Increased magnetic moment induced by lattice expansion from a-Fe to a' -Fe8N

Buffer-free and epitaxial a-Fe and a' -Fe8N x thin films have been grown by RF magnetron sputtering onto MgO (100) substrates. The film thicknesses were determined with high accuracy by evaluating the Kiessig fringes of X-ray reflectometry measurements allowing a precise volume estimation. A gradual increase of the nitrogen content in the plasma led to an expansion of the iron bcc unit cell along the [001] direction resulting finally in a tetragonal distortion of about 10% corresponding to the formation of a' -Fe8N. The a-Fe lattice expansion was accompanied by an increase in magnetic moment to 2.61 ± 0.06m B per Fe atom and a considerable increase in anisotropy. These experiments show that-without requiring any additional ordering of the nitrogen atoms-the lattice expansion of a-Fe itself is the origin of the increased magnetic moment in a'-Fe8N.