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Applications of Spin-Polarized Photoemission

Applications of Spin-Polarized Photoemission. P. D. Johnson, Annual Rev. Mater. Sci. 25 (1995) 455-85 Combined spin –integrated/resolved detector: Giringhelli, et al., Rev. Sci. Inst. 70 (1999) 4225.

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Applications of Spin-Polarized Photoemission

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  1. Applications of Spin-Polarized Photoemission P. D. Johnson, Annual Rev. Mater. Sci. 25 (1995) 455-85 Combined spin –integrated/resolved detector: Giringhelli, et al., Rev. Sci. Inst. 70 (1999) 4225

  2. Most spintronic devices involve materials interfaces, and depend on polarization both adjacent to the interface (direct space) and Fermi level (inverse space). From Velev, et al Example: A slight oxidation of a FM (ferromagnetic ) surface can yield huge changes in spin transport properties. Why is this?

  3. Consider the oxidation of Fe: Fe Pauli Magnetism (delecalized electrons): Ferromagnetic (FM) FeOx We start to induce localized spins on Fe cations, and these interactions are antiferromagnetic (AF) A- A- A- A- A- A- dz2, dx2-y2 Δ dxy dyz dxz M+x Fe+3 = 3d5

  4. In transition metal and lanthanum oxides, the magnetic ions are typically separated by oxygen anions. That’s a very long distance. Metal ions can interact with each other via an intervening anion thru superexchange: O(2p) M 3d M 3d Antiferromagnetic Ordering via Superexchange: Shared covalency of metal centers with the oxygen leads to M-O spin pairing (see Cox, Electronic Structure and Chemistry of Solids (Oxford Press)

  5. ferromagnetic Two isolated atoms/ions (Curie model) with unpaired spins Si,j have a spin-spin interaction energy defined as : U = -2KSi•Sj J = Exchange Integral J = <φa (1) φb(2)1/r12φa (2) φb(1)> or antiferromagnetic When the exchange energy U is < kT, the spins become disordered complex behavior χ=C/T χ=C/(T-θ) χ=C/(T-θ) χ F AF T T T θ

  6. Therefore, we expect surface magnetism to depend heaviliy on: • Surface oxidation and other environmental factors • Temperature (below or above magnetic ordering temperature)

  7. Spin-polarized photoemission should therefore be a powerful probe of environmental effects on surface magnetic behavior P. D. Johnson, Ann. Rev. Mat. Sci. 25 (1995) 455 Review: spin-polarized detector

  8. Temp. dependence of Fe(100) magnetic polarization near Ferm. Level (Johnson, Ann. Rev. Mat. Sci.)

  9. Oxidation can then induce big changes in a FM surface! Metal/FM, P > 0 FM, P > 0 + O2 Meta ox., AF P = 0 Is this reflected in SP-photoemission??

  10. SP PES of clean Fe(100) shows high polarization near EF E. Vescovo, et al. Phys. Rev. B. 47 (1993) 13051 (Rapid Comm.)

  11. ~ 3 L of O2 exposure largely destroys polarization near EF

  12. Fe(100) + O2 @ RT Anneal to 650 C

  13. A real MTJ: (S. Tehrani, et al. IEEE Trans. on Magnetics 36 (2000) 272) Note, a key step is Al deposition and oxidation…

  14. Thicker oxide, attenuates CAP Too thick, oxidizes NiFe electrode Note, excessive oxidation decrease MR due to oxidation of the substrate electrode!

  15. In such a system, metallic behavior for T< Tc semiconducting behavior for T> Tc

  16. Spin-integrated PES: Magnetic ordering yields increase in DOS near Fermi level (consistent with model)

  17. Spin-polarized PES: Increased metallic nature associated with polarization near EF

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