RESUMEN
The perpendicular magnetocrystalline anisotropy, magnetoelastic properties as well as the Gilbert damping factor in Co2Fe0.4Mn0.6Si thin films were found to depend on a magnetic layer thickness, and they can be also tuned by the application of additional Ag buffer layer. The tetragonal distortion of a magnetic layer was found to increase with decreasing thickness, and after the application of an additional Ag buffer layer, the character of this distortion was changed from tensile to compressive in the plane of a film. A correlation between the tetragonal distortion and perpendicular magnetocrystalline anisotropy was found. However, the magnitude of the observed tetragonal distortion for most samples seems to be too small to explain alone the experimentally found large magnitude of the perpendicular magnetocrystalline anisotropy. For these samples, other mechanisms including both surface and volume effects must be taken into account.
RESUMEN
Co2Fe0.4Mn0.6Si (CFMS) and Co2FeGa0.5Ge0.5 (CFGG) Heusler alloys are among the most promising thin film materials for spintronic devices due to a high spin polarization, low magnetic damping and giant/tunneling magnetoresistance ratios. Despite numerous investigations of Heusler alloys magnetic properties performed up to now, magnetoelastic effects in these materials remain not fully understood; due to quite rare studies of correlations between magnetoelastic and other magnetic properties, such as magnetic dissipation or magnetic anisotropy. In this research we have investigated epitaxial CFMS and CFGG Heusler alloys thin films of thickness in the range of 15-50 nm. We have determined the magnetoelastic tensor components and magnetic damping parameters as a function of the magnetic layer thickness. Magnetic damping measurements revealed the existence of non-Gilbert dissipation related contributions, including two-magnon scattering and spin pumping phenomena. Magnetoelastic constant B11 values and the effective magnetic damping parameter αeff values were found to be in the range of - 6 to 30 × 106 erg/cm3 and between 1 and 12 × 10-3, respectively. The values of saturation magnetostriction λS for CFMS Heusler alloy thin films were also obtained using the strain modulated ferromagnetic resonance technique. The correlation between αeff and B11, depending on magnetic layer thickness was determined based on the performed investigations of the above mentioned magnetic properties.
RESUMEN
When an electron passes through a chiral molecule, there is a high probability for correlation between the momentum and spin of the charge, thus leading to a spin polarized current. This phenomenon is known as the chiral-induced spin selectivity (CISS) effect. One of the most surprising experimental results recently demonstrated is that magnetization reversal in a ferromagnet with perpendicular anisotropy can be realized solely by chemisorbing a chiral molecular monolayer without applying any current or external magnetic field. This result raises the currently open question of whether this effect is due to the bonding event, held by the ferromagnet, or a long-time-scale effect stabilized by exchange interactions. In this work we have performed vectorial magnetic field measurements of the magnetization reorientation of a ferromagnetic layer exhibiting perpendicular anisotropy due to CISS using nitrogen-vacancy centers in diamond and followed the time dynamics of this effect. In parallel, we have measured the molecular monolayer tilt angle in order to find a correlation between the time dependence of the magnetization reorientation and the change of the tilt angle of the molecular monolayer. We have identified that changes in the magnetization direction correspond to changes of the molecular monolayer tilt angle, providing evidence for a long-time-scale characteristic of the induced magnetization reorientation. This suggests that the CISS effect has an effect over long time scales which we attribute to exchange interactions. These results offer significant insights into the fundamental processes underlying the CISS effect, contributing to the implementation of CISS in state-of-the-art applications such as spintronic and magnetic memory devices.
RESUMEN
Kelvin-probe measurements on ferromagnetic thin film electrodes coated with self-assembled monolayers of chiral molecules reveal that the electron penetration from the metal electrode into the chiral molecules depends on the ferromagnet's magnetization direction and the molecules' chirality. Electrostatic potential differences as large as 100 mV are observed. These changes arise from the applied oscillating electric field, which drives spin-dependent charge penetration from the ferromagnetic substrate to the chiral molecules. The enantiospecificity of the response is studied as a function of the magnetization strength, the magnetization direction, and the handedness and length of the chiral molecules. These new phenomena are rationalized in terms of the chiral-induced spin selectivity (CISS) effect, in which one spin orientation of electrons from the ferromagnet penetrates more easily into a chiral molecule than does the other orientation. The large potential changes (>kT at room temperature) manifested here imply that this phenomenon is important for spin transport in chiral spintronic devices and for magneto-electrochemistry of chiral molecules.
