New magnetism of 3d monolayers grown with oxygen surfactant: Experiment vs. ab initio calculations

Transcription

1 New magnetism of 3d monolayers grown with oxygen surfactant: Experiment vs. ab initio calculations 1. Growth and structure 2. Magnetism and MAE 3. Induced magnetism at oxygen Klaus Baberschke Institut für Experimentalphysik Freie Universität Berlin Arnimallee 14 D D Berlin-Dahlem Germany Goal: Theory should calculate: XAS-, XMCD spectra, magnetic moments at Ni and O 1

2 Acknowledgement BESSY-crew: H. Wende, C. Sorg, N. Ponpandian, M. Bernien, (A. Scherz) Lab. experiments: K. Lenz, T.Tolinski, (J. Lindner, E. Kosubek, C. Rüdt, R. Nünthel) Support: BMBF (BESSY), DFG (lab.) 2

3 1. Growth and structure: Surfactant Activity Known: Improved growth R. Nünthel et al., Surf. Sci. 531, 53 (2003), Surf. Sci , 100 (2004). Shift of SRT to lower thickness J. Lindner et al., Surf. Sci. 523, L65 (2003). Open: Final evidence: O on top? NiO? Influence of O on spin and orbital moments of Ni? Induced magnetism of O? 3

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5 O( 2 2 2)R45 /Cu(100) missing row reconstruction c(2 2)O/Ni/Cu(100) 47 ev evaporate 5.5 ML Ni nm nm 2 nm nm [010] [001] nm U = V I = 0.65 na U = V I = 1.55 na nm [010] [100] oxygen top layer middle layer bottom layer [010] [100] from AES oxygen floats on top of Ni film oxygen top layer bottom layer R. Nünthel et al., Surf. Sci. 531, (2003) 5

6 2. Magnetic Anisotropy Energy (MAE) in ultra thin films There are only 2 origins for MAE: 1) dipol-dipol interaction (µ 1 r)(µ 2 r) and 2) spin-orbit coupling? L S (intrinsic K or E band ) Growth of artificial nanostructures bcc, fcc tetragonal, trigonal e 1 = +2.5% e 2 = -3.2% tetragonal Ni(001) (a p Ni bulk =2.49Å) fcc Cu(001) substrate (a p =2.55Å) Structural changes by 0.05 Å increase MAE by 2-3 orders of magnitude (~ µeV/atom) O. Hjortstam, K. B. et al. PRB 55, ( 97) R. Wu et al. JMMM 170, 103 ( 97) 6

7 For thin films the Curie temperature can be manipulated t=t/t C (d) P. Poulopoulos and K. B. J. Phys.: Condens. Matter 11, 9495 (1999) 7

8 SP-KKR calculation for rigit fcc and relaxed fct structures layer resolved E b =ΣK i at T=0 C. Uiberacker et al., PRL 82, 1289 (1999) R. Hammerling et al., PRB 68, (2003) The surface and interface MAE are certainly large (L. Néel, 1954) but count only for one layer each. The inner part (volume) of a nanostructure will overcome this, because they count for in n-2 layers. 8

9 Experiment X-ray absorption via total electron yield NEXAFS = µ(e) bond fingerprint of the chemical XMCD = µ + (E) - µ - (E) magnetic information element-specific method 9

10 Electronic Structure and Charge Transfer Strong angular dependence at O K edge oxygen on top NiO differs significantly from Ni on O/Cu no bulk-like NiO Enhanced XAS at Ni L 3 edge charge transfer from Ni to O 10

11 Interface K S ( µ ev/atom) d C (ML) Ni/vacuum Ni/Cu Ni/CO (van Dijken et al.) Ni/H 2 (van Dijken et al.) F 2 2 ( 2πM K ) cos θ ~ 2 K = K V + K S 1 + K d S 2 Ni/O (surfactant) V K film K (µev/atom) 2 V Kbulk d(ml) πM 2 t= Experiment Ni/Cu(001) Ni vacuum Ni + Cu cap Ni + O surfactant 1/d (1/ML) Theory (FLAPW) Jisang Hong et al., Phys. Rev. Lett. 92, (2004) 11

12 results of ab initio calculations Density of states MAE along ΓX axis O/Ni DOS (states/ev atom spin) surfactant grown Ni film clean Ni film E (ev) Γ clean Ni DOS shows that topmost Ni moment is basically unchanged O-induced surface state seen in the vicinity of X-point is responsible for change in MAE theory reveals induced moment in surfactant oxygen (~0.26µ B ) X Physik, FU Berlin KKR Jisang Bandstructure Hong et and al., Spectroscopy Phys. Rev. Calculations, Lett. 92, LMU München /(2004)

13 3. Induced magnetism at oxygen NEXAFS and XMCD at O K edge 13

14 Induced magnetism in oxygen? Ni on O/Cu(100) theory: group of H. Ebert (LMU) oxygen K-edge XMCD orbital moment µ L norm. XMCD (arb. units) hν M 15 ML Ni on O/Cu(100) 300K h=1.06 Å 0.88 Å 0.71 Å 0.53 Å 0.35 Å 0.18 Å 0 Å photon energy (ev) 14

15 Experiment & Theory I (AG Ebert) 15

16 lattice constant Ni-O distance height comment 3.52 (Ni) SEXAFS Aug. 2003, Ni on O/Cu(100) 3.61 (Cu) (Ni) SEXAFS, O/Ni(100) bulk, 3.61 (Cu) Wenzel et al. PRB 36, 7689 (1987) 3.52 (Ni) LEED, O/Ni(100) bulk, 3.61 (Cu) K. Heinz et al., PRB 41, (1990) Theory, T. S. Rahman in joint Surf. Sci. 531, 53 (2003), 2 different structural scenarios Theory, AG Wu in J. Hong et al. PRL 92, (2004) 16

17 Experiment & Theory II (R. Wu) Theory:Co = Ni Exp.: Co 3 Ni 17

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19 Summary The magnetism of nanostructures is a prototype case, which shows the close collaboration between theory and experiment Theory: H. Ebert, LMU; J.J. Rehr, UW; O. Eriksson UU; P. Weinberger, TU Vienna; R. Wu, UCI; D.L. Mills, UCI; K.H. Bennemann, FUB; W. Nolting, HUB magnetism changes dramatically with surfactant: surface anisotropy is strongly reduced in magnitude Fe, Co and Ni induce magnetic moment in surfactant fair agreement with ab initio calculations 19