MTJ RAM design

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MTJ RAM design
Takahiro Moriyama
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Magnetic tunnel junction (MTJ)
Ferromag. (soft)
Insulator (barrier)
Ferromag. (hard)
Ferromag. Co, Py, FeCo, etc. Barrier Al2O3,
MgO, etc.
TMR()(RAP-RP)/RP 100
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Whats the applications?
MRAM (Magnetic Random Access Memory) Read head
for HDD ? higher sensitivity Spin- transistor etc.
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Advantage of MRAM
DRAM Dynamic Random Access Memory Flash EEPROM
Electrically Erasable Programmable Read-Only
Memory FeRAM Ferroelectric RAM
MRAM will takeover DRAM !!
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Challenge Higher TMR ratio Improvement of bias
dependence overcome super-paramagnetic limit
High TMR
Bias dependence
Small MTJs
For highly integrated device
graph
Julliere model
kBTltKuV
TMR2P1P2/(1-P1P2)
Ku needs to be large
High polarization
TMR decrease with increasing bias voltage
Other factors
Flat interface Barrier material
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High spin polarization electrode material
3d ferromagnets
EF
Half metal P100
Spin polarization
LaSrMnO, Fe3O4, CrO2, etc.
P(k? - k?)/(k? k?)
Tc is not high enough
Magnetic semiconductor
k is Fermi wave number
MnGaAs, etc.
High P is observed.
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Film deposition scheme
Underlayers to make a flat interface induce
crystalline oriented structure
Antiferro layers to pin the hard layer
FeMn, IrMn, NiO..
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Barrier material
High barrier height and thinner width are better
for high TMR
Various kind of material have been tried.
Al2O3, MgO, ZrO2, Ta2O3, SrTiO, HfO2, etc.
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Epitaxial MTJs
Epitaxy the growth of crystals of one material
on the crystal face of another material
Fe/ MgO/ Fe, MnGaAs/AlAs/MnGaAs MTJs ,etc.
Atomically flat interface Coherent tunneling
electrode
electrode
barrier
In theory, Fe/MgO/Fe 1000 TMR!
J. Mathon et al. PRB 59 220403 (2001)
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MTJs TMR
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In 2005 MRAMs will be put into commercial
production
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