EuTe Layers Research Articles

MAGNETIC RESONANT X-RAY DIFFRACTION APPLIED TO THE STUDY OF EuTe FILMS AND MULTILAYERS

In this work we use magnetic resonant x-ray diffraction to study the magnetic properties of a 1.5 µm EuTe film and an EuTe / PbTe superlattice (SL). The samples were grown by molecular beam epitaxy on (111) oriented BaF 2 substrates. The measurements were made at the Eu L 2 absorption edge, taking profit of the resonant enhancement of more than two orders in the magnetically diffracted intensity. At resonance, high counting rates above 11000 cps were obtained for the 1.5 µm EuTe film, allowing to check for the type II antiferromagnetic order of EuTe . An equal population of the three possible in-plane magnetic domains was found. The EuTe / PbTe SL magnetic peak showed a satellite structure, indicating the presence of magnetic correlations among the 5 ML (monolayers) EuTe layers across the 15 ML PbTe non-magnetic spacers. The temperature dependence of the integrated intensities of the film and the SL yielded different Nèel temperatures T N . The lower T N for the SL is explained considering the higher influence of the surface atoms, with partial bonds lost.

MAGNETIC FIELD INDUCED NEAR-BAND-GAP OPTICAL PROPERTIES IN EuTe LAYERS

The magnetic response of the near-band-edge optical properties is studied in EuTe layers. In several magneto-optical experiments, the absorption and emission are described as well as the related Stokes shift. Specifically, we present the first experimental report of the photoluminescence excitation (PLE) spectrum in Faraday configuration. The PLE spectra shows to be related with the absorption spectra through the observation of resonance between the excitation light and the zero-field band-gap. A new emission line appears at 1.6 eV at a moderate magnetic field in the photoluminescence (PL) spectra. Furthermore, we examine the absorption and PL red-shift induced by the magnetic field in the light of the d - f exchange interaction energy involved in these processes. Whereas the absorption red-shift shows a quadratic dependence on the field, the PL red-shift shows a linear dependence which is explained by spin relaxation of the excited state.

Stabilization of PbSe quantum dots by ultrathin EuTe and SrTe barrier layers

Overgrowth of self-assembled quantum dots usually changes their shape and composition due to surface exchange reactions and redistribution of adatoms. As shown for PbSe dots, this can be completely suppressed by covering the dots with ultrathin EuTe or SrTe barrier layers, based on the large EuTe and SrTe binding energies. The model is supported by annealing experiments that show that these barrier layers also suppress the usual coarsening and Ostwald ripening process.

Magnetic ordering of EuTe∕PbTe multilayers determined by x-ray resonant diffraction

In this work we use resonant x-ray diffraction combined with polarization analysis of the diffracted beam to study the magnetic ordering in EuTe∕PbTe multilayers. The presence of satellites at the (12 12 12) magnetic reflection of a 50 repetition EuTe∕PbTe superlattice demonstrated the existence of magnetic correlations among the alternated EuTe layers. The behavior of the satellites intensity as T increases toward the Nèel temperature TN indicates that these correlations persist nearly up to TN and suggests the preferential decrease of the magnetic order parameter of external monolayers of each EuTe layer within the superlattice.

Molecular beam epitaxial growth of EuTe/SnTe strained superlattices

In this work, we report on the growth of EuTe/SnTe superlattices (SLs) on BaF 2 ( 1 1 1 ) substrates by molecular beam epitaxy. Reflection high energy electron diffraction allowed to find the growth modes at each growth stage, while high resolution X-ray diffraction (HRXRD) was employed to evaluate the heterostructure crystalline quality. In four subsequent stages, we studied the growth of SnTe buffer layers, the growth of EuTe on SnTe, the growth of SnTe on EuTe, and finally undertook the SLs growth. EuTe grows in the Stranski–Krastanov mode on SnTe, with a critical thickness for island formation that increases for lower temperatures and higher Te/Eu flux ratios. On the other hand, SnTe starts growing in the Volmer–Weber mode on EuTe, with evolution to layer-by-layer growth after the SnTe islands merge, which occurs faster for higher temperatures. As confirmed by HRXRD ω / 2 θ scans measured along the (2 2 2) SL Bragg peak, high structural quality EuTe/SnTe SLs were grown with EuTe layers below the critical thickness for island formation and SnTe layers thick enough for layer-by-layer growth to be restored. The X-ray spectra were fitted with those calculated using dynamical diffraction theory in order to find accurately the individual layer thicknesses and strain status.

