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

2. Contents Introduction to: Oxygen and Nitrogen on Co/Cu {001}
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 Ferromagnetism (FM): Antiferromagnetism (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

5. Giant Magnetoresistance (GMR)
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 FM NM
* Baibich et al. Phys. Rev. Lett. 61 (21), 2472 (1988)

6. Spintronics
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
FM NM FM
FM NM FM

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 ( ML) , 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)
* Kopper et 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, ultrasoft pseudopotentials
340 eV cutoff, 6x6x1 Monkhorst-Pack k-point mesh

11. Co/Cu{001} – Theoretical Part II
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)
n: number of layers
: information depth
p: magnetic moment
pads
Surface
pint (µB)
pbulk (µB)
psurf (µB)
pads (µB)
Clean
1.693
1.730
1.905 -
O-adsorbed
1.652
1.715
1.932
0.290
N-adsorbed
1.639
1.705
0.881
0.006
psurf
pbulk
pint

12. Co/Cu{001} – Theoretical Results
P/P0 Expt.
P/P0 Theor. O
98% ±2%
104% N
84% ±3%
83%
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} 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
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

14. Co{110} 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
Previous theoretical results in our group have highlighted a trend in the coupling between adsorbates and the Fe{211} surface1
1 Jenkins et al Surf. Sci. 600, 1431 (2006)

15. Co{110} – Theoretical Model
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:
CASTEP code, ultrasoft pseudopotentials
340 eV cutoff, 4x6x1 Monkhorst-Pack k-point mesh

16. Co{110} – Theoretical Model
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:
CASTEP code, ultrasoft pseudopotentials
340 eV cutoff, 4x6x1 Monkhorst-Pack k-point mesh

17. Co{110} – Adsorption sites
Preferred adsorption site for each adsorbate and coverage:
Adsorbate
Site O
0.5ML 3f
1.0ML 4f N lb C

18. Co{110} – Representative Spin Moment Values
Atom
Clean C N O
Adsorbate
-0.145
-0.018
0.263 12
1.866
0.644
0.812
1.876 11
1.863
2.033
2.015
1.878 10
1.636
1.628
1.505
1.799 9
1.626
1.286
1.495
1.707 8
1.694
1.738
1.762
1.678 7
1.697
1.600
1.631
1.673
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} Co{110}
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
Co/Cu/Co{001} systems, investigating interlayer exchange coupling in the ultrathin regime
Blue regions represent ferromagnetic coupling, white regions
represent anti-ferromagnetic coupling
* Figure reproduced from Kawakami et al Phys. Rev. Lett. 82, 4098 (1999)

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