The quantum well states in MgObased magnetic tunneling junctions

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The quantum well states in MgO-based magnetic
tunneling junctions

Zhong-Yi LU
Institute of Theoretical Physics
Chinese Academy of
Sciences
Beijing 100080, China
and Department
of Physics, Renmin University of China
Beijing 100872,
China
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In collaboration with X.-G. Zhang S.T.
Pantelides Oak Ridge National Laboratory and
Vanderbilt University Yan Wang Xiufeng
Han Institute of Physics Chinese Academy of
Sciences Beijing, China
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Outline

• Magnetic tunnel junctions based on MgO
• Quantum wells in single MgO barrier MTJs
• Quantum wells in double MgO-barrier MTJs
• Summary

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Magnetic Tunnel Junctions
T. Miyazaki and N.J. Tezuka J. Magn. Magn.
Mater. 139, L231 (1995) J.S. Moodera, et al.
PRL 74, 3273 (1995). smooth and pinhole-free
Al2O3 deposition technique (TMR of 3050)
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Motivation

• TMR GMR read-head
• Capacity of MRAM DEMO
• Spin Transistor
• Magneto-Logic Devices

MRAM DEMO
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Antiparallel alignment
Parallel alignment
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TMR is intrinsic property of the two leads in the
model
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• Jullieres model indicates increasing P will
  increase TMR
• The fact is spin polarization P for ferromagnetic
  metals and alloys experimentally no more than 0.6
  at low temperatures, thus
• TMR 100 ( low temperatures)
• 70 (room
  temperature)
• Half-metallic electrodes with P1
• A new approach MgO as barrier layer, then
  effectively P close to 1 with new mechanism.

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Coherent Tunneling through Magnetic Junctions
Fe(001)
MgO(001)
Fe(001)
TMR could be over 1000, predicted by W.H.
Butler, X.-G. Zhang, T. Schulthess, J. Maclaren,
PRB 63, 054416 (2001) J. Mathon
and A. Umerski, PRB 63, 220403 (2001).
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Coherent tunnling with MgO barriers in
Experiments I
S. Yuasa et. al., Nature Materials 3, 868
(2004). TMR180 of the single-crystal
Fe/MgO/Fe MTJ at room temperature.
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Coherent tunnling with MgO barriers in
Experiments II
S.S Parkin et. al., Nature Materials 3, 862
(2004). TMR220 of the polycrystalline
CoFe/MgO/CoFe MTJ at room temperature.
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Symmetry of the tunnel barrier
The crystalline MgO is a filter, only allowing
?1states pass, which makes the electrodes behave
as half-metals.
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Quantum Well States Formed within the Junction
bcc(001) Fe/MgO/Fe/Cr
Fe
MgO
Fe
Cr
Lu, Zhang, and Pantelides, PRL 94,207210 (2005)
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Minority
Majority
bcc Fe
bcc Cr
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An effective free electron model for majority sp
electrons at k//0
MgO
EF
Cr
Fe
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KKR method for electronic structure in solids

• Korringa-Kohn-Rostoker (KKR) method
• Spherical potential around each atom, electron
  wave function in this region expanded in
  spherical waves
• Constant (zero) potential between the spheres,
  electron wave function in this region expanded in
  plane waves
• Multiple scattering theory used to connect the
  wave function in all spheres
• Direct calculation of the Greens function
• Implemented under density functional theory (LDA
  and LSDA)

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Layer KKR for surfaces and interfaces
Surface
Interface

• No need for periodic boundary conditions
• No need for slabs
• Arbitrary number of inhomogeneous layers
• 2d periodicity and epitaxial structure required

