Néel temperature of zinc‐blende, MBE‐grown MnTe layers: modification by the crystal growth conditions

Abstract

We present evidence of the influence of the substrate (GaAs and CdZnTe) on the magnetic properties and on the surface structure of (001) zinc-blende MnTe grown by molecular beam epitaxy. High-resolution Xray diffraction and atomic force microscopy were employed in the characterisation of the MnTe structure. The temperature dependence of collective spin excitations (magnons) was determined via Raman scattering and subsequently analysed in order to study selected magnetic properties. Differences in both the Néel temperature and magneto-elastic coupling between the MnTe layer and the given substrate were demonstrated, and mechanisms contributing to these effects are discussed.1 Introduction The preparation method of thin semiconductor layers as well as their mechanical parameters can influence various properties, including their magnetic characteristics. In particular, this phenomenon is well known for insulating ferromagnets which crystallize in the rock salt structure (e.g., europium chalcogenides). An important enhancement of the Curie temperature of undoped EuS epitaxial films, depending on the growth conditions, has been observed in Ref. [1]. As has been demonstrated, this effect is associated with indirect exchange through additional charge carriers that were created in the film due to non-stoichiometry caused by dislocations. Recently, the influence of the substrate (via strain effects) on the Curie temperature of EuS/PbS multilayers was also observed in Ref. [2]. This effect was attributed to stress resulting mainly from the difference of thermal expansion coefficients between the substrate and the structure. In addition, for extremely thin EuS layers -below 10 monolayers (ML) -a systematic reduction of the phase transition temperature with decreasing EuS layer thickness was observed. The phenomena described above are less well known in other groups of magnetic (or diluted magnetic) semiconductors, e.g., II-VI or III-V based materials. In particular, to our knowledge such data do not exist for crystals which, at low temperature, are antiferromagnets. Zinc-blende MnTe (for which the Néel temperature T N is about 65 K) is a good representative of this group of materials. Due to its metastable character, MnTe in this crystal phase can be obtained only by non-equilibrium growth techniques, e.g., molecular beam epitaxy (MBE). The goal of the present work was to demonstrate a relationship between the surface structure and magnetic properties of zinc-blende MnTe layers.

Are values of parameters describing magnetic properties of crystal really fixed?

“…The bulk MnTe (crystallizing in NiAs structure) exhibits T N = 310 K [36], whereas T N = 284 K was observed for MnTe layer with the same structure grown on Al 2 O 3 substrate [37]. The T N value determined by the temperature dependence of the magnon frequency (which is given by the combination of the exchange integrals) observed by Raman scattering for zinc blende (ZB) modification of MnTe is also different for thin layers of this compound grown on GaAs and Cd 1−x Zn x Te substrates, as it was shown in [38]. The difference equal to a few K was found in this particular case.…”

Raman scattering as a tool to characterize semiconductor crystals, thin layers, and low-dimensional structures containing transition metals

“…In order to demonstrate this effect, the results of Raman scattering measurements performed on two MnTe layers grown epitaxially on different substrates: Cd 0.96 Zn 0.04 Te and GaAs were compared in Ref. . Careful analysis of the results demonstrated a difference as high as 5 K in the T N value corresponding to both investigated samples (see the original paper for details).…”