Magnetic Structure of Europium

Abstract

Neutron diffraction experiments on metallic europium have been conducted over a temperature range from room temperature to 4\ifmmode^\circ\else\textdegree\fi{}K using a sample in the form of europium filings. The data show that the metal undergoes a transition to an antiferromagnetic state at a N\'eel temperature ${T}_{N}$ of 91\ifmmode^\circ\else\textdegree\fi{}K and that this ordering process continues until saturation develops at about 20\ifmmode^\circ\else\textdegree\fi{}K. A model which conforms to the data consists of a helical spin structure with the magnetic moments lying parallel to a cube face and with the rotation axis directed perpendicular to the moments or along the [100] direction. The pitch of the helix was observed to change very slightly with temperature; the period was $3.5a$ at $\frac{T}{{T}_{N}}=1$ and increased to $3.6a$ at $\frac{T}{{T}_{N}}=0.05$ where $a$ is the lattice spacing. On the basis of the above model and the intensities of the magnetic reflections, the calculated ordered moment at each atom site is 5.9\ifmmode\pm\else\textpm\fi{}0.4 ${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ per atom. This measured moment is somewhat less than the maximum theoretical value of 7 ${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ expected from divalent europium characterized by a spin only ground state of $^{8}S_{\frac{7}{2}}$. Values of magnetic moment times magnetic form factor, $\ensuremath{\mu}f$, as calculated from Eu coherent magnetic reflections are in agreement with similar values of $\ensuremath{\mu}f$ derived from the compound EuO. The intensities of the magnetic diffraction peaks deviate from a Brillouin behavior and instead follow a temperature dependence proportional to ${({T}_{N}\ensuremath{-}T)}^{\frac{1}{2}}$ over a considerable region below the N\'eel temperature. The Debye temperature, as derived from the nuclear peak intensities, exhibited a variation from 70 to 120\ifmmode^\circ\else\textdegree\fi{}K over a corresponding sample temperature range of 100 to 293\ifmmode^\circ\else\textdegree\fi{}K.