Onset of non-colinear coupling in Fe/Cr trilayers

observed at high temperatures

S.M. Van Eek

D. Aernout

B. Croonenborghs

J. Meersschaut

M. Rots

Instituut voor Kern- en StralingsfysicaKatholieke Universiteit Leuven,

Belgium

Special position of Fe/Cr due to the peculiar antiferromagnetism of Cr.

Why Fe/Cr thin films? :

•Oscillatory exchange coupling between Fe layers

•Giant Magnetoresistance effect (GMR)

•Non-collinear coupling between Fe layers

QS Q

S

Bulk Cr is antiferromagnetic. The moments form a periodical variation (spin density wave, SDW) with a wavelength of ~ 20 lattice constants below Néel temperature, TN=311K.

Dr.

Ste

lla

Mar

is V

an E

ek

-0.02 -0.01 0.00 0.01 0.02

-0.8

-0.4

0.0

0.4

0.8

magnetic field strength [T]

M/M

sat

Msat

MR

M

Hsaturation

Also known as biquadratic BQ coupling, first observed by Rührig et al., it’s origin is still being

debated. (Phys. Stat. Sol. A 125 (1991) 635).

We focus on the non-collinear coupling

Fe

Fe

Cr

BQ coupling and Cr-Néel temperature

* From magnetization vs. resistivity measurements,

Fullerton et al. showed the suppression of BQ coupling in Fe/Cr(001) Superlattices below the Néel transition of Cr.Fullerton et al. Phys. Rev. Let. 75,2,1995.

TN (K)

(resistivity)

200

Coupling(magnetization)

BQ coupling

No BQ coupling

0

Wide range of TN for thin Cr films

Bulk Cr-TN = 311 K

70 K<TN<295 K

AFM Cr order increase upon reduction of Cr-thickness. Proximity effect.

Schmitte, Schreyer et al. Europhys. Lett. 48,(6), 692 (1999)

Fullerton et al. Phys. Rev. Let. 75,2,1995.

Scaling law

300 K<TN

AFM Cr order well above room temperature

Pierce, Unguris, Celotta J.M.M.M. 200 (1-3), 290 (1999).

TN at 200 K

Meersschaut et al. Phys. Rev. Lett. 87, 107201 (2001)

TN ~ 500 K for Ag/Cr

Demuynck et al. Phys. Rev. Lett. 81, 12, 2562 (1998)

What we want to do:

understand why this different TN …

•Grow trilayers with same thickness but on different conditions

•Magnetization measurement

•Cr magnetic state measurement

MBE

VSM

PAC

?

epitaxially grown Fe/Cr/Fe trilayerson MgO(001).

Growing temperature:

•TG=RT•TG= 450K•TG= 575K

Samples grown

MgO(100)/Fe(40 Å)/Cr(80Å)/Fe(40Å)/Au(40Å)

BQ coupling

No BQ coupling

0 50 100 150 200 250 300 350 4000,4

0,5

0,6

0,7

0,8

0,9

1,0

1,1

ToTo

rem

anen

ce m

agne

tiza

tion

T (K)

TG = 450 K TG = RT TG = RT

T=270 KTG=450 K

Results 1: VSM

The TN (onset temperature of BQ coupling) can be modified changing the growth temperature of trilayers.

Remanence magnetization

TN=150 K

TN=380 K

Au

Fe

Fe

Cr

MgO

R(t)= a0 + a1 cos(wt)a2 cos(2 wt)

Introduction to perturbed angular correlations (PAC) technique

in case of ferromagnetic or antiferromagnetic order

R(t)= a0 + a1 J0(wt)a2 J0(2 wt) in case of spin density wave order

Being J0 the 0th order Bessel function

0 5 10 15 20

-0 .3

0 .0

0 .3

0 .6

0 .9

Jo(x)

X

111In was implanted at 60keV.

a0 , a1 ,a2 depend on the orientation of the magnetic field at the radioactive nucleus place.

w proportional to the magnetic field at the radioactive nucleus place.

0 60 120 180

-0.04

-0.02

R(t)

t[ns]

G22 Bessel Fe irrdamage wq

PAC at 140 K on sample TG= 575 K

SDW

Cr spins direction in the sample plane

Results 2: PAC

0 30 60 90

-0.06

-0.03

0.00

h orizontal geo metry 77KTG=R T

R(t)

t[ns]30 60 90

-0.08

-0.06

-0.04

perpendicular geo metry 77KTG=R T

R(t)

t[ns]

PAC at T=77 K on sample TG=RT

SDW

Cr spins out of plane

-10

-5

0

-10

-5

0

(c)

298 K

0 100-10

-5

0

(d)

t(ns)

400 K

R(t

) *

10-2

(b)

77 K

(e)

70 K

(f) 288 K

-8 0 8

(g)

(a)

H(T)* 10-3

430 K

Onset of BQ at high temperatures for TG=RT sample.

TN~380 K

Think-schema

BQCoupling

Different Cr spins direction

•Stress in Cr layers?

•Interface roughness?

Different onset temperature

Because of

Possible causes for the modification of Cr magn.

0 50 100 150 200 250 300 350 4000,4

0,5

0,6

0,7

0,8

0,9

1,0

1,1

ToTo

rem

anen

ce m

agne

tiza

tion

T (K)

TG = 450 K TG = RT TG = RT

T=270 KTG=450 K

53 54 55 56 57

[111]

TG = RT TG = 450 K TG = 575 K

(°)

Results 4 : RBS

[001][111]

RBS-channelling data show that the sample TG=RT presents more strain.

Strain

Results 5: Reflectivity

X-Ray reflectivity data show that the sample TG=RT has rougher interface.

Roughness

1 2 3 4 5 61E-6

1E-5

1E-4

1E-3

0,01

0,1

1 TG = RT TG = 450 K TG = 575 K

Log

(I)

(°)

ConclusionsMagnetic state of Cr in Fe/Cr-trilayers measured with PAC technique.

Lowering the growth temperature, the TN increased ~200 K.

The enhancement of the TN is seen together with the Cr spins having orientation out of the plane of the sample.

Strain and roughness are higher for samples grown at low temperature.

Instituut voor Kern- en StralingsfysicaKatholieke Universiteit Leuven,

Belgium

Thanks

T. Slęzak, D.Wilgocka, H. Guérault, S. Cottenier (IKS-BE)

H. Bender (IMEC-BE)

P. Eversheim, C. Noll and S. Hinderlich (ISKP-DE)

Instituut voor Kern- en StralingsfysicaKatholieke Universiteit Leuven,

Belgium