Xiaomin Cheng

Thermal conductivity of chalcogenide material with superlatticelike structure

Thermal conductivity of chalcogenide material with superlatticelike (SLL) structure is investigated using the 3ω method and the molecular dynamics method. Both the measured and calculated results show that the thermal conductivity of SLL is lower than those of conventional chalcogenide materials and will decrease to a minimum as the number of interfaces increases. The Raman spectrum is introduced to study the phonon behavior of SLL and the “phonon mode vanishing” is proposed to explain its lower thermal conductivity. Finite-element analysis and phase change memory cell testing confirm the enhancement of cell performance for SLL with minimum thermal conductivity.

Effective method to identify the vacancies in crystalline GeTe

X-ray diffraction and photoelectron spectroscopy of different Co-doped concentration GeTe have been made to identify the vacancies in rock-salt GeTe. The results show that Co occupies germanium vacancy and forms Co–Te bond, but the alloy retains a rock-salt structure when the concentration of doped Co is less than the vacancy ratio in crystalline GeTe. If we construct 8% germanium vacancy and relative distortion in rock-salt GeTe, the magnetism calculation based on spin-polarized density functional theory of Co-doped GeTe agrees well with superconducting quantum interference device magnetometer result. From experiment and calculation results, we can confirm the existence of vacancies in rock-salt GeTe and the ratio is 8%.

Insulator-metal transition in GeTe/Sb2Te3 multilayer induced by grain growth and interface barrier

Unlike its two components, the temperature coefficient of resistivity (TCR) of GeTe/Sb2Te3 multilayer (ML) increases from negative to positive on annealing, indicating an insulator-metal transition (IMT). Impedance spectroscopy measurements demonstrate that the grain boundary resistance (negative TCR) determines the total resistance of initial ML. As grain grows, which is confirmed by x-ray diffraction, scanning electron microscope, and optical reflectivity measurements, the contribution of grain resistance (positive TCR) increases gradually to the leading part and finally accomplishes the IMT in a sufficiently crystallized film. Furthermore, the artificially introduced interfaces form additional potential barrier in ML and also modulate its IMT behavior.

Amorphization and amorphous stability of Bi2Te3 chalcogenide films

Si doping is proposed to be an effective way to improve the amorphous stability of Bi2Te3 thin film. Structural changes in doped crystalline phases are revealed by the bonding nature given by x-ray photoelectron spectroscopy results. Based on ab initio simulations, the energy, electronic and structural changes induced by Si doping are studied. The results show that both nucleation and growth of crystallite are suppressed by Si doping, so that the stability of amorphous Bi2Te3 is improved.

Effect of metal-to-metal interface states on the electric-field modified magnetic anisotropy in MgO/Fe/non-magnetic metal

The impact of metal-to-metal interface on electric-field modified magnetic anisotropy in MgO/Fe/non-magnetic metal (Ta, Pt, Au) is revealed by density functional calculations. We demonstrate that the contribution from the metal-to-metal interface can be strong enough to dominate the electric fieldeffect on magnetic anisotropy of Fe/MgO-based films, and the strain could also effectively tune the electric fieldeffect. By analyzing the interface states by density of states and band structures, the dependence of the magnetoelectric effect on metal-to-metal interface is elucidated. These results are of considerable interest in the area of electric field controlled magnetic anisotropy and switching.

Logic gates realized by nonvolatile GeTe/Sb2Te3 super lattice phase-change memory with a magnetic field input

Nonvolatile memory devices or circuits that can implement both storage and calculation are a crucial requirement for the efficiency improvement of modern computer. In this work, we realize logic functions by using [GeTe/Sb2Te3]n super lattice phase change memory (PCM) cell in which higher threshold voltage is needed for phase change with a magnetic field applied. First, the [GeTe/Sb2Te3]n super lattice cells were fabricated and the R-V curve was measured. Then we designed the logic circuits with the super lattice PCM cell verified by HSPICE simulation and experiments. Seven basic logic functions are first demonstrated in this letter; then several multi-input logic gates are presented. The proposed logic devices offer the advantages of simple structures and low power consumption, indicating that the super lattice PCM has the potential in the future nonvolatile central processing unit design, facilitating the development of massive parallel computing architecture.

