Evangelos Papaioannou

Recent achievements in ultrafast spin physics have enabled the use of heterostructures composed of ferromagnetic (FM)/non-magnetic (NM) thin layers for terahertz (THz) generation. The mechanism of THz emission from FM/NM multilayers has been typically ascribed to the inverse spin Hall effect (ISHE). In this work, we probe the mechanism of the ISHE by inserting a second ferromagnetic layer in the form of an alloy between the FM/NM system. In particular, by utilizing the co-sputtering technique, we fabricate Fe/L1(0)-FePt/Pt ultra-thin heterostructures. We successfully grow the tetragonal phase of FePt (L1(0)-phase) as revealed by X-ray diffraction and reflection techniques. We show the strong magnetic coupling between Fe and L1(0)-FePt using magneto-optical and Superconducting Quantum Interference Device (SQUID) magnetometry. Subsequently, by utilizing THz time domain spectroscopy technique, we record the THz emission and thus we the reveal the efficiency of spin-to-charge conversion in Fe/L1(0)-FePt/Pt. We establish that Fe/L1(0)-FePt/Pt configuration is significantly superior to the Fe/Pt bilayer structure, regarding THz emission amplitude. The unique trilayer structure opens new perspectives in terms of material choices for the future spintronic THz sources.

Terahertz (THz) spintronic emitters represent a novel class of heterostructures composed of ferromagnetic (FM) and non-magnetic (NM) metallic layers that strongly emit terahertz (THz) radiation upon femtosecond laser pulse excitation. The optimal geometric configuration to maximize the strength of the emission is currently considered a trilayer structure, NM1/FM/NM2, where the FM layer is confined between two NM layers with opposite spin Hall angles. To investigate this, ultrathin Ta/Fe/Pt trilayers are fabricated and their THz emission profiles are analyzed. These results show that the highest THz emission is achieved for the sample of Ta (1.5 nm)/Fe (2 nm)/Pt (2 nm), demonstrating a significant enhancement compared to standard FM/NM bilayers. Furthermore, the thickness dependence of the THz emission is modeled in Ta (t1 nm)/Fe (2 nm)/Pt (t2 nm), varying t1 and t2 from 1 nm to 3 nm. From this analysis, spin diffusion lengths of lambda Pt = 1.2 nm and lambda Ta = 0.85 nm are extracted. The structure-property relationship is assessed via transmission electron microscopy, revealing that an epitaxial single-crystalline Ta layer covers the MgO surface with Ta adopting a high-resistivity fcc allotropic phase with a lattice parameter of a = 0.436 nm. This phase, together with the prerequisite for low Ta+Pt thickness, emerges as a key factor in achieving high THz emission from trilayer structures.

We show in theory and experiment that in periodically patterned spintronic terahertz emitters (STEs), charge dynamics can modify the emission spectrum in a well-controlled way. Characterization of nanopatterned STEs at frequencies up to 30 THz shows that the STE emission spectrum systematically changes with emitter size. The spectral intensity exhibits significant reductions at frequencies below 4 THz, accompanied by pronounced dips at around 15 and 24 THz. While reduction of the STE size enhances the modulation of all features, it does not alter the dip frequencies. The effect originates from the charging of the structure's edges by terahertz currents, causing a backflow that interferes with the initially induced current pulse. An analytical model quantitatively reproduces these results and agrees well with the findings of control experiments. Our findings enable a detailed investigation of the charge dynamics in STEs and provide additional means for controlled shaping of STE emission spectra by nanopatterning.

THz radiation emitted by ferromagnetic/non-magnetic bilayers is a new emergent field in ultra-fast spin physics phenomena with a lot of potential for technological applications in the terahertz (THz) region of the electromagnetic spectrum. The role of antiferromagnetic layers in the THz emission process is being heavily investigated at the moment. In this work, we fabricate trilayers in the form of Co/CoO/Pt and Ni/NiO/Pt with the aim of studying the magnetic properties and probing the role of very thin antiferromagnetic interlayers like NiO and CoO in transporting ultrafast spin current. First, we reveal the static magnetic properties of the samples by using temperature-dependent Squid magnetometry and then we quantify the dynamic properties with the help of ferromagnetic resonance spectroscopy. We show magnetization reversal that has large exchange bias values and we extract enhanced damping values for the trilayers. THz time-domain spectroscopy examines the influence of the antiferromagnetic interlayer in the THz emission, showing that the NiO interlayer in particular is able to transport spin current.

Recent developments of lithographic techniques as well as improved chemical synthesis methods allow researchers to engineer novel nanostructured materials consisting of arrays of self-organized nanocrystals and multilayers grown as patterns on different substrates. In our case, the magnetic nanostructures consist either of multilayers directly deposited on pre-patterned substrates to form regular arrays of stripes and grooves or colloidal solutions of self-organized bimetallic Ag/Co nanoparticles on patterned and nonpatterned substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed in order to study the surface morphology of the 2D patterning arrays and the 3D nanostructures. The development of periodic arrays of magnetic patterns of micrometer size is strongly dependent on technological parameters such as: film, thickness, distances, size and shape of the patterns. Moreover, it is shown that the substrate morphology significantly affects the colloidal crystallization of magnetic nanoparticles and leads to different growth modes. This will ultimately affect the overall magnetic behavior of the nanostructures. Consequently, the combination of self-assembly and patterning allows for the controlled fabrication of the novel magnetic nanostructures at a macroscopic level and the study of fundamental aspects in magnetism such as quantum tunneling magnetization and magneto-transport properties along well-defined nanosized patterns. (C) 2003 Elsevier B.V. All rights reserved.