Neutron scattering studies of magnetic semiconductor thin films and superlattices

1. Neutron scattering studies of magneticsemiconductor thin films and superlattices T. M. Giebultowicz1and Henryk Kępa2,1 1. Dept. of Physics, Oregon State university, Corvallis, OR,USA 2. Institute of Experimental Physics, Warsaw University, Poland Subtitle: Doing neutron scattering in Oregon?

3. Well, Oregonians still can doneutron scattering, if they have: • Generous support for their research projects from CNR NIST, and from National Science Foundation; • Good collaborators and good friends!

5. Outline • magnetic neutron diffraction and interlayer spin correlations in all-semiconductor antiferromagnetic systems, e.g. EuTe/PbTe • Interlayer exchange coupling in ferromagnetic EuS/PbS and EuS/YbSe superlattices • magnetic domain structure of EuS/PbS, EuS/YbSe and Ga(Mn)As/GaAs superlattices studied by polarized neutron reflectometry.

6. Magnetic semiconductors - historical remark: MOTHER NATUREmade them almost exclusivelyantiferromagnetic! When we started our research long time ago, we had no other choice than to study Antiferromagnetic / NonmagneticSuperlattices (e.g., EuTe/PbTe)

7. AFM Bragg reflection bulk TN = 9.6 K EuTe 3-m epilayer

9. EuTe/PbTe superlattices NIST Center for Neutron Research NG1reflectometer = 4.75 Ǻ

10. Advent of semiconductor spintronics • End of 1990s: successes in the sythesis of new epitaxial ferromagnetic semiconductors, most notably Ga(Mn)As (Curie T may be as high as 175 K). • Much new excitement about magnetic semiconductors and their superlattices. ‘All-semiconductor’ heterostructures are expected to play a crucial role in second-generation spintronics devices (first-generation: metallic systems)

11. New epitaxial systems: Theoretically predicted Curie temperatures for strongly p-type alloys with 5% Mn. • Neutrons can be used for: • Investigating the new systems (transition temps., exchange parameters, etc.); • Studying effects of current • interest, such as, e.g., • interlayer magnetic coupling • or the domain structure in • superlattices (not necessarily • using new materials – intere-sting things can be done on “old” systems, e.g.,EuS. Year 2000: We find Dr. A. Sipatov from Ukraine, who grows very interesting EuS/PbS and EuS/YbSe superlattices!

12. FerromagneticEuS/PbSandEuS/YbSeSL’s EuS – Heisenberg ferromagnet TC = 16.6 K (bulk), Eg=1.5 eV PbS – narrow-gap (Eg=0.3 eV) semiconductor (n≈ 1017 cm-3) YbSe – wide-gap (Eg=1.6 eV) semiconductor (semiinsulator) all NaCl-type structure with lattice constants: 5.968Ǻ 5.936 Ǻ 5.932Ǻ (lattice mismatch ≈ 0.5%) 4-200 Ǻ 30-60 Å number of repetitions 10-20 (001) a=6.29 Å

15. Unpolarized neutron reflectivity experiments on the EuS/PbS system (NG-1 reflectometer, NIST Center for Neutron Research)

16. Electronic band structure in EuS

17. Interlayer exchange coupling mediated by valence band electrons J.Blinowski & P.Kacman, Phys. Rev. B 64 (2001) 045302. P.Sankowski & P.Kacman, Acta Phys. Polon. A 103 (2003) 621

18. Alternative explanations... • PbS is a narrow-gap material. At low T the concentrations of carriers may be still pretty high. Perhaps the effect seen in EuS/PbS is a carrier-mediated coupling? • Crucial test: make a EuS/XY system, in which XY is a wide-gap semiconductor or an insulator • An ideal material, YbSe was found for that purpose.

19. Unpolarized neutron reflectivity experiments on the EuS/YbSe system(NG-1 reflectometer, NIST Center for Neutron Research)

20. EuS/PbS EuS/YbSe

23. EuS/PbS EuS/YbSe

25. Advantages of using polarized neutrons… FM domains in Ga(Mn)As/GaAs superlattices Here are data from a Ga(Mn)As/GaAs superlattice (6% of Mn) obtained using an unpolarized beam:

26. Polarized neutron beam experiments – the principles Atomic spins S and the applied magnetic field H lie in the x-z reflecting planes. External field H and the neutron polarization P are parallel to z-axis Parallel to H projections of S  non-spin-flip (NSF) scattering (nuclear-magnetic interference) Perpendicular ones  purely magnetic spin-flip (SF)

33. Polarized neutron reflectivity profiles for the EuS/YbSe (46/20) Å. in-plane [110] axis horizontal EuS/PbS (30/4) Å - polarization analysis of the 1-st AFM SL Bragg peak

34. Polarized neutrons and the in-plane magnetic anisotropy in EuS/PbS & EuS/YbSe x=1 x=1

35. EuS/PbS superlattices on (111)BaF2 Polarized neutron reflectivity studies

36. Unpolarized neutron reflectivity measurements

37. Unpolarized neutron reflectivity measurements

38. Unpolarized neutron reflectivity measurements

39. The first SL-peak for different sample rotation angles

40. The model …. etc., where: and is the angle of rotation of the sample about Q

42. Different flipping ratios vs. angle of rotation of the sample about Q

45. CONCLUSIONS • Neutron reflectometry offers insight into some properties of magnetic semiconductor superlattices that cannot be obtained by other techniqes (interlayer coupling, domain structure); • Although the experiments, essentially, offer support for the model of intelayer coupling conveyed by the valence electrons, they also reveal that some ‘ingredients’ may still be missing in that theory; • In all investigated systems, polarization analysis reveals a significant asymmetry in the population of domain states - the reasons are not yet clearly understood, but this phenomenon may have important practical implications and studying it is certainly worth pursuing.

46. Neutron polarization analysis

47. For saturated sampleα=90º Calculatedratio: Experimental value: