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Student: Peter Knapp Research Advisor: Professor Jeremiah Abiade

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1 Student: Peter Knapp Research Advisor: Professor Jeremiah Abiade
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 (A) (B) 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 KrF Excimer Laser Thickness LSM0 (nm) Pulses for LSMO Deposition Thickness BFO (nm) Pulses for BFO Deposition Order of layers on substrate (bottom/top) 150 10,580 BFO BFO/LSMO 200 14,100 250 17,630 LSMO LSMO/BFO Films deposited on both LaAlO3 and SrTiO3

15 TEM Results – 150nm_BFO_LaAlO3

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 Rigaku-ATXG diffractometer
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 Slit to 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 20o to 600 If resulting graph has Single Peak Single Crystal Multiple Peaks Polycrystalline No clear Peaks Amorphous Amorphous Polycrystalline Nanocrystaline

20 Approaching Single Crystal
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
Film  (radians) B(2) (radians) Crystallite Width (nm) 150nm_BFO_SrTiO3 0.263 0.111 5 150nm_LSMO_SrTiO3 0.256 0.0803 8 150nm_BFO_150nm_LSMO_LaAlO3 0.265 200nm_BFO_150nm_LSMO_SrTiO3 0.259 0.0986 6 250nm_LSMO_150nm_BFO_SrTiO3 0.254 0.116 Possible to determine the size of crystallites using the Scherrer Eqn. B(2) = Peak Width (radians) λ = 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*106 photons/s Sample : 5mmX5mmX0.5mm substrates Layer Thickness (Å) SLD (Real) SLD (Imaginary) Roughness (Å) Air INF Residue 84.3 4.27*10-6 3.32*10-8 30.1 BFO 1450 6.57*10-5 7.90*10-6 77.5 LSMO 1550 5.09*10-5 1.59*10-5 51.5 SrTiO3 Substrate 4.49*10-5 1.95*10-6 49.8

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 Peak Position BE (eV) FWHM (eV) Raw Area (CPS) RSF
Atomic Mass Atomic Conc. (%) Mass Conc. (%) Bi 4f 156 2.767 9.140 208.98 21 68 Fe 2p 708 4.572 2.957 55.846 19 17 O 1s 527 3.162 0.780 15.99 60 15 XPS Results for original 150nm_BFO_SrTiO3: Proper Stoichiometry Observed Peak Position BE (eV) FWHM (eV) Raw Area (CPS) RSF Atomic Mass Atomic Conc. (%) Mass Conc. (%) Bi 4f 157 2.881 9.140 208.98 24 74 Fe 2p 708 5.099 2.957 55.846 13 11 O 1s 527 3.380 0.780 15.99 63 15 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 # 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.

34 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, (2011), L. 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, X. 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), M. 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, S. 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, (2007), Esteve, 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: /pssb G-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. Phys Lett., (2008), 1-3 8D. B. Chrisey, G.K. Hubbler: ‘Pulsed Laser Deposition of Thin Films’, 13-56; 1994, New York, John Wiley & Sons.


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