R.S Sarthour

Magnetism in lanthanide superlattices

Abstract Neutron diffraction studies of heavy rare-earth superlattices have revealed the stabilization of novel magnetic phases that are not present in bulk materials. The most striking result is the propagation of the magnetic ordering through nonmagnetic spacer materials. Here we describe some recent X-ray magnetic resonant scattering studies of light rare-earth superlattices, which illuminate the mechanism of interlayer coupling, and provide access to different areas of Physics, such as the interplay between superconductivity and magnetism. Magnetic X-ray diffraction is found to be particularly well suited to the study of the modulated magnetic structures in superlattices, and provides unique information on the conduction-electron spin–density wave responsible for the propagation of magnetic order.

Surface magnetic phase transitions in Dy/Lu superlattices

Abstract Dy/Lu superlattices comprising ferromagnetic Dy blocks coupled antiferromagnetically across the Lu blocks may be modelled as a chain of XY spins with antiferromagnetic exchange and six-fold anisotropy. We have calculated the stable magnetic phases for the cases of large anisotropy and a field applied along an easy direction. For an infinite chain an intermediate phase 〈1,xa05,…〉 is predicted, where the notation gives the angle between the moment and the applied field in units of π/3. Furthermore, the effects of surface reconstruction are determined for finite chains. A [Dy 20 Lu 12 ] 20 superlattice has been studied using bulk magnetization and polarized neutron reflectivity. The 〈1,xa05,…〉 phase has been identified and the results provide direct evidence in support of the theoretical predictions. Dipolar forces are shown to account for the magnitude of the observed exchange coupling.

The magnetism of Ce–Y alloys

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.

X-ray resonant scattering study of spin-density waves in a superlattice

The magnetic structures of Nd/Pr superlattices have been investigated using x-ray magnetic resonant scattering. Magnetic dipole resonances were observed at the edges of both components of the superlattices, proving that a coherent spin-density wave is established in the 5d bands of both elements. Multi- q magnetic structures were induced in the Pr, and the two components of the superlattice were found to adopt identical magnetic modulation vectors. Magnetic superlattice peaks were observed in Q-scans at both edges and from these we were able to determine the magnetization profiles in each component of the superlattice separately. From the width of the magnetic peaks in the wave-vector, , we were able to measure the magnetic coherence of the Nd, and the results demonstrate that the observed spin-density wave in the Pr is responsible for the coupling between successive Nd blocks.

X-RAY RESONANT SCATTERING STUDY OF AN INTERMEDIATE-VALENCE HO-CE ALLOY

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.

The magnetic structures of HoxTm1-x thin film alloys

Abstract Single-crystal alloys of Ho x Tm 1− 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 x c =0.325 and T c =67.85xa0K, which was also predicted by the mean-field model was confirmed by the experimental results.

Diffuse magnetic scattering from DHCP HoxCe1-x alloys

Abstract The magnetic ordering in Ho x Ce 1− x alloys has been investigated by neutron diffraction and X-ray magnetic resonant scattering (XMRS). The DHCP alloys display a rich phase diagram, and highly unusual, diffuse neutron magnetic scattering is observed over the composition range 0.35⩽ x ⩽0.5. The diffuse intensity may be the magnetic response of the conduction band, and this interpretation is supported by the XMRS measurements, which show that there is hybridisation of the Ce 4f electron with the conduction electrons.

Induced Magnetic Order in Nd/Pr Superlattices

Abstract Single-crystal Nd/Pr superlattices have been studied using resonance-enhanced magnetic X-ray scattering. Magnetic resonances were observed at the L II edges of Nd and Pr, so it was possible to study the magnetism of both components of a superlattice separately for the first time using synchrotron X-rays. The results demonstrate that the 5d bands are magnetically polarised in both elements, and suggest that the induced ordering in the Pr is localised. The induced moment profile across the Pr blocks has been determined and the results correlated with the observed magnetic coherence of the Nd. Thus new insight is obtained on the nature of the induced ordering of the Pr and on the interlayer coupling mechanism.

MAGNETISM AND SUPERCONDUCTIVITY IN NEODYMIUM/LANTHANUM SUPERLATTICES

Abstract A single-crystal Nd30La10 superlattice grown using molecular beam epitaxy is found to consist of alternating antiferromagnetic and superconducting layers at low temperature. The superlattice has the DHCP crystal structure, and the stacking sequence of close-packed planes is coherent over many bilayers. Magnetization studies reveal the onset of superconductivity at a temperaturre comparable to bulk DHCP La, and the results suggest coupling across the antiferromagnetic Nd layers. The magnetic structures, investigated using neutron diffraction techniques, resemble those found in bulk Nd. For the cubic sites of theDHCP structure the magnetic order is confined to individual Nd blocks. However, the magnetic order on the Nd hexagonal sites propagates coherently through the La, even when it becomes superconducting.