Adsorbate Influence on the Magnetism of Ultrathin Co/Cu

Systems

David Gunn

ContentsIntroduction to:

◦ Magnetism◦ Spintronics

Oxygen and Nitrogen on Co/Cu {001}

Adsorbate trends on Co {110}

Conclusions

Magnetism

Ferromagnetism and antiferromagnetism

Giant Magnetoresistance (GMR)

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

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

FMNMFM

* Baibich et al. Phys. Rev. Lett. 61 (21), 2472 (1988)

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

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

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

Co/Cu{001} – Experimental Results

• O suppresses P to 98% (±2%) of P0

• Slight delay in onset of ferromagnetism

• N suppresses P to 84% (±3%) of P0

• Slight delay in onset of ferromagnetism

* Kopper et al J. Appl. Phys. 103, 07C904 (2008)Co thickness (ML)

Rela

tive P

ola

risa

tion

(P/P

0)

Co thickness (ML)

Rela

tive P

ola

risa

tion

(P/P

0)

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

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)

1

0)(

n

i

i

iads epp

nP

n: number of layers : information depthp: magnetic moment

Surfacepint (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

pads

psurf

pbulk

pint

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

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

o 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

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

o 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)

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

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

Co{110} – Adsorption sites Preferred adsorption site for each adsorbate and

coverage:

Adsorbate Site

O

0.5ML

3f

1.0ML

4f

N

0.5ML lb

1.0ML lb

C

0.5ML lb

1.0ML 3f

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

ConclusionsCo/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

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)

Acknowledgements

Dr. Stephen Jenkins Klaus Peter Kopper & David

Küpper EPSRC (departmental quota) HPC facility (Darwin) The Surface Science group