STRUCTURE AND MAGNETIC PROPERTIES OF ULTRA-THIN MAGNETIC LAYERS This work supported by ONR/DARPA N00014-02-01-0627 Justin M. Shaw Optical Sciences Center,

STRUCTURE AND MAGNETIC STRUCTURE AND MAGNETIC PROPERTIES OF ULTRA-THIN PROPERTIES OF ULTRA-THIN

MAGNETIC LAYERSMAGNETIC LAYERS

This work supported by ONR/DARPA N00014-02-01-0627

Justin M. ShawOptical Sciences Center, University of Arizona, Tucson

Department of Physics and Astronomy, Arizona State University, Tempe

Sungkyun ParkLos Alamos National Laboratory

Sukmock LeeDepartment of Physics, Inha University, Inchon, 402-751, Korea

Charles M. FalcoOptical Sciences Center, University of Arizona, Tucson

Outline of TalkOutline of Talk

• Introduction and Motivation;

• Sample Growth

•Annealed Fe layers on GaAs(001);

• Description of Spin-waves and Brillouin Light

Scattering (BLS);

• Results;

• Summary.

Introduction and MotivationIntroduction and Motivation

•Why Fe LAYERS ON GaAs?

• Spin injection has been demonstrated in Fe on GaAs;

• High quality BCC Fe layers can be easily grown on GaAs aFe/aGaAs = 1/2 (1.4% mismatch);

• These properties make Fe on GaAs(001) a candidate for FM metal-semiconductor contacts in spintronic devices;

Spintronics: What and Why?Spintronics: What and Why?

We want to exploit the spin degree of freedom in electrons.

Simple ‘Spin-Valve’spin injection “valve”

non-FMnon-FM FMFM

M M

non-FM

non-FMnon-FM FMFM

M M

non-FM

CurrentOut

CURRENT

NoCurrentOut

• Lower power;

• Higher speeds;

• Non-Volatile Memory;

• Quantum

Computing;

Spin is a quantum state.

Molecular Beam Epitaxy (MBE)

Growth Pbase= 4 x 10–11 TorrAnalysis Pbase= 3 x 10–11 Torr

Sample GrowthSample Growth

•SUBSTRATE PREPARATION;

•1 keV Ar+ at 600 °C;

•Streaky 4×6 surface;

•Fe DEPOSITION (15 Å);•Knudsen cell (0.088 Å/s);

•Tsubstrate = 30–35 °C;• ANNEALED (150–350 °C for 10

min.);

•OVERLAYER (Al );•E-beam evaporation;•Thickness = 20–90 Å.

GaAs(001)

15 Å Fe

50Å Al

Sample Structure

[110]

Al overlayer

We deposited all samples in a Perkin-Elmer 4-chamber MBE system with a base pressure of ~5 x 10–11 Torr

[110]

Spin-waves (Magnons)Spin-waves (Magnons)

• Spins (magnetic moments) oscillate slightly about their equilibrium position;

• Electron spins interact via exchange (short range) and dipole-dipole (long range);

Ferromagnetic crystals form quantized “spin-waves”.

From Kittel “Intro to Solid State Physics”

Inte

nsi

ty (

cou

nts

)

Spin Wave Frequency Shift (GHz)

Brillouin Light Scattering (BLS)Brillouin Light Scattering (BLS)

Annealing Temperature Dependence on Annealing Temperature Dependence on the Saturation Fieldthe Saturation Field

• Saturation fields are given by the minima in the spinwave frequencies along the hard axis;

• Saturation field changes behavior at Tanneal~ 225 °C.

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Sa

tura

tion

Fie

ld (

kOe

)

Annealing Temperature (°C)0 1 2 3 4 5 6

0

5

10

15

20

25

30

Sp

inw

ave

Fre

qu

en

cy (

GH

z)

Magnetic Field (kOe)

RT Fe Annealed 200°C

Location of minima

H=0 H Hsat

MspontaneosM M

Anisotropy Dependence of TAnisotropy Dependence of Tannealanneal

Magnetic Field = 3kOe

0 45 90 135 180 225 270 315 36015

20

25

Spi

nwav

eF

requ

ency

(G

Hz)

Azimuthal Angle (deg.)

300°C

275°C

250°C

225°C

200°C

150°C

RT0 45 90 135 180 225 270 315 360

15

20

25

Spi

nwav

eF

requ

ency

(G

Hz)

Azimuthal Angle (deg.)

0 45 90 135 180 225 270 315 36015

20

25

0 45 90 135 180 225 270 315 36015

20

25

Spi

nwav

eF

requ

ency

(G

Hz)

Azimuthal Angle (deg.)

300°C

275°C

250°C

225°C

200°C

150°C

RT

300°C

275°C

250°C

225°C

200°C

150°C

RT

• At RT the magnetic anisotropy is in-plane uniaxial;

• Annealing up to 225°C results in:

• decreasing anisotropy;

• mixed uniaxial and cubic form.

