Ferromagnetism Inhomogenous magnetization Magnetic vortices Dynamics Spin transport

Magnetism on the Move

US-Spain Workshop on Nanomaterials

Ferromagnetism is rare……

B = 25 nm (<3 nm), W=150 nm, t = 14 nmdata rate ~ GHz

Direction of Disk Motion

Inductive Write Element

GMR Read Sensor

t

W

B

“Compass” that responds to local magnetic field and varies the resistance

…. but useful

Write Coil

Write Pole2

Read Head

<D> = 8.5 nm+/- 2.5 nm100 nm

Disk Head

Courtesy of Eric Fullerton

Recording Media

<D> = 8.5 nm+/- 2.5 nm100 nm

1000 nm

NSNR # grains/bitCourtesy of Eric Fullerton

Why is ferromagnetism neither common nor “perfect”?

Microscopic

Macroscopic

R. Schaefer, Dresden

Magnetostatics (Not as bad as it looks)

Magnetostatics: equilibrium condition

Variational method to find the equilibrium condition

, where

Torque

= 0

W. F. Brown, Jr., Micromagnetics (Interscience Publishers, New York, 1963)

Micromagnetics Simulation

Excitations

[Equilibrium State] [Excited State] [Dynamic motion]

Landau-Lifshitz-Gilbert Equation

Bg

= 17.6 GHz/kOe

Spin waves

q

Uniform precession (q = 0 spin wave)

Spin Waves

ij

nn

jijiexch JSSJE cos2

2Aq

The simple case (no magnetocrystalline anisotropy)

Magnetic vortex

Of course there are intermediate cases - such as the S-state

K.L. Metlov et. al., J. Magn. Magn. Mater. 242-245 (2002) 1015

LE = exchange length = 22/ sMA

Four different configurations of the vortex state

Schematic illustration of four different vortex states

P= Polarity (the magnetization direction of the vortex core)

C= Chirality (the winding direction of in-plane magnetization)

The magnetostatic energies are obviously identical….

Magnetic vortices

1) Lorentz Microscopy on 200 nm Co disk

2) MFM on 1 m Permalloy disk

3) SP-STM on 200 nm wideand 500 nm long Fe island

Observation of magnetic vortices

1)J. Raabe et al. J. Appl. Phys. 88, 4437 (2000)

2)T.Shinjo et al. Science 289, 930 (2000)

3)A. Wachowiak et al. Science 298, 577 (2002)

What about the dynamics?

Vortex-core dynamics (gyrotropic motion)

A.A. Thiele, Phys. Rev. Lett. 30, 230 (1973).See also B. Argyle et al., Phys. Rev. Lett. 53, 190 (1984).

Gyromagnetic force acting on a shifted vortex

where = static force for an applied field H = gyrovector (antiparallel to the direction of vortex polarity P) = magnetic energy dissipation dyadic

W

G

D�

01

dt

MdM

MHM

dt

Md

seff

Landau-Lifshitz-Gilbert Equation:

Equivalent force equation

0)()(

)(

))((

dt

tXdD

dt

tXdG

tX

tXW

�

When P changes sign, changes sign!G

Gyrotropic Mode

16

The lowest frequency excitation: Gyrotropic mode

1 s 1 ns

[Will be replaced with a movie:Gyrotropic motion in simulation]

10-14 sec(chemical reaction

dynamics)

10-12 sec(semiconductors)

10-9 sec(magnetism)

10-7 sec

Time Scales

How do you make a movieon picosecond time scales?

Time-resolved Kerr microscopy (stroboscopic)

[Freeman et al. J. Appl. Phys. 79, 5898 (1996)]

What we measure:

Polar Kerr Rotation Mz

as a function of time delay, probe-beam position, and applied field

Also Back, Hicken, and others.....This is a stroboscopic technique.

Experimental Setup

20

Pinning potential

Different equilibrium positions

Not at a pinning site

At a pinning site

Excitation off

Large Amplitude: Core SwitchingLarge Amplitude: Core Switching

21

Counterclockwise orbit

Clockwise orbit

B. Van Waeyenberge et al., Nature 444, 461 (2006)1 s 0.5 ns

22

Core reversal

Phase Diagram of Vortex DynamicsPhase Diagram of Vortex Dynamics

23

Pinned & Depinned

Magnetic Heterostructures

New Technologies:

• Magnetic Random Access Memory• Magnetic tunnel junction sensors• Patterned media• Semiconductor spintronics • Highly polarizable materials

Diskdrives

MagneticRandom AccessMemory

Field sensing(medical devices,security)

• The electrical response of the device depends on the magnetic state of two or moreelectrodes (field sensors, read heads)

• The magnetic state of the device can be changedby an electrical current (memory, oscillators)

F F

Integration of ferromagnets with insulators, semiconductors, and normal metals

Example: the spin valve

Magnetic Heterostructures

Read Head Technology

Pole2

Pole1

Gap

Read

WriteShield2

Shield1

Scale: 50 nm

MRLeads Leads

Compound PtMn

Free Layer

Pinned Layer

Cu

Magnetic Tunnel Junctions

FM 1

FM 2

Insulator

Spin transfer torque oscillators

• MgO-based tunnel junction devices for maximizing signal and reducing threshold current

• Built-in hard-axis polarizer enhancesoutput power and allows for zero-fieldoperation

• Influence of CoFeB on damping (withData Storage Institute, Singapore)

• Modification of CoFeB/MgO interfaceanisotropy (with DSI)

• Spin transfer torque FMR (with DSI)

J. Appl. Phys. 109, 07D307 (2011)J. Appl. Phys. 109, 07C714 (2011)Appl. Phys. Lett. (accepted, 2012)Wang, Crowell

Materials science of magnetic heterostructures

Co2MnGe

GaAs

TEM

Electronic StructureCalculations

Growth and characterization

Interfacial characterization

Transport

Simulations

Spin dynamics

Epitaxial Fe/InxGa1-xAs heterostructures

• Epitaxial structures: low temperaturegrowth to minimize interfacial reactions

• Transport and modeling techniques developed by the IRG

• Increase spin-orbit coupling by shiftingto InxGa1-xAs

Palmstrøm, Crowell

Summary

• Magnetism is ubiquitous, although ferromagnetism is relatively rare

• Ferromagnetism is useful if not always easy to understand

• Imperfect magnets are more interesting than perfect ones

• Dynamics are accessible by new tools

• Integration of ferromagnets with other materials yields new physicsand new devices