Applications: Spintronic Devices
Nanomagnetism for Biology and Spintronics group (NaBiS) Dipartimento Fisica, Politecnico di Milano
Via G. Colombo 81, 20133 Milano [email protected]
Lectures by Matteo Cantoni
Thursday 21/4/2016, h. 12:15-13:15
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Summary
2
I will discuss two significant examples of spintronic devices and technology: non volatile magnetic memories
(MRAMs) and semiconductor spintronics.
1. Magnetic memories
a. state of the art and perspectives
b. the writing issue: new strategies (magneto-electric coupling)
c. the density issue: new strategies (antiferromagnet spintronics)
2. Semiconductor spintronics
a. the Datta and Das spin-FET
b. the four problems: injection, transport, manipulation, detection
c. the conductivity mismatch issue
d. optical spin injection and spin photodiodes
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Summary
3
I will discuss two significant examples of spintronic devices and technology: non volatile magnetic memories
(MRAMs) and semiconductor spintronics.
1. Magnetic memories
a. state of the art and perspectives
b. the writing issue: new strategies (magneto-electric coupling)
c. the density issue: new strategies (antiferromagnet spintronics)
2. Semiconductor spintronics
a. the Datta and Das spin-FET
b. the four problems: injection, transport, manipulation, detection
c. the conductivity mismatch issue
d. optical spin injection and spin photodiodes
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Magnetic memories (MRAMs) – state of the art
4
MRAM = magnetoresistive random access memory
What is it?
A MRAM chip is a bidimensional array of magnetoresistive devices with stable remanent states (0 and 1),
integrated on a silicon complementary metal–oxide semiconductor (CMOS) circuit allowing to separately
address each memory element.
What does it employs?
• hysteretic properties of ferromagnetic materials (FMs) for data storage
• magnetoresistive phenomena (AMR, GMR, TMR) for data reading [see Spintronics I & II, Riccardo Bertacco,
20/4/2016 h. 9:00]
A brief history:
• 1980: birth of the MRAM concept
• 1975: TMR discovery by M. Jullière
• 1988: GMR discovery by A. Fert and P. Grünberg (Nobel prize for physics in 2007)
• 2008: record TMR value (604%@300K) by S. Ikeda and H. Ohno
• 2012: first commercial STT-based MRAM by Everspin
• …
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Magnetic memories (MRAMs) – working principle
5
P
PAP
R
RRMR
AP = antiparallel state ( )
P = parallel state( , )
high density (128 Gbit/in2 )
[Courtesy of J.P. Nozieres (Spintec)]
Nonvolatile fast read and write
(1 Tbit/s) low power consumption unlimited write endurance radiation hardness
high density (128 Gbit/in2 )
x large writing currents (107 A/cm2 )
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Magnetic memories (MRAMs) – comparison
6
Memory Advantages Drawbacks Main fields of application
Hard
drive
High density (1 Tbit/in2); very low
cost per byte stored (0.004 $/Gbit)
Moderate read and write
speeds (1 Gbit/s); bulky
moving parts
Secondary data storage device in computers
SRAM Fast read and write speeds
(35 Gbit/s); low power consumption
Large memory cells taking up
considerable space; low density
(4.5 Gbit/in2); volatile
Cache memory in computers
DRAM/S
DRAM
High density (>500 Gbit/in2; low cost
(1 $/Gbit); superfast read and write
speeds (200 Gbit/s)
Volatile; constant refreshing of
data draining power
Primary memory in computers
Flash Nonvolatile; very high density
(>1 Tbit/in2); low cost (<1 $/Gbit)
Power consuming; moderate
read and write speeds
(6 Gbit/s); limited endurance
Long-term external storage, firmware, SSD drives
MRAM Nonvolatile; fast read and write
speeds (1 Tbit/s); high density (128
Gbit/in2); low power consumption;
unlimited write endurance; radiation
hardness
High writing currents Military and space applications; memory in computers
R. Bertacco and M. Cantoni, New trends in magnetic memories, chapter book in «Ultra-high density magnetic recording», G. Varvaro and F. Casoli eds., CRC Press (Taylor & Francis), 2016
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Summary
7
I will discuss two significant examples of spintronic devices and technology: non volatile magnetic memories
(MRAMs) and semiconductor spintronics.
1. Magnetic memories
a. state of the art and perspectives
b. the writing issue: new strategies (magneto-electric coupling)
c. the density issue: new strategies (antiferromagnet spintronics)
2. Semiconductor spintronics
a. the Datta and Das spin-FET
b. the four problems: injection, transport, manipulation, detection
c. the conductivity mismatch issue
d. optical spin injection and spin photodiodes
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Magnetic memories (MRAMs) – the writing issue
8
Writing = setting the free layer magnetization direction: how to do?
Bit «0» Bit «1»
M1 M2
anti-parallel M1 M2
parallel
M1
M2
• Electric writing
J 107 A/cm7 • Magnetic field generated by current • Spin Transfer Torque (STT) • Spin Orbit Torque • Optical writing
Applied voltage instead of current
R. Bertacco and M. Cantoni, New trends in magnetic memories, chapter book in «Ultra-high density magnetic recording», G. Varvaro and F. Casoli eds., CRC Press (Taylor & Francis), 2016
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Electric writing of Magnetic Information: MCA
9
1) Voltage-controlled magnetic anisotropy
FM layer with Perpendicular Magnetic Anisotropy (PMA) the electric field (E) induces a variation of the MagnetoCrystalline Anisotropy (MCA) and thus of the coercive field
No current needed no power dissipation x Bias magnetic field needed x Reversible only after reversing the bias field
Electrical swithing of the MTJ from antiparallel to parallel configuration
E. Y. Tsymbal, Electric toggling of magnets, Nat. Mater. 11, 12 (2012)
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Electric writing of Magnetic Information: MEC (1)
10
2) Magneto-Electric Coupling (MEC)
M. Bibes, Nanoferronics is a winning combination, Nat. Mater. 11, 354 (2012)
Coupling of magnetization (M) and polarization (P)
Efficient, low power electrically controlled spintronic devices: Low cost writing (FE) Robustness and
durability of stored information (FM)
Single-phase multiferroics
unfortunately often present low Tc
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Electric writing of Magnetic Information: MEC (2)
11
2) Magneto-Electric Coupling (MEC)
M. Bibes, Nanoferronics is a winning combination, Nat. Mater. 11, 354 (2012)
Aiming at large scale technological impact multiferroics heterostructures, made of RT conventional FM and traditional FE, seem more promising.
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives 12
P↑ P↓
Transition of Interfacial Fe (oxidized) from ferromagnetic to antiferromagnetic related to a polarization switch of the ferroelectric BaTiO3 (BTO)
1 nm
2 ML
150 nm
Fe
BaTiO3
M
P
G. Radaelli et al., Electric control of magnetism at the Fe/BaTiO3 interface, Nat. Comm. 5, 3404 (2014)
X-ray absorption (XAS) spectra and X-ray magnetic circular dichroism (XMCD) of Fe-L2,3 at 300 K after BTO polarization with +5V (Pup) and -5V (Pdn)
XAS and XMCD measured at the APE beamline, Elettra (Trieste)
DFT calculations by S. Picozzi et al., CNR-SPIN (L’Aquila)
The artificial multiferroic Fe/BaTiO3 system
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Multiferroic Fe/BTO/LSMO tunneling junctions
13
M. Asa et al., Electric field control of magnetic properties and electron transport in BaTiO3-based multiferroic heterostructures, J. Phys.: Condens. Matter 27, 504004 (2015)
BaTiO3 (BTO)
La0.67Sr0.33MnO3 (LSMO)
Fe
Ferroelectric Tunneling Junction (FTJ)
Magnetic Tunneling Junction (MTJ)
Four resistance states New-generation memory cells based on a four-logic state
𝑇𝑀𝑅 =𝑅𝐴𝑃 − 𝑅𝑃
𝑅𝑃
TMR changes sign (RAP>RP becomes RAP<RP, and vice-versa) when the BTO polarization is reversed Magnetoelectric coupling
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Summary
14
I will discuss two significant examples of spintronic devices and technology: non volatile magnetic memories
(MRAMs) and semiconductor spintronics.
