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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 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
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?
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
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)
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/r12φa (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
Therefore, we expect surface magnetism to depend heaviliy on: • Surface oxidation and other environmental factors • Temperature (below or above magnetic ordering temperature)
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
Temp. dependence of Fe(100) magnetic polarization near Ferm. Level (Johnson, Ann. Rev. Mat. Sci.)
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??
SP PES of clean Fe(100) shows high polarization near EF E. Vescovo, et al. Phys. Rev. B. 47 (1993) 13051 (Rapid Comm.)
Fe(100) + O2 @ RT Anneal to 650 C
A real MTJ: (S. Tehrani, et al. IEEE Trans. on Magnetics 36 (2000) 272) Note, a key step is Al deposition and oxidation…
Thicker oxide, attenuates CAP Too thick, oxidizes NiFe electrode Note, excessive oxidation decrease MR due to oxidation of the substrate electrode!
In such a system, metallic behavior for T< Tc semiconducting behavior for T> Tc
Spin-integrated PES: Magnetic ordering yields increase in DOS near Fermi level (consistent with model)
Spin-polarized PES: Increased metallic nature associated with polarization near EF