Exchange Bias: Interface vs. Bulk Magnetism

Miyeon Cheon Hongtao Shi

Zhongyuan Liu Jorge EspinosaDavid Lederman

Elke Arenholz

Department of Physics

Hendrik OhldagJoachim Stöhr

Optical and Vibrational Spectroscopies Symposium:A Tribute to Manuel Cardona

August 20, 2010

-1.0 -0.5 0.0 0.5 1.0-6

-4

-2

0

2

4

6

m (

10-4em

u)

H (kOe)

HC

MR

Exchange Bias

FM

AF

MR: “Remanent” magnetization- Maximum value of M- Depends on FM

HC: Coercivity- Depends on FM magnetic anisotropy- Represents energy required to reverse magnetic domain

HE: Exchange Bias-Absent in pure FM, results from AF-FM interaction

HE

Application: Magnetic Tunnel Junction /GMR

SensorsFerromagnetic layers~1.0-5.0 nm thick

Insulator/NM Metal ~1.0-2.5 nm

Antiferromagnet~10 - 50 nm

1 - 100 m

cosRRR o

Free magnetic layer (analyzes electron spin)

Pinned magnetic layer (polarizes tunneling electrons)

Pinning Antiferromagnet

Albert Fert & Peter Grünberg2007 Nobel Prize in Physics

“for the discovery of Giant Magnetoresistance”

(www.research.ibm.com)

How does the pinning of bottom FM layer work?

Key Questions• Given that:

– All EB models require presence of uncompensated magnetization in the antiferromagnet (interface)

– Details of EB behavior (e.g. temperature dependence, magnitude) depend strongly on AF anisotropy (bulk)

• Some key questions are:– Can uncompensated moments in the AF be detected?

– Can the effects of uncompensated moments in the AF be studied systematically?

– Can the magnetic anisotropy be studied systematically?

MF2 Antiferromagnets

FeF2• Rutile structure (a = 0.4704 nm, c = 0.3306 nm) • Antiferromagnetic, TN=78 K• Magnetization along the c-axis

[001]

NiF2 • Rutile structure (a = 0.4651 nm, c = 0.3084 nm)• Antiferromagnetic, TN= 73 K• Weak ferromagnetic • Magnetization lies in the a-b plane

[001]

ZnF2• Rutile structure (a = 0.4711 nm, c = 0.3132 nm) • non-magnetic

weak anisotropy antiferromagnet

[001]

dilute antiferromagnet

So… where does Manuel Cardona fit in?

Naïve graduate student asks: can antiferromagnetic superlattice magnons be observed?

Two-magnon Raman line for 1.3 m FeF2 thin film

• MBE co-deposition of FeF2 (e-beam) and ZnF2, NiF2 (K-cell), Pbase = 7 x 10-10 Torr, Pgrowth < 4 x 10-8 Torr

• TS (AF) = 297 0C, poly-Co @125 0C, poly-MgF2 @RT

• Growth along (110)• Twin sample holder – simultaneous growth of

underlayer, different overlayers• In-situ RHEED, AFM• X-ray diffraction and reflectivity

• Cooling field (HCF = 2 kOe) in the film

plane along the c-axis of FexZn1-xF2

• M vs H via SQUID magnetometer,

horizontal sample rotator

Growth and Characterization

Key Questions

• Can uncompensated moments in the AF be detected?

• Can the effects of uncompensated moments in the AF be studied systematically?

• Can the magnetic anisotropy be studied systematically?

e-e-

Magnetic Dichroism in X-ray Absorption

700 710 720 730

0

2

4

6

Ele

ctro

n Y

ield

(a.

u.)

Photon Energy (eV)

e-e-

Antiferromagnetic Domains

876 879 882

100

200

300

Ele

ctr

on

Yie

ld (

a.u

.)

Photon Energy (eV)

X-ray magnetic circular dichroism

sensitive to FM order.

Fe L3, L2

NiO L2a, L2b

X-ray magnetic linear dichroism

sensitive to AF order.

Element specific technique sensitive to antiferromagnetic as well as ferromagnetic order.

Antiferromagnetic Order of FeF2(110)

Stronger XMLD signal for Co/FeF2(110) compared to bare FeF2(110) indicates an increase in antiferromagnetic order caused by exchange to the FM Co layer.

718 721 724

0

1

2

3Co/FeF

2(110)

E || [001] E || [110]

bare FeF2(110)

E || [001] E || [110]

Ele

ctro

n Y

ield

[a.u

.]

