Neutron scattering studies of short-period MnTe/ZnTe superlattices: magnetic order, magnon propagation and confinement
|Wojciech Szuszkiewicz 1, Bernard Hennion 2, Sylvain Petit 2, Elzbieta Dynowska 1, Elżbieta Janik 1, Grzegorz Karczewski 1, Tomasz Wojtowicz 1|
1. Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland
MnTe is well-known magnetic semiconductor which can crystallize it two phases. The stable MnTe crystal phase is the hexagonal one of the NiAs type. The metastable MnTe phase with the zinc blende (ZB) structure can be obtained with the use of non-equilibrium growth techniques only, such as MBE. This phase exhibits an antiferromagnetic (AF) order of type-III at low temperatures (Néel temperature of about 65 K). This kind of magnetic order persists also in MnTe layers in MnTe/ZnTe superlattices (SLs). Because of the distortion of the fcc lattice an energy-minimizing magnetic configuration with the unit cell doubling direction along the SL growth axis is realized in such structures. Due to this property the SLs above mentioned are characterized by the single orientation of magnetic domains. The presence of an interlayer exchange coupling has been reported for selected MnTe/ZnTe short-period SLs previously [1,2] but details of this coupling and an anomalous temperature behavior of features observed by the neutron diffraction were not fully understood. The goal of the current research was to get more information on the magnetic properties of various SLs mentioned above.
Several MBE-grown, MnTe/ZnTe short period SLs with various numbers of monolayers (MLs) in magnetic and non-magnetic slabs were grown by MBE at the Institute of Physics PAS in Warsaw. Prior to the growth of the SL the ZnTe buffer layer was deposited onto (001)-oriented 2°off GaAs substrate. The typical SL single bilayer was composed of 15 or 20 MnTe MLs and of 4 to 10 ZnTe MLs. In every case the SL contained 200 periods, so its thickness was close to 1 micrometer. The separate set of SLs corresponded to structures with similar period values as those mentioned above but with much smaller number of MnTe MLs (7, 8 or 9) and 18 ZnTe MLs. The crystal quality of grown samples and their structure parameters were checked by the high-resolution XRD measurements with the use of Cu Ka1 radiation. The typical diffraction pattern obtained at RT for one from our samples is shown in Fig. 1. The real and the nominal values of parameters describing typical SL bilayer were the same (possible difference in the case of the highest number of MnTe MLs does not exceed 0.5 ML).
Next, the samples were analyzed by elastic and inelastic neutron scattering performed at Laboratoire Léon Brillouin in Saclay. The first type of measurements tooks advantage of 4T2 spectrometer installed on the cold neutron source. The results of these measurements demonstrated a magnetic coherence in selected SLs for a distance as high as 900 Å at low temperatures. The temperature behavior of spectra observed in our experiments was found to be quite different from that previously reported in the literature. Due to careful analysis of the form of neutron diffraction structures at a few Brillouin zones we were able to propose the model of local magnetic order in SL. Possible physical mechanisms responsible for observed magnetic correlation in AF-ordered MnTe layers of these SLs given in the literature are mentioned and discussed.
In our earlier studies we have determined the collective magnetic excitations (magnons) dispersion in quasi-bulk ZB-MnTe slab at low temperature (in AF-III phase) by inelastic neutron scattering . In current research we have observed not only a long-range coherency between AF layers for ZnTe spacer thickness up to ~25 Å, but also the propagation of collective magnetic excitations (magnons) along the SL-stacking direction in short-period SLs. For large enough ZnTe spacers, on the other hand, the AF MnTe layers are no longer correlated and size quantization effects (confinement) for magnons take place. The effective magnetic volume for investigated SLs varied between 0.15 mm3 and 0.30 mm3 so the counting time was equal to about 20 minutes per step. The experimental evidence of both effects mentioned above was found in our inelastic neutron scattering measurements, performed with the use of thermal neutron beams. A similarity of theoretically predicted and experimentally observed magnon spectra was also shown. To the best of our knowledge presented results constitute the first clear evidence of magnon propagation and magnon confinement in SLs containing magnetic semiconductor that was demonstrated with the use of inelastic neutron scattering technique.
Fig. 1. The X-ray diffraction pattern collected for the (MnTe)18/(ZnTe)7 SL.
This work was partially supported by the research grant N N202 128639 from Ministry of Science and Higher Education (Poland) and by the European Commission under the 7th Framework Programme through the 'Research Infrastructures' action of the 'Capacities' Programme, NMI3-II Grant number 283883.
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Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 2, by Wojciech Szuszkiewicz
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-04-15 18:51 Revised: 2013-07-17 23:00