Effect of Sputtering Parameters on the Magnetic Properties of SmCo5/Cu Films

SmCo5 film is an emerging candidate for the recording material of future high density magnetic recording because of its huge uniaxial magneto-crystalline anisotropy. In this article, SmCo5 films were fabricated by radio frequency (RF) magnetron sputtering system onto quartz glass substrate, and a Cu underlayer was proposed in order to improve magnetic properties of SmCo5 films. The dependences of sputtering power, thickness and annealing temperature on the magnetic properties of SmCo5/Cu films were studied. Using the optimal sputtering parameters of sputtering power of 60 w, thickness of 65 nm, and annealing temperature of 650°C, the coercivity of SmCo5/Cu films is improved significantly.

Disorder-induced anomalously signed Hall effect in crystalline GeTe/Sb2Te3 superlattice-like materials

Opposite to the almost persistent p-type conductivity of the crystalline chalcogenides along the GeTe-Sb2Te3 tie line, n-type Hall mobility is observed in crystalline GeTe/Sb2Te3 superlattice-like material (SLL) with a short period length. We suggest that this unusual carrier characteristic originates from the structural disorder introduced by the lattice strain and dangling bonds at the SLL interfaces, which makes the crystalline SLLs behave like the amorphous chalcogenides. Detailed structural disorder in crystalline SLL has been studied by Raman scattering, X-ray photoelectron spectroscopy, as well as Variable-energy positron annihilation spectroscopy measurements. First-principles calculations results show that this structural disorder gives rise to three-site junctions that dominate the charge transport as the period length decreases and result in the anomalously signed Hall effect in the crystalline SLL. Our findings indicate a similar tetrahedral structure in the amorphous and crystalline states of...

Fabrication and Magnetic Properties of Ag/FePt Thin Films

The effects of Ag under layer on the micro structural and magnetic properties of the FePt films were investigated. It was found that the thickness of the Ag under layer is very important factor to induce the L10 ordered FePt (001) films with high perpendicular anisotropy. The chemical ordering increased with increasing Ag layer thickness, reached its maximum at 40 nm, and then slightly decreased with further increasing Ag thickness to 50 nm. Moreover, the initial magnetization curve and the scanning electrode microscopy (SEM) results showed that the magnetization reversal mechanism of the Ag/FePt films changed from nucleation mode to pinning mode/S-W rotation with increasing the Ag thickness. The increase of the chemical ordering and the change of magnetization reversal mechanism from nucleation mode to pinning mode/S-W rotation led to the increase in out-of-plane coercivity when Ag thickness increased from 0 to 40 nm. For the sample with 50 nm Ag layer, the decrease in out-of-plane coercivity could be attributed to the decrease in the chemical ordering.

Gold fillings unravel the vacancy role in the phase transition of GeTe

Phase change memory (PCM) is an important candidate for future memory devices. The crystalline phase of PCM materials contains abundant intrinsic vacancies, which plays an important role in the rapid phase transition upon memory switching. However, few experimental efforts have been invested to study these invisible entities. In this work, Au dopants are alloyed into the crystalline GeTe to fill the intrinsic Ge vacancies so that the role of these vacancies in the amorphization of GeTe can be indirectly studied. As a result, the reduction of Ge vacancies induced by Au dopants hampers the amorphization of GeTe as the activation energy of this process becomes higher. This is because the vacancy-interrupted lattice can be “repaired” by Au dopants with the recovery of bond connectivity. Our results demonstrate the importance of vacancies in the phase transition of chalcogenides, and we employ the percolation theory to explain the impact of these intrinsic defects on this vacancy-ridden crystal quantitatively....