SPINTRONICS

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SPINTRONICS
Tomáš Jungwirth
University of Nottingham
Fyzikální ústav AVCR
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1. Current spintronics in HDD read-heads and
memory chips 2. Physical principles of
operation of current spintronic devices 3.
Research at the frontiers of spintronics 4.
Summary
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Current spintronics applications
First hard disc (1956) - classical electronics
for read-out
1 bit 1mm x 1mm
MByte
From PC hard drives ('90) to micro-discs -
spintronic read-heads
1 bit 10-3mm x 10-3mm
GByte
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HARD DISKS
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HARD DISK DRIVE READ HEADS
spintronic read heads
horse-shoe read/write heads
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Anisotropic magnetoresistance (AMR) read head
1992 - dawn of spintronics
Appreciable sensitivity, simple design,
scalable, cheap
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Giant magnetoresistance (GMR) read head
1997
High sensitivity
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MEMORY CHIPS
.DRAM (capacitor) - high density, cheep x slow,
high power,
volatile .SRAM (transistors) - low power, fast
x low density,
expensive, volatile .Flash (floating gate) -
non-volatile x slow, limited life,
expensive
Operation through electron charge manipulation
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MRAM universal memory fast, small, non-volatile
First commercial 4Mb MRAM
Tunneling magneto-resistance effect (TMR)
RAM chip that won't forget ? instant on-and-off
computers
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(No Transcript)
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MRAM universal memory fast, small, non-volatile
First commercial 4Mb MRAM
Tunneling magneto-resistance effect (TMR)
RAM chip that won't forget ? instant on-and-off
computers
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1. Current spintronics in HDD read-heads and
memory chips 2. Physical principles of current
spintronic devices operation 3. Research at the
frontiers of spintronics 4. Summary
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Electron has a charge (electronics) and spin
(spintronics) Electrons do not actually
spin, they produce a magnetic moment that is
equivalent to an electron spinning clockwise or
anti-clockwise
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quantum mechanics special relativity ?
particles/antiparticles spin


Dirac eq.



Ep2/2m E? ih d/dt p? -ih d/dr . . .
E2/c2p2m2c2 (Emc2 for p0)
high-energy physics
solid-state physics and microelectronics
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Resistor
classical
spintronic
external manipulation of charge
spin
internal communication between
charge spin
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Non-relativistic (except for the spin) many-body
Pauli exclusion principle Coulomb repulsion ?
Ferromagnetism

• Robust (can be as strong as bonding in solids)
• Strong coupling to magnetic field
• (weak fields anisotropy fields needed
• only to reorient macroscopic moment)

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Relativistic "single-particle"
Spin-orbit coupling (Dirac eq. in external field
?V(r) 2nd-order in v /c around
non-relativistic limit)
Produces an electric field
Ingredients - potential V(r)
- motion of an electron
E
In the rest frame of an electron the electric
field generates and effective magnetic field
- gives an effective interaction with the
electrons magnetic moment

• Current sensitive to magnetization
• direction

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Spintronics
Ferromagnetism Coulomb repulsion Pauli
exclusion principle
Spin-orbit coupling Dirac eq. in external field
?V(r) 2nd-order in v /c around
non-relativistic limit
Fermi surfaces
(k . s)2 Mx . sx
(k . s)2
Mx . sx
FM without SO-coupling
SO-coupling without FM
FM SO-coupling
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Fermi surfaces
(k . s)2 Mx . sx
(k . s)2
Mx . sx
FM SO-coupling
FM without SO-coupling
SO-coupling without FM
AMR
M
Ferromagnetism sensitivity to magnetic
field SO-coupling anisotropies in Ohmic
transport characteristics 1-10 MR sensor
scattering
ky
kx
M
ky
kx
hot spots for scattering of states moving ? M ?
R(M ? I)gt R(M I)
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Diode
classical
spin-valve
TMR
Based on ferromagnetism only 100 MR sensor or
memory
no (few) spin-up DOS available at EF
large spin-up DOS available at EF
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1. Current spintronics in HDD read-heads and
memory chips 2. Physical principles of current
spintronic devices operation 3. Research at the
frontiers of spintronics 4. Summary
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Removing external magnetic fields (down-scaling
problem)
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EXTERNAL MAGNETIC FIELD
problems with integration - extra wires,
addressing neighboring bits
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Current (instead of magnetic field) induced
switching
Angular momentum conservation ? spin-torque
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magnetic field
current
Myers et al., Science '99 PRL '02
local, reliable, but fairly large currents needed
Likely the future of MRAMs
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Spintronics in the footsteps of classical
electronics from resistors and diodes to
transistors
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AMR based diode
- TAMR sensor/memory elemets
TAMR
TMR
no need for exchange biasing or spin coherent
tunneling
Au
FM
AFM
Simpler design without exchange-biasing the fixed
magnet contact
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Spintronic transistor based on AMR type of effect
Huge, gatable, and hysteretic MR
Single-electron transistor
Two "gates" electric and magnetic
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Spintronic transistor based on CBAMR
magnetic
electric
SO-coupling ? ?(M)
control of Coulomb blockade oscillations
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CBAMR SET

