Magnetic Structure of Europium

Abstract

Neutron diffraction experiments on metallic europium have been conducted over a temperature range from room temperature to 4°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 Néel temperature $T_N$ of 91°K and that this ordering process continues until saturation develops at about 20°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 $\frac{T}{T_N}=1\mathrm{}$ and increased to $3.6a$ at $\frac{T}{T_N}=0.05\mathrm{}$ 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±0.4 ${\mathrm{\mu }}_{\mathit{B}}$ per atom. This measured moment is somewhat less than the maximum theoretical value of 7 ${\mathrm{\mu }}_{\mathit{B}}$ 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, $\mu f$, as calculated from Eu coherent magnetic reflections are in agreement with similar values of $\mu f$ 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 ${(T_N-T)}^{\frac{1}{2}}$ over a considerable region below the Néel temperature. The Debye temperature, as derived from the nuclear peak intensities, exhibited a variation from 70 to 120°K over a corresponding sample temperature range of 100 to 293°K.