Dy Layer Research Articles

MAGNETISM AND SUPERCONDUCTIVITY IN NANOSTRUCTURED Nb-Dy SYSTEMS

We have prepared Nb/Dy/Nb trilayers to study size effects on the magnetic and superconductor properties and to look at the influence of the magnetic layer on a superconductor layer in a nanoscale system. We find size effects on the magnetic properties of the Dy layer for thickness less than 10 nm and size effects associated with the superconductor layer for Nb layers of thickness less than 50 nm. In addition increasing the thickness the Dy layer reduces the superconductor transition temperature of the Nb layer.

Heterogeneous bilayer films NiFe (Fe)-Dy: magnetic circular dichroism and Dy spin ordering

Results of the magnetic circular dichroism (MCD) and Auger electron spectroscopy (AES) investigations in the 3d transition metal-Dy bi-layer films are presented. It is shown that even at room temperature the Dy layer makes a contribution to MCD of the bi-layer film, which corresponds to the MCD value in the single-layer Dy film measured below T C =85 K . According to the AES data there is no sharp interface between 3d and Dy layers in these films. Some amount of Ni and Fe atoms is dispersed in the Dy layer and some amount of Dy atoms is dispersed in the 3d layer. The comparison of the MCD and AES data allows one to suppose the Dy layer in the bi-layer films to be magnetically ordered at room temperature under the influence of the 3d-layer spin system. The influence spreads to long distances inside Dy layer through the 3d-ions dispersed in it.

The growth of pinhole-free epitaxial DySi2−x films on atomically clean Si(111)

The growth of pinhole-free epitaxial DySi2−x films on atomically clean Si(111) has been achieved by depositing a 2-nm-thick Dy layer onto Si(111) with a 1.5-nm-thick capping amorphous Si (a-Si) layer at room temperature followed by annealing at 700 °C in ultrahigh vacuum. The thickness of the a-Si was selected to be such that the consumption of Si atoms from the substrate is minimized by taking into account the formation of an amorphous interlayer at the Dy/Si(111) interface. Based on our experimental findings, a new mechanism for the formation of pinhole is proposed. The Stranski–Krastanov growth behavior of epitaxial DySi2−x on Si(111) by solid phase epitaxy leads to the apparently random formation of a high density of recessed regions at the initial stage of silicidation. Polycrystalline DySi2−x was found to be present at the areas inside and epitaxial DySi2−x outside the recessed regions. Large numbers of Si atoms from the substrate can therefore diffuse through the recessed regions. As a result, the depth and size of the recessed regions increase with annealing time. Finally, the DySi2−x thin layer inside the recessed regions with higher interface energy is thermally unstable and breaks apart to form pinholes.

Magnetic properties of (11̄02) Dy/Y superlattices

Epitaxial (11̄02) Dy/Y rare earth superlattices and a thick (11̄02) Dy film, grown on sapphire with Y/Ta buffer layers, have been prepared by molecular beam epitaxy. Neutron diffraction and SQUID magnetization measurements on a single 200 nm thick (11̄02) Dy layer showed nearly bulk behavior. However, for the (11̄02) Dy/Y superlattices we found different magnetic behavior depending on the relative Dy to Y-layer thickness. The superlattices exhibit both ferromagnetic and helical phases, but with the Néel and Curie temperatures significantly different from bulk Dy. These results differ from previous findings in Dy/Y superlattices grown along [0001].

Structure and magnetism of dysprosium/zirconium multilayers

Dy/Zr bilayers and multilayers were grown on oxydized Si(100) substrates by evaporation under ultrahigh vacuum conditions. X-ray and electron diffraction shows that both Zr and Dy layers are hexagonal, strongly textured with the c-axis along the growth direction. The grains can extend over several layers and have an in-plane diameter of about 200 Å. The Dy lattice displays an in-plane compression of about 1.5%. Interface profiles determined from high-resolution electron microscopy studies are sharp, extending over less than 3 lattice spacings. Magnetization measurements and polarized neutron reflectometry reveal a strong enhancement of T C, accompanied with a suppression of the helical antiferromagnetic phase in the multilayer samples. This effect can be understood as the result of the change in the balance between exchange and magnetoelastic energies caused by in-plane epitaxial compression of the Dy lattice by Zr. A large out-of-plane magnetic anisotropy energy is observed when the Dy layers are made thinner than 15 Å. Volume and interface contributions to this anisotropy compensate each other for a Dy layer thickness of about 8 Å. Comparison is made between the results of crystal-field calculations based on the point-charge approximation and the measured anisotropy energy.

The magnetic properties of DyFeCo thin films

A dc magnetron co-sputtering system incorporating a rotating substrate table has been used to produce Dy-FeCo films with a multilayered structure. For this study a series of films were produced in which the periodicity was varied whilst the overall composition and film thickness were kept constant. The magnetic, thermo-magnetic and magneto-optic properties of the films have been measured in an attempt to establish the best layer structure and periodicity for good thermo-magnetic recording characteristics. For the two compositions studied, Dy22Fe68.5Co9.5 and Dy28Fe52Co20, the optimum characteristics were found for films in which the rare earth Dy layer is approximately one atomic layer thick.

Linear-dichroism studies of thin Dy overlayers on Ni(110) and Cu(110) substrates.

