Dereje Seifu

Transition metal doping in two-dimensional transition metal dichalcogenides offers a promising route for tailoring magnetic and structural properties for spintronic applications. In this work, Co-doped WSe2 thin films were synthesized using a combination of DC, pulsed DC, and RF magnetron sputtering, followed by rapid thermal annealing at 600 °C. X-ray diffraction confirms the formation of the hexagonal 2H-WSe2 phase with a preferential c-axis orientation and no detectable secondary phases, indicating successful incorporation of Co into the lattice. Magnetic measurements reveal a significant enhancement in saturation magnetization and coercivity after annealing, attributed to improved crystallinity, reduced defect density, and strengthened exchange interactions between Co dopants and the WSe2 matrix. Angular-dependent studies demonstrate the emergence of magnetic anisotropy, while temperature-dependent measurements show stable coercivity over the range of 10–300 K, indicating robust ferromagnetic behavior. Raman spectroscopy further confirms lattice distortion and modified bonding environments due to Co incorporation, with annealing improving spectral sharpness and structural ordering. These results highlight the critical role of post-deposition annealing in optimizing the magnetic and structural properties of Co-doped WSe2, underscoring its potential for future spintronic and multifunctional device applications.

A thin film of Tbx(Co0.4Fe0.6)1−x, with x ≅ 0.2, was synthesized using magnetron DC and RF sputtering, which offers industry-friendly benefits, including faster deposition at medium energy, cost-effectiveness, and scalability. The surface morphology of the film was characterized using a transmission electron microscope; identification and quantification of the elements were carried out using energy-dispersive x-ray spectroscopy (EDS), which confirmed the Tb–Co–Fe film is Fe-rich with clear Tb and Co incorporation, consistent with Fe2RE-type phases that support strong magnetic behavior. The lattice was scanned using selected area electron diffraction, showing an hcp lattice with lattice spacing of 2.25 ± 0.05 Å; however, Tb–(Co, Fe) at the composition found by EDS is typically amorphous or nanocrystalline, so “lattice spacing” refers to the nearest-neighbor/short-range order distance inferred from the first diffuse ring, not a true crystal lattice parameter. Magnetic properties studied using a vibrating sample magnetometer at several angles between the sample normal and the magnetic field show magnetic anisotropy with an easy axis at 0° and a hard axis at 90°, typical for magnetically doped thin films with a strong twofold anisotropy with an anisotropy ratio of 2.2. This magnetic alloy is promising for potential applications in low-power ultrafast spintronic technologies due to its low symmetry and large spin–orbit coupling.

2H-WSe2 films were synthesized by magnetron co-sputtering of tungsten and selenium targets onto SiO2/Si(100), SiO2/Si(111), and Al2O3(0001) substrates in an Ar atmosphere at 150 °C, without any additional vacuum selenization step. After rapid thermal annealing at 700 °C in Ar, x-ray diffraction confirmed the formation of the optimal crystal phase. Films grown on SiO2/Si exhibited lower strain compared to those deposited on Al2O3. We demonstrated that (002)-textured WSe2 films with a thickness of approximately 50 nm can be achieved, and that planar-like growth is maintained for films thinner than about 60 nm. Raman spectroscopy showed the characteristic E2g and A1g modes of 2H-phase WSe2 at 248 cm−1 and 251 cm−1, corresponding to in-plane and out-of-plane vibrational phonon modes, respectively. The indirect optical bandgap of few-layer WSe2 was determined to be between 1.43 eV and 1.49 eV, with the bandgap varying as a function of film thickness—an expected property of 2D materials such as WSe2.

Bismuth selenide (Bi2Se3) is a promising 2D quantum nanomaterial for thermoelectric and optoelectronic devices; however, several growth techniques lack crystalline uniformity and scalability. Through multiple corroborating methods, we study the optical properties of Bi2Se3 in conjunction with its structural properties for a uniquely comprehensive description. The Raman spectra of films of Bi2Se3 prepared using a radio frequency (RF) source of magnetron sputtering reveal the characteristic Eg2 and A1g2 peaks of Bi2Se3 at ∼120 and 175 cm−1, which undergo spectral blue shifts as film thickness is increased due to the quantum size effect. Numerical analysis of the width of the Raman peaks suggests that the films are crystalline with nanocrystal sizes of 13–22 nm, confirmed by Scherrer's analysis of complementary x-ray diffraction. A strong optical absorption was observed in the visible–near infrared (NIR) range with an optical direct bandgap Eg determined from visible–NIR spectra within a 1.8–1.4 eV range, depending on film thickness and nanoparticulate structure. The interplay between film thickness and surface roughness strongly influences the optical response of Bi2Se3, where thinner, rougher films exhibit enhanced scattering and Urbach tailing, while thicker, smoother films display sharper absorption edges and bulk-like behavior. The deposition of nanometer Bi2Se3 films using RF magnetron sputtering constitutes a promising scalable platform for quantum thermoelectric and optoelectronic devices.