Photon Energy [eV]

Fe Fe

Fe FeFe

F F

F F

Einc || [001]

Einc || [110]

FeF2 L2 absorption edge

Room T: “Free” uncompensated moments follow FM

Low T: Additional “pinned” uncompensated moments antiparallel to easy direction.

Measy

Mpinned

Ferromagnet

Interface-3 -2 -1 0 1 2 3

-10

0

10

Applied Field [kOe]

Co

XM

CD

[%]

-0.5

0.0

0.5

Fe

XM

CD

[%]

-10

0

10

-0.1

0.0

0.1

RT

15K

Interface Coupling and Exchange Bias

MgF2(110) sub.

68 nm FeF2

2.5 nm Co

2 nm Pd cap

Results

0 40 80 120 300

0

200

400

Co XMCD HC

Co XMCD HE

Fie

ld [

Oe]

Temperature [K]

0.0

0.5

1.0

Fe XMCD Fe M XMCD

XM

CD

[%

]

0.0

0.5

1.0 Fe XMLD

XM

LD [

arb.

u.]

• Fe in FeF2/Co interface, despite being non-metallic, has– Unpinned magnetization to RT

– Pinned magnetization to TB

– AF order verified to TN via XMLD

• Co at interface– TB~TN

– HC peak near TB

Ohldag et al., PRL 96, 027203 (2006)

0 20 40 60 80 100 120 300

0.0

0.5

1.0

Temperature [K]

XM

CD

[%

]

0.0

0.5

1.0

XM

LD [

a.u.

]

1.) XMLD and long range AF order vanish at TN.

2.) XMCD is indication of

interfacial magnetic order at RT.

Parallel Interface Coupling and Exchange Bias

Related to enhancement of coercivity for T >> TN

(Grimsditch et al, PRL 2003)

Also, see Roy et al, PRL 2006

Key Questions

• Can uncompensated moments in the AF be detected?– Uncompensated moments exist in AF, not due to “metallization”– Pinned uncompensated moments in AF vanish near TN

– Unpinned uncompensated moments exist up to RT, well above TN

• Can the effects of uncompensated moments in the AF be studied systematically?

• Can the magnetic anisotropy be studied systematically?

Systems

[001][001]

FexNi1-xF2FexZn1-xF2

Dilute antiferromagnet Random anisotropy antiferromagnet

Systematic study of uncompensated M

Effects of Dilution

• Domain state model: dilute AF should make small domain creation easier due to nonmagnetic impurities (Malozemoff model)

• Net magnetization of AF domains should increase effective interface interaction

P. Miltényi, et al., Phys. Rev. Lett., 84, 4224 (2000)

Co1-xMgxO/ CoO (0.4 nm) /Co

Previous Results

Sample Profile

5 nm MgF2 Cap

(110)-MgF2 Sub

5 nm MgF2 Cap

(110)-MgF2 Sub

1.0 nm FeF2

Magnetic interface changes with x in FexZn1-xF2

65 nm (110)FexZn1-xF2 (AF) 65 nm (110)

FexZn1-xF2 (AF)

18 nm Cobalt (F) 18 nm Cobalt (F)

Pure interfacelayer (PIL)

HE, HC Dependence on T

PIL affects HE, HC; no effect on TB

-400

-300

-200

-100

0

0 10 20 30 40

0

100

200

300

400

T (K)

HC (

Oe)

x = 0.34H

CF = 2 kOe

TB

With PIL Without PIL

HE (

Oe)

-500

-400

-300

-200

-100

0

0 10 20 30 40 50 60 70 80 900

100

200

300

T (K)

HC (

Oe)

x = 0.57H

CF = 2 kOe

TB

Without PIL With PIL

HE (

Oe)

HE, HC vs. Temperature for x = 0.75

• HE changes sign as T increases to TB.• HC has two peaks corresponding to HE = 0.• Therefore AF ground state is not unique

TB vs. x in FexZn1-xF2

TB agrees reasonably well with bulk TN data

Interface Energy Dependence on x

• No large HE enhancement observed• Small AF domains not formed at large x ?

