Oscillating Antiferromagnetism of Ultrathin EuTe Layers

Abstract

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.PACS numbers: 75.50. Εe, 75.50.Ρp, 76.50.+g Usually it is believed that in a macroscopic, 3-dimensional antiferromagnet (AF) each magnetic sublattice is statically magnetised, but the magnetic moments compensate each other. Α more sophisticated discussion of the nature of AF, however, shows that the AF coupling leads rather to an AF correlαtion of the sublattices, but not necessarily to their static magnetization [1][2][3]. Someadditional coupling, e.g., to a nuclear spin system, can lead to a symmetry breakdown and thus to the formation of static staggered magnetization. So far, there is no solid experimental evidence whether an AF is of static or of oscillatory character. In macroscopic systems the oscillation frequency tends to zero. In finite systems, the socalled macroscopic quantum coherence (MQC) rate is discussed. Some resonances, with their frequency depending on the size of the AF, were found in AF nanoclusters [4], but their interpretation induced some controversy [1].Twodimensional AF's are intensively investigated since some classes of high temperature superconductors are 2D antiferromagnets [5]. AF correlations are well evidenced but no static staggered magnetization is proven in these materials. Moreover, there is a concept according to which coupling of conduction carriers to "oscillating" AF planes leads to the formation of superconducting pairs. Thin AF MnTe and MnSe layers were investigated by neutron diffraction. It was shown

Correlations in Magnetic-Nonmagnetic Semiconductor Superlattices - Simple Model

“…This means that whatever the total magnetic moment of the single barrier is (i.e., zero as for even m or finite as for odd m) the lower energy corresponds to the superlattice with zero average magnetic moment, in agreement with the results of SQUID measurements of magnetization and magnetic susceptibility of EuTe/PbTe superlattices [7].…”

IV-VI Semimagnetic Semiconductors: Recent Developments

“…The best studied structure is EuTe-PbTe/BaF 2 (antiferromagnet-diamagnet) system grown by molecular beam epitaxy [22][23][24][25]. The neutron diffraction studies of these structures have demonstrated the presence of the specific satellite magnetic diffraction peaks indicating the existence of the exchange interaction between antiferromagnetic EuTe layers via the diamagnetic PbTe layer.…”

Theoretical studies of model thin EuTe films with surface elastic stresses

“…For the thinnest¯lm (with n = 1), however, the approximation (although crude) is still acceptable. For instance, the experimental measurements for EuTe with n = 1 show [19] that there exists a ferromagnetic ordering, which fact is in contrast to the Mermin-Wagner theorem, but is in agreement with MFA predictions [18]. In order to increase the approximation of the description, one has to go beyond MFA, and to use the methods incorporating the spin-pair correlations.…”

“…On the other hand, for n = 1, a single plane of EuTe (which should be surrounded by the substrate containing Te) is always ferromagnetic in the ground state, because of the lack of antiferromagnetic neighbours [10].…”

“…For the set of n linear equations (19), by putting the set determinant equal to zero, the continuous phase transition temperatures can be found. It is worth to stress here, that among n formal solutions resulting from the determinant, the physical one must be chosen, which corresponds to the lowest Gibbs energy.…”