Manipulation Of Microstructure And Magnetic Porperties Of FePt Thin Films

The L10 phase FePt thin film is proposed to be a potential candidate for next generation high density perpendicular magnetic recording media for its high magnetocrystalline anisotropy (MCA) and chemical stability. With the continual increasing recording area density, the storage system raises a series of critical requirements for the recording media, such as high perpendicular magnetic anisotropy, low ordering temperature and uniformity small particles with proper exchange coupling. To fulfill these requirements, it is extremely important to control the microstructure and magnetism of the recording media. Firstly, we need to fabricate L10 phase FePt recording media with high perpendicular anisotropy by using non-epitaxial method. Secondly, a suitable cap layer is essential in magnetic recording media because it can affect the morphology, microstructure and magnetic property of the magnetic films greatly. Additional, the high MCA of the L10 phase FePt thin film leads to an ultrahigh coercivity, which exceeds the writing ability for a recording head. According to the first principle calculation, researchers predicted that the MCA energy of L10 phase FePt can be changed by external electric field or trapping electrons, and proposed the trapping electron assisted magnetic recording technology. In this dissertation, we focus on the manipulation of microstructure and magnetic properties of L10 phase FePt thin films. The results can be summarized as follow:Ⅰ. Non-epitaxial growth of perpendicular anisotropy FePt thin filmsFePt thin films of different thickness were deposited on the thermal oxide Silicon substrates by using magnetron co-sputtering, after the deposition, the samples were annealed in vacuum for an hour with a base pressure less than 1.2×10-5 Torr. The microstructure and magnetic properties of the FePt thin films have been investigated by using XRD, SEM, and VSM. The results indicate that FePt thin films less than 10nm are composed of isolated small particles with (001) structure; whereas the films thicker than 20nm become continuous particle structure with a dominant (111) texture. It can be attributed to the surface energy of the FePt particles formed during annealing process. The small particles in FePt films less than 10nm have a smaller top surface area than side surface; whereas the larger particles in FePt films thicker than 20nm have a larger top surface area. To minimize the total surface energy of the FePt particles, the larger area surface prefers to be (111) texture with lower surface energy.II. Effects of Ru and Ag cap layers on microstructure and magnetic properties of FePt filmsFePt (t nm, t=3,10) single layers and FePt (t nm,t=3,10)/X (5nm, X=Ru/Ag) bilayers were deposited on the thermal oxide Silicon substrates by using magnetron co-sputtering. After the deposition, the samples were annealed in vacuum for an hour with a base pressure less than 1.2×10-5 Torr. The effects of Ru and Ag cap layers on the microstructure and magnetic properties of the FePt thin films have been investigated by using XRD, SEM, VSM and anomalous Hall effect (AHE). The results indicate that:The Ag cap layer segregates from the FePt/Ag bilayer, lowers the FePt ordering temperature, promotes the FePt thin film to form island structure, enhances the coercivity; Whereas the Ru cap layer shows completely inverse behavior, the Ru cap layer increases the FePt ordering temperature, helps to maintain smooth continuous structure film, and restrains the FePt (001) orientation and perpendicular magnetic anisotropy. The effects become more pronounced for the 3nm-thick FePt thin films. The effects can be mainly attributed to the different melting point, and thermal expansion stress between the cap layer and FePt thin film.III. Electric field magnetism manipulation of FePt thin films on dielectric and ferroelectric substratesFePt thin films of different thickness were deposited on the dielectric SrTiO3 and ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT) substrates by using magnetron co-sputtering, and the electric field manipulation of magnetism in FePt/SrTiO3(001), FePt/PMN-PT(111) and FePt/PMN-PT(001) heterostructures were investigated by AHE. The results can be summarized as follow:1. FePt/SrTiO3(001) heterostructures:The coercivity variation resulted from the electric field is very small, and no significant charge effect has been observed, this can be mainly attributed to the low charge accumulation (dissipation) in the FePt/SrTiO3(001) heterostructures.2. FePt/PMN-PT(111) heterostructures:The PMN-PT(111) substrate shows mainly nonvolatile tensile strain proerty with in-plane anisotropic, and the strain is very low with only strain peaks at ±2kV/cm, therefore the coercivity variation resulted from strain effect is low; whereas hysteresis loop-like He vs E-field behavior has been observed in 1.5nm and 2nm FePt thin film, which can be attributed to the dominant charge effect in FePt/PMN-PT(111) heterostructures.3. FePT/PMN-PT(001) heterostructures:For the FePt films of different thickness, the He vs E-field curves show typical asymmetrical butterfly-like loop behavior, further results indicate that the coercivity variation is composed of the volatile symmetrical butterfly-like loop and nonvolatile hysteresis loop-like parts, which originate from the volatile and nonvolatile strains induced by the E-field in the PMN-PT(001) substrate, respectively. No significant difference has been observed after inserting a 2-nm W interlayer, suggesting that the charge-mediated coercivity variation is negligible in FePt/PMN-PT(001) heterostructures.