Xinyi Liu

Single laser pulse induced dynamic magnetization reversal mechanism of perpendicularly magnetized L10 FePt films

In order to measure photoinduced dynamic magnetization reversal by single laser pulse, alternating magnetic field is synchronized with the femtosecond laser pulse such that the magnetization state is reset before each single laser pulse. For perpendicularly magnetized L10 FePt films, the dynamic magnetization reversal process is accompanied by the nucleation of reversed domains and the barrierless transient domain wall motion at low magnetic fields and subsequent pinned domain wall motion at the switching field. The switching field does not change with the pump-probe delay time.

Photoinduced magnetic softening of perpendicularly magnetized L10-FePt granular films

Ultrafast spin dynamics has for the first time been studied in perpendicular magnetized granular films. For FePt continuous films and FePt–MgO and FePt–Ag granular films with femtosecond laser excitations, the coercivity HC and the saturation Kerr rotation θKS are dramatically reduced, accompanied by a sharp increase in the reflectivity R. Afterward, these physical quantities are slowly recovered. The changes in HC, θKS, and R are all different among FePt, FePt–MgO, and FePt–Ag films. The difference is caused by different film thicknesses and in particular by the surface plasmon resonance of metallic nanoparticles.

TixSb100-x thin films as candidates for phase-change memory application

Tellurium-free TixSb100−x phase-change materials were systematically investigated. The amorphous-to-crystalline transition was studied by in situ resistance measurements. The crystallization temperature, crystalline resistance, and optical bandgap of TixSb100−x thin films were enhanced significantly with the increase in the titanium concentration. The phase structure and microstructure were confirmed by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM), respectively. The chemical state of the elements was detected by X-ray photoelectron spectroscopy (XPS). The phase transition speed between the amorphous and crystalline states was obtained by picosecond laser pulses. Phase change memory cells based on the Ti27Sb73 thin film were fabricated to evaluate the electrical characteristics as well. The results indicate that the TixSb100−x thin films have the great potentiality in phase change memory applications.

Multi-level storage and ultra-high speed of superlattice-like Ge50Te50/Ge8Sb92 thin film for phase-change memory application

Superlattice-like Ge50Te50/Ge8Sb92 (SLL GT/GS) thin film was systematically investigated for multi-level storage and ultra-fast switching phase-change memory application. In situ resistance measurement indicates that SLL GT/GS thin film exhibits two distinct resistance steps with elevated temperature. The thermal stability of the amorphous state and intermediate state were evaluated with the Kissinger and Arrhenius plots. The phase-structure evolution revealed that the amorphous SLL GT/GS thin film crystallized into rhombohedral Sb phase first, then the rhombohedral GeTe phase. The microstructure, layered structure, and interface stability of SLL GT/GS thin film was confirmed by using transmission electron microscopy. The transition speed of crystallization and amorphization was measured by the picosecond laser pump-probe system. The volume variation during the crystallization was obtained from x-ray reflectivity. Phase-change memory (PCM) cells based on SLL GT/GS thin film were fabricated to verify the multi-level switching under an electrical pulse as short as 30 ns. These results illustrate that the SLL GT/GS thin film has great potentiality in high-density and high-speed PCM applications.

Exploring mechanism on nano-structuring manipulation of crystallization temperature of superlattice-like [GeSb/Ge]3 phase-change films

Superlattic-like phase-change films are considered a promising phase-change material because it provides more controllable degree of freedoms for the simultaneous optimization of multiple parameters of phase-change films. However, the mechanism on the effect of superlattice-like structure on parameters of phase-change films is still controversial. At present there are two opinions: interfacial effect and the reduction of thermal conductivity. Here four superlattice-like phase-change films, [Ge8Sb92(15 nm)/Ge (x nm)]3, are fabricated. Their behaviors of crystallization are investigated using the measurements of sheet resistance and coherent phonon spectroscopy. Two measurements show the crystallization temperature of the four superlattice-like films increases with the thickness of Ge layers. However, this increase cannot be explained by both the interfacial effect and the reduction of thermal conductivity. It is proposed that true superlattice effect should be considered to explain the effect of superlattice-like structure. Electron diffusion between two different constituent layers should be considered, as done in semiconductor superlattice structures. Electron diffusion can lead to the establishment of built-in electric field inside the superlattice-like films, which causes the change of band structures of two constituent materials and long-range coupling of superlattice-like films, further change of physical parameters. Based on this long-range coupling, the effect of cycle number in superlattice-like films on crystallization temperature can be explained. Some primary evidence on electric field effect on crystallization temperature of phase-change films is provided.

Understanding the crystallization behavior and structure of titanium addition in germanium antimony phase change thin films

The effects of a titanium dopant on the phase transition behavior and crystallization mechanism of Ge8Sb92 films were systematically investigated. The crystallization behavior induced by heat was studied by in situ resistance measurements. With the incorporation of titanium atoms, both the crystallization activation energy and electrical resistance increase, resulting in a higher amorphous thermal stability and a lower of the programming energy consumption. A broadening of the optical bandgap causes the enhancement in the amorphous resistance. X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy analysis reveal that a small amount of titanium addition can retard the grain growth and refine the crystal size, owing to the formation of amorphous Ge and Sb components. The shift in the Raman modes associated with Sb upon crystallization was observed. X-ray reflectivity and atomic force microscopy results illustrate that the volume fluctuation becomes smaller and the surface morphology becomes smoother after titanium doping. A reversible phase transition can be achieved by picosecond laser pulses. Phase change memory cells based on a titanium-doped Ge8Sb92 film were also fabricated to evaluate the electrical characteristics. The results indicate that the suitable incorporation of titanium in Ge8Sb92 thin films is an effective way to tune and optimize the crystallization performance for phase change memory applications.

Periodic cycle number modulating effect on crystallization temperature in superlattice-like [Ge/Ge8Sb92]n phase-change films and exploration of mechanism

Superlattice-like (SLL) phase-change films provide more controllable parameters for the optimization of the performance of phase-change films, including the thickness of each constituent layer, the thickness ratio of two constituent layers and cycle number of periodicity. The effects of the first two parameters on the performance of SLL films have been studied widely. However, the influence of last parameter, cycle number of periodicity, was studied sparsely. In this study, we have studied the period number effect on crystallization temperature of SLL [Ge/Ge8Sb92]n films, and designed and fabricated a series of superlattice-like (SLL) [Ge/Ge8Sb92]n phase-change films. Their crystallization behaviors are studied by the measurement of temperature-dependent sheet resistance. We find that crystallization temperature decreases with increasing cycle number of periodicity, revealing period-cycle-number modulation effect. However, such the effect cannot be explained by current interface effect model. We test the ...

Fast switching and low power of superlattice-like SnSe2/Sb thin films for phase change memory application

Two non-promising phase change materials (SnSe2 and Sb) were prepared through the superlattice-like (SLL) method to explore the suitable phase change layer for phase change memory (PCM) application. The crystallization temperature, activation energy, and 10-year data retention of the SLL [SnSe2(10 nm)/Sb(2 nm)]4 ([SS(10)/S(2)]4) thin film are 185 °C, 3.03 eV, and 116 °C, respectively. The volume change of the SLL [SS(10)/S(2)]4 thin film during the crystallization is as small as 3.5%. The phase transition speed of the SLL [SS(10)/S(2)]4 thin film for crystallization is only about 11.9 ns. PCM cell based on the SLL [SS(10)/S(2)]4 thin film shows high operation speed (20 ns for SET/RESET) and lower power consumption (2.75 × 10−11 J for RESET operation).