1. 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 Stralingsfysica Katholieke Universiteit Leuven, Belgium

2. 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 Q S 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, T N =311K.

3. Dr. Stella Maris Van Eek M sat MRMR M H saturation 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 Cr

4. 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. T N (K) (resistivity) 200 Coupling (magnetization) BQ coupling No BQ coupling 0

5. Wide range of T N for thin Cr films Bulk Cr-T N = 311 K 70 K<T N <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<T N AFM Cr order well above room temperature Pierce, Unguris, Celotta J.M.M.M. 200 (1-3), 290 (1999). T N at 200 K Meersschaut et al. Phys. Rev. Lett. 87, 107201 (2001) T N ~ 500 K for Ag/Cr Demuynck et al. Phys. Rev. Lett. 81, 12, 2562 (1998)

6. What we want to do: understand why this different T N … Grow trilayers with same thickness but on different conditions Magnetization measurement Cr magnetic state measurement MBE VSM PAC ?

7. epitaxially grown Fe/Cr/Fe trilayers on MgO(001). Growing temperature: TG=RT TG= 450K TG= 575K Samples grown MgO(100)/Fe(40 Å)/Cr(80Å)/Fe(40Å)/Au(40Å)

8. BQ coupling No BQ coupling Results 1: VSM The T N (onset temperature of BQ coupling) can be modified changing the growth temperature of trilayers. Remanence magnetization T N =150 KT N =380 K

9. Au Fe Cr MgO R(t)= a 0 + a 1 cos(wt)  a 2 cos(2 wt) Introduction to perturbed angular correlations (PAC) technique in case of ferromagnetic or antiferromagnetic order R(t)= a 0 + a 1 J 0 (wt)  a 2 J 0 (2 wt) in case of spin density wave order Being J 0 the 0 th order Bessel function 111 In was implanted at 60keV. a 0, a 1,  a 2 depend on the orientation of the magnetic field at the radioactive nucleus place. w proportional to the magnetic field at the radioactive nucleus place.

10. PAC at 140 K on sample TG= 575 K SDW Cr spins direction in the sample plane Results 2: PAC PAC at T=77 K on sample TG=RT SDW Cr spins out of plane

11. Onset of BQ at high temperatures for TG=RT sample. T N ~380 K

12. Think-schema BQ Coupling Different Cr spins direction Stress in Cr layers? Interface roughness? Different onset temperature Because of Possible causes for the modification of Cr magn.

13. Results 4 : RBS [001] [111]  RBS-channelling data show that the sample TG=RT presents more strain. Strain

14. Results 5: Reflectivity X-Ray reflectivity data show that the sample TG=RT has rougher interface. Roughness

15. Conclusions Magnetic state of Cr in Fe/Cr-trilayers measured with PAC technique. Lowering the growth temperature, the T N increased ~200 K. The enhancement of the T N 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 Stralingsfysica Katholieke Universiteit Leuven, Belgium

16. 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 Stralingsfysica Katholieke Universiteit Leuven, Belgium