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

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?

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

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)

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

Therefore, we expect surface magnetism to depend heaviliy on:

1.Surface oxidation and other environmental factors

2.Temperature (below or above magnetic ordering temperature)

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

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

+ O2

FM, P > 0

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.)

~ 3 L of O2 exposure largely destroys polarization near EF

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…

Note, excessive oxidation decrease MR due to oxidation of the substrate electrode!

Thicker oxide, attenuates CAP

Too thick, oxidizes NiFe 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