• Dr. Ovidiu Crisan - Director proiect
• Dr. Victor Kuncser
• Dr. Alina Crisan
• Ing. Aurel Leca
• Dr. Anda Stanciu
• Dr. Simona Greculeasa
• Dr. Liviu Nedelcu
• Dr. Adrian Crisan

Etapa 1 (2022):

Proiectul propune dezvoltarea unei tehnologii pentru emisie de radiatie THz si realizarea unui demonstrator al unei heterostructuri magnetic-spintronice capabila sa emita radiatie THz. Heterostructura este alcatuita din bistraturi magnet de faza dura / metal non-magnetic, iar proiectul isi propune sa demonstreze ca la stimularea optica cu un puls laser femtosecunda, heterostructura produce o tranzitie a curentului de spin in curent de sarcina prin efect spin Hall invers iar aceasta produce o emisie de radiatie THz cu parametri reproductibili. Primul obiectiv specific, preconizat de atins in prima etapa a fost legat de sinteza straturilor FM/NM, realizate din filme subtiri din sistemele CoFe-B/(Pt,Pd,Ta, Cu-Ir), Fe/(Pt,Au,Ru,W), fabricate prin co-depunere in fascicul de ioni respectiv in camp de radiofrecventa, in instalatia dedicata, existenta in INCDFM. In decursul primei activitati au fost realizate specificatiile in ce priveste natura, geometria si grosimile pentru tehnologia de depunere a filmelor subtiri pentru conceperea heterostructurilor magnetice. Pentru cea de-a doua activitate preconizata, au fost realizate sinteza stratului model liber feromagnetic, drept component ulterior al heterostructurilor magnetice FM/NM precum si sinteza de straturi antiferomagnetice din aliaj CrAlC. S-a procedat de asemenea cu succes la optimizarea procedurilor de depunere si a parametrilor acestora, pentru un control mai bun al grosimii si al compozitiei. In continuare , in cadrul aceleiasi Etape 1 s-au realizat caracterizarea structurala a straturilor realizate utilizand XRD, SEM si TEM respectiv caracterizarea magnetica si de magnetotransport a stratului model liber feromagnetic, utilizand magnetometria in proba vibranta VSM si SQUID. Pentru determinarea comportamentului de transport dependent de spin al stratului liber, a starii si orientarii spinilor, s-a efectuat o analiza detaliata a structurilor prin spectrometrie Mossbauer. Toate activitatile preconizate a se desfasura in aceasta Etapa s-au relizat in intregime si rezultatele au fost atinse in concordanta cu cele propuse in proiect.

Stage 1 (2022):

The project proposes the development of a technology for THz radiation emission and the realization of a demonstrator of a magnetic-spintronic heterostructure capable of emitting THz radiation. The heterostructure is made up of hard phase magnet / non-magnetic metal bilayers, and the project aims to demonstrate that when stimulated optically with a femtosecond laser pulse, heterostructure produces a transition of spin current into charge current through reverse spin Hall effect and this produces a THz radiation emission with reproducible parameters. The first specific objective, expected to be achieved in the first stage, was related to the synthesis of FM/NM layers, made of thin films from CoFe-B / (Pt, Pd, Ta, Cu-Ir), Fe / (Pt, Au, Ru, W) systems, manufactured by co-deposition in ion beam respectively in radio frequency field, in the dedicated installation, existing in INCDFM. During the first activity, specifications in terms of nature, geometry and thicknesses for thin film deposition technology for designing magnetic heterostructures were realized. For the second expected activity, the synthesis of the ferromagnetic free model layer was performed, as a subsequent component of FM/NM magnetic heterostructures, as well as the synthesis of antiferromagnetic layers of CrAlC alloy. We have also successfully optimized the deposition procedures and their parameters for better thickness and composition control. Next, within the same Step 1, the structural characterization of the layers made using XRD, SEM and TEM was performed, respectively the magnetic and magnetotransport characterization of the ferromagnetic free model layer, using magnetometry with VSM and SQUID. To determine the spin-dependent transport behavior of the free layer, the state and orientation of the spins, a detailed analysis of the structures by Mossbauer spectrometry was performed. All the activities planned to take place in this Stage have been fully realized and the results have been achieved in accordance with those proposed in the project.