Magnetic tunnel junctions (MTJs) have been widely utilized in sensitive sensors, magnetic memory, and logic gates due to their tunneling magnetoresistance. Moreover, these MTJ devices have promising potential for renewable energy generation and storage. Compared with Si-based devices, MTJs are more tolerant to electromagnetic radiation. In this review, we summarize the functionalities of MgO-based MTJ devices under different electromagnetic irradiation environments, with a focus on gamma-ray radiation. We explore the effects of these radiation exposures on the MgO tunnel barriers, magnetic layers, and interfaces to understand the origin of their tolerance. This review enhances our knowledge of the radiation tolerance of MgO-based MTJs, improves the design of these MgO-based MTJ devices with better tolerances, and provides information to minimize the risks of irradiation under various irradiation environments. This review starts with an introduction to MTJs and irradiation backgrounds, followed by the fundamental properties of MTJ materials, such as the MgO barrier and magnetic layers. Then, we review and discuss the MTJ materials and devices’ radiation tolerances under different irradiation environments, including high-energy cosmic radiation, gamma-ray radiation, and lower-energy electromagnetic radiation (X-ray, UV–vis, infrared, microwave, and radiofrequency electromagnetic radiation). In conclusion, we summarize the radiation effects based on the published literature, which might benefit material design and protection.

Smart materials usually are either mounted on surfaces or embedded inside; in this work, we study magnetic properties of smart materials that are integrated into the structure. Magnetostrictive particles of Tb_xDy_1−xFe₂ (x ∼ 0.3), commonly known as Terfenol-D, incorporated into carbon fiber reinforced polymers showed a 3000% enhancement in the coercive field compared to the reference pristine bulk Terfenol-D. Torque magnetometer measurements revealed a magnetic anisotropy with twofold symmetry. The lowest applied field at which a reversible torque without hysteresis occurred was ∼5.0 kOe, which yields 5 Oe anisotropic field for a carbon fiber reinforced polymer magnet. This value of the anisotropic field is confirmed using the Miyajima method. Angle dependent coercive field and saturation/remanent magnetization also confirmed the twofold symmetry of magnetization in this system. The high magnetostrictive coefficient, together with the enhanced coercive field, makes Terfenol-D integrated into carbon fiber reinforced polymers serve as an excellent candidate for in situ sensing of damage in various polymer composites under several mechanical loading conditions.

Nanocomposites of cobalt ferrite (CFO) magnetic nanoparticle intercalated carbon nanotubes (CNTs) are evaluated as a nanomagnetic ultrasensitive sensor for an environmental toxin, hexavalent chromium (Cr-VI). Specifically, the structural and magnetic changes that accompany the infiltration of the CFO/CNT composite by Cr-VI are presented. The extended x-ray absorption fine structure shows that the atomic spacing within the CFO structure changes in the presence of Cr, suggesting that the Cr is incorporated into the nanoparticles. Vibrating sample magnetometry (VSM) reveals that the CFO/CNT composite infiltrated with Cr-VI has a 71% enhancement in saturation magnetization compared with the uninfiltrated CFO/CNTs, while small-angle neutron scattering (SANS) suggests that this magnetic enhancement is not associated with the nanoparticle length-scales, but may arise from longer-ranged clusters. Both VSM and SANS clearly demonstrate that the Cr-doped CFO/CNTs are hysteretic with a net magnetization at remanence that is about 1/3 of saturation, while this hysteresis is absent in the undoped CFO/CNTs. These magnetic differences in either remanence or saturation are promising for the magnetic detection of Cr-VI using CFO/CNT sensors.

Nanowires of tunneling magnetoresistance (TMR) were synthesized using magnetron DC/RF sputtering by filling Fe/MgO/Fe inside vertically grown and substrate-supported carbon nanotubes. Nanocolumns of Fe/MgO/Fe TMR were synthesized using glancing angle deposition. The magnetic properties of nanowires, nanocolumns and planar nanometric thin films of Fe/MgO/Fe showed similarities including twofold magnetic symmetry. Nanowires of Fe/MgO/Fe showed improved magnetic properties, in particular its coercive field, which is 754% higher than planar thin films of Fe/MgO/Fe. A macroscopic phenomenon that can be explained only by quantum mechanics is TMR, where electrical current can flow across a nanometric thin insulator layer between two electrodes when an external magnetic field is applied parallel to the trilayer system. Coherence in the TMR effect is paramount to make spintronic devices. Nanowires possess shape anisotropy, which can play an important role in coherence.