We investigated spin transfer torque magnetization precession in a nanoscale pillar spin-valve under external magnetic fields using micromagnetic simulation. The phase diagram of the magnetization precession is calculated and categorized into four states according to their characteristics. Of the four states, the precessional state has two different modes: steady precession mode and substeady precession mode. The different modes originate from the dynamic balance between the spin transfer torque and the Gilbert damping torque. Furthermore, we reported the behavior of the temporal evolutions of magnetization components in steady precession mode at the condition of the applied magnetic field using the orbit projection method and explaining perfectly the magnetization components evolution behavior. In addition, a result of a nonuniform magnetization distribution is observed in the free layer due to the excitation of non-uniform mode. 1. Introduction There has been intensive interest in spintronics for the past two decades [1]. An important example is giant magnetoresistance (GMR), proposed by Fert et al. [2] and Grünberg et al. [3], which has led to commercial products [4]. GMR effect has many potential applications such as magnetic sensor, magnetic read head, data memory and spin transistor, and so forth. Spin-valve [5], as a simple GMR device, also exhibits the switching effect. The merit of spin-valve is that the magnetization reversal can be realized under a small driving magnetic field. Besides being driven by the magnetic field, the magnetization may also be driven by a torque, spin transfer torque (STT), originated from the electron spins in the conducting current. The angular momentums may be transferred from the electrons of the spin-polarized current to the ferromagnet, giving rise to a switching of magnetization or stable precession of magnetizations [6] that leads to the generation of spin waves [7]. This mechanism of STT was initially proposed by Berger et al. [8] and Slonczewski [9] in 1996 and attracted significant interests because of the great potential for direct current-induced spintronic devices [10–12]. The role of STT in magnetization switching has been verified by numerous experiments in spin-valve nanopillars [13–15], magnetic nanowires [16, 17], point contact geometry [18–20], and magnetic tunnel junctions [21–24]. The most attractive application of current-induced magnetization switching is magnetic random-access memory (MRAM), which has the advantages of nonvolatile, high addressing speed, low-energy consumption, and avoidance of
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