Study Of Electric Manipulation Of Magnetocrystalline Anisotropy In L1₀FePt And Related Film Systems

In recent years, with the conspicuous development of the information industry, the demand for the capacity of the information memory has increased greatly, which directly pushes the related magnetic memory devices improving towards the ones with small size and high density. In order to achieve this aim, one of the critical points is to study and prepare the materials with high magnetocrystalline anisotropy as the recording medium. Among the magnetic materials, the L10FePt film material has been considered as a good candidate of the medium for the next generation ultrahigh density memory device because of its high perpendicular magnetocrystalline anisotropy energy (MAE). However, application of materials with too high MAE would bring about the increase in the energy consumption. Thus it becomes the focus of attention that how effectively tunes the MAE of the FePt and related film systems. Meanwhile, in the area of spintronics, as a novel magnetic information writing method, the electric control of magnetism has developed remarkably. It is found that manipulation of MAE by the electric field can change the magnetization orientation of materials. Comparing with the existing writing methods of the magnetic field and spin current, this method can reduce the energy consumption for the information writing obviously, and therefore contain the great application value in the future research of information devices. Based on these research backgrounds, we apply the first principles method to calculate the MAE of the L10FePt composite film systems and explore the multiple effects on them. Additionally, combining with the LLG macrospin simulation method, we study the magnetization switching based on the electric field pulse modulation of MAE in a FePt magnetic tunnel junction. Also we analyze the microcosmic mechanism for control of MAE by external electric field in a simple Fe/Cu film system. All the work has been shown in the following:Chapter1mainly gives the basic aim and research motivation. We simply introduce the background of studying the FePt and other high perpendicular magnetocrystalline anisotropy materials and focus on the recent progress of manipulation of MAE by external electric field. Chapter2mainly contains the basic theories and methods of the first principles calculation.In Chapter3, we calculate the MAE of the Cu/(FePt)n/MgO system and discuss the film thickness, strain and interface hybridization effects on the MAE. It is shown that the MAE doesn’t change much with the increase of the FePt layer thickness, while the increase of the lattice constant induced by the strain effect can reduce the MAE greatly; even the easy axis is changed. By analyzing the projected DOS of the Fe atoms in the system, we find that the occupation number of the minority spin channel in Fe3dx2-y2orbital decreases clearly under the increasing lattice constant, which leads to the reduction of the orbital moment anisotropy and thus MAE. Meanwhile, effects of the Cu/FePt and FePt/MgO interface can decrease the MAE, and the Cu/FePt interface effect on MAE is more obvious, which comes from the reason that the occupation number of the minority spin channel in Fe dz2orbital is improved by the hybridization between the Cu and Fe atom at the interface.In Chapter4, we consider the MgO/FePt/Pt(001) system and explore the dependence of MAE on the external electric field. The results show that there exists a linear relationship between the applied electric field and MAE. Based on this linear tuning effect, we apply the LLG macrospin simulation method to discuss the coherent magnetization switching by the electric field pulse in a FePt-based magnetic tunnel junction and find that the final state of the magnetization switching depends on the pulse time width r. When the pulse amplitude is enhanced, the minimal critical pulse width τmin for triggering the switching decreases and the maximal pulse width τmax increases. In addition, by increasing the initial precessional angle of the magnetization, it would restrict the irregular switching under the situation of short pulse width. Besides, the applied multiple pulse can induce a successive magnetization switching.In Chapter5, we choose a simple Fe/Cu structure to understand the physical mechanism of electric control of surface magnetism and MAE. The first principles calculations indicate that the external electric field can induce the spin-dependent screening at the surface of the system and change the magnetic moment of the surface Fe atom and MAE linearly. Following these results, we utilize the orbital selective external potential (OSEP) method to apply potential on the3d orbitals of one Fe atom layer, which changes the relative occupation of the minority spin electron. Combining with the existing analytical theory, it is shown that the influence of the dz2orbital is most pronounced when considering the same change of the electron occupation number. Through designing the new alignment structure of the surface Fe atom, we adjust its electron occupation on3dz2orbital and achieve the control of electric field effect on MAE. Also by applying the OSEP method to change the electron occupation of the3dz2orbital, we have enhanced the response of MAE to electric field theoretically.Finally, the conclusion of this thesis and some feasible issues in the future are given in Chapter6.