Neutron Diffraction Investigation of the 119°K Transition in Magnetite

Neutron diffraction measurements on synthetic single crystals of magnetite confirm the orthorhombic ordering scheme proposed by Verwey and co-workers to explain the changes in physical properties which magnetite undergoes when cooled below 119°K. A magnetic field suitably applied during cooling prevents the multiple twinning which occurs in the absence of a field and allows one to make the following deductions: The spins of all magnetic ions align themselves parallel to the cubic [001] direction nearest the direction of the external field. The face diagonals of the cube orthogonal to this c axis become the orthorhombic a and b axes. If these are labeled such that a is magnetically harder than b, the ferric ions in octahedral sites lie in rows parallel to a and the ferrous ions in rows parallel to b. Although the average oxygen position is unchanged, there are presumably shifts in the four oxygen parameters which compensate for the effect of the ordering on the ionic sizes at the various sites. In attempting to reproduce the results on natural crystals, which had a transition temperature some five to ten degrees lower, it was found impossible to remove the [a,b] twinning by use of a magnetic field; furthermore, failure to observe an (002) reflection would seem to indicate that the twinning persists on a micro basis, that is, that there is only short-range order. A least-squares fit was made to the intensities of the cubic phase at 296°K; a value of $u=0.2548\mathrm{}\pm 0.0002$ was obtained for the oxygen parameter in the spinel structure. The severe primary and secondary extinction encountered was satisfactorily accounted for by methods previously described by the author.