Atomic layer deposition of void-free ZnFe₂O₄ thin films and their magnetic properties

Atomic layer deposition (ALD) is an excellent method for depositing single-component thin films, but it is not a suitable technique for directly depositing multi-component films because different precursor sources usually require incompatible growing conditions. Therefore, an effective method to grow multi-component films by ALD is to prepare multi-layers films and then post-anneal them to form multi-component films. However, as has been reported so far, Kirkendall voids always inevitably form during post-annealing of ZnO/Fe₃O₄ thin films to form pure-phase zinc ferrite (ZnFe₂O₄) films, making it hard to obtain a compact zinc ferrite films without voids. Therefore, it is still challenging to prepare high-quality spinel films without voids by ALD. In this work, we used ALD to grow pure-phase and Kirkendall void-free ZnFe₂O₄ spinel thin films. The systematic transmission electron microscopy and selected area electron diffraction investigations prove that the as-grown ZnO and Fe₃O₄ thin films are composed of a number of nano-crystalline particles embedded in an amorphous matrix. This kind of unique microstructure is favorable for the thermal diffusion of elements during the solid-state reaction to form high-quality spinel films at a low temperature. The magnetic characterizations indicate that the obtained ZnFe₂O₄ thin films have a net magnetization, exhibiting ferrimagnetic behavior at room temperature and a very low blocking temperature T_B=17K±2K.