Presentation on theme: "Analysis of Ferromagnetic-Multiferroic interfaces in Epitaxial Multilayers of LSMO and BFO Student: Peter Knapp Research Advisor: Professor Jeremiah Abiade."— Presentation transcript:
Analysis of Ferromagnetic-Multiferroic interfaces in Epitaxial Multilayers of LSMO and BFO Student: Peter Knapp Research Advisor: Professor Jeremiah Abiade
Overview I.Bilayers were fabricated from ferromagnetic (FM) LSMO (La 0.7 Sr 0.3 MnO 3 ) and anti- ferromagnetic (AFM) BFO (BiFeO 3 ) via Pulsed Laser Deposition (PLD) II.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 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 Materials where electric polarization influences ferromagnetic polarization, allowing manipulation of electric/magnetic order 1 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 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: ABO 3
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 These experiments will focus on achieving structural control during deposition Substrate will be varied between LaAlO 3 or SrTiO 3 The thickness of the layers will be varied Layer order will be varied
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 I.PLD for synthesis of the Bilayers. II.TEM to observe local characteristics II.XRD to observe interlayer interaction and structural characteristics III.XPS to confirm composition
Pulsed Laser Deposition 1.Physical Vapor Deposition Technique 2.High Powered (Excimer Laser) focused on target (material to be deposited) in vacuum 3.Material is vaporized into plasma plume which extends from target 4.Proceeds to land on substrate forming a thin film 5.Highly Advantageous
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 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
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 Target Substrate Distance=4.5 cm Deposition Temp=650 0 Celsius O 2 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) 0015010,580BFO 15010,58015010,580BFO/LSMO 20014,10015010,580BFO/LSMO 25017,63015010,580BFO/LSMO 15010,58000LSMO 15010,58015010,580LSMO/BFO 15010,58020014,100LSMO/BFO 15010,58025017,630LSMO/BFO Films deposited on both LaAlO 3 and SrTiO 3
TEM Results – Contd. Clean Diffraction Pattern Indicates highly crystalline film Growth rate of BFO twice what was expected LaAlO 3 BFO Glue Unknown
TEM - Results 1.PLD Allowed for deposition of films that are highly crystalline 2.At the interface there is a slight rotation (30 o to 40 o ) between the crystalline plane of the substrate and film 3.Growth Rate of BFO is twice that of LSMO
XRD Preliminary Work Slit Collimation Geometry S 1 = 0.5 mm (h) 2 mm (v) S 2 = 0.1 mm (h) 2 mm (v) S 3 and X Replaced with Soller Slit to lock out reflection from excess crystal planes/substrate Sample : 5mmX5mmX0.5mm substrates Rigaku-ATXG diffractometer X S3S3 S2S2 S1S1
Crystallinity Scans Hold Omega at 0.5 degrees Scan 2Theta from 20 o to 60 0 If resulting graph has – Single Peak Single Crystal – Multiple Peaks Polycrystalline – No clear Peaks Amorphous PolycrystallineNanocrystaline Amorphous
Sample Scans PolycrystallineNanocrystaline AmorphousApproaching Single Crystal
Crystallinity Scan Contd. AmorphousNanocrystalline or Amorphous Nanoctystalline or AmorphousPolycrystaline
Crystallinity Scans Contd. Amorphous Amorphous or Nanocrystalline
Crystallinity Scans Contd. AmorphousAmorphous 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 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) Film (radians) B(2 ) (radians) Crystallite Width (nm) 150nm_BFO_SrTiO 3 0.2630.1115 150nm_LSMO_SrTi O 3 0.2560.08038 150nm_BFO_150n m_LSMO_LaAlO 3 0.2650.1115 200nm_BFO_150n m_LSMO_SrTiO 3 0.2590.09866 250nm_LSMO_150 nm_BFO_SrTiO 3 0.2540.1165
New 150 nm BFO Film on SrTiO3 Used standard Laser Fluence and Pulse Frequency Modified Annealing Process Deposition at 670 o C at.02 Torr Cool to 390 o C, anneal for 1 hour Cool to room temperature at 5 o C/min Data indicates Amorphous film. XPS analysis used to confirm composition allowing us to draw a more accurate conclusion.
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 GE111 Compressor Crystal S 1 = 0.5 mm (h) 2 mm (v) S 2 = 0.1 mm (h) 2 mm (v) S 3 = 0.2 mm (h) 5 mm (v) X = 0.2 mm (h) Flux: ~ 2.1*10 6 photons/s Sample : 5mmX5mmX0.5mm substrates LayerThickness (Å)SLD (Real) SLD (Imaginary) Roughness (Å) AirINF000 Residue84.34.27*10 -6 3.32*10 -8 30.1 BFO14506.57*10 -5 7.90*10 -6 77.5 LSMO15505.09*10 -5 1.59*10 -5 51.5 SrTiO 3 SubstrateINF4.49*10 -5 1.95*10 -6 49.8
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 PeakPosition BE (eV) FWHM (eV) Raw Area (CPS) RSFAtomic Mass Atomic Conc. (%) Mass Conc. (%) Bi 4f1572.88114818709.140208.982474 Fe 2p7085.099245382.52.95755.8461311 O 1s5273.380331482.50.78015.996315 PeakPosition BE (eV) FWHM (eV) Raw Area (CPS) RSFAtomic Mass Atomic Conc. (%) Mass Conc. (%) Bi 4f1562.76713028509.140208.982168 Fe 2p7084.572378805.02.95755.8461917 O 1s5273.162318230.00.78015.996015 XPS Results for original 150nm_BFO_SrTiO 3 : Proper Stoichiometry Observed XPS Results for New 150nm_BFO_SrTiO 3 : Proper Stoichiometry not Observed
XPS - Results Stoichiometry of films very similar to target material Currently no explanation for iron deficiency in the new BFO film
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 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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