Abstract
The influence of the grain boundary (GB) specific area sGB on the appearance of ferromagnetism in Fe-doped ZnO has been analysed. A review of numerous research contributions from the literature on the origin of the ferromagnetic behaviour of Fe-doped ZnO is given. An empirical correlation has been found that the value of the specific grain boundary area sGB is the main factor controlling such behaviour. The Fe-doped ZnO becomes ferromagnetic only if it contains enough GBs, i.e., if sGB is higher than a certain threshold value sth = 5 × 104 m2/m3. It corresponds to the effective grain size of about 40 μm assuming a full, dense material and equiaxial grains. Magnetic properties of ZnO dense nanograined thin films doped with iron (0 to 40 atom %) have been investigated. The films were deposited by using the wet chemistry “liquid ceramics” method. The samples demonstrate ferromagnetic behaviour with Js up to 0.10 emu/g (0.025 μB/f.u.ZnO) and coercivity Hc ≈ 0.03 T. Saturation magnetisation depends nonmonotonically on the Fe concentration. The dependence on Fe content can be explained by the changes in the structure and contiguity of a ferromagnetic “grain boundary foam” responsible for the magnetic properties of pure and doped ZnO.
Highlights
The possibility of ferromagnetism (FM) in oxides has been widely debated since 2000
The Fe-doped ZnO becomes ferromagnetic only if it contains enough grain boundary (GB), i.e., if sGB is higher than a certain threshold value sth = 5 × 104 m2/m3
We were able to observe the FM behaviour even in pure ZnO due to the extremely small grain size in our films deposited by the original method of so-called “liquid ceramics”, which is based on the application of organic acids for the solution of metallic ions for pure and Mn- and Co-doped ZnO [6,17,18]
Summary
The possibility of ferromagnetism (FM) in oxides has been widely debated since 2000 In their theoretical work, Dietl et al discussed the chances for oxides to possess saturation of magnetisation in an external magnetic field, coercivity, and a Curie temperature above room temperature (RT) [1]. That FM behaviour does not appear in bulk ZnO (even doped by Mn or Co), but only in polycrystalline samples with very high specific area sGB of grain boundaries (GBs), i.e., the ratio of GB area to grain volume [6]. We were able to observe the FM behaviour even in pure ZnO due to the extremely small grain size in our films deposited by the original method of so-called “liquid ceramics”, which is based on the application of organic acids for the solution of metallic ions for pure and Mn- and Co-doped ZnO [6,17,18]
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