Introduction to Spintronics and Spin Caloritronicsusers.physik.fu-berlin.de/~nunner/Teaching/ws1415/Introduction.pdf2 Outline Format of seminar How to give a presentation How to search

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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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)


source: evoravip.de

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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


source: trainingsoutheast.blogspot.com

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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)


source: netrafic.com

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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)


source: mrsstancilsclass.pbworks.com

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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


source: crystalgraphics.com

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Scientific literature

  Textbooks

  Articles

•  Regular research article

•  Review article

•  Popular article


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)

  Google


source: ru.nl

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Introduction to Spintronics …


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


charge current

heat current

spin current

spintronics thermoelectric effects

spin caloritronics

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Spintronics materials


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

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Talk 1: GMR effect


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

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Talk 2: TMR effect


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

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Talk 3: (Thermal) spin torque


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


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


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

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Talk 6: Spin diffusion with SOI


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


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

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Talk 8: Ferromagnetic semiconductors


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)


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

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Talk 10: Berry phase in spin transport


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

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Talk 11: Quantum computation


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

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Conclusions


…

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Acknowledgements


Thank you for your attention !

Cooperations ….

Funding …

Documents

Introduction to Spintronics and Spin Caloritronicsusers.physik.fu-berlin.de/~nunner/Teaching/ws1415/Introduction.pdf2 Outline Format of seminar How to give a presentation How to search