• Annealing above 250 °C results in:

• gradually increasing anisotropy;

• becoming uniaxial again at 300 °C;

• the onset of a second mode.

Anisotropy Dependence of TAnisotropy Dependence of Tannealanneal

Magnetic Field = 3kOe

0 45 90 135 180 225 270 315 36015

20

25

Spi

nwav

eF

requ

ency

(G

Hz)

Azimuthal Angle (deg.)

300°C

275°C

250°C

225°C

200°C

150°C

RT0 45 90 135 180 225 270 315 360

15

20

25

Spi

nwav

eF

requ

ency

(G

Hz)

Azimuthal Angle (deg.)

0 45 90 135 180 225 270 315 36015

20

25

0 45 90 135 180 225 270 315 36015

20

25

Spi

nwav

eF

requ

ency

(G

Hz)

Azimuthal Angle (deg.)

300°C

275°C

250°C

225°C

200°C

150°C

RT

300°C

275°C

250°C

225°C

200°C

150°C

RT

• At RT the magnetic anisotropy is in-plane uniaxial;

• Annealing up to 225°C results in:

• decreasing anisotropy;

• mixed uniaxial and cubic form.

• Annealing above 250 °C results in:

• gradually increasing anisotropy;

• becoming uniaxial again at 300 °C;

• the onset of a second mode.

Anisotropy Dependence of TAnisotropy Dependence of Tannealanneal

Magnetic Field = 3kOe

0 45 90 135 180 225 270 315 36015

20

25

Spi

nwav

eF

requ

ency

(G

Hz)

Azimuthal Angle (deg.)

300°C

275°C

250°C

225°C

200°C

150°C

RT0 45 90 135 180 225 270 315 360

15

20

25

Spi

nwav

eF

requ

ency

(G

Hz)

Azimuthal Angle (deg.)

0 45 90 135 180 225 270 315 36015

20

25

0 45 90 135 180 225 270 315 36015

20

25

Spi

nwav

eF

requ

ency

(G

Hz)

Azimuthal Angle (deg.)

300°C

275°C

250°C

225°C

200°C

150°C

RT

300°C

275°C

250°C

225°C

200°C

150°C

RT

• At RT the magnetic anisotropy is in-plane uniaxial;

• Annealing up to 225°C results in:

• decreasing anisotropy;

• mixed uniaxial and cubic form.

• Annealing above 250 °C results in:

• gradually increasing anisotropy;

• becoming uniaxial again at 300 °C;

• the onset of a second mode.

-20 0 20100

104

108

1012

1016

1020

Nor

mal

ized

Yie

ld

Frequency Shift (GHz)

90°

100°

110°

120°

135°

150°

160°

170°

180°

hard axis

easy axis0 45 90 135 180 225 270 315 360

15

20

25

Sp

inw

ave

Fre

qu

ency

(G

Hz)

Azimuthal Angle (deg.)

15 Å Fe Annealed 10 min. 275 °C

Additional Spinwave ModeAdditional Spinwave Mode• An additional spinwave mode develops for annealing

temperatures at or above 250 °C;

• This new mode is present only near a <110> direction (easy and hard axes).

Magneto-Optical Kerr Effect Magneto-Optical Kerr Effect (MOKE)(MOKE)

-400 -200 0 200 400

Ker

r E

llipt

icity

(ar

b. u

nit)

Magnetic Field (Oe)

RT

150 °C

200 °C

225 °C

250 °C

275 °C

300 °C

0 50 100 150 200 250 3000

50

100

150

200

250

300

Annealing Temperature (°C)

Co

erc

ivity

(O

e)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Sa

tura

tion

Fie

ld (kO

e)

• Coercivity increases rapidly for annealing temperatures above 225 °C.

Scanning Tunneling Microscopy (1 Scanning Tunneling Microscopy (1 μμm × 1 m × 1 μμm )m )

• Rectangular pits faceted along <110> directions form for annealing temperatures between 200 °C and 250 °C;

• X-ray Reflectometry 16% increase in Fe thickness for 300 °C anneal.

[110

]

[110

]

[110

]

[110

]

RT FeRMS roughness =0.890 ±0.02 nm

200 °C Anneal0.940 ±0.05 nm

250 °C Anneal0.883 ±0.04 nm

300 °C Anneal0.887 ±0.01 nm

(terraces: =0.484 ±0.07 nm)

400×400 nm

SummarySummary

•Ultra-thin ferromagnetic layers interfaced with semiconductors are critical to future Spintronic technology;

•Magnetic properties can be studied with high sensisitivity using BLS;

•The form and magnitude of the magnetic anisotropy is highly dependent on annealing temperature;

•Magnetic and structural changes occur at annealing temperatures at ~ 225 °C.