X-ray photoelectron spectroscopy study of MBE-grown Gd/EuTe multilayers

Photoelectron spectroscopy has been used to study the Gd/EuTe multilayers structure covered with a thin (100 Å) Al film. The study of the main core level and valence band photoemission spectra has shown no interaction between Gd and EuTe layers. Gadolinium was in pure metallic state and the ions of Eu were found divalent in the Gd/EuTe multilayers structure. The europium ions spectra analysis indicates that Ar + ion sputtering of the sample surface leads to a deviation from stoichiometry in the near surface region of the EuTe layers and to the appearance of Eu 3+ species in uppermost layers of the Gd/EuTe multilayers structure. The long-time exposure to the XPS ambient gases has been characterized from the analysis of europium 3d core level spectra.

Magnetic polaron induced near-band-gap luminescence in epitaxial EuTe

Epitaxial EuTe layers exhibit narrow luminescence peaks with a large Stokes shift and a large magnetic-field dependence. At low temperatures, the band-gap energy as well as the energy of the narrow luminescence peaks decreases with increasing field. The Stokes shift, in contrast, increases with magnetic field from $340\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ up to $500\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. From a comparison of the magnetic-field dependence of the band gap, the luminescence peaks, and the Stokes shift with density plots of the quasiparticle bandstructure of EuTe, it is concluded that the luminescence originates from a deeper conduction band than that causing the absorption edge. The existence of this deeper band is confirmed by an analysis of the dispersion of the refraction index and by a faint absorption onset. Time-resolved photoluminescence experiments and the continuous-wave photoluminescence intensity as a function of magnetic field and temperature indicate a change of the selection rule for the luminescence transition from optically allowed to optically forbidden, either when the magnetic field exceeds the critical field or when the temperature increases considerably above the phase transition temperature. This change is attributed to the presence of magnetic polarons, which breaks the selection rules at low temperatures and below the critical field and thus enables the luminescence. In the parameter range where no magnetic polarons exist, at high temperatures and above the critical field, almost no luminescence appears.

Magnetic interactions in EuTe epitaxial layers and EuTe/PbTe superlattices

The magnetic properties of antiferromagnetic (AFM) EuTe epitaxial layers and short period EuTe/PbTe superlattices (SLs), grown by molecular beam epitaxy on (111) BaF$_2$ substrates, were studied by magnetization and neutron diffraction measurements. Considerable changes of the N\'eel temperature as a function of the EuTe layer thickness as well as of the strain state were found. A mean field model, taking into account the variation of the exchange constants with the strain-induced lattice distortions, and the nearest neighbor environment of a Eu atoms, was developed to explain the observed $T_{\text N}$ changes in wide range of samples. Pronounced interlayer magnetic correlations have been revealed by neutron diffraction in EuTe/PbTe SLs with PbTe spacer thickness up to 60 \AA. The observed diffraction spectra were analyzed, in a kinematical approximation, assuming partial interlayer correlations characterized by an appropriate correlation parameter. The formation of interlayer correlations between the AFM EuTe layers across the nonmagnetic PbTe spacer was explained within a framework of a tight-binding model. In this model, the interlayer coupling stems from the dependence of the total electronic energy of the EuTe/PbTe SL on the spin configurations in adjacent EuTe layers. The influence of the EuTe and PbTe layer thickness fluctuations, inherent in the epitaxial growth process, on magnetic properties and interlayer coupling is discussed.

Studies of Interlayer Magnetic Coupling in All-Semiconductor Superlattices by Means of Neutron Scattering Techniques

An overview of neutron scattering studies of ferromagnetic and antiferromagnetic all-semiconductor superlattices is presented. Diffraction experiments on MnTe/CdTe, MnTe/ZnTe and EuTe/PbTe superlattices show pronounced correlations between the MnTe and EuTe layers across the non-magnetic spacers, even though these layers are antiferromagnetic and the systems are nearly-insulating. Current theory status of these systems is discussed. Diffractometry and reflectometry data from EuS/PbS superlattices reveal pronounced antiferromagnetic coupling between the ferromagnetic EuS block. First polarized neutron reflectometry data from superlattices prepared of a novel ferromagnetic spintronics material, Ga(Mn)As, are also presented.

