1. AN INTRODUCTION TO SPINTRONICS
रास्ट्रीय प्रद्योगिकी संस्थान हमीरपुर
NATIONAL INSTITUTE OF TECHNOLOGY
HAMIRPUR
Centre for Materials Science and Engineering
AN INTRODUCTION TO SPINTRONICS
BY: SAMIR KUMAR
10M601
M.TECH 1ST YEAR
Center for Materials Science and Engineering

2. Outline Introduction What do we mean by spin of an electron
Why Spintronics
Spintronic Effects
Phases in Spintronics
Materials of Spintronics
Conclusions
Acknowledgments
Outline

3. Electron has : Mass Charge Spin
INTRODUCTION
Electron has : Mass Charge Spin

4. One can picture an electron as a charged sphere rotating about an axis.
The rotating charged sphere will produce magnetic moment in that can be either up or down depending upon whether the rotation is anticlockwise or clockwise
What is spin?

5. Electron Spin is a Quantum phenomenon
A spinning sphere of charge can produce a magnetic moment.
Considering Electrons size to be of the order of m at that size a high spin rate of some 1032 radian/s would be required to match the observed angular momentum that is velocity of the order of 1020 m/s.

6. The component Sz along z axis:
Electron Spin
The component Sz along z axis:

7. SPINTRONICS = SPIN + ELECTRONICS
Conventional electronic devices ignore the spin property.
Random spins have no effect on current flow.
Spintronic devices create spin-polarized currents and use the spin to control current flow.
Spintronics=spin based electronics
What is Spintronics?

8. Moore’s Law Why Spintronics?
Moore’s Law states that the number of transistors on a silicon chip will roughly double every eighteen months
Why Spintronics?

9. Power dissipation=greatest obstacle for Moore’s law!
Can Moore’s law keep going?
Power dissipation=greatest obstacle for Moore’s law!
Modern processor chips consume ~100W of power of which about 20% is wasted in leakage through the transistor gates.
The traditional means of coping with increased power per generation has been to scale down the operating voltage of the chip but voltages are reaching limits due to thermal fluctuation effects.

10. Advantages of Spintronics Devices
Non-volatile memory
Performance improves with smaller devices
Low power consumption
Spintronics does not require unique and specialised semiconductors
Dissipation less transmission
Switching time is very less
Compared to normal RAM chips, spintronic RAM chips will:
– increase storage densities by a factor of three
– have faster switching and rewritability rates smaller
Promises a greater integration between the logic and storage devices
Advantages of Spintronics Devices

11. Spintronics Effects GMR (Giant Magneto- Resistance) FM-Metal-FM
MTJ (Magnetic Tunnel Junction)
FM-Insulator-FM

12. Giant Magneto-Resistance (GMR)
The 2007 Nobel prize for physics was award jointly to Fert and Grunberg for giant magnetoresistance (GMR) discovered independently in 1988.
This discovery led to development of the “spin valve” and later the tunnel magnetoresistance effect (TMR) which found application in advanced computer hard drives, and more recently magneto-resistive random access memory (MRAM) (which is non-volatile).

13. Giant Magneto-Resistance (GMR)
Discovered in 1988 France
A multilayer GMR consists of two or more ferromagnetic layers separated by a very thin (about 1 nm) non-ferromagnetic spacer (e.g. Fe/Cr/Fe)
When the magnetization of the two outside layers is aligned, resistance is low
Conversely when magnetization vectors are antiparallel, high R
Condition for GMR: layer thickness ~ nm

14. Parallel Current GMR
Current runs parallel between the ferromagnetic layers
Most commonly used in magnetic read heads
Has shown 200% resistance difference between zero point and antiparallel states

15. Perpendicular Current GMR
Easier to understand theoretically, think of one FM layer as spin polarizer and other as detector
Has shown 70% resistance difference between zero point and antiparallel states
Basis for
Tunneling
MagnetoResistance

16. Concept of the Giant Magnetoresistance (GMR)
1) Iron layers with opposite magnetizations : spin up and spindown are stopped → no current (actually small current only)
2) If a magnetic field aligns the magnetizations: spins go through

17. Applications of GMR It is used in Hard Drives 0.5 MB ← 1975
100 GB hard disc (Toshiba), →
soon in portable digital audio-players
1997 (before GMR) : 1 Gbit/in2 , 2007 : GMR heads ~ 300 Gbit/in2

18. Magnetic Tunnel Junction
A magnetic tunnel junction (MTJ) consists of two layers of magnetic metal, such as cobalt-iron, separated by an ultrathin layer of insulator.
Ferromagnetic
electrodes
Tunnel Magnetoresistive effect combines the two spin channels in the ferromagnetic materials and the quantum tunnel effect

19. Magnetic Tunnel Junction
Device
Ferromagnetic leads L & R
Insulating spacer S
Parallel alignment (P)
Antiparallel alignment (AP)
Measured: tunneling current I, conductance G
Tunneling magneto-resistance (TMR)

20. Applications The read heads of modern hard disk drives.
Is also the basis of MRAM, a new type of non-volatile memory.

