Microstructure, Magnetic And Transport Properties Of Magnetic Nanogranular Films

Novel properties such as giant magnetoresistance (GMR), giant Hall effect (GHE) and huge coercivity have made ferromagnetic nanogranular films promising candidates for the applications of magnetic sensors, high density magnetic recording materials, read-out magnetic head and magnetic random access memory. Recently, L10 ordered FePt alloy, half-metals and ferromagnetic metal-semiconductor composite materials have become the focuses in the field of condensed matter physics and materials science.Series of ferromagnetic nanocomposite films, including soft ferromagnetic metal ?C granular films (Co-C, Fe-C, FeN-CN), hard ferromagnetic metal–C granular films (FePt-C, FePtCu-C, FePtN-CN), half metal–semiconductor granular films (Fe₃O₄-Ge), ferromagnetic metal–semiconductor granular films (Fe-Ge) were fabricated using magnetron sputtering. Their chemical composition, microstructure, magnetic properties, reversal mechanism, and transport properties were studied systemically.It was found that, for the soft ferromagnetic metal–C granular films (Co, Fe, FeN)-C(N), the phase segregation between metal particles and C matrix, particle size control and interparticle interaction determine the magnetic properties and reversal mechanism of samples. The better the phase segregation and the weaker the interparticle interaction are, the larger the coercivity is, and the reversal mechanism is single domain rotation. Contrarily, the coercivity will decrease, and the reversal mechanism becomes domain wall motion. The magnetic percolation of Fe-C system was directly observed using magnetic force microscopy, which clarifies the changes of interparticle interaction in the nanogranular system. We also found that the amorphous C in the as-deposited Co-C granular films transforms into graphitized nanostructured C (carbon nanostripes and nanoneedles) after being exposed in the electron beam for several minutes.For the hard ferromagnetic metal–C granular films (FePt-C, FePtCu-C, FePtN-CN), we found that appropriate Cu doping can improve the transformation of ordered-L10 FePt, but excessive Cu suppresses the formation of L10 phase. Particularly, we noted that the evaporation of N and decompounding of Fe-N bonds in FePtN-CNgranular films promote the formation of L10 phase during annealing, and improves its chemical ordering. Besides, the size of FePt particles can be effectively controlled by N doping at high N₂ partial pressure, which is benefit to the practical application of FePt-based granular films in high-density magnetic recording media.The mechanism of electron transport in polycrystalline Fe₃O₄ films is tunneling. Although the moments at the surface and/or interface of Fe₃O₄ grains slightly affect the magnetization of films, the magnetoresistance of polycrystalline Fe₃O₄ films changes obviously in the high field range when a small part of the moments at the grain surface and/or interface was aligned. This observation explains the weak saturation of MR with applied field in the polycrystalline Fe₃O₄ films. For Fe-Ge composite films, when the Fe atomic fraction is 50%, the Hall resistivity (ρ_xy) reaches its maximum value of 126μΩcm, which is 139 times larger than that of pure Fe films. In the magnetic field range of -10?10 kOe,ρ_xy changes linearly with the applied magnetic field and the slope almost keeps constant at temperatures rangeing from 2 to 300 K, making the practical application of Fe-Ge composite films possible in the field of microelectronic devices.