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.     

Properties of PbTe/EuTe short period superlattices and their application to laser diodes

Short period superlattices (SPSLs) consisting of PbTe (E_g=0.3eV) and EuTe (E_g-2.0eV) were prepared on KCl (100) substrates by alternate growth of the PbTe (100 Å) and Eu (less than a few atomic layers) using a hot-wall technique. The structural properties were studied through x-ray diffraction and electron-probe micro analysis. The band gap of the SPSL, depending on average Eu (EuTe) composition, was determined by optical transmission measurement. It was found that the band gap of the SPSL of a small EuTe composition increases very sensitively with the EuTe composition, while that of a large EuTe composition depends only on the period length, resulting from the quantum size effect due to large band gap of EuTe (barrier). Results of magneto resistance measurements of the SPSL are also presented. A multiple quantum-well (MQW) laser was fabricated using the SPSL for barrier layers in the MQW active region and cladding layers. The laser operated up to 160 K (pulsed).     

Author index to volume 25

The effect of pressure up to 18.6 kbar on the magnetic hyperfine field of EuTe has been studied using the 21.6 keV Mössbauer resonance of ¹⁵¹Eu. The antiferromagnetic EuTe has been ferromagnetically oriented applying an external magnetic field of 12.0 T. In contrast to the antiferromagnetic phase the ferromagnetically aligned spin phase shows an increase of the hf field at the Eu-nuclei to more negative values under pressure. This pressure effect can be attributed mainly to the pressure dependence of the hf field contributions transferred from the nearest Eu²⁺-neighbours.     

Einfluß der magnetischen Ordnung auf die optische Absorption von ferro- oder antiferromagnetischen Halbleitern

The optical absorption of magnetically ordered semiconductors EuO, EuS, EuSe and EuTe was measured and a forbidden energy-gap was determined. The width of the gap decreases by 0.2 to 0.3 eV below the Curie- and Néel-temperatures respectively. It is shown that the decrease of the gap is proportional to the magnetization. This yields a new method to determine the spontaneous magnetization. A rather long tail of the spontaneous magnetization is found above the magnetic ordering temperature. It is shown that EuSe is antiferromagnetic above 4°K, but it becomes spontaneously ferromagnetic below 3°K. Differences in the behaviour of antiferromagnetic EuTe and metamagnetic EuSe are discussed.     

Biquadratic exchange interactions in the Europium monochalcogenides

We have examined the biquadratic exchange interaction strengths in the Europium monochalcogenides EuO, EuS, EuSe and EuTe using magnetization data of the paramagnetic phase and elaborate the consequences this additional interaction mechanism has on the magnetic phase diagrams of EuSe and EuTe. It is shown that the cubic susceptibility χ₃ obeys a Curie-Weiss law at suffciently high temperatures and that the associated Curie-Weiss temperature θ₃ is a measure for the biquadratic interaction strength. For all these materials the biquadratic interaction is ferromagnetic (θ₃ > 0). This leads to a conflicting situation in the case of EuTe for which θ₁ < 0. We attribute the peculiar observation, that the MnO superstructure reflection intensities as observed with neutron scattering correspond only to 0.6 of that moment expected for perfect magnetic order, to the presence of biquadratic interactions. The critical field B_c follows a T² law in the spin-wave regime (T < 0.8 K) for EuTe and EuSe but for these two materials with an antiferromagnetic ground state the cubic susceptibility χ₃ diverges at a temperature T* which is 2.5 K and 1.2 K above the ordering temperature, respectively. In the temperature range T_c < T < T* the magnetization curves exhibit some weak but definite anomaly which might be interpreted as a field-induced transition into the ferromagnetic state. A new multicritical point has been identified along the critical field curve B_c of EuSe.     

