Anisotropy Of Gd Research Articles

On the possibility of anisotropic heat flow in the inner core

We consider the possibility of anisotropic heat flow in the inner core by examining the potential for anisotropic thermal conductivity of hexagonal close-packed (hcp-)Fe. Because hcp-Fe exists only at pressures above 13 GPa at room temperature, we investigate thermal conductivity anisotropy in analog material Gd by measuring the electrical conductivity and applying the Wiedemann–Franz Law to determine thermal conductivity (k). The electrical conductivity anisotropy of Gd was measured at pressures up to 1.4 GPa and temperatures up to 873 K in the hcp phase range. At elevated pressure, the variation with temperature of anisotropic thermal conductivity of Gd single crystal resembles the anisotropic behavior at high temperature and 1 atm observed in earlier work. The temperature range of anisotropy of thermal conductivity of Gd, where kc > ka, is extended by pressure, but the anisotropy disappears before the high temperature hcp[Formula: see text]bcc (body-centered cubic) transformation. Our results on hcp-Gd lead us to raise the question of the possibility of hcp-Fe exhibiting anisotropy of thermal conductivity. Together with the known seismic anisotropy of the inner core, and the inferred textural alignment of hcp crystals causing it, we suggest some implications that an anisotropy of thermal conductivity of hcp-Fe, and a concomitant anisotropy of inner core heat flow, could have on thermally driven core processes.

Electrical Resistivity Anisotropy of Gd at High Pressure

Measurements of electrical resistivity of single- and polycrystalline Gd were performed in a large volume cubic anvil press in the h.c.p. phase region at room temperature up to 16 kbar. The anisotropy of electrical resistivity of h.c.p. Gd, observed at room pressure and temperature, is maintained at elevated pressure for the entire h.c.p. range. When the low pressure dependence of anisotropy is extrapolated to higher pressure beyond the stability region of the h.c.p. phase, the anisotropy disappears at approximately 205 kbar. The pressure-induced decrease in anisotropy is discussed in terms of related anisotropies of compressibility and the spin-disorder component of electrical resistivity.

Magnetic anisotropy in Gd/(FeCo) and Gd/Fe multilayers for high density magneto-optical recording

Very recently, new techniques in the magneto-optical recording were proposed which promise the readout of domains smaller than the optical diffraction limit. For this purpose Gd–(Fe,Co) alloy films are used, which change their magnetization direction with temperature. In this experimental study for the first time spin reorientation effects in multilayers of Gd/(FeCo) and Gd/Fe were investigated with varying composition and periodicity. Both Gd/(FeCo) and Gd/Fe films show a compositional dependence of the anisotropy similar to a phase transition. An in-plane to perpendicular spin reorientation near the compensation temperature Tcomp can be obtained in rare earth rich films for small periodicities λ∼1.1 nm, but for increasing λ the anisotropy change occurs at temperatures different from Tcomp. The opposite effect, i.e., spin reorientation from perpendicular to in-plane, can be observed in transition metal rich Gd/(FeCo) multilayers but not in Gd/Fe multilayers. The anisotropy behavior and the reorientation effects are explained qualitatively within a new model.

A new theory for magnetic anisotropy in the heavy rare earth metals

A theory of bandwidth anisotropy in metallic ferromagnets developed previously is specialised to the case of 5d electrons in a hexagonal close-packed lattice. This theory is combined with a model for 5d electrons in the heavy rare earth metals to give a new theory for the low temperature values of the magnetic anisotropy coefficientsκ 2 0 andκ 4 0 in Tb, Dy, Ho, Er and Tm. In this theory the magnetic anisotropy is due to a combination of (i) crystal fields acting on 5d and 4f electrons and (ii) bandwidth anisotropy associated with a dependence of 5d bandwidths on magnetization direction. After use is made of empirical upper limits on the eighth order magnetic anisotropy in Gd, there remain four partially adjustable parameters of importance in the theory. These can be chosen to give a good fit to the six observed values forκ 2 0 andκ 4 0 in Tb, Dy and Ho. Crystal fields corresponding to negative point charges are seen by 5d electrons, but because of 4f – 5d interactions effective fields of larger magnitude and opposite sign act on 4f shells. Bandwidth anisotropy gives a significant contribution toκ 4 0 of opposite sign to that due to crystal fields, and dominates the latter in Tb and Er.