Adsorbate Influence on the Magnetism of Ultrathin Co/Cu Systems

1. Adsorbate Influence on the Magnetism of Ultrathin Co/Cu Systems David Gunn

2. Contents • Introduction to: • Magnetism • Spintronics • Oxygen and Nitrogen on Co/Cu {001} • Adsorbate trends on Co {110} • Conclusions

3. Magnetism Ferromagnetism and antiferromagnetism Giant Magnetoresistance (GMR)

4. FM and AFM • Alignment of electron spins along a preferred direction • Co, Ni, Fe native 3-d ferromagnetic elements • Regular alternating alignment of neighboring spins • Both FM and AFM only occur below a particular temperature Ferromagnetism (FM): Antiferromagnetism (AFM):

5. Giant Magnetoresistance (GMR) FM NM FM * Baibich et al. Phys. Rev. Lett. 61 (21), 2472 (1988) Discovered independently in 1988 by Peter Grünberg and Albert Fert Awarded Nobel prize in physics in 2007 GMR materials consist of two or more FM layers separated by a non-magnetic (NM) spacer: Decrease in electrical resistance in the presence of a magnetic field

6. Spintronics FM NM FM FM NM FM Devices that utilize the quantum spin state of the electron to transfer information (extra degree of freedom) Spin valves Commercial uses: hard drives, MRAM Increased scattering, and therefore resistance, occurs as spin-polarized current passes through a layer that is aligned anti-parallel to the polarization

7. Co/Cu{001} Key model system for studying magnetism Epitaxially grown fcc Co on a Cu{001} substrate Gaseous adsorbates are known to significantly alter structural and magnetic properties of systems Study of a well-defined quantity of gas adsorbate on cobalt layers of increasing thickness

8. Co/Cu{001} – Experimental Background Experimental work completed by Klaus-Peter Kopper and David Küpper Pre-dosed Cu{001} surface with O at 510K Leads to an initial (√2 x 2√2)R45o-O reconstruction Co is then deposited in steps (1.1-2.8ML) , O acts as surfactant and migrates to the top layer O on top of Co{001} forms a c(2x2) reconstruction occupying the four-fold hollow site Polarisation measurements taken at each step

9. Co/Cu{001} – Experimental Results • O suppresses P to 98% (±2%) of P0 • Slight delay in onset of ferromagnetism Relative Polarisation (P/P0) Co thickness (ML) • N suppresses P to 84% (±3%) of P0 • Slight delay in onset of ferromagnetism Relative Polarisation (P/P0) * Kopperet al J. Appl. Phys. 103, 07C904 (2008) Co thickness (ML)

10. Co/Cu{001} – Theoretical Model • 6 copper layers simulating the substrate • 1-6ML of cobalt epitaxed • 0.5ML O and N adatoms placed in four-fold hollow position on top of cobalt layer • Vacuum region of ~15Å • Bader topological analysis enables atomically resolved spin-moments • CASTEP code, ultrasoftpseudopotentials • 340 eV cutoff, 6x6x1 Monkhorst-Pack k-point mesh

11. Co/Cu{001} – Theoretical Part II n: number of layers : information depth p: magnetic moment pads psurf pbulk pint Spin moments can be resolved into four distinct groups, pint , psurf, pbulk, pads Secondary electron spin polarisation is a strongly surface-oriented technique, can fit to exponential relationship: Calculated magnetic moments (from 6ML Co values)

12. Co/Cu{001} – Theoretical Results • Theoretical results show remarkable agreement with experiment • Oxygen has little impact on polarization, compared to nitrogen • Now have an accurate method of predicting polarisation of systems of this type

13. Co{110} • Interesting experimentally observed effects such as: • Change in the coercive field of Co{110} on adsorption of O, H • Spin reorientation transition of Co{110} on adsorption of CO • fcc {110} surface • Simple atomic adsorbates: C, N, O • Can be produced experimentally e.g. through dissociation of CO, N2, O2 • Surface localised effect on magnetic moment

14. Co{110} • Previous theoretical results in our group have highlighted a trend in the coupling between adsorbates and the Fe{211} surface1 1Jenkins et al Surf. Sci. 600, 1431 (2006) • fcc {110} surface • Simple atomic adsorbates: C, N, O • Can be produced experimentally e.g. through dissociation of CO, N2, O2 • Surface localised effect on magnetic moment

15. Co{110} – Theoretical Model • CASTEP code, ultrasoftpseudopotentials • 340 eV cutoff, 4x6x1 Monkhorst-Pack k-point mesh Previous calculations have established that our 6ML slab is of sufficient thickness to simulate the surface termination of the bulk substrate Adsorbates (C, N, O) are modelled at two coverages (0.5ML and 1.0ML), and at five high-symmetry sites:

16. Co{110} – Theoretical Model • CASTEP code, ultrasoftpseudopotentials • 340 eV cutoff, 4x6x1 Monkhorst-Pack k-point mesh Previous calculations have established that our 6ML slab is of sufficient thickness to simulate the surface termination of the bulk substrate Adsorbates (C, N, O) are modelled at two coverages (0.5ML and 1.0ML), and at five high-symmetry sites:

17. Co{110} – Adsorption sites Preferred adsorption site for each adsorbate and coverage:

18. Co{110} – Representative Spin Moment Values • All moments are in µB • Values shown are for 0.5ML adsorption • Increasingly FM coupling between adsorbate and surface as we go from C-N-O • Trend holds across other ferromagnets • (Fe, Ni) and for greater coverage

19. Conclusions • Co/Cu{001} • Excellent agreement of theory and experiment • N-induced polarization decrease of ~17% • O has little effect on polarization • Co{110} • Increasing FM character of bonding from carbon-nitrogen-oxygen • Strongly surface localized effect • Trend continues for higher coverage and for other 3d-ferromagnets

20. Future work * Figure reproduced from Kawakami et al Phys. Rev. Lett. 82, 4098 (1999) Co/Cu/Co{001} systems, investigating interlayer exchange coupling in the ultrathin regime Blue regions represent ferromagnetic coupling, white regions represent anti-ferromagnetic coupling

21. Acknowledgements Dr. Stephen Jenkins Klaus Peter Kopper & David Küpper EPSRC (departmental quota) HPC facility (Darwin) The Surface Science group