ΔE = -tCo*HE*MS

T = 5K

Net AF Magnetization

M

Key Questions

• Can uncompensated moments in the AF be detected?– Uncompensated moments exist in AF, not due to “metallization”– Pinned uncompensated moments in AF vanish near TN

– Unpinned uncompensated moments exist up to RT, well above TN

• Can the effects of uncompensated moments in the AF be studied systematically?– Uncompensated M does not necessarily lead to HE enhancement;

critical concentration of impurities must be achieved– However, uncompensated M dependent on defect concentration

• Can the magnetic anisotropy be studied systematically?

Systems

[001][001]

FexNi1-xF2FexZn1-xF2

Dilute antiferromagnet Random anisotropy antiferromagnet

Systematic study of AF anisotropy

Magnetic Order

FeF2 Rutile structure (a = 0.4704 nm, c = 0.3306 nm) Antiferromagnet, TN=78 K Magnetization along the c-axis

NiF2 Rutile structure (a = 0.4651 nm, c = 0.3084 nm) Antiferromagnetic, TN= 73 K (80 K in films) Weak ferromagnet Magnetization lies in the a-b plane

[001]

[001]

Growth and measurements

magnetic anisotropy changes with x.

MgF2(110) sub.

50 nm FexNi(1-x)F2

18 nm Co5 nm Al,Pd cap

MBE Growth MgF2 (110) substrate Growth temperature 210 ˚C Fe concentration: 0.0, 0.05, 0.21, 0.49, 0.55 1.0

[001]

x=1.0

[001]

x=0.0

Expectations

For nearest neighbor interactions

cos)1(cos)1(cos 2222FeNiFeNiNiNiNiFeFeFe SSxzxJSxzJSzxJE

)(cos)1(cos 2222 NiNiFeFe SxDxSD

For small , there is a critical Fe concentration xc beyond which spins will lie along the c-axis:

22

2

FeFeNiNi

NiNic SDSD

SDx

For FeF2 and NiF2 xc = 0.14

[001]

FexNi1-xF2

NiF2/CoFeF2/Co

• No exchange bias along c-axis • Exchange bias along c-axis • TB ~ 81 K

0 30 60 90 120 150

-400

-300

-200

-100

0

HE (

Oe

)

T (K)

-2 -1 0 1 2-3

-2

-1

0

1

2

3

m (

10-4

em

u)

H (kOe)

5 K

H┴ c

H || c

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-1.0

-0.5

0.0

0.5

1.0

T = 5 K T = 90 K

HCF

= 2 kOe _H, H

CF || NiF

2 [110]

MR/M

S

H (kOe)

49 nm NiF2 / 16 nm Co

H. Shi et al., Phys. Rev. B 69, 214416 (2004).

Fe0.05Ni0.95F2/Co

For T ≤ 45 K• Negative exchange bias along the c-axis• Asymmetric saturation magnetization

For 50 K ≤ T ≤ 70 K• No exchange bias• Wide hysteresis loop

For 75 K ≤ T• No exchange bias

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-6

-4

-2

0

2

4

6 5 K 20 K 35 K

m(1

0-4em

u)

H(kOe)-10 -8 -6 -4 -2 0 2 4 6 8 10

-6

-4

-2

0

2

4

6 50 K 80 K

m(1

0-4em

u)H(kOe)

-10 -5 0 5 10-6

-4

-2

0

2

4

6

m(1

0-4

emu)

H (kOe)

50 K 55 K 60 K 65 K 70 K

Large coercivity loops of Fe0.05Ni0.95F2/Co

50 55 60 65 70

-10

-5

0

5

10

H(k

Oe

)

T(K)

-H' +H'

• For 50 K ≤ T ≤ 70 K, large coercivity loops appear for the scanning field range -10 kOe to 10 kOe.

• Negative exchange bias (HE ~ -500 Oe) for T = 50 K and 55 K

Fe0.21Ni0.79F2/Co

• Similar behavior to Fe0.05Ni0.95F2/Co• Negative HE along the c-axis at T≤ 40 K• Asymmetric saturation magnetization

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-6

-4

-2

0

2

4

6 5 K 20 K 35 K

m(1

0-4em

u)

H(kOe)For 45 K ≤ T ≤ 70 K

• No exchange bias effect• Wide hysteresis loop

For 75 K ≤ T• HE = 0

-10 -8 -6 -4 -2 0 2 4 6 8 10

-6

-4

-2

0

2

4

6 50 K 75 K

m(1

0-4em

u)H(kOe)

Large HC loops of Fe0.21Ni0.49F2/Co

-10 -5 0 5 10

-6

-4

-2

0

2

4

6 40 K 50 K 60 K 70 K

m(1

0-4e

mu

)