Optical properties of antiferromagnetic/ion-crystal superlattices

Transmission, refraction and absorption properties of an antiferromagnetic/ion-crystal superlattice are investigated. The transmission spectra based on FeF₂/TlBr superlattices reveal that there exist two intriguing guided modes in a wide stop band. Additionally, FeF₂/TlBr superlattices possess either the negative refraction or the quasi left-handedness, or even simultaneously hold them at certain frequencies of two guided modes, which require both negative magnetic permeability of antiferromagnetic layers and negative permittivity of ion-crystal layers. Frequency regimes of the guided modes will be dependent on the magnitude of the external magnetic field. Therefore, handedness and refraction properties of the system can be manipulated by modifying the external magnetic field. Absorption spectra exhibit that absorption corresponding to guided modes is noticeable.     

Thermoelectric power in superlattices of nonparabolic semiconductors with graded interfaces under magnetic quantization

Summary We study the thermoelectric power of the electrons under magnetic quantization in III–V, II–VI, PbTe/PbSnTe and strained layer superlattices with graded interfaces and compare the same with the corresponding bulk specimens of the constituent materials by formulating the respective expressions incorporating the broadening. It is found, by taking GaAs/Ga_1−x Al _x As, CdS/CdTe, PbTe/PbSnTe and InAs/GaSb superlattices with graded interfaces as examples, that the thermoelectric power exhibits oscillatory dependence with the inverse quantizing magnetic field due to Shubnikov-de Hass effect and increases with decreasing electron concentration in an oscillatory manner in all the aforementioned cases. The thermopower in graded superlattices is greater than that of constituent bulk materials together with the fact that the oscillations in superlattices show up much more significantly as compared to the respective constituent materials. In addition, the well-known expressions for bulk specimens of wide-gap semiconductors have also been obtained as special cases from our generalized expressions under certain limiting conditions.     

IV–VI compound compositional and doping superlattices

Structural, electronic, optical and magnetooptical investigations on compositional superlattices (mainly PbTe/PbSnTe, PbTe/PbEuSeTe) are summarized. Recent results on transport and magnetooptical effects in PbTe doping superlattices are presented and examples for an excellent photoconductivity response of these structures in the 5μm region are given.     

Effective-medium approach for surface-localized magnetic polaritons in magnetic (ferromagnetic and antiferromagnetic) superlattices

The general effective-medium dispersion relations are derived for surface-localized magnetic polaritons which propagate parallel to the surface between a superlattice and semi-infinite bulk material, as applied to ferromagnetic and antiferromagnetic superlattices, in the situation when a static magnetic field is applied in the plane of the layers and parallel to the magnetization. The dependence of the energy of the surface waves on the volume fraction of the ferromagnetic superlattice component and the influence of the external magnetic field on the spectrum of the surface magnetic polaritons for the antiferromagnetic superlattice are investigated. The spectrum of the surface-localized magnetic polaritons which appear at the junction of the magnetic (ferromagnetic and antiferromagnetic) superlattice with the magnetic material are more complex, in contrast to the cases of semi-infinite magnetic material or semi-infinite magnetic SL. It is essential that in all cases in the presence of the external magnetic field the spectrum of the magnetic polaritons are non-reciprocal. The properties of surface polaritons are discussed in detail for the system ferromagnetic superlattice (YIG/non magnet)/YAG and the antiferromagnetic superlattice (MnF₂/ZnF₂)/FeF₂.     

High-field cooperative transport phenomena and screening in phase-separated degenerate antiferromagnetic semiconductors

Response to a strong electric field is investigated of degenerate antiferromagnetic semiconductors EuTe spontaneously separated into antiferromagnetic and ferromagnetic phases. Whereas in weak fields their response is Ohmic, in strong fields a sequence of very high peaks appears over the Ohmic-type background beginning from a certain threshold field strength. Their spacing decreases with increasing field strength. These peaks are explained by cooperative motion of charged ferromagnetic microregions inside the antiferromagnetic host. The screening by these microregions is investigated theoretically.     

Recoilless fractions calculated with the nearest-neighbour interaction model by Kagan and Maslow

The recoilless fraction is calculated for a number of Mössbauer atoms that are natural constituents of HfC, TaC, NdSb, FeO, NiO, EuO, EuS, EuSe, EuTe, SnTe, PbTe and CsF. The calculations are based on a model developed by Kagan and Maslow for binary compounds with rocksalt structure. With the exception of SnTe and, to a lesser extent, PbTe, the results are in reasonable agreement with the available experimental data and values derived from other models.     