MacLaren, Crampin, Vvedensky, and Pendry PRB
40,12164 (1989)
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Layer KKR for transport at a finite bias
Zhang, et al. PRB 69,134406 (2004)
Layer KKR further parallelized in 2005 at ITP, CAS
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Quantum Well States Formed within the Junction
bcc(001) Fe/MgO/Fe/Cr
Fe
MgO
Fe
Cr
Lu, Zhang, and Pantelides, PRL 94,207210 (2005)
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Majority s-resolved partial DOS at k//0 for the
Junction Fe/MgO/FeO/8Fe/Cr
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Minority s-resolved partial DOS at k//0 for the
Junction Fe/MgO/FeO/8Fe/Cr
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I-V for Fe/MgO/FeO/8Fe/Cr
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TMR versus bias for Fe/MgO/FeO/8Fe/Cr
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Spin-dependent resonant tunneling through
metallic QW states in MgO MTJs
Majority s-resolved partial DOS at k//0 for the
Junction Fe/MgO/FeO/8Fe/Cr
Lu, Zhang, and Pantelides, PRL 94,207210 (2005)
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Co/MgO/9Fe/Cr
Fe/MgO/FeO/8Fe/Cr
The oxidized interface severely damages TMR
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Quantum Well States Formed within the double MTJ
Fe/MgO/Fe/MgO/Fe
Fe
MgO
Fe
MgO
Fe
Wang, Lu, Zhang, and Han, PRL 97, 087210 (2006)
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First observation of the quantum size effect in
MgO-DBMTJ
T. Nozaki, et al. PRL 96, 027208 (2006)
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• s-partial DOS at k//0 for the middle Fe film
  in Fe/MgO/9Fe/MgO/Fe DBMTJs

majority spin QW states
Wang, Lu, Zhang, and Han, PRL 97, 087210 (2006)
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QW state resonances (from layer-KKR calculations)
compared with experimental resonant voltages
VQW ? VEXP ! Why? Coulomb blockade effect in
the middle Fe layer (island)! a capacitance
model VCB Ec/e e/C 2e/CFe-MgO 2e/eMgOe0
(dMgO/A). VCB ? dMgO/D2 e0
electrical permittivity constant eMgO
dielectric constant of MgO dMgO thickness of
the MgO barrier A area of the Fe island D
diameter of Fe island
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Coulomb blockade effect the primary source of
smearing at low temperatures
linear fit y x 0.013.
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An effective free electron model for sp electrons
in majority-spin channel
parameters 1. Effective mass m 2. Barrier
height V
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Middle Fe film thickness dependence of QW state
energies in DBMTJs
phase accumulation model (PAM)
phase shift on reflection at two Fe/MgO interface
additional phase shift
m 4.0 V_barrier 4.7 eV
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Summary for Single-MgO barrier

• Large resonant tunneling through metallic QW
  states predicted in Fe/MgO/FeO/Fe/Cr and
  Co/MgO/Fe/Cr
• TMR decreases quickly with biases
• Tunneling current from QW states above Fermi
  energy much greater than from QW states below
  Fermi energy
• Majority spin QW states contribute to a large
  positive TMR, minority spin QW states contribute
  to a large negative TMR
• The oxidation at the interface severely reduces
  TMR
• Due to long MFP of minority spin electrons,
  resonant tunneling through minority spin QW
  states may be easier to observe, but requires
  larger bias windows and thicker films

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Perspectives and conclusions

• Spin-dependent resonant tunneling through MgO
  DBMTJ
• Reduce the effect of Coulomb blockade
• continuous and smooth middle Fe layer
• The significance of utilizing the
  first-principles electronic structure and
  electronic transport calculations on studying TMR
  nanostructures

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Thanks!
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Brief History of MTJ

• 1974, M. Julliere (a graduate student) published
  an experiment article which claimed 14 TMR in
  Fe/Ge/Fe trilayers. A simple model was proposed
  (the paper became a sleeping giant).
• 1982, IBM reported 2 TMR on Ni/AlO/Ni.
• 1995, Moodera (MIT) and Miyazaki (Japan) reported
  10 TMR for Co/AlO/Co.
• 1998, DARPA launched MRAM solicitation
• 1999, Motorolas 128kB MRAM demo
• 2003, IBM, Motolora, 4Mb MRAM chip demo
• More than 10 startup MRAM companies formed.
• MRAM becomes internationally recognized future
  technology

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Why DBMTJ?

• The reduction of the TMR with increasing the
  bias voltage, a decrease of the
  sensitivity
• The annealing temperature dependence of the MR
  ratio because standard backend technology for
  metallization of CMOS circuits requires
  annealing at 400450 C
• Recent theoretical works show that a DBMTJ
  yields higher TMR than SMTJ
• TMR of DBMTJ decreases more slowly than that of
  SMTJ as a function of a bias voltage.

J. S. Moodera, et al,PRL74, 3273(1995). Sheng,
et al, PRB 59, 480(1999) X. Zhang,et al, PRB 56,
5484(1997) K. Inomata, JAP 87, 6064(2000).