Etapa 2 (2023)

In cadrul Etapei a 2-a a proiectului a fost realizata sinteza si caracterizarea heterostructurilor magnetice complexe FM/NM. In speta, in continuarea activitatilor realizate in etapa anterioara, a fost realizata activitatea privind depunerea straturilor de metal non magnetic cu compozitii variabile. Materialele ce pot produce emisie de radiatie THz cu aplicabilitate spintronica reprezinta o noua clasa de surse de radiatii THz. Acestea se bazeaza pe fenomene de transport ultrarapid al spinului, care sunt induse atunci cand o heterostructura compusa dintr-un strat feromagnetic (FM) si non-magnetic (NM) este iradiata cu un puls de radiatie laser femtosecunda. In acest moment, in stratul non-magnetic se propaga un current de spin creat de inversarea magnetizarii in stratul feromagnetic. Prin asa-numitul efect Hall de spin invers (ISHE) se converteste curentul de spin intr-un curent de sarcina ultrarapid care este capabil sa emita radiatii THz. Asa-numitele emitatoare spintronice THz (STE) pot fi o noua generatie de tehnologii THz, deoarece sunt capabile sa ofere o intensitate ridicata a campului si un spectru foarte larg, ajungand pana la 30 THz. In continuare a fost realizata caracterizarea structurala a heterostructurilor FM/NM utilizand tehnicile de difractie de radiatie X (XRD), microscopie electronica in transmisie (TEM), precum si microscopie electronica in baleiaj (SEM). Ulterior, a fost verificata compozitia straturilor si heterostructurilor prin spectroscopie de dispersie de energie (EDX) si s-au studiat procesele de dinamica de spin. Pentru studiul dinamicii spinului, am efectuat masuratori prin rezonanta feromagnetica (FMR). Rezonanta feromagnetica a fost probata de campul magnetic cu microunde indus de un ghid de unda coplanar, care este alimentat de un semnal de unda continua la frecventa constanta f in intervalul 5-19 GHz. Un camp magnetic extern reglabil H a fost aplicat si modulat cu o amplitudine de 0,2-0,3 mT la o frecventa de 197 Hz pentru un raport semnal/zgomot mai bun. Transmisia RF este masurata folosind o dioda, iar tehnica de blocare este utilizata pentru a imbunatati raportul semnal-zgomot. Esantionul este plasat deasupra ghidului de unda, iar stratul de Pt a fost orientat spre antena folosind un strat izolator subtire. Experimentele THz au fost efectuate folosind un sistem standard THz-TDS, unde tristraturile au fost utilizate ca emitatoare THz. In continuarea studiului privind optimizarea caracteristicilor magnetice ale magnetilor nanocompoziti s-a procedat la investigatii magnetice detaliate a probelor din sistemele de heterostructuri FM/NM. Pentru urmatoarea activitate antamata, pregatirea bancului optic pentru detectia radiatiei THz, prin spectroscopie THz in domeniu temporal, in vederea detectiei radiatiei THz, a fost necesara realizarea unui traseu optic. Cest traseu este astfel conceput incat furnizeaza atat elementele de excitare optica a magnetizarii cat si elementele ce permit stimularea emisiei de radiatie THz. Sistemul este bazat pe secventele realizate in cadrul spectroscopiei THz. Principiul emiterii radiatiei THz in structurile spintronice este bazat pe conversia intre curentul de spin si curentul transversal de sarcina, conversie datorata efectului de cuplaj spin orbita la interfata dintre elementul magnetic (FM) si cel non-magnetic (NM). Acest efect se numeste effect Hall de spin inversat (inverse spin Hall effect ISHE). Miscarea de precesie a vectorului magnetizare in stratul magnetic cauzeaza astfel generarea unui curent de spin. In prezenta etapa a proiectului au fost efectuate toate activitatile preconizate a fi realizate in aceasta perioada, conform planului de lucru initial.Un alt indicator de rezultat a fost realizat conform planului de lucru initial, si anume un articol publicat in jurnal indexat ISI cu factor de impact ridicat, Q1.

