Thickness-dependent magnetism variation for ferrimagnetic rare-earth/transition metal Fe1-xGdx films

The magnetic compensation effect in rare-earth/transition metal alloys, in which the magnetization of the antiferromagnetically aligned rare-earth and transition metal sublattices cancel each other out, can be utilized in a number of novel applications. However, nanoscale composition variations of the rare-earth/transition metal ratio broaden the temperature range of magnetic compensation (i.e., near zero net magnetization) and can even result in the reversal of the dominant magnetic sublattice at a given temperature. We observe a spin reorientation in sputter-grown Fe1-xGdx thin films with nominal x=0.28 as a function of film thickness. Thicker films (> 50 nm thickness) exhibit an in-plane anisotropy near room temperature, while robust out-of-plane anisotropy is observed in thin films (∼16 nm) at all temperatures. Correspondingly, the compensation temperature for thick films is near 25 K, while that for thin films is above room temperature. A film with intermediate thickness (∼35 nm) displays a more complicated evolution of magnetic configurations. X-ray and polarized neutron scattering indicate that while structurally the intermediate thickness film appears homogeneous along the film normal direction, its depth-dependent magnetization profile is unusual, with two phases that have different compensation behavior. Elemental mapping using high-resolution electron microscopy reveals column formation leading to lateral variations in the Fe:Gd ratio near the top of the film, while Fe and Gd are uniformly distributed near the bottom of the layer. Our results identify an unexpected transitional thickness region in Fe1-xGdx films where the nonuniform elemental distribution leads to complex multistep magnetization behavior that should be controlled or could be leveraged in spintronic and magneto-optical applications.