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Introduction to Spintronics and Spin Caloritronics
Tamara Nunner Freie Universität Berlin
2
Outline
Format of seminar
How to give a presentation
How to search for scientific literature
Introduction to spintronics and spin caloritronics
Possible topics for presentations
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
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3
Format of the Seminar
Presentation (45 min) about a topic in the field of Spintronics/Spincaloritronics
Preparation involves:
• search for own literature (some initial references will be given) • abstract (1 week before talk, will be announced on webpage)
Participation in discussion after each talk
One meeting with instructor before talk (optional)
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
source: evoravip.de
4
Giving a talk Tell a story
Motivation: why is your research important
Be qualitative
• Use simple physical pictures and graphs • Keep formulas simple
Fair citation
Understand everything you present
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
source: trainingsoutheast.blogspot.com
5
Consider the audience Adjust necessary introduction & level of technical details
• Give the audience the pleasure of learning new or already forgotten “standard knowledge”
Engage the audience
• Move
• Make eye contact
• Make the audience think and not just listen (e.g. ask a question, pause, then give an answer)
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
source: netrafic.com
6
Structure of Talk/Slides Timing of a 30 - 60 min talk (approx 2-3 min per slide):
Title 1 min Contents Introduction 5 - 20 min (20 - 30% of talk) Body Conclusions 2 - 4min Questions 5 - 10 min
And:
Beginning: Thank organizers for invitation or opportunity to present work
End: thank for attention
Acknowledgements (if applicable)
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
source: mrsstancilsclass.pbworks.com
7
Preparation of slides/talk Slides
Limit amount of text
No need for complete sentences
Never over-crowd slides
Make images and text large enough
Label all plots
Talk
Practice (transition between slides)
Do not read from slides (only occasionally)
Anticipate questions • You appear competent when you know how to answer questions • But be honest if you don’t know the answer
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
source: crystalgraphics.com
8
Scientific literature
Textbooks
Articles
• Regular research article
• Review article
• Popular article
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
source: cheshireruraltouringarts.co.uk
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How to search for scientific literature General scientific data base: www.webofknowledge.com
Search engines of individual journals
Common journals
• Nature (Nature physics, Nature materials, Nature nanotechnology)
• Science
• APS (PRL, PRB, Rev. Mod. Phys.)
• Applied Physics Letters (APL)
• ….
preprint server arXiv (http://arXiv.org)
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
source: ru.nl
10
Introduction to Spintronics …
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
Electronic spin (magnetic moment) Charge currents → spin currents:
hσz
h ≈ niJ�Sz�
σxy ∝ h ∝ �Sz�
H =�
k
�k2
2m+ α(σxky − σykx)
�+�
i
�V + J �Si · �σ
�δ(r −Ri)
H =�
k
�k2
2m+ α(σxky − σykx)− hσz
�+�
i
V δ(r −Ri)
0 =�
p
�1
τpk(T ph
p − Tm
k )− 1
τ̃pk(T ph
p − Tm
k−p)
�
Tm
k = Tm
Tph
diffusive − Tm
∆T≈ −10−5 (VISHE ≈ .5µV )
ds
dt= �∇ ·�jspin = −νµspin
τsf
�jspin = �j↑ −�j↓
�jspin = − σ
e2�∇µspin
�j = σ �E + eD�∇n = −σ�∇φ+ eD�∇n
�jspin = −σ�∇φspin − σspin�∇φ+ eD�∇nspin + eDspinn
Tm(k) = Tph(k)
−�kT
∂fk∂�
(vk ·∇T) + vk ·∇fk = −δfkτmk
−ωp
T
∂np
∂ω(up ·∇T) + up ·∇np = −δnp
τphp
2
Advantages: • less dissipation • fast
Prominent example: Giant magnetoresistance (GMR)
Application: read head in hard disks
Nobel prize 2007: Albert Fert and Peter Grünberg
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… and Spin Caloritronics
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
charge current
heat current
spin current
spintronics thermoelectric effects
spin caloritronics
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Spintronics materials
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
Metal spintronics: • generic for ferromagnets
Semiconductor spintronics: • easy to integrate into exisiting semiconductor logic • spin manipulation via spin-orbit interaction • BUT: spin dephasing? ferromagnetic semiconductors?
