Showing papers by "W. C. Koehler published in 1968"

Magnetic Structures of Er 2 O 3 and Yb 2 O 3

Abstract: The magnetic structure of ${\mathrm{Er}}_{2}$${\mathrm{O}}_{3}$ and ${\mathrm{Yb}}_{2}$${\mathrm{O}}_{3}$ have been determined by analysis of neutron diffraction data on powders and single crystals. Noncollinear antiferromagnetic structures were found, with the moment direction related to the local symmetry axis. For ${\mathrm{Er}}_{2}$${\mathrm{O}}_{3}$, the moment on the ${C}_{2}$ site is $(5.36\ifmmode\pm\else\textpm\fi{}0.08){\ensuremath{\mu}}_{B}$ and at the ${C}_{3i}$ site the moment is $(6.06\ifmmode\pm\else\textpm\fi{}0.23){\ensuremath{\mu}}_{B}$. For ${\mathrm{Yb}}_{2}$${\mathrm{O}}_{3}$, the corresponding moments are $(1.86\ifmmode\pm\else\textpm\fi{}0.06){\ensuremath{\mu}}_{B}$ and $(1.05\ifmmode\pm\else\textpm\fi{}0.06){\ensuremath{\mu}}_{B}$. The N\'eel points are 3.4\ifmmode^\circ\else\textdegree\fi{}K for ${\mathrm{Er}}_{2}$${\mathrm{O}}_{3}$ and 2.3\ifmmode^\circ\else\textdegree\fi{}K for ${\mathrm{Yb}}_{2}$${\mathrm{O}}_{3}$. Calculations of the dipole-dipole energies for the observed structures indicate that the dipole forces are not sufficient to explain the structures. The dependence of the long-range magnetic order on the reduced temperature is the same in both systems. Good agreement with this temperature dependence was obtained by assuming a biquadratic exchange interaction and making a simple molecular-field approximation.

Magnetic Properties of Rare Earth‐Lanthanum Alloys

Abstract: Alloys of the heavy rare earth metals with La have been studied by neutron‐diffraction methods. For alloys with 15 at.% or less of La the alloys have the simple hcp structure. The influence of La on the magnetic structures of the metals is such as to enhance the stability of a ferromagnetic configuration relative to an oscillatory one. The transition temperature is lower and the initial turn angle (when it exists) smaller in a lanthanum alloy than in the corresponding yttrium alloy. In the lanthanum‐rich region, the system La–Tb has been most extensively studied with five compositions ranging from Tb0.10La0.90 to Tb0.75La0.25 having been investigated. Of these, the compositions Tb0.20La0.80, Tb0.40La0.60 and Tb0.50La0.50 formed a single phase alloy with the double hexagonal lanthanum structure (dhcp). The 4.2°K diffraction patterns of these three alloys exhibited a number of broad diffraction maxima rather similar to those found for Nd in the antiferromagnetic region. Neel temperatures of approximately 50°, 68°, and 75°K, respectively, were estimated for the three specimens. The specimen Tb0.75La0.25 exhibited lines from the hcp and dhcp structures as well as of a third phase which is probably the δ phase. In the Tb0.10La0.90 alloy both a dhcp and fcc phase were observed at room temperature; the low‐temperature pattern exhibited magnetic reflections characteristic of the dhcp alloys. The La‐rich Tb alloys thus are antiferromagnetic, rather than ferromagnetic as had been proposed previously.