Abstract
Electric field modulation of magnetic characteristics has drawn much attention due to its potentials for novel spintronic or magnetoelectric devices, such as voltage-driven magnetic random access memories, logic circuits, and so on, with lower power consumption and higher processing speed. In addition, spintronic devices with perpendicularly magnetized ferromagnetic layers have been extensively studied recent years because of their advantages of low critical current density and high thermal stability for current-induced magnetization switching. Therefore, this dissertation focuses on growth of ferromagnetic thin films with perpendicular magnetic anisotropy (PMA) and electric field controlled anisotropy change by using single crystalline piezoelectric substrates. In the first topic, we fabricated perpendicularly magnetized ordered L10-FePt thin films with a buffer layer of CrMoMn/Pt bi-layers, which were out-of-plane (002) pre-ferred orientation and prepared by in-situ heating deposition on thermally-oxidized silicon substrates. For the purpose of being an electrode in perpendicular magnetic tunnel junc-tions, the plasma treatment with O2/Ar mixture was applied to reduce the surface rough-ness of the L10-FePt thin films. Combined with a MgO barrier and Co/Pt multilayers, the typical magnetic characteristics of pseudo spin valve structure were well demonstrated. In the second topic, we fabricated a single CoFeB layer with PMA on thermal-ly-oxidized silicon substrates or MgO buffer layers. After annealing, the PMA of the CoFeB layer strongly depended on the oxidation degree of the MgO buffer layers. To in-vestigate the origin of PMA in the CoFeB thin films, x-ray magnetic circular dichroism and sum rule calculation were used to extract the orbital moments per 3d hole (morb/Nh) of Fe in annealed CoFeB layers. The correlation between morb/Nh, depending on the oxidationn time of MgO, and the PMA variation of CoFeB revealed that the PMA of CoFeB was mainly contributed from the interfacial orbital hybridization of the Fe-3d and O-2p. In the third topic, a perovskite-type single crystalline piezoelectric substrate, (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3(x: 0.28~0.32), was used to grow epitaxial L10-FePt and polycrystalline CoFeB thin films. Since strain would be propagated from the piezoelectric substrate, the out-of-plane coercivity (Hc) of L10-FePt showed a typical butterfly-like loop, companying with an abnormal asymmetric phenomenon, related to the applied electric fields due to the magnetostriction. The thickness dependence of FePt on the out-of-plane Hc revealed that the asymmetric behavior resulted from the ferroelectric field effect of the electric polarized substrate. The Hc difference at zero electric field, back from opposite direction of electric fields, showed a non-volatile behavior and was of significance for applications of voltage-driven solid state devices.