H(kOe)

40 45 50 55 60 65 70 75-10

-8

-6

-4

-2

0

2

4

6

8

10

H (

kOe)

T (K)

-H' +H' H

E

• For 40 K ≤ T ≤ 70 K, large HC loops appear for the scanning field range ±10 kOe • Negative exchange bias effect (HE ~ - 1000 Oe) for 40 K ≤ T ≤55 K

Fe0.49Ni0.51F2/Co

For T ≤ 15 K• Negative exchange bias • Asymmetric saturation magnetization

For 50 K ≤ T ≤ 65 K• No exchange bias • Wide hysteresis loop

For 70 K ≤ T• No exchange bias

For 25 K ≤ T ≤ 50 K• Positive exchange bias • Asymmetric saturation magnetization

-10 -8 -6 -4 -2 0 2 4 6 8 10

-6

-4

-2

0

2

4

6 55 K 75 K

m(1

0-4em

u)H(kOe)

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-6

-4

-2

0

2

4

6 5 K 10 K 30 K 35 K

m(1

0-4em

u)

H(kOe)

Large HC loops of Fe0.49Ni0.51F2/Co

-60 -40 -20 0 20 40 60

-6

-4

-2

0

2

4

6 5 K 10 K 15 K 20 K

m(1

0-4em

u)

H(kOe)

• For 5 K ≤ T ≤ 55 K, large HC loops appear for H=± 70 kOe • Positive exchange bias effect with HE ≥10 kOe

• For 55 K ≤ T ≤ 70 K, large HC loops appear for H = ±10 kOe

0 10 20 30 40 50 60 70

-40

-20

0

20

40

60

+H' from 10 kOe

H (

kOe)

T(K)

-H' +H' HE

Is it Possible to Control the Sign of HE? Magnetization measurement Exchange bias studies after field cooling with 2000 Oe from 95 K with SQUID Measurement direction: c-axis Measurement sequence: 70 kOe → -70 kOe → 70 kOe, ( ) 70 kOe → -20 kOe → 70 kOe, ( ) -70 kOe, 20 kOe → -70 kOe → 20 kOe ( )

H

M

-70 kOe -20 kOe 20 kOe 70 kOe

Fe0.49Ni0.51F2/Co•Tunable exchange bias (reversal of wide hysteresis loop)

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

-6

-4

-2

0

2

4

6 70 kOe, -70 kOe 70 kOe, -20 kOe -60 kOe, 20 kOe

m

(10

-4em

u)

H(kOe)

5 K

Reversible Exchange Bias• MCo favors parallel exchange coupling with Muncompensated

MCo

Muncompensated

M. Cheon, Z. Liu, and D. Lederman, Appl. Phys. Lett. 90, 012511 (2007)

Consistent with micromagnetic modeling

-1.0

-0.5

0.0

0.5

1.0

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-1.0

-0.5

0.0

0.5

1.0

-60 -40 -20 0 20 40 60

-1.0

-0.5

0.0

0.5

1.0

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-1.0

-0.5

0.0

0.5

1.0

(a)

M/M

S

H (kOe)

(b)

M/M

s

H (kOe)

M/M

s

H (kOe)

Summary for FexNi(1-x)F2/Co bilayers

Note sign change of HE correlated with M(same as in FeZnF2 samples)

Note low TB

TN

0.05

0.10

0.15

0.20

0 20 40 60 80 100

-400

-300

-200

-100

0

T(K)

HE(O

e)

M

s/M

s

0.00 0.05 0.21 1.00

-0.10

-0.05

0.00

0.05

0.10

0 20 40 60 80 100-400

-200

0

200

400

0.49HE(O

e)

M

s/M

sT(K)

What about FeZnF2? Can HE be Reversed at Low T?

Fe0.36Zn0.64F2/Co

Fe0.05Ni0.95F2/Co

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-4

-2

0

2

4

m(1

0-4

em

u)

H(kOe)

30 K

Fe0.21Ni0.79F2/Co

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-8

-6

-4

-2

0

2

4

6

8

m(1

0-4

em

u)

H(kOe)

30 K

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-8

-6

-4

-2

0

2

4

6

8

m(1

0-4

em

u)

H(kOe)

20 K-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-6

-4

-2

0

2

4

6

m(1

0-4

emu)

H(kOe)

20 K

1 nm FeF2

no effect at 5K

Key Questions• Can uncompensated moments in the AF be detected?

