Tianshu Lai

Superlattice-like Sb50Se50/Ga30Sb70 thin films for high-speed and high density phase change memory application

Multi-level phase change character of superlattice-like (SLL) Sb50Se50/Ga30Sb70 thin films was investigated through in-situ film resistance measurement. SLL structure of the thin films was confirmed by using transmission electron microscopy. Three resistance states were observed during heating process, and their thermal stability was also examined. A picosecond laser pump-probe system was used to measure phase-change time of the SLL Sb50Se50/Ga30Sb70 thin films. Phase change memory cells based on the SLL [SS(5 nm)/GS(10 nm)]3 thin films were fabricated to test and verify multi-level switch between set and reset states.

Spin waves and small intrinsic damping in an in-plane magnetized FePt film

Laser-induced spin-wave dynamics in an in-plane magnetized FePt film is studied using all-optical pump-probe magneto-optical Kerr spectroscopy under different external fields and pump fluences. Uniform precession spin wave is observed. Nonlinear external field dependence of its frequency is found and well explained by the macrospin model of uniform precession. The effective damping shows a significant external-field dependence. Calculation taking account for magnetic inhomogeneity fits the external-field dependence well, revealing main magnetic inhomogeneity origin of the extrinsic damping. An intrinsic Gilbert damping parameter of <0.028 is inferred and shows potential applications of this film in magnonics.

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.

Evolution of spin coherence dynamics and g factor with electron excess energy in bulk intrinsic GaAs

Circularly polarized absorption quantum-beat spectroscopy is used to study excess-energy dependence of electron-spin coherence dynamics in intrinsic GaAs. Absorption quantum beats are observed. The quantum-beat oscillatory frequency is used as a high precision measure of electron g factor. Double linear energy dependence of g factor is obtained and disagrees with the prediction of k∙p theory. A theoretical calculation reveals the double linear energy dependence reflects the energy dependence of g factor of electrons measured through light-hole- and heavy-hole-electron transitions, respectively. The phase of the quantum beats provides the key information to distinguish the two transitions experimentally.

Elliptically polarized pump-probe spectroscopy and its application to observation of electron-spin relaxation in GaAs quantum wells

An elliptically polarized pump-probe spectroscopy is developed. A theoretical model is derived from the rate equations of two-level system with small signal approximation to describe the experimental signal of the spectroscopy. The spectroscopy eliminates the systematic errors between theories and experimental results in present circularly polarized pump-probe spectroscopy, and is implemented easily in experiments. The initial degree of spin polarization of carriers in an excited state can be extracted by means of the spectroscopy, which is an important parameter in the investigation of spatial transport of spin polarization. The electron-spin relaxation in GaAs quantum wells is studied by using this spectroscopy.

Dynamics of magnetization, reversal, and ultrafast demagnetization of TbFeCo amorphous films

Static and dynamic hysteresis loops of TbFeCo amorphous films are measured by vibrating sample magnetometry (VSM) and magneto-optical Kerr rotation. The static VSM loop shows a faster magnetization reversal than the static Kerr loop does. The dynamic Kerr hysteresis loop in a sinusoidal alternating magnetic field at 1.14kHz shows a large increase in coercivity with respect to the static coercivity. The ultrafast dynamics of laser-induced demagnetization and magnetization recovery is studied by femtosecond time-resolved polar Kerr rotation spectroscopy, and shows a subpicosecond demagnetization and several hundreds of picosecond magnetization recovery process, which suggests gigahertz writing rate possible.

Temperature dependence of electron-spin coherence in intrinsic bulk GaAs

Temperature dependence of electron-spin coherence dynamics is investigated for an intrinsic bulk GaAs in the Voigt geometry using the elliptically polarized absorption quantum-beat spectroscopy. Temperature dependences of spin coherence and recombination lifetimes as well as g factor of electrons are reported over a temperature range from 8.1to260K. The temperature dependence of spin coherence lifetime (T2*) agrees well with a reported theoretical calculation and can be fitted well by a relationship T2*∼T−1∕2, which provides an evidence to support electron-spin decoherence dominated by the Bir-Aronov-Pikus mechanism. The temperature dependence of g factor also agrees well with reported results.

Origin of anomalous hysteresis loops induced by femtosecond laser pulses in GdFeCo amorphous films

A controllable pump-pulse-number magneto-optical Kerr technique combined with an initializing field scanning approach is developed to eliminate and identify memory and accumulation effects, respectively, from external field history and multiple pulse excitations. A series of anomalous loops of GdFeCo films are measured for different amount of pump pulses using this technique, revealing that serious memory and accumulation effects exist in continuous-pulse-pumped anomalous hysteresis loops which show illusory information of hot coercivity and degree of magnetization reversal. Single-pulse-induced anomalous loop reveals that the hot coercivity shown by continuous-pulse-pumped anomalous loops is not the minimum external field that drives real magneto-optical recording.

Femtosecond laser excitation of multiple spin waves and composition dependence of Gilbert damping in full-Heusler Co2Fe1-xMnxAl films

Spin-wave dynamics in 30 nm thick Co2Fe1−xMnxAl full-Heusler films is investigated using time-resolved magneto-optical polar Kerr spectroscopy under an external field perpendicular to films. Damon-Eshbach (DE) and the first-order perpendicular standing spin-wave (PSSW) modes are observed simultaneously in four samples with x = 0, 0.3, 0.7, and 1. The frequency of DE and PSSW modes does not apparently depend on composition x, but damping of DE mode significantly on x and reaches the minimum as x = 0.7. The efficient coherent excitation of DE spin wave exhibits the promising application of Co2Fe0.3Mn0.7Al films in magnonic devices.

Field-dependent ultrafast dynamics and mechanism of magnetization reversal across ferrimagnetic compensation points in GdFeCo amorphous alloy films

Ultrafast dynamics of genuine magneto-optical recording across ferrimagnetic compensation points is demonstrated in GdFeCo films using time-resolved polar Kerr spectroscopy combined with a laser-synchronized sinusoidal alternating magnetic field which can reinitialize irreversible initial magnetization state to laser radiating. The external field dependence of magnetization reversal dynamics is measured and shows that reversal rate accelerates with increasing external fields. Analysis of the magnetization reversal dynamics with Bloch equation shows the magnetization reversal rate is linearly dependent on the external fields within experimental errors, which supports quantitatively that the mechanism of magneto-optical recording in rare earth-transition metal ferromagnetic films is related to the formation and growth of the reversed domains. It is also shown that nucleation field is obviously larger than hot coercivity shown in the anomalous hysteresis loop.