0.2MnO3 buffer layer effects on microstructure, leakage current, polarization, and magnetic properties of BiFeO3 thin films’, J. Appl. Phys., 2007, 102 044113 (2007), 1-6.

6Esteve, D., Postava, K., Gogol, P., Niu, G., Vilquin, B. and Lecoeur, P. (2010), In situ monitoring of La0.67Sr0.33MnO3 monolayers grown by pulsed laser deposition. Phys. Status Solidi B, 247: 1956–1959. doi: 10.1002/pssb.200983960

7G-Z. Liu, C. Wang, C-C. Wang, J. Qiu, M. He, J. Xing, K-J Jin, H-B Lu, G-Z. Yang: ‘Effects of interfacial polarization on the dielectric properties of BiFeO3 thin film capacitors’, Appl. PhysLett., 2008. 92 122903 (2008), 1-3

8D. B. Chrisey, G.K. Hubbler: ‘Pulsed Laser Deposition of Thin Films’, 13-56; 1994, New York, John Wiley & Sons.