The Importance of Controlling Composition to Tailor the Properties of Magnetic Thin Films

Many physical properties, for example structural or magnetic, of a material are directly dependent on elemental composition. Tailoring of properties through highly accurate composition control is possible in thin films. This work exemplifies such tailoring. A short review is given of the current status for research in the area of permanent magnets, focusing on rare earth element free alternatives, where FeNi in the L10 phase is a possible candidate. Epitaxial FeNi L10 thin films were successfully synthesized by magnetron sputtering deposition of monoatomic layers of Fe and Ni on HF-etched Si(001) substrates with Cu or Cu100-xNix/Cu buffers. The in-plane lattice parameter aCuNi of the Cu100-xNix buffer layer was tuned by the Ni content. Through matching of aFeNi to aCuNi, the strain state (c/a)FeNi was controlled, where c is the out-of-plane lattice parameter. The 001 reflection indicative of chemical order, as measured by resonant x-ray diffraction, was in most cases split in two peaks due to a composition modulation of Fe and Ni. This chemical disorder contributed to that the uniaxial magnetocrystalline anisotropy energy, KU≈0.35 MJ/m3, was smaller than predicted. In later experiments the composition modulation could partly be compensated for. Remaining discrepancies with respect to predicted KU values were attributed to additional disorder induced by surface roughness of the buffer layer. The interface sharpness between Fe and Ni was explored by producing epitaxial symmetric multilayers with individual layer thicknesses n = 4-48 monolayers (ML). For n ≤ 8 ML the films had pure fcc structure, with antiferromagnetic Fe layers. For n ≥ 8 ML the Fe layers relaxed to bcc structure. A combinatorial sputter chamber, which has the capability to deposit samples with composition and thickness gradients, was assembled. A model for simulation of composition and thickness across large substrates, for the conditions in this chamber, is presented. The model is verified by comparison to experimental data. Some challenges inherent in combinatorial sputtering are discussed, and two experimental studies employing the technique are presented as examples. These investigated magnetic and structural properties of Tb-Co films, with 7-95 at.% Tb, and of amorphous and crystalline ternary gradient Co-Fe-Zr films, respectively