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Neutron scattering studies of magnetic semiconductor thin films and superlatticesPowerPoint Presentation

Neutron scattering studies of magnetic semiconductor thin films and superlattices

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### Neutron scattering studies of magneticsemiconductor thin films and superlattices

### Magnetic semiconductors - historical remark: MOTHER NATUREmade them almost exclusivelyantiferromagnetic!

### EuS/PbS superlattices on (111)BaF EuS/PbS & EuS/YbSe2

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?

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!

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.

When we started our research long time ago, we had no other choice

than to study

Antiferromagnetic / NonmagneticSuperlattices (e.g., EuTe/PbTe)

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)

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!

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

Unpolarized neutron reflectivity experiments on

the EuS/PbS system

(NG-1 reflectometer, NIST Center for Neutron Research)

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

Alternative explanations... electrons

- 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.

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

EuS/PbS electrons

EuS/YbSe

EuS/PbS electrons

EuS/YbSe

Advantages of using polarized neutrons… electrons

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:

Polarized neutron beam experiments – the principles electrons

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)

Polarized neutron reflectivity profiles for the electrons

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

Polarized neutron reflectivity studies

Unpolarized neutron reflectivity measurements EuS/PbS & EuS/YbSe

Unpolarized neutron reflectivity measurements EuS/PbS & EuS/YbSe

Unpolarized neutron reflectivity measurements EuS/PbS & EuS/YbSe

The first SL-peak for different sample rotation angles EuS/PbS & EuS/YbSe

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

CONCLUSIONS sample about Q

- 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.

Neutron polarization analysis sample about Q

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