Applications: Spintronic Devices - Milan...

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

mailto:[email protected]

Matteo Cantoni Applications: Spintronic devices 21/4/2016 h12:15-13:15

Conclusions and perspectives Summary

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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



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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

• …


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 )


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



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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


Conclusions and perspectives Electric writing of Magnetic Information: MCA

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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)


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


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.


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


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



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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


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


Conclusions and perspectives Spintronic paradigms

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Magnetic and magneto-transport anisotropy effects present in AFMs with spin-orbit equally well as in FMs

AFM


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)


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]


Conclusions and perspectives Storing information in an IrMn/MgO/Ta-based memory cell

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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”



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Conclusions and perspectives Semiconductor spintronics

22


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


Conclusions and perspectives The four problems

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Conclusions and perspectives Spin injection and detection

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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


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


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)



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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


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


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


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


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


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


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

Applications: Spintronic Devices - Milan...

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