– Uncompensated moments exist in AF, not due to “metallization”– Pinned uncompensated moments in AF vanish near TN

– Unpinned uncompensated moments exist up to RT, well above TN

• Can the effects of uncompensated moments in the AF be studied systematically?– Uncompensated M does not necessarily lead to HE enhancement;

critical concentration of impurities must be achieved– However, uncompensated M dependent on defect concentration

• Can the magnetic anisotropy be studied systematically?– Low magnetic anisotropy leads to reversible HE, in addition to low TB,

as a result of reversal of “pinned” uncompensated M in the AF– Low TB ≠ low TN

– Reversible HE requires uncompensated M in the AF– Dilute AF system can also be reversed, but only at higher temperatures

due to coupling of H to uncompensated magnetization

Remaining Questions• How universal is the effect of uncompensated

moments in the AF?– Can it explain, e.g., low TB , in other AFs?– Is it possible to engineer desirable interface exchange

properties by manipulating AF anisotropy?

• What is the size of the AF domains? And does their size really matter?– If they don’t matter, what is the coupling mechanism and

where does the uncompensated magnetization come from?• Strain (piezomagnetism)?• Defects?

– Update: surprisingly, domain size does not seem to matter much – see Fitzsimmons et al., PRB 77, 22406 (2008).

Group

Areas of Interest

-60 -40 -20 0 20 40 60

-6

-4

-2

0

2

4

6 5 K 10 K 15 K 20 K

m(1

0-4em

u)

H(kOe)MgF2(110) sub.

50 nm FexNi(1-x)F2

18 nm Co

5 nm Al,Pd cap

Biomolecular Electronics

Magnetic Nanostructures and Interfaces

Hybrid Multifunctional Heterostructures

Exchange bias GMR in anisotropic structures Self-assembly and surface dynamics

Myoglobin Single Electron Transistor

YMnO3/GaN 3210-1-2

-140

-120

-100

-80

-60

-40

-20

0

20

40

T~5.7K-5.8KMyoglobin

Bia

s V

olta

ge (

mV

)

Gate Voltage (V)

T = 565 °C

Areas of Interest

-60 -40 -20 0 20 40 60

-6

-4

-2

0

2

4

6 5 K 10 K 15 K 20 K

m(1

0-4em

u)

H(kOe)MgF2(110) sub.

50 nm FexNi(1-x)F2

18 nm Co

5 nm Al,Pd cap

Biomolecular Electronics

Magnetic Nanostructures and Interfaces

Hybrid Multifunctional Heterostructures

Exchange bias GMR in anisotropic structures Self-assembly and surface dynamics

Myoglobin Single Electron Transistor

YMnO3/GaN 3210-1-2

-140

-120

-100

-80

-60

-40

-20

0

20

40

T~5.7K-5.8KMyoglobin

Bia

s V

olta

ge (

mV

)

Gate Voltage (V)

T = 565 °C

Uncompensated M, x=0.75

Sign change of HE due to reversal of AF structure

H. Shi and D. Lederman, Phys. Rev. B 66, 094426 (2002)

Measurement Procedure

F

AFJint

HCF

Jint

H

1. Cool in HCF from above T = TN

2. Measure M vs. H at T < TN

-1.0 -0.5 0.0 0.5 1.0-6

-4

-2

0

2

4

6

m (

10-4em

u)

H (kOe)

FjAi SSJE ,,intintConventional view:

Interface exchange interaction sets low T antiferromagnet configuration

Direct Exchange Mechanism• Direct exchange mechanism

(Meiklejohn and Bean, 1956) predicts– a) wrong magnitude (~100 times too large)

– b) no exchange bias in compensated or disordered surfaces

F

AF

Ideal Uncompensated Compensated Roughness

FFE tMaJH 2int / HE = 0

Jint

Random Exchange at Interface

• Due to interface roughness, defects, etc.• Antiferromagnetic domains created with local exchange

satisfied during cooling

FFE taMLJH /2 int

L = domain size in AF

Malozemoff, 1987

AF Domain Wall Formation• AF or F domain walls created during

cool-down procedure

Jint

H H

FFE tMaAKH 2/2

Malozemoff, 1987; Mauri et al. 1987

Correct order of magnitudeaJA FAF /,Exchange stiffness

Magnetic anisotropy energy KLattice parameter a