Laser-induced spin dynamics of in-plane magnetized CoFeB films has been studied by using time-resolved magneto-optical Kerr effect measurements. While the effective demagnetization field shows little dependence on the pump laser fluence, the intrinsic damping constant has been found to be increased from 0.008 to 0.076 with the increase in the pump fluence from 2 mJ/cm2 to 20 mJ/cm2. This sharp enhancement has been shown to be transient and ascribed to the heating effect induced by the pump laser excitation, as the damping constant is almost unchanged when the pump-probe measurements are performed at a fixed pump fluence of 5 mJ/cm2 after irradiation by high power pump pulses.
We have investigated the magnetic damping of precessional spin dynamics in defect-controlled epitaxial grown Fe 3 O 4 (111)/Yttria-stabilized Zirconia (YSZ) nanoscale films by all-optical pump-probe measurements. The intrinsic damping constant of the defect-free Fe 3 O 4 film is found to be strikingly larger than that of the asgrown Fe 3 O 4 film with structural defects. We demonstrate that the population of the first-order perpendicular standing spin wave (PSSW) mode, which is exclusively observed in the defect-free film under sufficiently high external magnetic fields, leads to the enhancement of the magnetic damping of the uniform precession (Kittel) mode. We propose a physical picture in which the PSSW mode acts as an additional channel for the extra energy dissipation of the Kittel mode. The energy transfer from Kittel mode to PSSW mode increases as in-plane magnetization precession becomes more uniform, resulting in the unique intrinsic magnetic damping enhancement in the defect-free Fe 3 O 4 film.The photo-induced precessional spin dynamics in various magnetic materials has attracted significant attention since the observation of the uniform magnetic precession (Kittel mode) and the corresponding first-order perpendicular standing spin wave (PSSW mode) in Ni films by the all-optical pump-probe technique. 1,2 After excitation by a femtosecond laser pulse, besides the uniform Kittel mode, different spin wave modes can be stimulated including first-order PSSW and Damon-Eshbach dipolar surface spin waves (DE modes). 3 At the same time, all-optical pump-probe measurements allow determination of the magnetic Gilbert damping α, 4,5 which is a key parameter for magnetic data recording and the nextgeneration spintronic memory devices such as Magnetoresistive Random Access Memory (MRAM) 6,7 . Therefore, understanding and controlling the magnetic damping is of crucial importance. Among many factors affecting the magnetic damping, structural defects are crucial because they are generally inevitable when preparing films or devices. It was proposed theoretically that defects scatter the Kittel mode into short wavelength spin waves via two magnon scattering, producing an extrinsic contribution to magnetic damping. 8 This extrinsic mechanism was verified by the fact that in thin NiFe films the FMR linewidth increases with decreasing thickness. 9
Shales from the Yanchang Formation Chang 7 Member in the Ordos Basin are among the most important shale reservoirs in China and have been investigated due to their great potential. Knowledge of pore structure is important for understanding the storage capacity and flow mechanism in shale reservoirs. In this study, eight shale samples were collected from the Yanchang Formation Chang 7 Member in the southwestern Ordos Basin, and their geochemistry, mineral compositions, pore structure, and fractal characteristics were investigated based on X‐ray diffraction (XRD) analysis, total organic carbon (TOC) analysis, low‐pressure adsorption/desorption analysis, thermal maturity analysis, and fractal analysis. The results indicated that the TOC content ranged between 0.48% and 2.37%, and the Ro values varied from 0.826% to 1.217%. The major mineral compositions were quartz and clay minerals. Nitrogen adsorption/desorption analysis indicated that the isotherms were similar for all collected shale samples from the Chang 7 Member and resembled the type IV isotherm. Narrow slit‐like pore was the dominant pore type, and the pore size distribution appeared to be unimodal with its peak mainly around 40 nm. Investigation of factors for pore structures showed that the TOC content was the controlling factor for the Chang 7 shales. The Frenkel‐Halsey‐Hill (FHH) method was applied to determine fractal dimensions, which were calculated as the D1 (relative pressure >0.96), D2 (0.96 >relative pressure >0.45), and D3 (0.45 >relative pressure) values, ranging in the intervals of 2.788~2.854, 2.547~2.688, and 2.410~2.567, respectively. The relatively high fractal dimensions indicated that pore structures were complicated, and the higher D1 values suggested that pores with larger sizes showed a rougher pore surface and more complex pore structure. Fractal dimensions showed positive correlations with the contents of TOC and clay minerals, and a negative relationship with the quartz contents.
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