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

• Quantum chaos

• Spintronics en.wikipedia.org/superconductivity; graphene.nus.edu.sg; computerhistory.org;

http://userweb.eng.gla.ac.uk/douglas.paul/SiGe/SET.html

~ 2 cm

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

• Less heat dissipation

• Higher information density

• Faster read/write speed

en.wikipedia.org

current

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)

≈

slideshare.net

N

S

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

• Electron sees magnetic field due to nucleus “orbiting” around it

(accelerating charge → magnetic field)

Nucleus

reference frame Electron

reference frame

Image: http://www.pha.jhu.edu/~rt19/hydro/img87.gif

Spin-orbit coupling

• 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

Er3+ energy level splitting

B

damping

SOT

precession

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

on the magnetization

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

http://cronodon.com/images/magnetic_spin_precession.jpg

D. Fang et al., Nat. Nanotechnol. 6, 413 (2011).

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

D. Fang et al., Nat. Nanotechnol. 6, 413 (2011).

IAC Sample bar

VDC

Bias

tee

My experiments

• Sending microwave-frequency (GHz)

current through a NiMnSb bar,

measuring voltage across the bar

5 mm

4 μm

Why study SOT?

Image: sciencecareers.sciencemag.org/career_magazine/previous_issues/articles/2013_09_25/caredit.a1300209

“Why should this be funded by grants instead of bake sales?”

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.html

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!

A. Fert, V. Cros, & J. Sampaio, Nat. Nanotechnol. 8, 152 (2013).

C. Pfleiderer & A. Rosch, Nature 465, 880 (2010).

Spin Transfer Torque

• Spin-polarized current changes direction of

magnetization of a layer of magnetic material

1. Electrons pass through layer with fixed

magnetization, become spin-polarized

2. Spin-polarized current travels through non-

magnetic buffer layer

3. Electrons pass through thinner (“free”)

magnetic layer

4. Angular momentum transfer (i.e. torque) from

electrons to free layer

J. Sankey, Ph.D. thesis, 2007.

Fixed layer

“Free” layer

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

www.pcworld.fr en.wikipedia.org