Other (exotic) materials: • e.g. topological insulators
source: azonano.com
13
Talk 1: GMR effect
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
Reproduced from: nobelprize.org
Nobel prize 2007: Albert Fert and Peter Grünberg
In ferromagnet: scattering rate depends on spin direction
parallel spins → small resistance
antiparallel spins → large resistance
→ GMR effect
14
Talk 2: TMR effect
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
Similar to GMR effect but based on tunnel junctions (i.e. insulating layer between ferromagnets)
Application: read-heads of modern hard disk drives
Reproduced from Wikipedia
15
Talk 3: (Thermal) spin torque
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
A. Brataas et al., Nature Mater. 11, 372 (2012)
• Spin polarized current exerts torque on ferromagnet
• Inverse effect: rotating magnetization injects spin current (spin pumping)
• Spin torque can also be generated by thermal spin currents (thermal spin torque)
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Talk 4: Domain walls
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
Magnetic domain wall racetrack memory S.S.P. Parkin et al., Science 320, 190 (2008)
Magnetic domains can be manipulated by spin polarized currents (current induced torque)
O. Boulle et al. Materials Science and Engineering R 72 (2011), 159
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Talk 5: Spin caloritronics
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
K. Uchida et al., Nature 455, 778 (2008)
Temperature gradient in metal → thermoelectric voltage
Seebeck effect: voltage at contact between two different conductors
Application to ferromagnet:
Different transport properties for spin ↑ and spin ↓ → Spin voltage µspin= µ↑ - µ↓
K. Uchida et al., J. Appl. Phys. 111, 103903 (2012)
→ spin dependent Seebeck effect
Similar effect also in ferromagnetic insulators: → spin Seebeck effect
18
Talk 6: Spin diffusion with SOI
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
Systems with spin-orbit interaction: + : allows active manipulation of spin - : spin dephasing (spin is not conserved)
Effective magnetic field (ΩSOI) changes after each collision
k k’
k’’ e- e- e-
ΩSB(k) ΩSB(k’) ΩSB(k’’)
Strong disorder enhances spin lifetimes (random walk, D’yakonov Perel regime)
Search for long lived spin polarized states, e.g. persistent spin helix
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Talk 7: Spin Hall effect / Anomalous Hall effect
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
B
E vE v
B
In systems with spin-orbit interaction
asymmetric scattering rate for spin ↑ and ↓ → transversal spin current Spin Hall effect In ferromagnets: majority spin polarization → transversal spin- and charge currents: Anomalous Hall effect
Reproduced from J.-I. Inoue and H. Ohno, Science 309, 2004 (2005)
n =�
m
nm �= 0 with nm =1
2π
�d2kFm
Fm = i∇× �um|∇k|um�
HSOI = − 1
4m2c2�σ · (�p× �∇V )
1
20
Talk 8: Ferromagnetic semiconductors
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
Reproduced from: T. Jungwirth et al., Rev. Mod. Phys. 78, 809 (2006)
For semiconductor spintronics: magnetic semiconductors highly desirable
Origin of ferromagnetism in diluted magnetic semiconductors (Ga,Mn)As Conduction electron vs. impurity model
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Talk 9: Applications (semiconductor spintronics)
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
Datta-Das Spin transistor
Reproduced from: J.Fabian et al., arXiv:0711.1461
Bipolar spintronic devices based on magnetic pn-junction: • magnetic diode • magnetic dipolar transistor
22
Talk 10: Berry phase in spin transport
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
Berry: Quantum system transported adiabatically around a closed circuit C
acquires phase depending only on the geometry of the circuit C → Berry phase
Reproduced from: P. Bruno, Berry phase effects in magnetism
Berry phase enters semiclassical transport equations, e.g. anomalous Hall effect
23
Talk 11: Quantum computation
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
Reproduced from: G. Burkard, arXiv:cond-mat/0409626
Quantum information theory: • Qubit: quantum state includes superposition between classical bits |0> and |1> • Quantum operations
Applications: Cryptography (Shor’s algorithm)…
Physical realizations: e.g. quantum dot arrays
24
Conclusions
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
…
25
Acknowledgements
Introduction to Spintronics and Spin Caloritronics, Berlin, 17.10.2014
Thank you for your attention !
Cooperations ….
Funding …
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