In this paper we present polarization-, temperature-, and thickness-dependent x-ray-absorption measurements on extremely thin Dy overlayers. For Dy overlayers deposited at room temperature on Cu(110) and (magnetic) Ni(110) we observe strong linear dichroism. We deduce that the nature of the dichroic effects is predominantly electrostatic. When Dy is evaporated onto Ni(110) and kept at 50 K almost no dichroism is observed. Deposition of Dy on Cu(110) at 50 K, on the other hand, yields strong dichroism. We discuss these effects in greater detail and formulate a model for the overlayer system that seeks to explain the observed differences. Finally, a strong decrease of dichroism is observed as the Dy layer thickness increases, a phenomenon which is independent of the substrate. We propose a mechanism to explain this that involves counteracting contributions to the dichroism from both the overlayer-substrate and surface-vacuum interfaces. We illustrate this with an experiment involving a Ho probing layer.

Neutron scattering studies of rare earth magnetic multilayers

Single crystal artificial superlattices prepared by molecular beam epitaxy and consisting of Dy, Er, and Gd layers alternated with Y all exhibit long-range magnetic order that is propagated through intervening non-magnetic Y layers as shown by neutron diffraction studies. In the case of Dy systems, the phase and chirality of the helical order are preserved from one Dy layer to another. The phase shift across the Y layer is proportional to the Y layer thickness, and so is in general a non-integral multiple of π, suggesting that the coupling is propagated by a spin-density-wave mechanism. The magnetic coherence range, obtained from a series of samples of constant Dy thickness and varying number of Y layers, shows a linear variation with the inverse of the Y layer thickness.

Neutron Diffraction Studies on Magnetic Properties of Fe/Dy Artifical Superstructured Films

Neutron diffraction technique is applied to the study of magnetic/magnetic artificial superstructured films (ASF's) composed of alternate stackings of Fe and Dy layers. Neutron and conventional magnetization experiments are combined, and the contributions of the Fe and Dy layer to the magnetizations are separately estimated. Based on temperature and magnetic field dependences of the magnetizations, magnetic properties of Fe/Dy ASF's are discussed. It is indicated that, besides a strong magnetic anisotropy of Dy atoms, an antiferromagnetic coupling of Fe and Dy moments at the interface plays an important role on the magnetic behavior of Fe/Dy ASF's.

Magnetic structure of Dy-Y superlattices.

Two samples of Dy-Y superlattices produced by molecular-beam-epitaxy techniques are shown by neutron diffraction to order magnetically in a helix which is incommensurate with the bilayer thickness. One sample consists of 64 bilayers, each bilayer made up of about 15 growth planes (42 A) of Dy atoms followed by 14 planes (38 A) of Y atoms. The second sample has 90 layers, each layer consisting of 9 Dy atomic planes and 8 Dy/sub 0.5/Y/sub 0.5/ alloy planes. The phase coherence of this ordering extends over several bilayers, and is especially striking in the sample where the layers of localized Dy spins are separated by 14 atomic planes of nonmagnetic Y. The fact that the helix chirality propagates across several bilayers rules out a simple scalar Ruderman-Kittel-Kasuya-Yosida coupling between the Dy planes on either side of an Y layer, but suggests instead that a helical spin density wave is induced in the Y conduction electrons. A simple model for the superlattice structure factor demonstrates that observed asymmetries in the magnetic diffraction-peak intensities can be ascribed to the existence of different magnetic modulation wave vectors in each layer type (Dy and Y). In these superlattices the strain clamping by the intervening non-Dymore » layers and the substrate suppresses the first order ferromagnetic transition found in bulk Dy in both zero and finite fields. Although the planar magnetostriction is clamped, it is observed that the application of a magnetic field in the basal plane produces at low temperatures a second order irreversible transition to a metastable ferromagnetic state. At high temperature the magnetization process is initiated by a reduction of the helical coherence length due to a random-field coupling to the uncompensated Dy layer moment.« less

Fast Neutron TL Dosimetry by Knock-on Protons: Energy Deposition in Thin Dosemeter Layers

Thin layer dosemeters are suitable for rapid laser heating and for the detection of knock-on protons produced in hydrogenous materials by collisions with fast neutrons. In an attempt to establish the optimum design parameters for a fast neutron dosemeter based on such a proton radiator, we calculate the proton conversion efficiencies and the resulting normalised TL signals for CaSO4:Dy layers of various thicknesses. On the basis of the known neutron energy spectra of 252Cf, moderated 252Cf, and Pu-Be neutron sources, the generation of protons in polyethylene foils, placed in close contact with the TLD layer, and the energy deposited by these protons in the CaSO4:Dy layer are computed. The results are compared with experiments performed in mixed neutron-gamma fields with 9 mm2 spots of this TL phosphor on borosilicate glass substrates heated with a computer-controlled CO2 laser beam of uniform intensity profile.

Fast Neutron TL Dosimetry by Knock-on Protons: Energy Deposition in Thin Dosemeter Layers

Abstract Thin layer dosemeters are suitable for rapid laser heating and for the detection of knock-on protons produced in hydrogenous materials by collisions with fast neutrons. In an attempt to establish the optimum design parameters for a fast neutron dosemeter based on such a proton radiator, we calculate the proton conversion efficiencies and the resulting normalised TL signals for CaSO4:Dy layers of various thicknesses. On the basis of the known neutron energy spectra of 252Cf, moderated 252Cf, and Pu-Be neutron sources, the generation of protons in polyethylene foils, placed in close contact with the TLD layer, and the energy deposited by these protons in the CaSO4:Dy layer are computed. The results are compared with experiments performed in mixed neutron-gamma fields with 9 mm2 spots of this TL phosphor on borosilicate glass substrates heated with a computer-controlled CO2 laser beam of uniform intensity profile.