Giant tunability of excitonic photoluminescence transitions in antiferromagnetic EuTe epilayers induced by magnetic polarons

EuTe layers with high purity, grown by molecular beam epitaxy, are investigated by magneto-optical spectroscopy. Low temperature photoluminescence (PL) experiments reveal narrow, exciton like emission peaks exhibiting a Stokes shift of 300 meV . These emission peaks can be tuned over a giant range of 160 meV by applying magnetic fields between zero and 5 T . The temperature dependence of the PL transition energies shows a kink at the antiferromagnetic-paramagnetic phase transition shifting to lower temperatures with increasing magnetic field. This unique magnetic field dependence as well as the temperature dependence of the PL transitions results from the formation of magnetic polarons, due to d–f exchange interactions between electrons in the conduction band and localized magnetic moments.

Interlayer correlations in ferromagnetic semiconductor superlattices EuS/PbS

Neutron diffraction and reflectivity data from all-semiconductor ferromagnetic/diamagnetic superlattices EuS/PbS provide direct evidence of antiferromagnetic coupling between EuTe layers across up to 90 A ̊ of diamagnetic PbS, despite the absence of mobile carriers that can support long-range RKKY interactions.

Giant tunability of exciton photoluminescence emission in antiferromagnetic EuTe

The photoluminescence properties of antiferromagnetic EuTe layers grown by molecular-beam epitaxy are reported. At low temperatures, two excitonic photoluminescence peaks are observed at 1.92 and 1.88 eV with a full width at half maximum of about 10 meV. With applied magnetic field, these excitonic transitions shift linearly by $\ensuremath{-}34 \mathrm{m}\mathrm{e}\mathrm{V}/\mathrm{T}$ to smaller transition energies with a total shift of more than 240 meV at 7.2 T. This is the largest tuning range observed in any semiconductor. The observed magnetic field and temperature dependence of the luminescence lines is explained by the formation of large magnetic polarons due to exchange interactions between the d-like electrons in the conduction band and localized $4f$ spins.

Observation of large-scale surface undulations due to inhomogeneous dislocation strain fields in lattice-mismatched epitaxial layers

The surface structure of highly relaxed IV–VI heteroepitaxial layers is studied using scanning tunneling and scanning force microscopy. For bilayer structures consisting of highly relaxed EuTe layers covered by thick PbTe buffer layers, huge surface undulations with amplitudes as large as 50 Å are observed. These undulations are completely decoupled from the epitaxial surface step structure and can be observed even for rather large cap layer thicknesses. The deconvolution of the surface profiles shows that the surface undulations are purely caused by the nonuniform misfit dislocation network at the EuTe/PbTe interface. On the contrary, no indication is found that these dislocation strain fields give rise to surface structures formed by preferential growth.

Antiferromagnetic phase transition and interlayer spin correlations in short-period EuTe/PbTe superlattices

Neutron diffraction and magnetization studies of short-period antiferromagnetic (AF) semiconducting (1 1 1) EuTe/PbTe superlattices are presented. Due to the symmetry-breaking lattice mismatch strain, only a single AF configuration forms in the EuTe layers, instead of four allowed by symmetry in bulk EuTe. The strain and the finite layer thickness also lead to a strong shift in the Néel temperature. Neutron-diffraction spectra exhibit pronounced patterns of satellites, indicating spin correlations between successive EuTe layers separated by PbTe layers, and the transfer of magnetic interactions across the diamagnetic spaces. Experiments on samples with doped PbTe layers and in external magnetic fields rule out that the coupling is caused by any of the mechanisms known to occur in metallic superlattices. Hence, our data strongly suggest the existence of a new interlayer coupling mechanism.

Interlayer spin coherence in antiferromagnetic EuTe/PbTe superlattices observed by polarized neutron diffraction

Neutron diffraction experiments on short-period EuTe/PbTe superlattices with a [111] growth axis reveal distinct magnetic correlations between the EuTe layers across up to 55 Â of non-magnetic PbTe spacers, even though EuTe is antiferromagnetic and there are not enough carriers to support RKKY interactions. With increasing in-plane external magnetic field we observe a decrease of the antiferromagnetic interlayer interactions. This decrease depends very strongly on whether the superlattices are cooled down through the Néel point in zero or non-zero field.