21. Magnetoresistive Random Access Memory
MRAM uses magnetic storage elements instead of electric used in conventional RAM
Tunnel junctions are used to read the information stored in Magnetoresistive Random Access Memory, typically a ”0” for zero point magnetization state and “1” for antiparallel state
Non volatile, instant-on computers

22. MRAM combines the best characteristics of Flash, SRAM and DRAM

23. Phases in Spintronics
SPIN INJECTION
SPIN MANIPULATION
SPIN DETECTION

24. Spin injection
It is the transport or creating a non-equilibrium spin population across interface
Using a ferromagnetic electrode
Effective fields caused by spin-orbit interaction.
Tunnel barrier could be used to effectively inject spins into a semiconductor
Tunneling spin injection via Schottky barrier
By “hot” electrons

25. Spin Manipulation
To control electron spin to realize desired physical operation efficiently by means of external fields
Mechanism for spin transfer implies a spin filtering process.
Spin filtering means that incoming electrons with spin components perpendicular to the magnetic moment in the ferromagnet are being filtered out.
Spin-polarized current can transfer the angular momentum from carriers to a ferromagnet where it can change the direction of magnetization This effect is equivalent to a spin transfer torque.

26. Spin Transfer Torque S v v
The spin of the
conduction electron
is rotated by its
interaction with the
magnetization.
This implies the magnetization exerts a torque on the spin. By Conservation of angular momentum, the spin exerts an equal and Opposite torque on the magnetization.

27. Spin Detection
To measure the physical consequences of spin coherent states in Spintronics devices.
The injection of non-equilibrium spin either induces voltage or changes resistance corresponding to buildup of the non-equilibrium spin. This voltage can be measured in terms of change in resistance by potentiometric method.

28. Spin Detection Technique
An ultrasensitive silicon cantilever with a SmCo magnetic tip positioned 125nm above a silica specimen containing a low density of unpaired electron spins. At points in the specimen where the condition for magnetic resonance is satisfied, the magnetic force exerted by the spin on the tip.

29. Materials of Spintronics
Problems
Currently used materials in conventional electronics are usually non-magnetic and only charges are controllable.
Existing metal-based devices do not amplify signals.
Whereas semiconductor based spintronic devices could in principle provide amplification and serve, in general, as multi-functional devices.
All the available ferromagnetic semiconductor materials that can be used as spin injectors preserve their properties only far below room temperature, because their Curie temperatures (TC) are low.

30. Spintronic Research and Applications
GMR - Giant magnetoresistance - HDD read heads
MTJ - Magnetic Tunnel Junction - HDD read heads+MRAM
MRAM - Magnetic RAM - nonvolitile memory
STT - Spin Transfer Torque - MRAM+oscillator

31. Solution Diluted Magnetic Semiconductor or (DMS). Add Fe or Mn to
Si/GaAs
Half-Metallic Ferromagnets
Fe3O4 magnetite
CrO2
Heusler FM
Ni2MnGa
Co2MnAl

32. Diluted Magnetic Semiconductor or (DMS)
One way to achieve FS is to dope some magnetic impurity in a semiconductor matrix. (Diluted Magnetic Semiconductor )
Semiconductor host atom
Magnetic impurity

33. Theoretical predictions
Various DMS displays room temperature ferromagnetism!
Science 287, 1019 (2000)
& PRB 63, (2001)
Theoretical predictions
by Dietl, Ohno et al.
Curie Temperature — The temperature above which a ferromagnetic material loses its permanent magnetism.

34. DMS materials I: (Ga,Mn)As
First DMS material, discovered in 1996 by Ohno et al.
Curie temperature 𝑻 𝒄 =𝟏𝟏𝟎 K at optimal doping
Max TC ~ 110K
x ~ .05
[Ohno et al., APL 69, 363 (1996)]

35. DMS materials II: (Ga,Mn)N
Highest Tc
in Dietl’s
prediction
First room temperature DMS discovered in 2001
High curie temperature
Experiment: up to Tc =800 K
Theory: up to Tc =940 K

36. DMS materials III: Transition metal doped oxide
Hysteresis curve at Room temperature
for Mn doped ZnO(Sn)
Room temperature ferromagnetism discovered in Mn doped ZnO in 2001
Material:
Mn doped ZnO
Co doped TiO
Reported Tc up to 400K

37. Half-Metallic Ferromagnets
Half metals are ferromagnets with only one type of conduction electron, either spin up, ↑, or spin down, ↓
The valence band related to one type of these electrons is fully filled and the other is partially filled. So only one type of electrons (either spin up or spin down) can pass through it.

38. Half-Metallic Ferromagnets
E.g.:
Chromium(IV) oxide
Fe3O4 magnetite
Heusler alloys

39. Future Outlook High capacity hard drives Magnetic RAM chips
Spin FET using quantum tunneling
Quantum computers

40. Limitations
Problems that all the engineers and scientists may have to overcome are:
To devise economic ways to combine ferromagnetic metals and semiconductors in integrated circuits.
To find an efficient way to inject spin-polarized currents, or spin currents, into a semiconductor.
To create long relaxation time for effective spin manipulation.
What happens to spin currents at boundaries between different semiconductors?
How long can a spin current retain its polarization in a semiconductor?

41. for your kind attention
THANK YOU
for your kind attention ☺