• Generic effect in FMs with SO-coupling
• Combines electrical transistor action
• with magnetic storage
• Switching between p-type and n-type transistor
  
• by M ? programmable logic

In principle feasible but difficult to realize at
room temperature
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Spintronics in the footsteps of classical
electronics from metals to semiconductors
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Spin FET spin injection from ferromagnet SO
coupling in semiconductor
Difficulties with injecting spin polarized
currents from metal ferromagnets to
semiconductors, with spin-coherence, etc. ? not
yet realized
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Ferromagnetic semiconductors all semiconductor
spintronics
More tricky than just hammering an iron nail in a
silicon wafer
GaAs - standard semiconductor Mn - dilute
magnetic element (Ga,Mn)As - ferromagnetic
semiconductor

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(Ga,Mn)As (and other III-Mn-V) ferromagnetic
semiconductor

• compatible with conventional III-V
  semiconductors (GaAs)
• dilute moment system ? e.g., low currents needed
  for writing
• Mn-Mn coupling mediated by spin-polarized
  delocalized holes ? spintronics
• tunability of magnetic properties as in the more
  conventional semiconductor electronic properties.
• strong spin-orbit coupling ? magnetic and
  magnetotransport anisotropies
• Mn-doping (group II for III substitution)
  limited to 10
• p-type doping only
• maximum Curie temperature below 200 K

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(Ga,Mn)As material
5 d-electrons with L0 ? S5/2 local
moment moderately shallow acceptor (110 meV) ?
hole
- Mn local moments too dilute (near-neghbors
cople AF) - Holes do not polarize in pure
GaAs - Hole mediated Mn-Mn FM coupling
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Mnhole spin-spin interaction
As-p
Mn-d
hybridization
Hybridization ? like-spin level repulsion ? Jpd
SMn ? shole interaction
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Ferromagnetic Mn-Mn coupling mediated by holes
heff Jpd ltSMngt x
Hole Fermi surfaces
Heff Jpd ltsholegt -x
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No apparent physical barriers for achieving room
Tc in III-Mn-V or related functional dilute
moment ferromagnetic semiconductors Need to
combine detailed understanding of physics and
technology
Impurity-band holes short-range coupl.
Delocalized holes long-range coupl.
Weak hybrid.
Strong hybrid.
InSb, InAs, GaAs
d5
GaP
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And look into related semiconductor host families
like e.g. I-II-Vs
III I II ? Ga Li Zn

• GaAs and LiZnAs are twin SC
• (Ga,Mn)As and Li(Zn,Mn)As
• should be twin ferromagnetic SC
• But Mn isovalent in Li(Zn,Mn)As
• no Mn concentration limit
• possibly both p-type and n-type ferromagnetic SC

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Spintronics in non-magnetic semiconductors way
around the problem of Tc in ferromagnetic
semiconductors back to exploring spintronics
fundamentals
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Spintronics relies on extraordinary
magnetoresistance
Ordinary magnetoresistance response in normal
metals to external magnetic field via classical
Lorentz force
Extraordinary magnetoresistance response to
internal spin polarization in ferromagnets often
via quantum-relativistic spin-orbit coupling
B
anisotropic magnetoresistance
_ _ _ _ _ _ _ _ _ _
_
FL

I
V
_
_
FSO
_
M
I
e.g. ordinary (quantum) Hall effect
and anomalous Hall effect
Known for more than 100 years but still
controversial
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Anomalous Hall effect in ferromagnetic
conductors spin-dependent deflection more
spin-ups ? transverse voltage
skew scattering
side jump
intrinsic
Spin Hall effect in non-magnetic
conductors spin-dependent deflection ?
transverse edge spin polarization
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Spin Hall effect detected optically in
GaAs-based structures
Same magnetization achieved by external field
generated by a superconducting magnet with 106 x
larger dimensions 106 x larger currents
SHE mikrocip, 100?A
supravodivý magnet, 100 A
SHE edge spin accumulation can be extracted and
moved further into the circuit
SHE detected elecrically in metals
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1. Current spintronics in HDD read-heads and
memory chips 2. Physical principles of current
spintronic devices operation 3. Research at the
frontiers of spintronics 4. Summary
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Downscaling approach about to expire
currently 30 nm feature size interatomic
distance in 20 years

• Spintronics from straighforward downscaling to
• more "intelligent" device
  concepts
• simpler more efficient realization for a given
  functionality (AMR sensor)
• multifunctional (integrated reading, writing,
  and processing)
• new materials (ferromagnetic semiconductors)
• fundamental understanding of quantum-relativistic
  electron transport (extraordinary MR)

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Anisotropic magneto-resistance sensor
Electromagnet

• Information reading

?

• Information reading storage

Tunneling magneto-resistance sensor and memory
bit

• Information reading storage writing

Current induced magnetization rotation
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• Information reading storage writing
  processing

Spintronic single-electron transistor magnetoresi
stance controlled by gate voltage

• New materials

Dilute moment ferromagnetic semiconductors

• Spintronics fundamentals

AMR, anomalous and spin Hall effects