Magnetic semiconductors attract a lot of interest for possible future use in spin electronics. Typical examples are Eu chalcogenide semiconductors EuX (X = O; S; Se; Te), which are classical Heisenberg magnets due to the direct and indirect nearest (NN) and next nearest (NNN) neighbour exchange interactions between the localised magnetic moments of Eu2+ ions (spin 7/2). In the Eu chalcogenides, the pronounced dependence of the nearest and next-nearest neighbour exchange interactions on the interatomic distances strongly affects their magnetic properties. Hence, the biaxial lattice distortions produced in strained multilayer structures also have a major effect on the magnetic properties. EuSe is particularly interesting due to its metamagnetic behaviour caused by the near balance between the ferromagnetic NN exchange interaction J1 and the antiferromagnetic NNN exchange interaction J2. As a result, bulk EuSe shows several magnetic phase transitions between different antiferromagnetic, ferrimagnetic, as well as ferromagnetic phases.

Fig. 101: (a) Anomalous XRD reciprocal space map around the (111) Bragg point of a 30 x [10 ML EuSe / 23 ML PbSe] SL. Clearly visible are the central superlattice peak SL0, the ±1 satellites, the PbSe buffer, and the BaF2 substrate peaks. The cross section of the RSM along qz shown in (b) clearly reveals all 28 thickness fringes corresponding to 30 SL periods.

We studied the growth and the magnetic properties of multilayers of strained EuSe intercalated with diamagnetic PbSe1-xTex spacers. Strained EuSe/PbSe1-xTex superlattices (SLs) were grown on top of µm-thick buffer layers in which the ternary composition was varied between x = 0 and 25% to adjust the strain state of the magnetic EuSe layers from -1.1 to +0.2%. In the SLs, the EuSe layer thickness was kept at 10 or 16 ML, whereas the spacer layer thickness varied from 10 to 100 ML. Thus, the final strain state in the EuSe layer is determined by the lattice constant of the spacer layers, as well as by the thickness ratio between the magnetic and nonmagnetic layers. To determine the structural quality and sharpness of the heterointerfaces, anomalous high resolution X-ray diffraction (XRD) was performed at the beamline ID01. By setting the X-ray wavelength to the Pb-Mv absorption edge around 5 Å, the chemical contrast between the EuSe and PbSe1-xTex layers can be drastically enhanced. Figure 101a shows the resulting reciprocal space map (RSM) recorded around the (111) reciprocal lattice point of a 30 period 10 ML EuSe/23 ML PbSe SL. Apart from the substrate, the buffer as well as central superlattice peaks, finite thickness fringes arising from the SL stack as a whole are clearly resolved between the ±1 order satellites. This is demonstrated in Figure 101b by the qz cross section extracted from the RSM, where in the enlarged region all (N – 2) = 28 maxima of the interference function of the 30 bilayers are clearly observed. This proves a coherent growth of the crystal lattice throughout the complete multilayer structure. From the detailed analysis of the anomalous XRD, the precise EuSe and PbSe1-xTex layer thicknesses were obtained. To determine the in-plane lattice constant a|| and strain of the SL structures, we have recorded reciprocal space maps around the (264) asymmetric reflection for all samples. For the strained EuSe/PbSeTe superlattices, pronounced changes of the Néel temperature TN were observed as a function of the in-plane strain in the layers. For the smallest -1.1% compressive strain, corresponding to a|| = 6.122 Å, TN increases up to 6.5 K whereas for a|| = 6.20 Å, i.e., +0:2% tensile strain, a TN value of only 3.5 K was found. Plotted versus a|| , the TN values for the whole series of samples show a linear behaviour with a slope ∂TN / ∂a|| = 37 K/Å (inset in Figure 102a), from which we obtain a linear change of the FM exchange constant J1. In order to map out the whole magnetic H-T phase diagrams of the samples, susceptibility (T, H) measurements at various static magnetic fields H of up to 1 T were performed. The observed peaks in the (T) curves, indicative for the phase transitions, are compiled as H-T phase diagrams, which are shown in Figure 102 for the bulk-like EuSe sample (a) and the -1.1% strained EuSe/PbSe SL (b). The H-T diagram of the thick EuSe epilayer (Figure 102a) shows similar phase boundaries as known for bulk EuSe, but with slightly increased transition temperatures and critical fields. For the -1.1% strained SL sample, the AFM phase in the H-T diagram of Figure 102b extends to significantly higher temperatures up to TN = 6.5 K, and depicts higher critical field values for transitions from the AFM II to the FiM and from the FiM to the FM phase, respectively.

Fig. 102: H-T phase diagrams for unstrained (a) and -1.1% strained (b) EuSe derived from susceptibility measurements (T,H) (black circles) at various static magnetic fields. inset: TN of all strained superlattices (circles) and one bulklike EuSe sample (square) plotted as a function of the in-plane EuSe lattice constant a||.

In summary, we have demonstrated that in EuSe/PbSe1-xTex multilayers the strain imposed on the magnetic layers influences considerably the magnetic properties of the semiconductor EuSe.


Principal Publication and Authors

R.T. Lechner (a), G. Springholz (a), T.U. Schülli (b), J. Stangl (a), T. Schwarzl (a) and G. Bauer (a), Phys. Rev. Lett. 94, 157201 (2005).
(a) Institut für Halbleiter- und Festkörperphysik, Johannes Kepler Universität, Linz (Austria)
(b) Commissariat à l’Energie Atomique, Grenoble (France)