Roger Ward

Abstract:

MBE methods have been used to grow single-crystal Laves-phase DyFe 2-YFe2 superlattice samples with a (110) growth direction. In this paper, magnetisation curves for the YFe2 dominated multilayer samples [wDyFe2/4wYFe2] × N, where w = 20, 30, 40, and 50 Å, and N= 40, 27, 20, and 16, respectively, are presented and discussed. It will be shown that the measured M-H loops are dominated by magnetic exchange springs, which reduce the magnetic coercivity, considerably. In particular, it is possible to engineer samples with a negative coercivity. © 2001 Elsevier Science B.V. All rights reserved.

Abstract:

We report on the magnetisation measurements with a magnetic field applied along the HCP-structure b-axis for Ho/Dy magnetic superlattices (SLs). The zero-field-cool susceptibility, χZFC, shows the magnetic-order coexistence of the Ho and Dy layers, below their respective ordering temperatures. The magnetic-phase diagrams (MPD) of Ho/Dy SLs have been obtained from magnetisation measurements, and we found that the magnetic phases of the Ho and Dy layers are very similar to the bulk ones. Although the analysis of the high-field magnetisation indicates that Ho and Dy do not interact magnetically, this coupling exists at low field. The increment of the Néel temperature in the Ho layers with respect to the bulk could reflect some kind of polarisation between Ho and Dy layers. In the MPD that we have proposed for Ho/Dy SLs, a huge region exists where an helifan phase remains stable for both rare-earth layers, especially for the Ho30/Dy15 SL. This could represent the first time that the helifan phase of Ho layers stabilises a similar phase for Dy layers, due to the coherent propagation of conduction band electrons through Ho and Dy layers. However, at low temperatures, we do not expect to find the ferro-cone phase in Ho layers as in the bulk, because of the epitaxial strain in between the Ho and Dy layers of the superlattice. © 2001 Elsevier Science B.V. All rights reserved.

Abstract:

Single-crystal alloys of HoxTm1-x were grown using the molecular beam epitaxy technique. The components of the magnetic moments along the c-direction and in the basal plane were obtained using neutron diffraction, and the magnetic structures of this system have been determined. The results were compared with calculations based on a mean-field model, and the theoretical predictions are in good agreement with the experimental results. The existence of the penta-critical point at xc = 0.325 and Tc = 67.85 K, which was also predicted by the mean-field model was confirmed by the experimental results. © 2001 Elsevier Science B.V. All rights reserved.

Abstract:

The magnetic ordering in Ce-Y alloys has been investigated using neutron diffraction and X-ray magnetic resonant scattering (XMRS). Neutron diffraction determined the magnetic structure to be antiferromagnetic with magnetic modulation vector [1/2, 0,0] and with moments pointing perpendicular to this direction in the basal plane. XMRS studies revealed that Ce-Y forms an ideal system with the localised 4f electrons characteristic of the other lanthanides. © 2001 Elsevier Science B.V. All rights reserved.

Abstract:

The magnetism of a DHCP Ho-Ce alloy has been investigated using X-ray magnetic resonant scattering. A self-consistent magnetic structure is proposed that agrees with diffraction data for three different cross sections (neutron, dipolar and quadrupolar). The spectroscopic valence of the Ce is found to be 3.27(5), and the Ce is in an intermediate-valence state in this alloy. © 2001 Elsevier Science B.V. All rights reserved.