Interlayer correlations in antiferromagnetic semiconductor superlattices EuTe/PbTe

We report neutron diffraction studies of short-period (111) [(EuTe) ξ|(PbTe) η] N superlattices (SLs), with a special emphasis on the interlayer spin coherence phenomena seen in these systems. We summarize the results of experiments on over thirty samples, in which we have investigated the dependence of interlayer coupling effects on the EuTe and PbTe layer thicknesses, on doping of the PbTe layers, and on external magnetic fields.

Oscillating Antiferromagnetism of Ultrathin EuTe Layers

We study magnetic resonance on EuTe/PbTe superlattices. Analysis of the magnetic dipole anisotropy in the superlattices and of the EPR amplitude of isolated Eu ions in the PbTe wells shows that the diffusion at interfaces is very small. The real thickness of EuTe can be evaluated with an accuracy better than one monolayer. Since for EuTe layers thicker than 2 monolayers there is no difference in the character of the antiferromagnetic resonance observed for even and odd numbers of monolayers, we conclude that there is no static magnetization of the antiferromagnetic sublattices. Nevertheless, long range antiferromagnetic order is clearly evident.

Magnetic susceptibility of EuTe/PbTe Heisenberg antiferromagnetic superlattices: Experimental and theoretical studies

We report results on the temperature dependence of the susceptibilities of a set of MBE-grown short-period EuTe/PbTe antiferromagnetic superlattices having different EuTe layer thicknesses. In-plane and orthogonal susceptibilities have been measured and display a strong anisotropy at low temperature, confirming the occurrence of a magnetic phase transition in the thicker samples, as seen also in neutron-diffraction studies. We suggest that dipolar interactions stabilize antiferromagnetic long-range order in an otherwise isotropic system and we present numerical and analytical results for the low-temperature orthogonal susceptibility.

Polarized neutron diffraction studies of antiferromagnetic EuTe/PbTe superlattices (abstract)

PbTe is a nonmagnetic semiconductor, and EuTe, also a semiconductor, is a type II fcc antiferromagnet below TN=10 K. Our earlier diffraction studies of [(EuTe)m|PbTe)3m]N samples (with m=2–7, N=200–600) showed that the type II ordering is preserved in the EuTe layers,1 but the lattice strain always selects an arrangement with the (111) ferromagnetic (FM) sheets (characteristic for the type II structure) parallel to the layers. In addition, samples with n⩽5 scans through the antiferromagnetic (AFM) maxima reveal pronounced patterns of satellite peaks—a clear signature of interlayer spin coherence, indicating that Eu–Eu interactions are transferred through PbTe barriers even as thick as 55 Å! The origin of this surprisingly long-range interaction cannot be understood on the grounds of the existing theories of interlayer coupling, in which high concentrations of conduction electrons always play a crucial role,2 because EuTe is insulating at low T and in PbTe n is as low as ∼1017 cm−3. In order to identify the coupling mechanism, we have begun new studies using polarized neutron diffraction. This technique is particularly useful for investigating the AFM domain arrangement. While the (111) symmetry allows six nonequivalent type II configurations, gradually increasing field should align all spins perpendicular to H, interfering with the coupling forces. Eventually, it should destroy the interlayer coherence—“monitoring” of this process may therefore reveal the much needed information about the interlayer coupling strength. Our experiments show a drastic differences in the system behavior depending on the “cooling history.” If the sample is cooled at H=0, then a modest field (∼0.5 T) indeed leads to the expected domain rotation. Further H increase induces a growing ferromagnetic component, and gradually suppresses the AFM one—however, the interlayer coherence in the AFM ordering can be still seen up to surprisingly high H values ∼5 T). On the other hand, if the sample is cooled through TN with the field on, then H as weak as 0.02 T already destroys the interlayer correlations. Explanation of the observed facts and estimates of the interaction strength based on model calculations will be presented, and some possible physical mechanisms undering the interlayer coherence (e.g., coupling by dipolar forces or coupling involving bound carrier states) will be discussed.

Interlayer coupling in (111) AFM multilayers

Abstract Neutron diffraction studies of MBE-grown (111) EuTe PbTe superlattices are reported. The EuTe layers exhibit an AF structure consisting of antiferromagnetically coupled (111) spin sheets. The data reveal interlayer magnetic coupling accross PbTe spacers with suprisingly large thicknesses (up to 55 A), despite the low carrier concentration which rules out any significant RKKY interactions.