1. Magnetic memories
a. state of the art and perspectives
b. the writing issue: new strategies (magneto-electric coupling)
c. the density issue: new strategies (antiferromagnet spintronics)
2. Semiconductor spintronics
a. the Datta and Das spin-FET
b. the four problems: injection, transport, manipulation, detection
c. the conductivity mismatch issue
d. optical spin injection and spin photodiodes
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Magnetic memories (MRAMs) – the density issue
15
Increasing density = reducing the memory cell size and distance: how to do?
S. S. P. Parkin et al., Magnetic domain-wall racetrack memory, Science 320, 190 (2008)
Problems:
• magnetic stray field lines prevent from reducing the distance between cells
• Joule heating by currents prevent from reducing the cell size
Potential solution #1:
• Moving from 2D arrays to 3D devices
Racetrack Memories based on magnetic domain motions, induced by current pulses, in U-shaped magnetic nanowires
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Antiferromagnet spintronics
16
Potential solution #2:
• Substituting ferromagnets (FMs) with antiferromagnets (AFMs)
Ferromagnets (FM) Antiferromagnets (AFM)
M M=0
Easy to manipulate: M can be rotated
via external magnetic fields
The magnetic cross talk
limits the density on a chip
Hard to manipulate
No magnetic interaction:
high density on a chip
Problem: how to store and read information in antiferromagnets ?
Easy to erase by external fields Very robust versus
external fields
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Spintronic paradigms
17
Magnetic and magneto-transport anisotropy effects present in AFMs with spin-orbit equally well as in FMs
AFM
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Reading the AFM configuration
18
Magnetic and magneto-transport anisotropy effects present in AFMs with spin-orbit equally well as in FMs
J J
Anisotropic Magneto Resistance (AMR)
AFM
AFM
J
AFM
Insulating
barrier Non magnetic
metal
J
Tunnelling AMR D. Petti et al., Storing magnetic information in
IrMn/MgO/Ta tunnel junctions via field-cooling, Appl. Phys. Lett. 102, 192404 (2013)
M. Cantoni et al., Towards Cr-based antiferromagnetic spintronics: growth and
magnetic anisotropy of chromium thin films, book of abstract of AIM 2016 (Bormio)
IrMn(AFM)/MgO/Ta
Cr(AFM)
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Writing the AFM configuration
19
[1] B. G. Park et al., A spin-valve-like magnetoresistance on an antiferromagnet-based tunnel junction, Nat. Mat. 10, 347 (2011) [2] D. Petti et al., Storing magnetic information in IrMn/MgO/Ta tunnel junctions via field-cooling, Appl. Phys. Lett. 102, 192404 (2013) [3] X. Marti et al., Room-temperature antiferromagnetic memory resistor, Nat. Mat. 13, 367 (2014) [4] P. Wadley et al., Electrical switching of an antiferromagnet, Science 351, 587 (2016)
Electric writing of
the magnetic state
[1]
FM-AFM
transition Spin Orbit Torque
Néel-ordered SOT [4]
Field Cooling
[2]
[3]
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Storing information in an IrMn/MgO/Ta-based memory cell
20
STO
Seed
TJ
Ta 20
MgO 2.5
IrMn 2
Ta 20/Ru 18/Ta 2
Field cooling assisted storing with antiferromagnets
Splitting onset at IrMn antiferromagnetic transition
RH
RL
cv cv TNéel
cv
Insensitive to strong external fields
D. Petti et al., Storing magnetic information in IrMn/MgO/Ta tunnel junctions via field-cooling, Appl. Phys. Lett. 102, 192404 (2013)
In perspective: • room temperature operation • CMOS-compatibility
M. Cantoni, Magnetic information storage in Antiferromagnetic spintronic devices (MAGISTER), project grant 2013-0726 of Fondazione Cariplo, bando “Ricerca scientifica e tecnologica sui materiali avanzati – 2013”
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Summary
21
I will discuss two significant examples of spintronic devices and technology: non volatile magnetic memories
(MRAMs) and semiconductor spintronics.
