1. ZnCo 2 O 4 : A transparent, p-type, ferromagnetic semiconductor relevant to spintronics and wide bandgap electronics Norton Group Meeting 4/1/08 Joe Cianfrone

2. Proposal Review ZnCo 2 O 4 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. ZnCo 2 O 4 : Structure and composition Spinel structure, A +2 (B +3 ) 2 O 4 Space group Fd3m Tetrahedral A sites Octahedral B sites (From O’Handley, from Woodward)

4. ZnCo 2 O 4 : 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 ZnCo 2 O 4 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 XRD θ-2θ scans Structural characterization Vary T, P to optimize Growth region for spinel phase epitaxy Tools used ExperimentGoal Hall effect, SQUID, optical absorption, SE, AFM XRD Ω-RCs, φ-scans, XPS Quality of epitaxy Vary Laser pulse E,f to optimize crystallinity Hall effect, PPMS, SQUID optical absorption, SE, AFM Magnetic, electronic, optical, surface properties Film growth at optimum temperature, vary growth pressure Magnetic, electronic, optical, surface properties Film growth at optimum temperature, vary growth pressure Hall effect, PPMS, SQUID optical absorption, SE, AFM Magnetic, electronic, optical, surface properties Film growth at optimum temperature, vary growth pressure

7. Structural Characterization: X-Ray Diffraction

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

9. Electronic Characterization: Hall effect

10. Optical Characterization: UV-Vis spectra

11. Magnetic Properties: SQUID Ferromagnetism exhibited in ZnCo 2 O 4 sample grown at 400 C, 150 mTorr O 2 T C >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 O 2 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. p O 2 –T C vs p O 2 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