STRUCTURAL STUDIES OF MULTIFERROIC THIN FILMS

Transcription

1 STRUCTURAL STUDIES OF MULTIFERROIC THIN FILMS Lisa Krayer (UCSD) Mentor: Daniel Pajerowski (NIST) Collaborating with: (University of Florida) Professor Amlan Biswas Daniel Grant

2 NCNR <

3 Ferromagnetism Ferroicity H < ck/mag_rock.html> Ferroelectricity E Magnetoelectric : Magnetic Susceptibility Tensor < rroelectricity> : Electric Susceptibility Tensor W. F. Brown, Jr., R. M. Hornreich and S. Shtrikman. Phys. Rev. 168, 2 (1968)

4 Perovskite Structure (ABO3) A-Site Atoms B-Site Atoms Oxygen Atoms <

5 BiMnO3 Distorted perovskite structure. Multiferroic (Magnetoelectric). Boukhvalov, D.W.; Solovyev, I.V. Phys. Rev. Serie 3. B Cond. Matt. 82, (2010) SrTiO3 (Substrate) Perovskite structure. Brous, J.; Fankuchen, I.; Banks, E. Acta Cryst. 6, (1953) K. Momma and F. Izumi, "VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data," J. Appl. Crystallogr., 44, (2011).

6 Bulk (Powder) BiMnO3 3GPa, 1100K for synthesis. Msat = 3.6 μb/mn at 5 K Tc = 105 K Ps = 62 μc/cm 2 at 87K A. Moreira dos Santos, S. Parashar, A.R. Raju, Y.S. Zhao, A.K. Cheetham, C.N.R. Rao, Solid State Commun. 122, (2002) 1 2.

7 Bulk (Powder) BiMnO3 3GPa, 1100K for synthesis. Msat = 3.6 μb/mn at 5 K Tc = 105 K Ps = 62 μc/cm 2 at 87K Film BiMnO3 Pulsed Laser Deposition. Msat = 1 μb/mn at 5 K Tc = reduced / smeared Ps = 16 μc/cm 2 at 87K A. Moreira dos Santos, S. Parashar, A.R. Raju, Y.S. Zhao, A.K. Cheetham, C.N.R. Rao. Solid State Commun. 122, 1 2, (2002)

8 Why is there a difference between the bulk and the film form of BiMnO3? Hypotheses 1. Strain from substrate SrTiO3 plane binds with BiMnO3 plane Magnetic moment reduced from change in geometric shape. 2. Unstoichiometric composition Magnetic Properties dependent on stoichiometry (BiMnO3+δ). Magnetic moment is reduced with increased oxygen content. Requires Polarized Neutron Reflectivity to find the magnetic moment as a function of depth. Alexei A. Belik, Katsuaki Kodama, Naoki Igawa, Shin-ichi Shamoto, Kosuke Kosuda and Eiji Takayama-Muromachi. J. Am. Chem. Soc. 132, (2010),

9 Diffraction Growth orientation Lattice parameters Attacking the Problem Atomic Force Microscopy Surface roughness Reflectometry Roughness Thickness Density as a function of depth. X-Ray Photoelectron Spectroscopy Atomic composition as a function of depth

10 X-Ray Diffraction Powder Diffraction Peaks of SrTiO Intensity \ \ \ Theta dd h kk ll λλ = 2 dd h kk ll sin θθ Brous, J.; Fankuchen, I.; Banks, E. Acta Cryst. 6, (1953)

11 SrTiO3 X-ray Diffraction (XRD) Data Calculated Lattice Parameter: a = Å Literature Value: a = Å a λλ = 2 aa sin θθ

12 BiMnO3 X-ray Diffraction Data SrTiO3 XRD Data SrTiO BiMnO SrTiO SrTiO BiMnO SrTiO BiMnO Calculated Cubic Lattice Parameter: a = Å λλ = 2 dd h kk ll sin θθ Bi2O / Bi2O BiMnO Bi2O Bi2O3 Bi2O3 BiMnO3 BiMnO Bi2O3 BiMnO3

13 X-Ray Diffraction Mesh Scan {2 1 1} SrTiO3 Plane Rocking Curves: λλ = 2 dd h kk ll sin θθ asrtio3 = Å (literature) abimno3 = Å in plane (calculated) Elongated out of plane. Epitaxial Growth.

