GMR Enhancement

1
GMR Enhancement

• Michael Teitelbaum
• Eleg 667
• Spintronics

2
Enhancement Techniques

• Back-Layer Effect
• Specular Scattering

3
Basic GMR
FL
S
P
AF
BL
? Basic GMR has a pinned layer a spacer and
a FL layer. In parallel alignment electrons
can penetrate deeper in the FL and in
anti-parallel electron scatter at interface
or shortly after penetration
? Adding Back Layer increases MFP of
majority spin electrons for parallel alignment
4
Back-layer Effect
?Minority spin carriers are turned away upon
entrance into FL
?Majority spin carriers continue into backing
layer
?Resistance for majority spin carriers is
decreased while it stays the same for minority
spin carriers, hence the MR of the structure is
increased
Gurney et al. PRL V71, pp4023
5
Back-layer Effect
? Gurney used this structure to measure MFP
of electrons in the backing layer
?G ?Gf ?Gb1-e-ß(td-tx)/?
?Gf is contribution from filter
?Gb is contribution from backing layer
Gurney et al. PRL V71, pp4023
6
Back-layer Effect
? Sensitivity of spin valve is dependent on
the product of the magnetization and the
thickness of the FL so decreasing FL
thickness is desirable
CL
AF
PL1
? Decreasing the thickness of the FL
decreases the MR ratio and increases the
effect of the stray field
NM
SAF
PL2
NM
? Spin filter spin valve is a structure used
to alleviate this problem
FL
? SAF structure helps reduce the effect of
the stray field
HCL
UL
? HCL increases MFP and reduces the field
from the sense current
7
Advantages of Back-layer Effect
? Field in FL is much smaller for SFSV
than for a CSV
8
Advantages of Back-layer Effect
? With Spin Filter Spin Valve Structure a
decreasing FL thickness allows one to
maintain a relatively constant MR
9
Specular Scattering
? The geometrical boundary of the layer
structure serves to decrease the MFP of
electrons
FL
? When the electron interacts with the
boundary it scatters
NM
PL
? By adding specular reflecting capping
layers we can effectively increase the MFP
and the conduction of the device for majority
spin carriers
10
Theory of Specular Scattering
Barnas et al PRB V42 pp8110
? Theoretical structure used by Barnas to
model effect of scattering in magnetic layers
? Solved Boltzmann Eq. using Fuchs boundary
conditions for the free interfaces
? pA and pD are specular scattering factors
for A and D interface
11
Specular Scattering
? Dependence of ?G and MR with various
specular scattering factors and thickness of
capping layer
? MR and ?G increase with increasing
specular scattering
? MR increases with t at first due a
back-layer effect but then starts to
decrease due to shunting
12
Specular Scattering Oxide Layers
? NiO and Fe2O3 layers have been used to make
single and double spin valve structures
NiO or Fe2O3
Single Spin Valve
Double Spin Valve
13
Oxide Specular Scattering
? Oxide based spin valve shows a much
higher MR ratio than the all metal spin
valve
Swagten et al, PRB V53, pp9108
14
Single and Dual Spin Valves
? Single Spin Valve with oxide capping layer
shows an 18 increase in the MR ratio
? Dual Spin Valve shows 28 increase in
the MR ratio
15
Thin Oxide Layer
? Oxide layers show great MR enhancement
but unfortunately NiO and Fe2O3 cannot be
used for commercial MR heads due to low
blocking temperature
? Natural oxidation layers inside the
pinned and free layers provide a feasible
method to enhance GMR using oxide layers to
increase specular scattering
16
Conclusion

• Back-Layer Effect increases MFP by adding a HCL
  after the FL
• Specular Scattering increases the MFP by adding
  capping layer to reflect electrons specularly
• Both effects increase conduction for the majority
  spin electrons while keeping it the same for
  minority spin electrons hence increasing the GMR
  of the layer structure
• Hirota, Sakakita, and Inomata, Theory of GMR