Adsorbate Influence on the Magnetism of Ultrathin Co/Cu Systems - PowerPoint Presentation

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

Contents Introduction 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 FM NM FM * 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 systemsStudy 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 510KLeads to an initial (√2 x 2√2)R45o-O reconstructionCo is then deposited in steps (1.1-2.8ML) , O acts as surfactant and migrates to the top layerO 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 P 0 Slight delay in onset of ferromagnetism N suppresses P to 84% (±3%) of P 0 Slight delay in onset of ferromagnetism * Kopper et al J. Appl. Phys. 103 , 07C904 (2008) Co thickness (ML) Relative Polarisation (P/P 0 ) Co thickness (ML) Relative Polarisation (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 , p bulk , padsSecondary 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 Surface p int (µ B ) p bulk (µ B ) p surf (µ B ) p ads (µB) Clean 1.693 1.7301.905- O-adsorbed 1.6521.7151.932 0.290 N-adsorbed 1.6391.705 0.8810.006 p ads p surf p bulk p int

Co/Cu{001} – Theoretical Results P/P 0 Expt. P/P 0 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, N 2 , O 2 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

Co{110} fcc {110} surface Simple atomic adsorbates : C, N, OCan be produced experimentally e.g. through dissociation of CO, N2, O2Surface localised effect on magnetic moment Previous theoretical results in our group have highlighted a trend in the coupling between adsorbates and the Fe{211} surface 1 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 µ B 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

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

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 KüpperEPSRC (departmental quota)HPC facility (Darwin)The Surface Science group