Enhancement of interlayer spin coupling in ZnTe/MnTe superlattices by Cl doping

doi:10.1016/S0304-8853(00)01083-0

Journal of Magnetism and Magnetic Materials

Volumes 226–230, Part 2, May 2001, Pages 1808-1810

https://doi.org/10.1016/S0304-8853(00)01083-0 Get rights and content

Abstract

Significant enhancement of the magnetic correlation range in [ZnTe|MnTe] magnetic semiconductor superlattices is produced by introduction of Cl during growth. Results of Cl added to the ZnTe layers alone, to both layers, and to a partial MnTe layer suggests that the magnetism arises from an exchange mechanism dependent on the electronic states in addition to conventional superexchange.

Introduction

II–VI magnetic semiconductor superlattices consisting of alternate antiferromagnetic MnTe and non-magnetic ZnTe layers exhibit long-range magnetic coupling extending over multiple bilayers. For example [(ZnTe)₅|(MnTe)₁₀] [1] (five monolayers of ZnTe, 10 monolayers of MnTe) exhibits a correlation range exceeding 200 Å at low temperature. This range is remarkable, considering that superexchange is expected to be the dominant exchange interaction. The addition of Cl during the molecular beam epitaxy (MBE) growth into both magnetic (MnTe) and non-magnetic (ZnTe) layers has been shown [2] to further enhance the range of magnetic correlations by almost a factor of 2 at low temperature. In this paper we extend these results to the case of Cl introduced into only the non-magnetic ZnTe layer [(ZnTe:Cl)₅|(MnTe)₁₀] at concentration level of 10¹⁷–10¹⁸ cm⁻³ and also into only the middle $13$ of the MnTe layers [(ZnTe)₅|(MnTe)_3.3|(MnTe:Cl)_3.3|(MnTe)_3.3]. The Cl-doping results strongly suggest that an additional exchange mechanism, possibly involving indirect exchange with electrons bound on orbits associated with deep electronic levels within the gap, is responsible for the long-range character of the spin interaction.

In bulk form MnTe crystallizes in the NiAs structure; however strains associated with MBE growth produce superlattices with the cubic zinc blende (ZB) structure. The relatively large NNN exchange interaction (≈10–20% of the NN exchange) removes the spin frustration normally associated with the tetrahedral atomic arrangement of the ZB structure and produces long-range type-III AFM order. The approximately 4% lattice mismatch between MnTe and ZnTe layers produces a compressive in-plane strain that favors the single-domain state for which the doubled AFM cell is oriented only along the growth axis [0 0 1].

Section snippets

Neutron scattering results

Neutron scattering has been used to study the temperature dependence of the order parameter and the range of magnetic spin correlations. The excellent crystal quality, with an atomic coherence extending essentially throughout the sample thickness, was confirmed from longitudinal scans [0, 0, Q] through the structural peak positions of the superlattice (e.g., [0, 0, 2]), using both neutrons and X-rays. These exhibited multiple bilayer harmonics that were essentially resolution-limited in width

Acknowledgements

The authors gratefully acknowledge the support of the National Science Foundation through Grant DMR98-03218.

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