RESUMEN
We propose a theoretical model to investigate the magnetic properties of cobalt-doped ZnO (ZnO:Co) thin films qualitatively. The model was built on the dilute Co dopants in the host of ZnO forming the magnetic Co+2 ions and the energy level of the magnetic ions crossing the band edge of ZnO resulting in a magnetic interaction between the Co+2 spins and the spins of the electrons from the conduction band of ZnO. The mechanism of the ferromagnetism revealed in the studied system is proposed here to be induced not only by the mediated conducting electrons via spin interactions but also by the Coulomb excitations, arising from the electrons localized by the oxygen vacancies. This approach of including Coulomb excitation in the modified carrier-mediated model could explain well the magnetic properties of ZnO:Co and solves the drawback of the carrier-mediated model in interpreting the appearance of ferromagnetism in the insulating ZnO:Co. We propose that the Coulomb excitations induced by the electrons captured by the oxygen vacancies are an essential element in the magnetic ZnO, which reveals the fact that the bound magnetic polaron model without considering the Coulomb excitation is insufficient to describe the magnetic properties of ZnO.
RESUMEN
CdTe nanowires with the average diameter of only 40 nm coated with (Cd,Mg)Te shells are grown using Au-catalyzed vapor-liquid-solid growth mechanism in a system for molecular beam epitaxy. High optical quality of individual nanowires is revealed by means of low temperature cathodoluminescence and micro-luminescence. It is found that, the optical emission spectrum consists mostly of the near band edge emission without any significant contribution of defect related luminescence. Moreover, the importance of surface passivation with (Cd,Mg)Te coating shells is demonstrated.
RESUMEN
We report on the growth, cathodoluminescence and micro-photoluminescence of individual radial and axial CdTe insertions in ZnTe nanowires. In particular, the cathodoluminescence technique is used to determine the position of each emitting object inside the nanowire. It is demonstrated that depending on the CdTe deposition temperature, one can obtain an emission either from axial CdTe insertions only, or from both, radial and axial heterostructures, simultaneously. At 350 °C CdTe grows only axially, whereas at 310 °C and 290 °C, there is also significant deposition on the nanowire sidewalls resulting in radial core/shell heterostructures. The presence of Cd atoms on the sidewalls is confirmed by energy dispersive X-ray spectroscopy. Micro-photoluminescence study reveals a strong linear polarization of the emission from both types of heterostructures in the direction along the nanowire axis.
RESUMEN
The absence of luminescence in the near band edge energy region of Te-anion based semiconductor nanowires grown by gold catalyst assisted molecular beam epitaxy has strongly limited their applications in the field of photonics. In this paper, an enhancement of the near band edge emission intensity from ZnTe/ZnMgTe core/shell nanowires grown on Si substrates is reported. A special role of the use of Si substrates instead of GaAs substrates is emphasized, which results in an increase of the near band edge emission intensity by at least one order of magnitude accompanied by a simultaneous reduction of the defect related luminescence. A possible explanation of this effect relies on the presence of Ga-related deep level defects in structures grown on GaAs substrates, which are absent when Si substrates are used. Monochromatic mapping of the cathodoluminescence clearly confirms that the observed emission originates, indeed, from the ZnTe/ZnMgTe core/shell nanowires, whereas individual objects are studied by means of microphotoluminescence.
RESUMEN
The self-assembled growth of epitaxial Au(111) islands on a Mo(110) buffer layer has been investigated as a function of growth temperature and amount of deposited material by reflection high energy electron diffraction and atomic force microscopy. At the growth temperature of 385 degrees C the dendrite-shaped islands coexist with the compact ones. The uniform islands formed at 500 degrees C adopt mostly a shape of truncated pyramids with a well developed (111) top plane and {111} and {100} side facets. As the growth temperature reaches 800 degrees C the Au islands take less regular shapes due to occurrence of coalescence. The averaged area and height of the islands increase with the deposition temperature and the amount of deposited material. The surface density of the islands decreases with increasing temperature. The epitaxial relations at the interface between the Au islands and the Mo buffer determined from the angular dependence of the electron diffraction pattern favour the Nishiyama-Wassermann growth mode. Factors responsible for the island-like growth and possible mechanisms of diffusion are discussed in details.
RESUMEN
Complementary multiscale magneto-optical studies based on the polar Kerr effect are carried out on an ultrathin cobalt wedge covered with a silver wedge and subsequently with the Au thick layer. A few monolayers of Ag are found to have a substantial effect on magnetic anisotropy, the coercivity field, and Kerr rotation. The silver overlayer thickness-driven magnetic reorientation from easy axis to easy plane generates a new type of 90 degrees magnetic wall for cobalt thicknesses between 1.3 and 1.8 nm. The tuning of the wall width in a wide range is possible. Tailoring of the overlayer structure can be used for ultrathin film magnetic patterning.