Detection of Spin-Polarized Electrons:

1
Detection of Spin-Polarized Electrons Spin-polari
zed Photoemission and Spin Polarized Inverse
Photoemission

1. Photoemission
2. Mott Detectors
3. Spin Polarized Inverse Photoemission
4. Other Techniques

2
Pass Energy C(V0-VI)
Conventional Detector/Spin Integrated
Photoemission
Only electrons with E Epass/- dE get thru the
analyzer dE increases with Epass
Outer Hemisphere (VO)
Inner Hemisphere VI
Note Intensity Increases with Pass energy,
resolution decreases!
e- E Epass
Detector Channeltron or Channelplate
Retards Electrons to Epass
Retarding/focussing lens
KE-Vretard Epass (Vretard varied, Epass
constant)
hv
e- E KE
3
Analyzer

• Exciting photon does NOT directly couple with
  electron spin
• Therefore, Spin is conserved during photoemission
  process
• (e.g., J. Osterwalder Spin-Polarized
  Photoemission, Lect. Notes Phys. 697, 95120
  (2006)
• Need a detector that detects electron spin
  direction The MOTT detector

e-
M
4
Detection of Spin-Polarized Electrons See T.
J. Gay and F. B. Dunning, Rev. Sci. Inst. 63
(1992) 1635 See also N. F. Mott., Proc. Roy. Soc.
A 135 (1932) 429
Consider Fig. 1 (Gay, Dunning). An unpolarized
electron beam with equal numbers of spins P and
AP to vector n1
Heavy atom nuclei
5
To a reasonable approximation, high energy
electrons will interact with the bare nucleus
charge Z. The motion of the electrons in the
presence of the field E due to the nuclear charge
sets up a magnetic field (B) on the electrons
v electron velocity
The nuclear field is (Gay, Dunning) given as
(Ze/r3)r. (r elec.-nucleus distance.) Since
rxv L, the orbital angular momentum of the
electron, we have
6
The interaction of B (L) with the electron spin S
introduces an asymmetry in the direction of
scattering to left or right in the plane of the
figure
For spin up (parallel to n1) the scattering
becomes s?(?1) I (?1) (1S(?1)) and s?(?1)
I (?1) (1-S(?1))
The net polarization (P) of the beam striking the
second target is
7
Scattering through the second target (also a high
z nucleus) will yield a similar asymmetry, given
by
For coplanar scattering, NL a N?(1S(?2))
N?(1-S(?2)), etc. and
Thus, if one knows (or independently measures) S
for a given scattering angle, measuring the
number of electrons scattered to the left and
right will give you the polarization of the
incoming beam
8
Conventional Detector/Spin Integrated
Photoemission
To detect spins of incoming polarized electrons,
we need a single heavy target (gold does not
oxidize) , and 2 (or 4 if fancy) channeltrons to
measure beam polariztion.
Outer Hemisphere (VO)
Inner Hemisphere VI
e- E Epass
R
L
Retards Electrons to Epass
Retarding/focussing lens
Detectors Channeltron or Channelplate
hv
e- E KE
9
Schematic of Mott Detector with retarding grids
in front of the Channeltrons. (Incoming electrons
are accelerated after energy selection in
hemispherical analyzer to provide good scattering
assymmetries, see Gay and Dunning)
The efficiency (?) of the detector is given by
Unfortunately, the efficiency of typical Mott
Detectors is 10-3 10-4
Patience is a virtue during spin polarized
photoemission!
10
Mott Detectors based on designs similar to this
are sold commercially
See Gray, Dunning and ref. therein
11
Because of low count rates in Spin polarized
measurements, it is possible to combine a
conventional detector for spin integrated
measurements with a Mott detetector and switch
back and forth between them!
H. Berntsen, et al., REVIEW OF SCIENTIFIC
INSTRUMENTS 81, 035104 2010
12
(E. Vescovo, et al)
13
Oxygen exposure Polarization of surface near
Fermi level disappears.(E. Vescovo, et al)
14
P0
e-
?
Au
Polarized beam
One can create a beam of polarized electrons for
inverse photoemission experiments by colliding an
unpolarized beam of a heavy (W, Au, etc) target.
15
Dowben Group Facility for spin-polarized inverse
photoemission
Dowben group uses photoelectrons from GaAs
16
Photoemission using circularly polarized light at
hv Eg gives heavily polarized light (direct
band gap) because of dipole selection rules (its
a bit complicated, see D. T. Pierce, F. Meier,
PRB 13 (1976) However, polarized photoelectrons
(P gt 50) can be made in this way.
17
Photoemission from GaAs is enhanced with a thin
coating of Cs to yield a negative electron
affinity (CBM below vacuum level)
18
Other techniques Magnetic circular dichroism
(change in polariztion of reflected light) Spin
polarized (SEM)looking at magnetic domains Spin
polarized LEEDhard to measure net polarization,
but can detect surface magnetic lattices Spin
polarized neutron detection? magnetic unit cells
of bulk lattices
19
From www.zurich.ibm.com
20
Spin polarized SEM useful for measuring magnetic
domains.
Browning, et al. J. Elect. Spect. Related Phen.
51(1990) 315
21
  from unit.aist.go.jp
Correlated electron research center



Figure 1-3. Magnetic domain image of the Fe(001), where magnetization direction is shown by the gray level.  The relationship between the magnetization direction and the gray level is given by the graded band above the domain image central gray shows a right direction, lighter gray shows a counterclockwise direction, and darker gray shows a clockwise direction.