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

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

2. 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

3. 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

4. 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)

5. 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

6. 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

7. 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

8. 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)

9. 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

10. 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

11. Transmission Electron Microscopy • Beam of Electrons fired through specimen • Electrons interact with material in film • Image created on photographic film or a CCD camera

12. 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

13. 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

14. 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

15. TEM Results – 150nm_BFO_LaAlO3 LaAlO3 LaAlO3 BFO BFO

16. TEM Results – Contd. Unknown LaAlO3 BFO Glue Clean Diffraction Pattern Indicates highly crystalline film Growth rate of BFO twice what was expected

17. 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

18. 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

19. 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

20. Sample Scans Approaching Single Crystal Amorphous Polycrystalline Nanocrystaline

21. Crystallinity Scan Contd. Amorphous Nanocrystalline or Amorphous Nanoctystalline or Amorphous Polycrystaline

22. Crystallinity Scans Contd. Amorphous Amorphous Amorphous Amorphous or Nanocrystalline

23. Crystallinity Scans Contd. Amorphous Amorphous Amorphous Amorphous

24. 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

25. 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)

26. 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.

27. 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

28. 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

29. 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

30. XPS Analysis XPS Results for original 150nm_BFO_SrTiO3: Proper Stoichiometry Observed XPS Results for New 150nm_BFO_SrTiO3: Proper Stoichiometry not Observed

31. XPS - Results • Stoichiometry of films very similar to target material • Currently no explanation for iron deficiency in the new BFO film

32. 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.

33. 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.

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