Ultrafast Magnetization Dynamics for Spin-Transfer Torque Based Nano-Device

在本論文中，我們主要的工作是研究外加磁場與自旋力矩交互影響下的自旋閥系統的物理特性。我們首先通過包含了Slonczewski自旋力矩項的Landau-Lifshitz-Gilbert方程，以自旋力矩項做為計算參數來分析了自由層磁化的一維等效自由能的形式。在磁場所加的平面上，臨界曲線不僅區分出了自由能含雙穩態，單穩態，或零穩態的區域，且曲線形狀變形的不同於以往傳統的Stoner-Wohlfarth星狀圖。我們也找出了在不同強度的自旋力矩下，結核場，矯頑場，以及進動臨界場與外場方向的相應關係。特別的是，在某些特殊的外場方向下，這些臨界場會有多重解，這導致了磁滯曲線會有多次的不連續跳躍，這在設計一系列可能的自旋電子學原件，諸如自旋力矩震盪器，自旋力矩隨機存取記憶體上，提供了一些有趣的應用。 另外，在我們的第二個工作中，研究了一個帶有垂直異向性的自由層，其磁化向量與外加的圓形自旋極化電流產生了同步的進動行為，並提出了一個可行的元件結構來完成這個機制。我們透過旋轉座標系下的Landau-Lifshitz-Gilbert方程來計算自由層磁化向量與自旋電流的動力學響應。我們發現了在一個頻率上限下，有兩個穩定的進動模式存在，並且可經由調整電流的強度，來轉換這兩個不同的模式，而且這個轉換是不可逆的。值得注意的是，這個垂直的自由層磁化向量，也能被這圓形自旋極化的電流給翻轉，依據右手定則能決定翻轉的方向與電流的極化關係。Recently most researches have been performed for a spin-valve system because of the great potential applications for new types of spintronics devices such as spin-transfer-torque (STT) random access memory and microwave generator. In experiments, one of the important critical parameters about magnetization reversal is the minimal switching field or current. For this, we first derive an analytical form of the critical current density for the gyromagnetic precession in STT devices. We find that the current from a perpendicularly magnetized pinned layer has a critical value needed to generate a large cone-angle precession motion, which depends on the Gilbert damping constant, in-plane anisotropy field, stray field, and demagnetizing filed. Second, we take a closer look at the Stoner-Wohlfarth (SW) asteroid and its dependence on current density. Under a perpendicularly magnetized pinned layer and in-plane (IP) external magnetic field, three-dimensional instability thresholds of the critical strength of STT and the field are derived from an effective one-dimensional free energy. The modified asteroid with effect of STT not only separates the region with two stable states from the region with only one stable state, but also delimits the region with a dynamical stable state when the amplitude of the STT is larger than a critical value. We also find that STT-induced multiple-steps magnetization switching in the spin valve with a perpendicular polarizer subjected to an IP magnetic field. The single-step, triple-steps, and double-steps magnetization reversals are observed at a certain range of the bias field angle and of the current density. For a given magnetic anisotropy, there is a critical value for the spin-transfer torque, above which the triple-steps magnetization switching disappears, and double-steps one appears. Also the nucleation field, coercivity, and precessional critical field for uniaxial anisotropy are rigorously determined including the effect of STT, and the hysteresis loops for various orientations of a bias field are computed and discussed. In our latest work, we investigated a phase-locking steady-state precession synchronized with the injected circularly polarized spin current for a perpendicular magnetized free layer. Response of the spin-wave dynamics with the injected spin-torque are calculated from the Landau-Lifshitz-Gilbert equation in a rotating coordinate frame. Two dynamical precessional modes are found below an upper frequency limit and can be reversibly tuned by the injected current. It is also noted that the perpendicular magnetization will reverse with a circularly spin-polarized current according to the right-hand rule