spin polarized current phenomena in magnetic tunnel junctions
Applications of Spin-Polarized Photoemission
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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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From Velev, et al
Most spintronic devices involve materials interfaces, and depend on polarization both adjacent to the interface (direct space) and Fermi level (inverse space).
Example: A slight oxidation of a FM (ferromagnetic ) surface can yield huge changes in spin transport properties. Why is this?
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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-
M+x
A-
A-
A-
A-
A-
dz2, dx2-y2
Fe+3 = 3d5
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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:
M 3dM 3d
O(2p)
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)
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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)>
When the exchange energy U is < kT, the spins become disordered
or
ferromagnetic
antiferromagnetic
χ=C/T χ=C/(T-θ)
T
χ
θT
complex behavior
χ=C/(T-θ)
T
AFF
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Therefore, we expect surface magnetism to depend heaviliy on:
1.Surface oxidation and other environmental factors
2.Temperature (below or above magnetic ordering temperature)
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Spin-polarized photoemission should therefore be a powerful probe of environmental effects on surface magnetic behavior
Review: spin-polarized detector
P. D. Johnson, Ann. Rev. Mat. Sci. 25 (1995) 455
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Temp. dependence of Fe(100) magnetic polarization near Ferm. Level(Johnson, Ann. Rev. Mat. Sci.)
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Oxidation can then induce big changes in a FM surface!
Metal/FM, P > 0
+ O2
FM, P > 0
Meta ox., AF P = 0
Is this reflected in SP-photoemission??
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SP PES of clean Fe(100) shows high polarization near EF
E. Vescovo, et al. Phys. Rev. B. 47 (1993) 13051 (Rapid Comm.)
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~ 3 L of O2 exposure largely destroys polarization near EF
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Fe(100) + O2 @ RT
Anneal to 650 C
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A real MTJ:
(S. Tehrani, et al. IEEE Trans. on Magnetics 36 (2000) 272)
Note, a key step is Al deposition and oxidation…
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Note, excessive oxidation decrease MR due to oxidation of the substrate electrode!
Thicker oxide, attenuates CAP
Too thick, oxidizes NiFe electrode
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In such a system, metallic behavior for T< Tc semiconducting behavior for T> Tc
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Spin-integrated PES: Magnetic ordering yields increase in DOS near Fermi level (consistent with model)
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Spin-polarized PES:Increased metallic nature associated with polarization near EF