Magnetism of Atoms and Nanostructures Adsorbed onto Surfaces

Transcription

1 Magnetism of Atoms and Nanostructures Adsorbed onto Surfaces Magnetism Coordination Small Ferromagnets Superlattices Basic properties of a permanent magnet Magnetization "the strength of the magnet" depends on the magnitude of the atomic magnetic moments. M H Magnetic Order "the essence of macroscopic magnetism" The individual atomic moments must be aligned to produce a net magnetization paramagnetic material M Magnetic Anisotropy "the stubborness of a magnet" is the tendency to maintain the magnet orientation along a fixed spatial direction. H ferromagnetic material

2 Magnetocrystalline anisotropy in 3d bulk metals Magnetocrystalline anisotropy is a property of bulk matter Fe bcc Co hcp Ni fcc easy axis: (100) easy axis: (0001) easy axis: (111) Magnetization (emu/cm 3 ) Magnetization (emu/cm 3 ) Magnetization (emu/cm 3 ) Magnetic field (Oe) Magnetic field (Oe) Magnetic field (Oe) The magnetic anisotropy energy K depends on the crystal structure K 1 = J / m 3 = 2.4 mev / atom K 1 = J / m 3 = 45 mev / atom K 1 = J / m 3 = -0.3 mev / atom S. Kaya, Sci. Reports Tohoku Univ. 17, 639 (1928) Magnetocrystalline anisotropy in bulk alloys

3 Permanent magnets Material Coercivity (Tesla) Remanence (Tesla) (BH) max (kj/m 3 ) Hexagonal ferrimagnetic oxides 3d- alloys BaO(6Fe 2 O 3 ) Alnico V SmCo d-4f intermetallics Nd 2 Fe 14 B M H ferromagnetic material Computer Hard Disks Superparamagnetic Limit retention time t ret = 10 ps e K/kT! 10 a, K! 1.2 ev bulk Co: K = 40 mev/atom 1 grain! 3 x 10 4 atoms 300 grains/bit 1 x 10 7 atoms/bit

4 Magnetic Random Access Memories (MRAMs) W: H-pulse R: TMR E: as W 256 kbits Cypress Problems: Crosstalk during W Small junctions: Improve on TMR? W with I-pulse? Metal clusters: a bridge system between the atomic and the solid state I. M. L. Billas, A. Châtelain, W. A. de Heer, Science 265, 1682 (1994).

5 self-assembled monolayer Colliod Particles K-distribution TEM, ø 8nm Co particles Co particle superlattices E a = 20!meV S. I. Woods et al., PRL 87 (2001). ø 4 nm Fe 52 Pt 48 particles, T ann = 850 K M. L. Plumer, J. v. Ek, and D. Weller (Springer, Berlin, 2001) From blocking to superparamagnetism Numerical solution Analytical solution: 1 c(t) = 1 + (wt ) 2 c eq (T) c eq (T) = M 2 È exp(k / k B T) Í pkk B TErfi( K / k B T ) - 1 Î 2K 1500 atoms K = 200 mev w = 0.3 Hz M macrospin K anisotropy w sweep frequency of field t relaxation time, t = t 0 exp(k/k B T) W. Kinzel et al., Sol. State Comm. 23, 687 (1977). R. W. Chantrell et al., JMMM 53, 199 (1985). O. Fruchart et al., JMMM 239, 224 (2002).

6 Magneto-Optical Kerr Effect - MOKE

7 Ramified islands Co/Pt(111) Size distribution Perimeter distribution 0.4 T = 130 K 200 Å MOKE: M z (H z ) for compact monolayer islands at T = 150 K size distribution mean size m = 2.1±0.2 m B m = 3.0±0.2 m B m = m Co, S + m Co, L + m Pt = 2.3 m B m Co, S = 1,8m B, Wu et al. JMMM 99, 71 (1991). m Co, L = 0.3m B, our XMCD-results m Pt = 0.2m B, Ferrer et al. PRB 56, 9848 (1997).

8 Compact monolayer islands perimeter atoms K p = 0.87 ± 0.10 mev/atom surface atoms K s = ± mev/atom (K shape = mev, K anis = mev) bulk atoms K b = mev/atom Bimetallic islands: Pt core & Co shell edge atoms K p = 0.87 ± 0.10 mev/atom 0.2 ML T = 180 K, T ann = 760 K; 0.2 ML T = 220 K, T ann = 300 K S. Rusponi et al. Nature Materials 2 (2003).

9 Oxygen adsorption onto Co-islands on Pt(111) clean 0.05 L O L O Å 100 Å 100 Å T = 300 K Enhanced c-max by O 2 adsorption T. Cren et al. J. Phys. Chem. B 108 (2004).

10 X-ray Magnetic Dichroism (XMD) Combines chemical selectivity of X-ray Absorption Spectroscopy (XAS) with magnetic (dichroic) sensitivity Determines orbital m L and spin m S magnetic moments and their anisotropies K in element specific way Synchrotron storage ring Absorption L E 3 : 2p 3/2 > 3d L 3 : 2p 3/2 > 3d variable energy, polarized X-rays L 2 : 2p 1/2 > 3d L Spin down Spin up E F R photoemitted electrons E (hn) monochromator 2p 3/2 2p 1/2 B E (hn) I s Sum Rules of XMCD 15 ML Co/Pt(997) m L = h d A 3 + A 2 L 3 + L 2 m S + 7m T = -h d 2A 3-4A 2 L 3 + L 2 m L m S + 7m T = 2 3 A 3 + A 2 A 3-2A 2 with m T magnetic spin dipole moment

11 Orbital moments m L Co n /Pt(111) normalized XMCD spectra at B = 7 T and T = 10 K m L m S + 7m T = 2 3 A 3 + A 2 A 3-2A 2 m S = 2.1 m B 7m T = m B Giant K- and m L - values for low-coordinated Co atoms 30 Å m L = 1.1 ± 0.1 m B and K = 9.3 ± 1.6 mev P. Gambardella et al. Science 300 (2003).

12 Au(111) (!3"x"22)-reconstruction 200 Å L. Bürgi et al. PRL (2002). J. V. Barth et al. PRB 42, 9307 (1990). Nucleation of phase-coherent lattices Co/Au(788) 26 T islands/in Å 300 Å V. Repain et al. PRL 84 (2002). 0.2 ML 130 K, ann 400 K V. Repain et al. EPL 58 (2002); Mat. Sci. Eng. B 96 (2002).

13 X-ray Magnetic Circular Dichroism (XMCD) Q = 0.35 ML, T = 10 K, B = 5 T, (B sat = 2.5 T) Co coverage 1.1 ML Dipolar Interactions? 100 Å K p = 0.90 ± 0.10 mev/atom N. Weiss et al. PRL 95 (2005).

14 0.75 ML narrow K-distribution with HWHM 17% 32% 100 Å 17% K p = 0.90 ± 0.10 mev/atom N. Weiss et al. PRL 95, (2005). Collaborators Stefano Rusponi Safia Ouazi Géraud Moulas Kirsten Halleux Pietro Gambardella* Tristan Cren* Nicolas Weiss*