Preparation, Structure And Magnetoelectric Properties Of Magnetic Nanogranular Films Using Carbon Or Si-N As The Matrix

Novel properties such as giant magnetoresistance （GMR）, giant Hall effect （GHE） and high 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, magnetic granular films, consisting of nanoscale ferromagnetic grains dispersed in a non-magnetic carbon matrix, have received much attention due to their potential applications as high-resistive soft magnetic materials and giant magnetoresistance materials. In addition, L1₀ ordered FePt and CoPt films embedded in non-magnetic matrix have become the focuses in the field of ultra-high density magnetic recording medium and materials science.Various series of ferromagnetic nanocomposite films, including soft ferromagnetic carbon-based granular films （Co-C, Fe-C）, hard ferromagnetic CoPt-C and FePt-SiN granular films were fabricated on Si（100） or glass substrates by dc magnetron sputtering. Their preparation, structure, magnetic properties, and transport properties were studied systemically. The main results are as follows:Firstly, the cobalt and carbon or graphite in the as-deposited films are in amorphous state, and the Co_xC_1-x films are diamond like carbon （DLC） films. The Co_xC_1-x films have smooth surface （Ra=0.152nm）, homogeneous grain size and film thickness. The nano-sized amorphous Co particles were homogeneously dispersed in the amorphous cross-linked carbon matrix. After doping cobalt into DLC film, the sp³-hybridized carbon content in DLC composite films almost had no change. The coercivity of Co_xC_1-x film is less than 180 Oe. The as-deposited Co_xC_1-x granular films with 80nm thickness had larger value of magnetoresistance than the amorphous carbon film. A very high positive MR, up to 36% at magnetic field B=5 T and x=2.5 at.% was observed in a Co_xC_1-x granular film at room temperature when the external magnetic field was perpendicular to the electric current and the film surface. With increase of the film thickness and Co-doped content, the MR decreased gradually. The MR effect of the Co_xC_1-x granular films may be interpreted by p-n heterojunction theory and interface scattering effect.Secondly, as-deposited Fe-doped amorphous Fe_xC_1-x granular films are also in amorphous state, and the Fe_xC_1-x films are diamond like carbon （DLC） films. After doping iron into DLC film, the Fe_xC_1-x films have smooth surface morphology and the surface roughness Ra is 0.231nm for x=18 at.%. Moreover, the sp³-hybridized carbon content in DLC composite films increases with Fe doping. The Fe_xC_1-x films have good soft magnetic properties, the coercivity of only around 20 Oe was obtained. The as-deposited Fe_xC_1-x granular films with 160nm thickness have larger value of magnetoresistance. A very high positive MR, up to 93% with x=1 at.% was observed in a Fe_xC_1-x granular film at 300 K. The MR effect may be interpreted by two-channel electric conduction model. The model is in accordances with the experiment results.Thirdly, the CoPt in as-deposited CoPt-C films had face-centered cubic （fcc） structure, which transforms into the face-centered tetragonal （fct） structure after thermal annealing at 700°C. The as-deposited films have smooth surface morphology and the average grain size is about 23.5 nm. Carbon components have played important effect in grain refinement. The saturation magnetization increases firstly and then decreases with the increase of C concentration, about 1000 emu/cm3 with 15 at.% C. The coercivity of CoPt-C films is up to 4200 Oe measured at 300 K when the films are annealed at 700°C for 1h. It is the first time that the CoPt-C films exhibited negative MR effect about -1% at 2T.Finally, FePtSiN films consisting of FePt nanoparticles embedded in a Si-rich matrix were successfully fabricated on silicon substrate by dc reactive magnetron sputtering. The as-deposited films had fcc structure, which transforms into fct structure after thermal annealing at 600°C. The grain size of FePt increased with the annealing temperature but decreased with increasing Si-N content. Increasing Si content led to the formation of Si-N rich amorphous phase distributed between the FePt nano-grains, which reduced the lattice distortion and increased the coercivity. The fct-FePt films annealed at 700°C exhibited very high coercivity, up to 13.6 kOe at room temperature and about 17.5 kOe at 100 K. The high coercivity mechanism depends largely on Si-N concentration. These FePtSiN films with novel structure have shown promise for high density magnetic recording medium.