onset of non-colinear coupling in fe/cr trilayers observed at high temperatures s.m. van eek
DESCRIPTION
Instituut voor Kern- en Stralingsfysica Katholieke Universiteit Leuven, Belgium. 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. S. S. Q. Q. Why Fe/Cr thin films? :. - PowerPoint PPT PresentationTRANSCRIPT
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