Properties of granular (CoFeB)x(Al2O3)100-x and (CoFeB)x(LiNbO3)100-x nanocomposites: Manifestation of superferromagnetic ordering effects

We present a comparative study of (CoFeB)x(Al2O3)100-x and (CoFeB)x(LiNbO3)100-x film nanocomposites (NCs) with various structure deposited onto a glass-ceramic substrate by the ion-beam sputtering (CoFeB = Co40Fe40B20). NCs consist of the CoFe nanogranules embedded into the nonstoichiometric oxide matrix. For (CoFeB)x(Al2O3)100-x the granules are rounded (2-4 nm in size), whereas for (CoFeB)x(LiNbO3)100-x the granules are strongly elongated along the NC growth axis up to15 nm with the same in-plane size 2-4 nm. In both cases, the anomalous Hall effect takes place and its behavior is similar to the one observed in films with easy-plane magnetic anisotropy. Moreover, the analysis of magnetization showed that for (CoFeB)x(LiNbO3)100-x film there is no any structurally induced magnetic anisotropy, i.e. the magnetic anisotropy of samples is practically entirely determined by their shape. For both NCs along with the ferromagnetic (FM) contribution, additional paramagnetic (PM) component in the total magnetization is observed at T < 20 K; in case of (CoFeB)x(LiNbO3)100-x PM component even exceeds the FM one in three times. We have shown that PM component is caused by Fe2+ and Co2+ ions dispersed between granules with the concentration reaching Ni ≈ 3·1022 cm−3 and explain our results by a crucial role of these ions in the FM exchange between granules forming percolation network as well as in the total magnetization of the system.