Preparation And Study The Magnetoelectric Properties Of The Fe₃O₄ And Its Composite Structure

The materials of high spin-polarization have recently attracted heightened interest because of their promising potential in spintronics. Fe₃O₄ is a kind of magnetic material which has high curie temperature, 100% polarization and metal-insulator transition that make it quite attractive in the applications for spintronics devices. So the research of magnetic, electronic transportation properties and the fabrication of Fe₃O₄ films have become one focus in the field of condensed matter physics. Firstly, we prepared the Fe₃O₄ film and nanodot arrays using porous anodic alumina（PAA） template and laser molecular beam epitaxy（LMBE） technique, and analyzed the origin of the giant magnetic moment and the properties of the nanodot arrays; Secondly, we prepared Fe₃O₄/LCMO heterostructure to investigate the effect of Fe₃O₄ on the MIT temperature for LCMO films with different thickness. We performed some works as follows:（1） We prepared Fe₃O₄ films with different substrate temperatures and thicknesses using LMBE, and systemically studied the influence of the substrate temperature and thickness on the structure, magnetic and transport properties of Fe₃O₄ films. Experimental results as follows: films prefer to grow at the layer by layer growth mode below 550 ℃ with poor crystallinity, and the films tend to the island growth mode above 550 ℃ with good crystallinity. Combined with film’s structure and magnetic test results, the optimal substrate temperature is 600 ℃. The saturation magnetic moment（Ms） of Fe₃O₄ films strongly depend on the films thickness. For the films with thickness below 15 nm, the Ms increase with the decrease in films thickness. The Ms of 3 nm Fe₃O₄ film is dramatically increased to 1017 emu/cm3. As for films thickness more than 15 nm, Ms is tending to be close to the Fe₃O₄ bulk value. According to XMCD result, and combine with the structure, magnetic, and transport properties of the Fe₃O₄ films, we propose that the giant magnetic moment most likely come from the spin of Fe ions in the tetrahedron site switching parallel to the Fe ions in the octahedron site.（2） We prepared Fe₃O₄ nanodot arrays using PAA template and LMBE technique. Experimental results as follows: The Ms of the nanodot arrays are smaller than that of the film. This characteristic could be explained by the accumulation of spin canting around the nanoparticle surface that reduce magnetic moment; the film exhibit magnetic in-plane easy axis anisotropy and the anisotropy is reduced with the dot size. Possibly the magnetostrictive was greatly reduced by fast relaxation strain from the dot edges, whereas the shape anisotropy and crystalline anisotropy were released by forming noncontinuous dots; Meanwhile, the coertivity is reduced with the dot size, indicated that Magnetization reversal may be dominated by the domain wall movement of the film and big dots, while small dots may reverse their magnetization by coherent rotation; Tv was undetectable at D = 40 nm, this may be due to the hopping energy barrier between different Fe sites within the nanodot was relatively small compared with the inter nanodot tunneling barrier.（3） We prepared LCMO single films and Fe₃O₄/LCMO heterostructures using LMBE technique. Experimental results as follows: The MIT temperature of the single-layer LCMO films increase monotonically with increasing film thickness at the magnetic field of 0 T and the MR response are very strong for different thickness LCMO films; the MR response of the 20 nm film can reach a maximum of 1600% at 5 T. Remarkably enhanced MIT temperature is observed for the decorated film with Fe₃O₄, the MIT temperature increase from 160 K to 200 K at 0 T and the MR response is preserved and can reach a maximum of 1500% at 5 T for 20 nm LCMO film with capped Fe₃O₄ film. According to analyze the resistance and magnetization behavior of the LCMO films before and after Fe₃O₄ coverage, we conclude the antiferromagnetic interaction between the Fe₃O₄ film and the LCMO film plays the critical role in the enhancing MIT temperature.In summary, we have prepared Fe₃O₄ ultrathin film with giant magnetic moment, and discussed the physical mechanism; we have prepared the Fe₃O₄ nanodot arrays firstly, compared and analyzed their structure and magnetic properties with corresponding film. We have successfully raised the MIT temperature of LCMO film by 40 K according to antiferromagnetic coupling between Fe₃O₄ and LCMO, which could act to push the LCMO film in to practical devices.