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.Comment: 17 pages, 19 figures, accepted to PR

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

Varying Calcium Abundances in Solar Flares Seen by the Solar Maximum Mission

We report on calcium abundance A (Ca) estimates during the decay phases of 194 solar X-ray flares using archived data from the Bent Crystal Spectrometer (BCS) on the Solar Maximum Mission (operational 1980–1989). The abundances are derived from the ratio of the total calcium X-ray line emission in BCS channel 1 to that in neighboring continuum, with temperature from a satellite-to-resonance line ratio. Generally, the calcium abundance is found to be about 3 times the photospheric abundance, as previously found, indicating a “first ionization potential” (FIP) effect for calcium, which has a relatively low FIP value. The precision of the abundance estimates (referred to hydrogen on a logarithmic scale with A (H) = 12), is typically ∼± 0.01, enabling any time variations of A (Ca) during the flare decay to be examined. For a total of 270 short time segments with A (Ca) determined to better than 2.3% accuracy, many (106; 39%) showed variations in A (Ca) at the 3 σ level. For the majority, in 74 (70%) of these 106 segments A (Ca) decreased with time, and for 32 (30%) A (Ca) increased with time. For 79 out of 270 (29%) we observed constant or nearly constant A (Ca), and the remaining 85 (31%) with irregular time behavior. A common feature was the presence of discontinuities in the time behavior of A (Ca). Relating these results to the ponderomotive force theory of Laming, we attribute the nature of varying A (Ca) to the emergence of loop structures in addition to the initial main loop, each with its characteristic calcium abundance

Interlayer Coupling in EuS-Based Superlattices Deduced from Neutron Scattering Experiments

The ferromagnetic/diamagnetic semiconductor superlattices, EuS/PbS and EuS/YbSe, were studied by neutron reflectivity. In order to determine the strength of the interlayer coupling, the intensity of the first magnetic Bragg peak vs. applied external magnetic field was measured. Additionally, the in-plane anisotropy and the domain structure were studied by polarized neutron reflectivity. The dependence of the intensity of the antiferromagnetic neutron reflectivity peak vs. magnetic field was simulated using a Stoner-Wohlfarth model. To reproduce the observed spectra it was necessary to take into account the presence of fluctuations of the nonmagnetic layers thickness, by assuming a Gaussian spread of the interlayer coupling constant $J$. For both EuS/PbS and EuS/YbSe superlattices, the best fit was obtained for the directions of the in-plane easy axes, which agree with those determined by polarized neutron reflectivity

Interlayer Coupling in EuS-Based Superlattices Deduced from Neutron Scattering Experiments

The ferromagnetic/diamagnetic semiconductor superlattices, EuS/PbS and EuS/YbSe, were studied by neutron reflectivity. In order to determine the strength of the interlayer coupling, the intensity of the first magnetic Bragg peak vs. applied external magnetic field was measured. Additionally, the in-plane anisotropy and the domain structure were studied by polarized neutron reflectivity. The dependence of the intensity of the antiferromagnetic neutron reflectivity peak vs. magnetic field was simulated using a Stoner-Wohlfarth model. To reproduce the observed spectra it was necessary to take into account the presence of fluctuations of the nonmagnetic layers thickness, by assuming a Gaussian spread of the interlayer coupling constant $J$. For both EuS/PbS and EuS/YbSe superlattices, the best fit was obtained for the directions of the in-plane easy axes, which agree with those determined by polarized neutron reflectivity

Laser-induced plasma emission: from atomic to molecular spectra

International audienceThe aim of this paper is the description of the optical emission spectral features of the plasma produced by laser matter interaction from a fundamental point of view. The laser induced plasma emission spectra are discussed in connection with the basic mechanisms that take place in the plasma phase at different time delays from the laser pulse. Being the laser induced plasma a dynamics system, the hierarchy of the elementary mechanisms changes continuously because the electron number density and the electron temperature decrease during the expansion. As a consequence of this, along the plasma's persistence time, the prevailing emitting species changes from ions to atoms and from atoms to molecules. Both atomic and molecular emission spectroscopy are discussed to convey a complete description of temporal evolution of the laser induced plasma. Current literature, as well as the traditional plasma's theories, are presented and discussed in order to give to the reader a general idea of the potentialities and of the drawbacks of emission spectroscopy for the study of laser induced plasma and the of the various applications. 1. Introduction. Laser Induced Plasma (LIP) is the plasma generated by laser-matter interaction when the laser irradiance exceeds a certain threshold that is a characteristic of any specific materials