1. Magnetic memories
a. state of the art and perspectives
b. the writing issue: new strategies (magneto-electric coupling)
c. the density issue: new strategies (antiferromagnet spintronics)
2. Semiconductor spintronics
a. the Datta and Das spin-FET
b. the four problems: injection, transport, manipulation, detection
c. the conductivity mismatch issue
d. optical spin injection and spin photodiodes
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Semiconductor spintronics
22
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Datta and Das spin-FET
23
Datta and Das spin transistor (1990)
S. Datta and B. Das, Electronic analog of electro-optical modulator, Appl. Phys. Lett. 56, 665 (1990)
FM injector FM detector (with magnetization
parallel to injector’s one)
Semiconductor channel (2D electron gas)
Gate voltage converted in an effective
magnetic field by Rashba effect
Spin precession due to spin-orbit coupling in the
semiconductor (or, equivalently, to presence of the effective magnetic field Heff)
Heff
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives The four problems
24
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Summary
25
I will discuss two significant examples of spintronic devices and technology: non volatile magnetic memories
(MRAMs) and semiconductor spintronics.
1. Magnetic memories
a. state of the art and perspectives
b. the writing issue: new strategies (magneto-electric coupling)
c. the density issue: new strategies (antiferromagnet spintronics)
2. Semiconductor spintronics
a. the Datta and Das spin-FET
b. the four problems: injection, transport, manipulation, detection
c. the conductivity mismatch issue
d. optical spin injection and spin photodiodes
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Spin injection and detection
26
FM NM
z>0 z<0
Ferromagnet Non-magnet
z
injection
detection
zeJ
lz
Je
S
S
S
S
SSSS
1
2
(Macroscopic transport from Boltzmann equation)
(Ohm’s law)
(S=,)
T. Valet and A. Fert, Theory of the perpendicular magnetoresistance in magnetic multilayers, Phys. Rev. B 48, 7099 (1993)
• FM metal / NM metal (es. Co/Cu)
• FM semiconductor / NM semiconductor (es. GaMnAs/GaAs)
• FM metal / NM semiconductor (es. Fe/Ge, Fe/GaAs) conductivity mismatch issue
A spin-polarized current in the FM remains spin-polarized in the NM?
A. Fert and H. Jaffrès, conditions for efficient spin ijection from a ferromagnetic metal into a semiconductor, Phys. Rev. B 64, 184420 (2001)
FM >> SC
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives The conductivity mismatch issue (1)
27
A simple explanation: electric model
a) FM/SC interface RFM RSC
RFM RSC
j
j
V
RFM << RSC
RSC=RSC
R=RFM+ RSC RSC
R = RFM +RSC RSC
I = V / R V / RSC
I = V / R V / RSC
I = I
b) FM/barrier/SC interface
Rb >> RSC >> RFM
RSC=RSC
R=RFM+ RSC + Rb Rb
R =RFM + RSC + Rb Rb
I = V / R V / Rb
I = V / R V / Rb
RFM RSC
RFM RSC
j
j
V
Rb
Rb
FM >> SC
I I
The introduction of an insulating barrier between FM and SC allows for overcoming the conductivity mismatch issue
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives 28 The conductivity mismatch issue (2)
FM metal/NM metal
FM/SC without Barrier
FM/SC with barrier
FM SC
z>0 z<0
Ferromagnet Barrier Semiconductor
z
injection
detection
B
totalI
II
• The barrier allows to obtain a spin polarized current (I-I0) in the semiconductor
• The spin polarization decays with the spin diffusion length in the material (m in GaAs and Ge)
A. Fert and H. Jaffrès, conditions for efficient spin ijection from a ferromagnetic metal into a semiconductor, Phys. Rev. B 64, 184420 (2001)
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Summary
29
I will discuss two significant examples of spintronic devices and technology: non volatile magnetic memories
(MRAMs) and semiconductor spintronics.
