A. Mesaros

Spin–orbit torques and magnetization switching in W/Co2FeAl/MgO structures

Magnetization switching by current induced spin–orbit torques (SOTs) in heavy metal/ferromagnetic metal/oxide structures is of great research interest due to its potential applications in the field of low power consumption spintronic devices. Here, we study the Slonczewski-like and the field-like SOT effective fields in β-W/Co2FeAl/MgO structures showing perpendicular magnetic anisotropy (PMA). We characterize the SOT effective fields using harmonic Hall voltage measurements and we point out the essential role of the planar Hall effect corrections. We estimate that for bulk β-W an effective spin Hall angle as large as 0.3 ± 0.03 and a spin diffusion length of 2.2 ± 0.3 nm. Moreover, we demonstrate SOT-induced magnetization switching for charge current densities of the order of 106 A cm−2.

Interface charge transfer process in ZnO:Mn/ZnS nanocomposites

ZnO:Mn/ZnS nanocomposites were prepared by seed-mediated growth of ZnS QDs onto the preformed ZnO:Mn nanoparticles. The formation of the nanocomposite structure has been evidenced by XRD, HRTEM, and XPS. The architecture of the nanocomposite with outer ZnS QDs around ZnO:Mn cores is sustained by the sulfur and oxygen depth profiles resulted from XPS. When the two components are brought together, the band gap of ZnS component decreases while that of ZnO:Mn increases. It is the result of interface charge transfer from ZnO:Mn to ZnS QDs. Here ZnO:Mn valence states are extended through the interface into unoccupied gap states of ZnS. The energy band setup is modified from a type II into a type I band alignment. The process is accompanied by enhancement of composite UV emission of PL spectra as compared to its counterparts. The charge transfer from valence band also determines the increase of the core-polarization effect of sshell electrons at Mn2+ nucleus, thus determining the increase of the hyperfine field through the reduction of the covalency degree of Zn(Mn)–O bonds. The quantum confinement in ZnS QDs promotes the ferromagnetic coupling of singly occupied states due to Zn vacancies determining a superparamagnetic behavior of the ensemble. When the nanocomposites are formed, due to interface charge transfer effects, an increased number of filled cation vacancies in ZnS QDs develop, thus disrupting the pre-existing ferromagnetic coupling between spins resulting in a significant reduction of the overall saturation magnetization. The possibility to modulate nanocomposite properties by controlling the interface interactions may be foreseen in these types of materials.

Magnetic and Electrical Properties of Undoped and Holmium Doped ZnO Thin Films Grown by Sol-Gel Method

Undoped and holmium-doped ZnO thin films were obtained, using the sol-gel method. The films were characterized by scanning electron microscopy (SEM), Hall effect measurements, electron paramagnetic resonance (EPR) spectroscopy and superconducting quantum interference device vibrating sample magnetometry (SQUID-VSM). The Hall effect measurements have indicated a n-type conduction with a resistivity of about 3.2 Ω.cm for the undoped ZnO and of about 4.5 Ω.cm for the 5 at. % Ho-doped ZnO thin films. The EPR measurements have indicated the presence of interstitial zinc in the undoped ZnO and the presence of zinc vacancies in 5 at. % Ho-doped ZnO. Ho (5 at. %)-doped ZnO films exhibit superparamagnetism at 5 K, while a low paramagnetic behavior was observed at room temperature.