Condensed Matter Physics: Electrons’
Adventures in Magnet-land
Laurel Anderson
MCR Seminar, 3 Dec 2014
Overview
• What’s “condensed matter?”
• Spin in condensed matter
• Spin-orbit torque
• “So what?” (AKA applications)
• A peek at some other cool condensed-matter physics (if we have time)
http://www.smbc-comics.com/?id=3541#comic
Title slide image: http://www.frm2.tum.de
“Condensed matter?”
• Solids, liquids, condensates…basically, “not gas or plasma”
• Coined at the Cavendish Laboratory! (1967)
• Basic questions:
• What internal factors affect the properties of a material?
• How can we manipulate a material to affect its properties?
• (Why are these hard questions? Quantum!)
• Often split into “soft” and “hard” condensed matter physics
http://www.phy.cam.ac.uk/history/years/croc
Soft condensed matter physics
• Liquids, gels, foams, colloids, grains
• Example: liquid crystals
• Biological systems
• Example: studying protein folding
soft-matter.seas.harvard.edu/
en.wikipedia.org
Hard condensed matter physics
• AKA “solid-state physics”
• Some research areas:
• Superconductivity
• Quantum information
• Strongly-correlated systems
• Graphene/carbon structures
~ 2 cm
• Quantum chaos
• Spintronics
en.wikipedia.org/superconductivity; graphene.nus.edu.sg; computerhistory.org;
http://userweb.eng.gla.ac.uk/douglas.paul/SiGe/SET.html
Spintronics
• Spin + electronics (or spin transport electronics): study of how electron
spin affects their interactions
• Giant magnetoresistance (1988)
• Advantages of spintronic devices:
• Lower power/current usage
current
• Less heat dissipation
• Higher information density
• Faster read/write speed
en.wikipedia.org
Spin: a crash course
• Spin: intrinsic magnetic moment of a particle
• Only takes discrete values (quantized)
• Electrons are spin-1/2 (up or down)
• Why call it “spin?” Analogous to classical electron
rotating (accelerating charge → magnetic field)
N
≈
S
slideshare.net
Spin: a crash course
• Magnetic moments precess in an applied magnetic field (B)
• Frequency depends on B, γ
• Application: magnetic resonance techniques
• NMR/MRI (nuclear magnetic resonance)
• ESR (electron spin resonance)
• FMR (ferromagnetic resonance)
γ
Spin-orbit coupling
Nucleus
reference frame
Electron
reference frame
• Electron sees magnetic field due to nucleus “orbiting” around it
(accelerating charge → magnetic field)
Image: http://www.pha.jhu.edu/~rt19/hydro/img87.gif
Spin-orbit coupling
Er3+ energy level splitting
• Consequence of spin-orbit coupling:
spin-up and spin-down electrons have
different energy
• In condensed matter: some
interactions between electrons,
atoms and fields become (even
more) spin-dependent
iopscience.iop.org
Spin-orbit torque
• Magnetization vector precesses in external B field (just like spin)
• Damping: energy loss from scattering, etc.
• Effectively a torque on the magnetization
• Spin-orbit torque: a charge current in
a magnetic material can induce a torque precession
on the magnetization
B
damping
SOT
D. Ralph & M.D. Stiles, J. Magn. Magn. Mater. 7, 1190 (2008).
Spin-orbit torque
• Cause: broken symmetry in the material’s structure (example: NiMnSb)
• Fun fact: NiMnSb is a half-metallic ferromagnet
• Exact mechanism of spin-orbit
torque unknown
http://www.ieap.uni-kiel.de/solid/ag-press/r/ag/magnet.htm
http://www.tcd.ie/Physics/
Measuring spin-orbit torque: FMR
• Magnetic resonance: with applied magnetic field B, magnetic moments
precess at a particular frequency ∝ B
• Ferromagnetic resonance (FMR): precession of
magnetization M of sample due to magnetic field
D. Fang et al., Nat. Nanotechnol. 6, 413 (2011).
http://cronodon.com/images/magnetic_spin_precession.jpg
Measuring spin-orbit torque: FMR
• SOT is a torque on the precessing magnetization that can either act to
increase (blue arrow) or decrease (yellow arrow) the precession
amplitude → change in resistance
• We monitor the resistance by measuring the “DC” voltage across the
bar as the external field is changed and AC current is applied
IAC
Sample bar
Bias
tee
VDC
D. Fang et al., Nat. Nanotechnol. 6, 413 (2011).
My experiments
• Sending microwave-frequency (GHz)
current through a NiMnSb bar,
measuring voltage across the bar
4 μm
5 mm
Why study SOT?
“Why should this be funded by grants instead of bake sales?”
Image: sciencecareers.sciencemag.org/career_magazine/previous_issues/articles/2013_09_25/caredit.a1300209
Why study SOT?
• Don’t know the fundamental physics behind SOT!
• Potential advantages of SOT-based spintronic devices:
• Simple fabrication
• More efficient/requires lower current
• Better device endurance
• Can be made smaller than existing
MRAM technology
A. Brataas & K. M. D. Hals, Nat. Nanotechnol. 9, 86 (2014).
Image: sciencecareers.sciencemag.org/career_magazine/previous_issues/articles/2013_09_25/caredit.a1300209
Acknowledgments
• Microelectronics Group
• Supervisor: Dr. Andrew Ferguson
• Special thanks: Chiara Ciccarelli,
Vahe Tshitoyan
• James B. Reynolds Scholarship
• You! (Any questions?)
Magnetization/spin texture
• Magnetization (or spin) varies in space within material
• Manipulated by applied field, current; also temperature-dependent
• Basic example: domains in a ferromagnet
hyperphysics.phy-astr.gsu.edu/hbase/solids/ferro.htm
Skyrmions
• Topologically stable “knots” in
magnetization
• Occur singly and in lattices
• Origin: spin-orbit coupling and
exchange interaction in magnets with
broken inversion symmetry
C. Pfleiderer & A. Rosch, Nature 465, 880 (2010).
A. Fert, V. Cros, & J. Sampaio, Nat. Nanotechnol. 8, 152 (2013).
“So what?”
• Stability = good for storing information
• Skyrmion-based racetrack memory?
• “Building blocks” for more complex spin textures
• REALLY COOL!
C. Pfleiderer & A. Rosch, Nature 465, 880 (2010).
A. Fert, V. Cros, & J. Sampaio, Nat. Nanotechnol. 8, 152 (2013).
Spin Transfer Torque
• Spin-polarized current changes direction of
magnetization of a layer of magnetic material
“Free” layer
1. Electrons pass through layer with fixed
magnetization, become spin-polarized
2. Spin-polarized current travels through nonmagnetic buffer layer
3. Electrons pass through thinner (“free”)
magnetic layer
4. Angular momentum transfer (i.e. torque) from
electrons to free layer
Fixed layer
J. Sankey, Ph.D. thesis, 2007.
Applications of STT
• Spin valves, magnetic tunnel junctions, STT-MRAM (“available” now!)
• Advantages: lower current required than present MRAMs
• Problems: power consumption still high, requires spin-polarized current
en.wikipedia.org
www.pcworld.fr