A del Moral

The reversible susceptibilities of dysprosium and terbium

The initial susceptibilities of Dy and Tb have been measured in the ferromagnetic and antiferromagnetic phases. In the ferromagnetic phase it is concluded that the susceptibility arises from domain wall displacements and that these are limited by an intrinsic restoring force resulting from the very narrow domain wall and not by local impedances due to imperfections and impurities. From measurements in the antiferromagnetic phase it is concluded that the often quoted two constant expression for the exchange coupling between sublattices cannot provide a quantitative explanation for the observed behaviour but that the susceptibility of Dy and the temperature variation is quantitatively consistent with the seven exchange constants and the turn angles determined from neutron scattering measurements. The reversible susceptibility measured in the presence of external fields shows a complicated behaviour and does not appear to be consistent with the predictions of the theory of Nagamiya et al. (1962).

Magnetostriction of some cubic rare earth-Co2 compounds in high magnetic fields

Magnetostriction measurements have been carried out in the cubic Laves phase compounds DyCo2, HoCo2 and ErCo2 from 10K to well above their respective Neel temperatures TN. The observed magnetostrictions are very large ( approximately 10-3). The measurements confirm the extremely high anisotropy of these materials. The expected values of the saturation magnetostriction at 0K are similar in sign and magnitude to those found in the corresponding rare earth metals. This fact and the scaling of magnetostriction with rare earth sublattice magnetization indicates that the rare earth ion is the main source of the magnetostriction. The metamagnetic transition above TN has been studied, the relation between critical field and temperature being nonlinear for HoCo2 and ErCo2. The compounds are highly anisotropic above TN and all the features indicate that the field-induced phases are likely to be ferrimagnetic.

Exchange force explanation for the antiferromagnet-ferromagnet transition in dysprosium

An explanation based on exchange forces is proposed for the antiferromagnet-ferromagnet transition in Dy which is in good agreement with the available experimental results. Contrary to the usual idea that the driving mechanism is the magnetostriction and anisotropy in the basal plane of the HCP structure it is shown that the exchange forces are mainly responsible for the transition. Magnetoelastic, anisotropy and applied magnetic field energies only provide the delicate balance of energy necessary to assist the exchange forces for the transition. These energies only amount to a relatively small fraction (<20%) of the total driving energy.

Magnetoelastic coupling in DyAl2

Abstract The magnetostriction of a single-crystal of DyAl 2 has been measured in the ordered and paramagnetic phases along [100] easy axis. Volume thermal expansion and forced magnetostriction have been measured simultaneously along three orthogonal directions on the same sample. We report a set of magnetoelastic CEF parameters using a single-ion model, incorporating high order CEF magnetoelastic contributions.

Magnetic moment and magnetostriction of ternary TbxGd1-xAl2 intermetallic compounds

The magnetostriction and magnetic moment of the pseudo-binary compounds TbxGd1-xAl2 (0.05<or=x<or=1) have been measured throughout the ordered state, on single crystals and polycrystals. A pair of crystal-field (CEF) parameters suffice to explain the behaviour of the whole series. The magnetoelastic constants M2gamma and M2E have been determined, and they remain practically constant across the series, and in reasonable agreement with the point charge model. The Tb3+ magnetic moment remains constant with Tb content and unquenched. The overall consequence is that the single-ion CEF model applies remarkably well to this series of compounds, in fact as well as does the mean-field approximation for random pseudo-binary alloys including Tb-Gd and Tb-Tb exchange interactions.

Effects of screening by conduction electrons on the crystal electric field in REAl2 intermetallic compounds

The fourth-order B4 and sixth-order B6 crystal electric field (CEF) parameters are evaluated for NdAl2, taking into account the contribution of the whole lattice of ions, by the use of reciprocal space according to the Duthie-Heine technique. Screening of the CEF by free conduction electrons gives a parameter B4 of opposite sign to the experimental value. This becomes correct if the screening is removed, the point-ion limit taken and the Al charge made zero. A simple explanation of this striking and unexpected result is offered by screening the rare-earth spheres with electrons with d character, while keeping the Al ones screened by free electrons. The situation for B6 is less satisfactory, the calculated value being one order of magnitude smaller than the experimental one. These conclusions can be extended to the rest of the REAl2 series (RE represents a rare earth). The effect of some internal structure on the ions is also considered.

Effects of screening on crystal-field magnetoelastic coupling in rare-earth intermetallic compounds

The effect of screening by conduction electrons on the magnetoelastic coupling due to the crystal field in the Laves-phase rare-earth (RE) intermetallics is calculated and applied to REAl2 compounds. The Thomas-Fermi and Hartree approximations for the screening are considered. Tetragonal, orthorhombic and rhombohedral strains are considered through the distortion of the lattice and of the screening and ligand-ion charge densities. The effect of the screening on magnetoelastic parameters is substantial. The introduction of conduction band exchange is able to modify the sign of the tetragonal parameter compared with the bare-ion point-charge model. The Friedel oscillations of the crystal potential modify the sign of the rhombohedral mode as well. A comparison with available magnetoelastic data for the compounds NdAl2, PrAl2 and TbAl2 is presented and good agreement with calculations is obtained if conduction electron band exchange is envisaged.

Magnetoelastic properties and magnetization in Gd1−xTbxAl2 intermetallics

Abstract The magnetic moment and magnetostriction of the intermetallic compounds Tb x Gd 1- x Al 2 (0.05≤ x ≤1) have been measured throughout the ordered state. The same couple of crystal field (CEF) parameters explain the beahviour of the whole series. The determined magnetoelastic coupling constants M 2 γ and M 2 ϵ remain constant across the series. The Tb 3+ magne tic moment does not change with Tb content and remains unquenched. The consequence is that the single-ion CEF model applies remarkably well, as well as the mean field approximation.

Point-charge crystal-field calculation of magnetoelastic and elastic constants for rare earth intermetallic compounds with the CsCl structure

Expressions for the first and second strain derivatives of the single-ion crystal-field Hamiltonian have been systematically calculated for the rare earth intermetallic compounds with the CsCl structure. The calculations were performed on the basis of the point-charge model considering the contributions of the rare earth and metal ions. The Hamiltonian is used to calculate the crystal-field contribution to the elastic constants and to the equilibrium strains.