Raman scattering in Europium chalcogenides near magnetic phase transitions

The inelastic light scattering spectra of the europium chalcogenides are analyzed in terms of a one phonon-one spin Raman process. Calculations are presented for the temperature dependence of the integrated intensity of the “spin-disorder” spectrum of EuS and compared with experiment. The observed temperature dependence of the Raman spectrum of EuSe in the two-sublattice antiferromagnetic phase is presented and examined in terms of the one phonon-one spin mechanism. Experiments are suggested in which the Raman spectra can be used to measure the magnetic field dependence of the order parameter exponent β in EuTe.     

GIANT MAGNETORESISTANCE OF MAGNETIC SUPERLATTICES: QUANTUM SIZE EFFECT

In this paper the quantum size effect in giant magnetoresistance of magnetic superlattices is studied. The electrons are considered to be confined in a set of quantum wells, which are different for the ferromagnetic and antiferromagnetic ordering in magnetic superlattices. The oscillation of giant magnetoresistance with increasing thickness of the nonmagnetic spacer layer is explained. It is shown that the influence of quantum size effects on the giant magnetoresistance of magnetic superlattices is considerable.     

Surface magnons and magnetic structures for the fcc anti-ferromagnetic lattice of the second kind: Application to EuTe

A two-sublattice model is presented and applied to the fee antiferromagnetic lattice of the second kind. It includes a Heisenberg hamiltonian for exchange interactions between first and second neighbor spins. Surface spin interactions are kept identical to bulk ones. Surface and bulk magnons are obtained and show some cases of instabilities (soft modes) which may account for surface superstructures. The model is applied to the case of EuTe.     

Anisotropic pinned/biased magnetization in SrRuO3/SrMnO3 superlattices

The exchange coupling at the interfaces of magnetic superlattices consisting of ferromagnetic SrRuO3 and antiferromagnetic SrMnO3 grown on (001) oriented SrTiO3 is studied with in-plane and out-of-plane orientations of the cooling magnetic field, with respect to the substrate plane. The magnetization of the in-plane, field cooled hysteresis loop is lower than the corresponding in-plane zero-field-cooled hysteresis loop. The out-of-plane field cooled hysteresis loop is shifted, from the origin, along the graphical magnetization axis. We attribute this irreversible rotation of the moment to the pinning/biasing of spin in the SrRuO3 layer in the vicinity of interfaces by the antiferromagnetic SrMnO3 layer.     

Exchange coupling of Co/Cr(0 0 1) superlattices for in-plane and perpendicular anisotropy

We have investigated the exchange coupling of Co/Cr(0 0 1) superlattices by polar and longitudinal magneto-optical Kerr effect measurements, by varying both the Co and Cr film thicknesses. At a Co thickness of ≈10 Å a nearly perpendicular anisotropy is found with antiferromagnetic order in the range of 5–15 Å of Cr thickness. For these superlattices the magnetization curve starting from remanence to saturation is characterized by a surface spin-flip transition at low field, followed by domain wall nucleation and motion, and finally by a coherent spin rotation with increasing field. Antiferromagnetic coupling is also observed for superlattices with thicker Co layers and with in-plane magnetic anisotropy.     

Magnetooptical investigation of PbTe/Pb1−xSnxTe superlattices

Intra- and interband magnetooptical experiments were carried out on PbTe/Pb_1?xSn_xTe superlattices (x=0.135) with layer thicknesses ranging from 13 – 225 nm. For the magnetic field parallel to the [111] growth direction, the Landau levels for the superlattice band structure are calculated within the framework of the envelope function approach for the [111] and the three obliquely oriented valleys. The experimental intra- and interband data agree qualitatively with the results of the model calculation and are consistent with the predictions of a LCAO calculation according to which the electrons or holes are confined in the Pb_1?xSn_xTe layers.