Norton Group Meeting 4/1/08 Joe Cianfrone

1. ZnCo2O4: A transparent, p-type, ferromagnetic semiconductor relevant to spintronics and wide bandgap electronics Norton Group Meeting 4/1/08 Joe Cianfrone

2. Proposal Review • ZnCo2O4 is ferromagnetic semiconductor • Can be p- or n-type • Applications to spintronics and wide bandgap electronics • Investigation of cation and anion substitution to better understand the optical, electronic and magnetic properties

3. ZnCo2O4: Structure and composition • Spinel structure, A+2(B+3)2O4 • Space group Fd3m • Tetrahedral A sites • Octahedral B sites (From O’Handley, from Woodward)

4. ZnCo2O4: Electronic and optical properties • Carrier type depends on oxygen pressure during growth • Indirect Bandgap of 2.63 eV (472 nm) (From Kim et al.)

5. Background: Origins of ferromagnetism • ZnCo2O4 can be both ferromagnetic and antiferromagnetic! • Antiferromagnetic Co-O-Co superexchange • Ferromagnetic Co-Co hole mediated exchange • For a large enough number of holes, films are ferromagnetic

6. Experiment Methodology: Characterization Scheme Tools used Experiment Goal Vary T, P to optimize Growth region for spinel phase epitaxy Structural characterization XRD θ-2θ scans Hall effect, PPMS, SQUID optical absorption, SE, AFM Hall effect, PPMS, SQUID optical absorption, SE, AFM Hall effect, SQUID, optical absorption, SE, AFM Magnetic, electronic, optical, surface properties Magnetic, electronic, optical, surface properties Magnetic, electronic, optical, surface properties Film growth at optimum temperature, vary growth pressure Film growth at optimum temperature, vary growth pressure Film growth at optimum temperature, vary growth pressure Film growth at optimum temperature, vary growth pressure Vary Laser pulse E,f to optimize crystallinity Quality of epitaxy XRD Ω-RCs, φ-scans, XPS

7. Structural Characterization: X-Ray Diffraction

8. Structural Characterization: X-Ray Photoelectron Spectroscopy Zn peaks Co peaks O peak O peak C peak

9. Electronic Characterization: Hall effect

10. Optical Characterization: UV-Vis spectra

11. Magnetic Properties:SQUID • Ferromagnetism exhibited in • ZnCo2O4 sample grown at • 400 C, 150 mTorr O2 • TC>300 K

12. Experiment status • Known: • Optimum temperature and substrate isolated (400 C, sapphire) • Film grown at 400 C, 150 mTorr was ferromagnetic • Unknown: • Effect of pressure on electronic, magnetic, optical properties • Origin of ferromagnetism • Effect of laser pulse frequency on crystal quality and how this affects electronic, magnetic, optical properties

13. Planned Experiments • Films to be grown at 400 C and pressures: • 10, 50, 100, 150, 200, 300 mTorr O2 • Characterization: • Electronic: Hall, PPMS (AHE, MR) • Magnetic: SQUID, PPMS (χ vs. H, χ vs. T) • Optical: Spectroscopic Ellipsometry, Optical Absorption • Surface: AFM

14. Goals • Electronic properties as a function of Oxygen growth pressure: • Carrier type • Resistivity • Carrier density • Optical properties dependence on growth pressure: • (α)1/2 and (αhν)2 vs. E • Size and type of bandgap • Refractive index, extinction coefficient vs. wavelength

15. Goals • Magnetic properties dependence on growth pressure: • M (per Co atom) vs. pO2 • TC vs pO2 • Origin of magnetic nature • Carrier dependent Co-Co exchange? • AHE? • Co clusters? • TEM to look for Co precipitates • XPS to look for Co-Co atoms