14 Atomic Force Microscopy < Crevices are calculated to make up 20% of the sample.

15 Refectometry Detector ρρ = nn ii=1 bb cc VV mm QQ cc = 4 ππρρ ρρ = nn ii=1 ZZ rr ee VV mm Q Q QQ = 4 ππ λλ sin θθ Δ 2ππ Modern Techniques for Characterizing Magnetic Materials. Ed. By Yimei Zhu. Kluwer Acad. Publishers (2005).

16 SrTiO3 X-ray Reflectometry Data Interface: 5.75 Å Chsq: 36.4 Q

17 BiMnO3 X-ray Reflectometry Data 1 Layer of BiMnO3 Chsq: Thickness: 376 Å Interface: 78.4 Å Q Δ

18 BiMnO3 X-ray Reflectometry Data 2 Layers of BiMnO3 Chsq: Total thickness: Å 1 st Layer thickness: 430 Å 1 st Layer interface: 43.4 Å 2 nd Layer thickness: Å 2 nd Layer interface: 42.2 Å Q Δ

19 BiMnO3 X-ray Reflectometry Data 3 Layers of BiMnO3 Chsq: 85.9 Total thickness: Å 1 st Layer thickness: 79.7 Å 1 st Layer interface: 77.5 Å 2 nd Layer thickness: Å 2 nd Layer interface: 71.4 Å 3 rd Layer thickness: Å 3 rd Layer interface: 32.2 Å Q Δ

20 BiMnO3 X-ray Reflectometry Data 4 Layers of BiMnO3 Chsq: 36.4 Total thickness: Å 1 st Layer thickness: Å 1 st Layer interface: Å 2 nd Layer thickness: Å 2 nd Layer interface: 66.4 Å 3 rd Layer thickness: Å 3 rd Layer interface: 61.7 Å 4 th Layer thickness: 75.6 Å 4 th Layer interface: 21.0 Å Q Δ

21 BiMnO3 X-ray Reflectometry Data 5 Layers of BiMnO3 Chsq: Total thickness: Å 1 st Layer thickness: Å 1 st Layer interface: Å 2 nd Layer thickness: Å 2 nd Layer interface: Å 3 rd Layer thickness: Å 3 rd Layer interface: 36.4 Å 4 th Layer thickness: Å 4 th Layer interface: 18.9 Å 5 th Layer thickness: Å 5 th Layer interface: Å Q Δ

22 X-Ray Photoelectron Spectroscopy X-Ray ee KKKK = hνν BBBBBBBBBBBBBB EEEEEEEEEEEE Bi 4p- Mn 2s Bi 4f 3s 2p 2s 1s Counts Per Second Bi 4p Mn 2p O 1s Bi 4d Ar 2p C 1s Mn 3s Mn 3p O2s Binding Energy (ev)

23 X-Ray Photoelectron Spectroscopy Expect BiMnO Observed Formula: BixMnOy Bi 4f O 1s Ti 2p Sr 3d Formula Unit Counts Per Second Bi 4p- Mn 2s Bi 4p Mn 2p Ti 2s O 1s Ti 2p Bi 4d Sr 3s Ar 2s Sr 3p C 1s Ar 2p Bi 4f Sr 3d Mn 3s Mn 3p Sr 4s O2s Etch Time (s) Binding Energy (ev)

24 Conclusions Data suggests unstoichiometric composition of BiMnO3. Diffraction: extra peaks show Atomic Force Microscopy: 20% of the surface has voids. Reflectometry: Reduced scattering length density on the surface and at the interface with the SrTiO3. Spectroscopy: May have an increase in oxygen content at the substrate. Large concentration of Bismuth and Oxygen at surface. On Going (Over the next 4 weeks): Neutron Reflectometry Data Scattering Length Densities X-Rays (electron density): Bi ~ 83, Mn ~ 25, O ~ 8 (atomic number) Neutrons (nuclear constrast): Bi ~ 8.5, Mn ~ -3.73, O ~ (bound coherent scattering lengths) Gives additional information on the chemical formula. Polarized Neutron Reflectometry: Calculate magnetic moment as a function of depth. Acknowledgments NIST/CHRNS. Julie A. Borchers, Robert D. Shull, and Terrell A. Vanderah (Directors, MML/NCNR Materials Science SURF Program). Dan Neumann, Rob Dimeo. Daniel Pajerowski, Professor Biswas and Daniel Grant.

25 Neutron Reflectometry Nuclear and magnetic scattering. Polarized Neutron Reflectometry Separates spin up and spin down. Red- Spin up Blue Spin down Roger Pynn. Neutron Reflectometry. Indiana U. and Spallation Neutron Source. <