Stage 2 (2023)

During Stage 2 of the project, synthesis and characterization of complex FM/NM magnetic heterostructures was performed. In this case, in continuation of the activities carried out in the previous stage, the activity on the deposition of non-magnetic metal layers with variable compositions was carried out. Materials that can produce THz radiation emission with spintronic applicability represent a new class of THz radiation sources. They are based on ultrafast spin transport phenomena, which are induced when a heterostructure composed of a ferromagnetic (FM) and non-magnetic (NM) layer is irradiated with a femtosecond laser pulse. At this point, a spin current created by the reversal of magnetization in the ferromagnetic layer propagates in the non-magnetic layer. The so-called reverse spin Hall effect (ISHE) converts the spin current into an ultrafast charge current that is capable of emitting THz radiation. The so-called THz spintronic transmitters (STE) can be a new generation of THz technologies, as they are capable of providing high field strength and a very wide spectrum, reaching up to 30 THz. Next, structural characterization of FM/NM heterostructures was performed using X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) techniques. Subsequently, the composition of layers and heterostructures was verified by energy dispersion spectroscopy (EDX) and spin dynamics processes were studied. For the study of spin dynamics, we performed ferromagnetic resonance (FMR) measurements. Ferromagnetic resonance was sampled by the microwave magnetic field induced by a coplanar waveguide, which is fed by a continuous wave signal at constant frequency f in the range of 5-19 GHz. An adjustable external magnetic field H was applied and modulated with an amplitude of 0.2–0.3 mT at a frequency of 197 Hz for a better signal-to-noise ratio. RF transmission is measured using a diode, and the locking technique is used to improve the signal-to-noise ratio. The sample is placed above the waveguide, and the Pt layer has been oriented towards the antenna using a thin insulating layer. THz experiments were conducted using a standard THz-TDS system, where trilayers were used as THz emitters. Following the study on optimizing the magnetic characteristics of nanocomposite magnets, detailed magnetic investigations of samples from FM/NM heterostructure systems were performed. For the next activity, the preparation of the optical bench for the detection of THz radiation, by THz spectroscopy in the temporal domain, in order to detect THz radiation, it was necessary to make an optical path. This route is designed so that it provides both the elements of optical excitation of magnetization and the elements that allow stimulation of THz radiation emission. The system is based on sequences performed during THz spectroscopy. The principle of THz radiation emission in spintronic structures is based on the conversion between spin current and transverse charge current, conversion due to the orbital spin coupling effect at the interface between the magnetic element (FM) and the non-magnetic element (NM). This effect is called the reverse spin Hall effect (ISHE). The precession motion of the magnetization vector in the magnetic layer thus causes the generation of a spin current. In the present phase of the project, all the activities expected to be carried out during this period were carried out, according to the initial work plan. Another result indicator was obtained according to the initial work plan, namely an article published in the ISI indexed journal with high impact factor, Q1.

1. Crisan, A.D.; Crisan, O. Morpho-Structural Investigations and Carbon Nanoclustering Effects in Cr-Al-C Intermetallic Alloys. Nanomaterials 2022, 12, 3225. Impact factor: IF: 5.719 (Q1) https://doi.org/10.3390/nano12183225
2. Crisan, O.; Crisan, A.D. Microcrystallization Effects Induced by Laser Annealing in Cr-Al-C Ion-Beam-Sputtered Films. Nanomaterials 2022, 12, 4136. Impact factor: IF: 5.719 (Q1) https://doi.org/10.3390/nano12234136
3. Crisan, O.; Crisan, A.D. Lithographically ordered FePt L10 dots with high coercivity for logic-conditioned magnetic nanostructures, Crystals, 2023, under review.

Dr. Ovidiu Crisan

email: ocrisan@yahoo.com