Introduction to Spintronics and Spin Caloritronics. Tamara Nunner Freie Universität Berlin

Transcription

1 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??? 2

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) source: evoravip.de 3

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 source: trainingsoutheast.blogspot.com 4

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 source: netrafic.com Make the audience think and not just listen (e.g. ask a question, pause, then give an answer) 5

6 Structure of Talk/Slides Timing of a min talk (approx 2-3 min per slide): Title Contents Introduction Body Conclusions Questions And: 1 min 5-20 min (20-30% of talk) 2-4min 5-10 min source: mrsstancilsclass.pbworks.com Beginning: Thank organizers for invitation or opportunity to present work End: thank for attention Acknowledgements (if applicable) 6

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) source: crystalgraphics.com 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 7

8 Scientific literature Textbooks Articles Regular research article Review article Popular article source: cheshireruraltouringarts.co.uk 8

9 How to search for scientific literature General scientific data base: 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 ( Google source: ru.nl 9

10 Introduction to Spintronics Advantages: less dissipation fast Electronic spin (magnetic moment) Charge currents spin currents: j spin = j j Prominent example: Giant magnetoresistance (GMR) Nobel prize 2007: Albert Fert and Peter Grünberg Application: read head in hard disks 10

11 and Spin Caloritronics charge current spintronics thermoelectric effects spin current heat current spin caloritronics 11

12 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 12

13 Talk 1: GMR effect In ferromagnet: scattering rate depends on spin direction parallel spins small resistance antiparallel spins large resistance Reproduced from: nobelprize.org GMR effect Nobel prize 2007: Albert Fert and Peter Grünberg 13

14 Talk 2: TMR effect Similar to GMR effect but based on tunnel junctions (i.e. insulating layer between ferromagnets) Reproduced from Wikipedia Application: read-heads of modern hard disk drives 14

15 Talk 3: (Thermal) spin torque Spin polarized current exerts torque on ferromagnet A. Brataas et al., Nature Mater. 11, 372 (2012) Inverse effect: rotating magnetization injects spin current (spin pumping) Spin torque can also be generated by thermal spin currents (thermal spin torque) 15

16 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),

17 Talk 5: Spin caloritronics Temperature gradient in metal thermoelectric voltage Application to ferromagnet: K. Uchida et al., Nature 455, 778 (2008) Seebeck effect: voltage at contact between two different conductors Different transport properties for spin and spin Spin voltage µ spin = µ - µ spin dependent Seebeck effect Similar effect also in ferromagnetic insulators: spin Seebeck effect K. Uchida et al., J. Appl. Phys. 111, (2012) 17

18 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 (Ω SOI ) changes after each collision Ω SB (k) k k e - e - Ω SB (k ) k Ω SB (k ) e - Strong disorder enhances spin lifetimes (random walk, D yakonov Perel regime) Search for long lived spin polarized states, e.g. persistent spin helix 18

19 Talk 7: Spin Hall effect / Anomalous Hall effect Reproduced from J.-I. Inoue and H. Ohno, Science 309, 2004 (2005) In systems with spin-orbit interaction H SOI = 1 4m 2 c 2σ (p V ) 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 v B E E B v 19

20 Talk 8: Ferromagnetic semiconductors For semiconductor spintronics: magnetic semiconductors highly desirable Reproduced from: T. Jungwirth et al., Rev. Mod. Phys. 78, 809 (2006) Origin of ferromagnetism in diluted magnetic semiconductors (Ga,Mn)As Conduction electron vs. impurity model 20

21 Talk 9: Applications (semiconductor spintronics) Datta-Das Spin transistor Bipolar spintronic devices based on magnetic pn-junction: magnetic diode magnetic dipolar transistor Reproduced from: J.Fabian et al., arxiv:

22 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 22

23 Talk 11: Quantum computation 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 Reproduced from: G. Burkard, arxiv:cond-mat/

24 Conclusions 24

25 Acknowledgements Cooperations. Funding Thank you for your attention! 25