Time-resolved ellipsometric measurements were made upon Au, Cu, Ag, Ni, Pd, Ti, Zr, and Hf thin films. Using an elliptically polarized pump beam, the decay of the optically induced polarization of the sample was observed. Characteristic relaxation times are extracted and interpreted in terms of scattering of linear and angular momentum of hot electrons. A systematic variation is observed between different metals that reflects their underlying band structure. DOI: 10.1103/PhysRevB.71.233104 PACS number͑s͒: 78.47.ϩp, 42.65.Ϫk, 78.20.Ls Investigations of ultrafast electron dynamics in metals are required to understand and implement phenomena such as ultrafast demagnetization, 1 ultrafast optical generation of magnons, 2 phonons, 3 and optical birefringence and dichroism. 4 The time scales for the linear and angular momentum dynamics of electrons and holes are of particular importance for transport phenomena, where the carrier energy, direction of motion, and spin polarization play a significant role. 5 The corresponding scattering times are determined by the interactions between electrons, holes, and phonons, and by scattering from defects, and are expected to be of the order of a few femtoseconds. 6 Hence, there is a need for time-resolved ͑TR͒ experiments 1-4,7 that have sufficient time resolution to resolve these processes.The interaction between light and matter can be viewed as the creation of a coherent quantum superposition of initial and final electron states that has an associated polarization. Coherence between states with different wave vector requires an intermediate virtual state and the presence of a coherent phonon. A transition may occur when the coherence of the system is broken either due to the finite width of the optical wave packet or by scattering from the environment. The transition results in the absorption of a photon and the creation of a hot electron-hole pair. Values for the decoherence time have been deduced from electronic spectroscopic techniques and TR two-photon photoemission ͑TR 2PPE͒, although their physical interpretation is not straightforward. 7,8 The polarization may persist for a longer time if the hot electron-hole pair retains some memory of the optically induced momentum after a scattering event.TR optical measurements have been used to study the specular inverse Faraday effect ͑SIFE͒ and specular optical Kerr effect 4,9 ͑SOKE͒ in metals, where it has always been assumed that the characteristic relaxation times are so short that the signal shape corresponds to the cross correlation of the pump and probe intensities. In this paper we show that pump-probe measurements of the incoherent part of the SIFE and SOKE may yield relaxation times for the transient polarization in metals that correspond to the relaxation times for the linear and angular momentum of the excited carriers. A correlation between the character of the TR polarization signal and the electronic structure of different groups of metals is revealed.Measurements 10 were performed with 90 fs ͑full width at hal...

Thermally assisted spin-transfer torque magnetization reversal in uniaxial nanomagnets Appl. Phys. Lett. 101, 262401 (2012) Thick CoFeB with perpendicular magnetic anisotropy in CoFeB-MgO based magnetic tunnel junction AIP Advances 2, 042182 (2012) Erase/restorable asymmetric magnetization reversal in polycrystalline ferromagnetic films

We report grazing incidence x-ray scattering evidence for sharpening of the interface between amorphous Co60Fe20B20 and AlOx during in situ annealing below the Co60Fe20B20 crystallization temperature. Enhancement of the interference fringe amplitude in the specular scatter and the absence of changes in the diffuse scatter indicate that the sharpening is not a reduction in topological roughness but a reduction in the width of the chemical composition profile across the interface. The temperature at which the sharpening occurs corresponds to that at which a maximum is found in the tunneling magnetoresistance of magnetic tunnel junctions.

No abstract

Optical pump-probe measurements of transient reflectivity were made on a sputtered polycrystalline thin film of hcp Zr. On top of a slow multiexponential decay, an oscillatory signal was observed with frequency varying from the initial value of f 0 = 2.24± 0.06 THz at zero time delay to a value of 2.56± 0.12 THz after 2 ps, which is similar to that of a transverse optical ͑TO͒ phonon near the ⌫ point of the Brillouin zone. From the data, the TO phonon relaxation time was determined to be 0.67 ps. The excitation of the coherent phonon was identified as being mediated by resonant coherent Raman scattering. DOI: 10.1103/PhysRevB.76.012301 PACS number͑s͒: 78.47.ϩp, 63.20.Ϫe, 78.20.Ϫe Recently, much attention has been drawn to the study of terahertz phonons in semiconductors, semimetals, and metals. The motivation for these studies arises from the need for efficient means by which to quantify the elastic properties of solids, on one hand, and to analyze the integrity of nanoscale structures, on the other. Atomic dynamics determines new phase formation in solids and the kinetics of chemical reactions. The latest advances in ultrafast laser technology have allowed the subpicosecond nonequilibrium dynamics of phonons to be investigated in the time domain. In most of these studies, acoustic modes were investigated. In such modes, neighboring atoms oscillate in phase, and so the characteristic length scale for associated nonuniformities is given by the phonon wavelength. At the same time, in crystals with two ͑or more͒ atoms in the unit cell, there also exist optical phonons. For these modes, the two basis atoms oscillate out of phase, and so by studying optical phonons, one may obtain information about interactions on the atomic length scale, even if the phonon wavelength is not small.In striking contrast to semiconductors and semimetals, 6 there have only been a few reports of the observation of coherent optical phonons by optical pump-probe measurements in metals. 12,13,17,21,25 The observation of coherent longitudinal optical ͑LO͒ phonons in Gd was reported in Refs. 12, 13, 21, and 25. Coherent transverse optical ͑TO͒ phonons in Zn and Cd were observed in Ref. 17. The samples in those studies were single crystals. At the same time, many of the metallic nanostructures that are important for applications are prepared by sputtering and so are polycrystalline. Therefore, a question arises as to whether terahertz optical phonons can be excited and detected and, hence, used for characterization of polycrystalline metallic samples. In this Brief Reports, we show that coherent TO phonons near the center of the Brillouin zone can be optically excited and detected in a sputtered polycrystalline film of Zr.Measurements were performed at room temperature with 90 fs ͑full width at half maximum͒ transform limited pulses with photon energy of 1.575± 0.005 eV from a Ti:sapphire laser with 80 MHz repetition rate. The pump and probe were focused to a 15 m spot size, had energies of 2.25 and 0.04 nJ, and were incident on the sample...