Instituut voor Kern- en StralingsfysicaKatholieke 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? : • Oscillatory exchange coupling between Fe layers • Giant Magnetoresistance effect (GMR) • Non-collinear coupling between Fe layers Special position of Fe/Cr due to the peculiar antiferromagnetism of Cr. 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.
Msat MR M Hsaturation We focus on the non-collinear coupling • 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). Dr. Stella Maris Van Eek Cr Fe Fe
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. Coupling (magnetization) TN (K) (resistivity) BQ coupling 200 No BQ coupling 0
Fullerton et al. Phys. Rev. Let. 75,2,1995. Wide range of TN for thin Cr films Bulk Cr-TN= 311 K 70 K<TN<295 K 300 K<TN AFM Cr order increase upon reduction of Cr-thickness.Proximity effect. Schmitte, Schreyer et al. Europhys. Lett. 48,(6), 692 (1999) AFM Cr order well above room temperature Pierce, Unguris, Celotta J.M.M.M. 200 (1-3), 290 (1999). Scaling law 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
Samples grown MgO(100)/Fe(40 Å)/Cr(80Å)/Fe(40Å)/Au(40Å) • epitaxially grown Fe/Cr/Fe trilayers • on MgO(001). • Growing temperature: • TG=RT • TG= 450K • TG= 575K
Results 1: VSM Remanence magnetization TN=150 K TN=380 K No BQ coupling BQ coupling The TN (onset temperature of BQ coupling) can be modified changing the growth temperature of trilayers.
Introduction to perturbed angular correlations (PAC) technique Au Fe Cr Fe MgO 111In was implanted at 60keV. R(t)= a0 + a1cos(wt)+ a2cos(2 wt) in case of ferromagnetic or antiferromagnetic order R(t)= a0 + a1J0(wt)+ a2J0(2 wt) in case of spin density wave order a0 , a1,a2 depend on the orientation of the magnetic field at the radioactive nucleus place. Being J0 the 0th order Bessel function w proportional to the magnetic field at the radioactive nucleus place.
Results 2: PAC PAC at 140 K on sample TG= 575 K • SDW • Cr spins direction in the sample plane • PAC at T=77 K on sample TG=RT • SDW • Cr spins out of plane
Think-schema Different onset temperature Possible causes for the modification of Cr magn. Because of • Stress in Cr layers? • Interface roughness? BQ Coupling Different Cr spins direction
  Results 4 : RBS Strain RBS-channelling data show that the sample TG=RT presents more strain.
Results 5: Reflectivity Roughness X-Ray reflectivity data show that the sample TG=RT has rougher interface.
Instituut voor Kern- en StralingsfysicaKatholieke Universiteit Leuven, Belgium • Conclusions • Magnetic 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)