1. Magnetic memories
a. state of the art and perspectives
b. the writing issue: new strategies (magneto-electric coupling)
c. the density issue: new strategies (antiferromagnet spintronics)
2. Semiconductor spintronics
a. the Datta and Das spin-FET
b. the four problems: injection, transport, manipulation, detection
c. the conductivity mismatch issue
d. optical spin injection and spin photodiodes
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives 30 Spin detection
How can we study the spin detection, i.e. how to measure the spin polarization of the current moving from the semiconductor to the ferromagnet?
FM SC
z>0 z<0
Ferromagnet Barrier Semiconductor
z
detection
B
We need to create in an independent way a spin polarized current in the semiconductor
Optical Orientation
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives 31 Optical orientation
A circularly polarized light resonant with the gap produces a spin polarized population at the point of the semiconductor bandstructure (Ge, GaAs)
-50 %
+50 %
Right circularly polarized light (σ+)
Left circularly polarized light (σ-)
NN
NNS
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives 32 Electrical detection of optical orientation
Circularly polarized light
• angular momentum L= σh with σ= ±1
Conservation of angular momentum
the spin of an electron changes when a photon is absorbed
1) Spin polarized carriers are generated in the semiconductor by light 2) The spin polarized current flows from the semiconductor to the ferromagnet 3) The residual spin polarization is detected into the ferromagnet
Photon angular momentum and spin
Semiconductors
Magnetism
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives 33 Spin filtering
Fe/MgO/Ge is a tunnelling junction with a tunneling probability that depends on the carrier spin orientation with respect to the magnetization of Fe
Spin-dependent density of states at the Fermi level
Spin-up and spin-down currents suffer different resistivities (current spin filtering)
An out-of-plane magnetization controls the behaviour of the device
Electrical spin detection of a polarized current
FM B SC
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives 34 Physical model of the spin photodiode
1. Absorption of light from the FM metal: Magnetic Circular Dichroism (MCD)
2. Photo-generation: electron-hole pairs creation.
The polarization of the photocurrent depends on the helicity of the light σ
3. Diffusion of electrons (V>0) or holes (V<0) towards the barrier.
4. Spin-dependent tunneling across the MgO barrier (spin filtering, SF). Left and right circularly polarized light produce
photocurrents of different magnitude. Helicity-dependent photocurrent:
SFMCD IIIII
ELECTRICAL MEASUREMENT OF LIGHT HELICITY
C. Rinaldi et al., Ge-Based Spin-Photodiodes for Room-Temperature Integrated Detection of Photon Helicity, Adv. Mat. 24, 3037 (2012)
FM
B
SC
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives 35 Fe/MgO/Ge spin photodiodes
SPIN-PHOTODIODE Integrated detector of light helicity
Spin-optoelectronics Novel communication systems
holes electrons
MCD
SF
H
C. Rinaldi et al., Ge-Based Spin-Photodiodes for Room-Temperature Integrated Detection of Photon Helicity, Adv. Mat. 24, 3037 (2012)
SFMCD IIIII
Left and right circularly polarized light produce photocurrents of different magnitude. Spin filtering efficiency:
photo
SF
I
ISF
1300 nm
Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15
Conclusions and perspectives Acknowledgements
36
C. Rinaldi, M. Asa, G. Radaelli, D. Petti, E. Albisetti, R. Bertacco
www.polifab.polimi.it
The NaBiS group @ Polifab