Studia Universitatis Babeș-Bolyai Physica

FOREWORD: ANNIVERSARY OF PROFESSOR ROMULUS TETEAN VINTELER

2023-05-10T06:10:55+00:00

I appreciate the opportunity offered by this anniversary issue of the scientific journal Studia Physica to note some important aspects related to the didactic and research activity of professor doctor Romulus Tetean. Profesor Romulus Tetean was born in Sântejude, Cluj County, Romania, on November 11, 1957. He graduated the Faculty of Physics at Babeș-Bolyai University in 1981. Thanks to the exceptional results obtained during his undergraduate studies, Romulus Tetean was among the most meritorious students who attended the Solid State Physics master's degree at Babeș-Bolyai University, graduating in 1982. He pursued his PhD studies between 1993 and 1997 at the same university, under the supervision of professor Emil Burzo. This period allowed him to advance his scientific competences in the field of materials science, more precisely in that of the magnetism of intermetallic compounds of 3d transition metals with rare earths. The scientific research of prof. Tetean was mainly directed to the study of the structural, electronic, magnetic and transport properties of different classes of materials, such as rare earth - 3d transition metal intermetallic compounds, permanent magnets, nanostructured materials, perovskites and superconducting materials with possible technical applications such as permanent magnets, sensors, biomedicine and magnetic refrigeration. He was involved in the preparation of new classes of materials and characterization by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), magnetic susceptibility, magnetization, specific heat, electronic properties, Mossbauer effect, and muon spin rotation and relaxation (μSR).

MAGNETOCALORIC EFFECT IN La1.2R0.2Ca1.6Mn2O7 (R=Tb, Dy, Ho, Er) PEROVSKITES SYNTHESIZED BY SOL-GEL METHOD

2023-05-10T06:39:14+00:00

Nanocrystalline double layered La_1.2R_0.2Ca_1.6Mn₂O₇ manganites with R = Tb, Dy, Ho, and Er were synthesized by sol-gel method. The XRD measurements indicate that all samples are single phase with a Sr₃Ti₂O₇-type tetragonal (I4/mmm) structure and mean crystallite sizes between 22 nm and 27 nm. The magnetic measurements evidence a spin-glass like behavior at low temperatures for all samples, which may be due to frustration of random competing ferromagnetic and antiferromagnetic interactions together with the anisotropy originating from the layered structure. A moderate magnetocaloric effect was found for all samples, with the maximum entropy change located at temperatures near the magnetic transition ones, but high RCP(S) values were obtained due to the broadened magnetic entropy curves.

INVESTIGATION OF THE PHYSICAL PROPERTIES OF MnNi(1-X)MX ALLOYS WHERE M=Ti, Al

2023-05-10T06:59:12+00:00

Mn₅₀Ni_50-xM_x, where M= Ti, Al and x=0, 2.5, 5 and 10, alloys with the L10 crystal structure were successfully synthesized. The martensite type transformation was fond to be strongly affected by doping. Both x=10 samples show a spin glass type behavior while the sample doped with Al also shows large exchange bias.

MAGNETIC PROPERTIES OF (Fe,Co)5SiB2 ALLOYS BY W DOPING

2023-05-10T07:10:47+00:00

The intrinsic magnetic properties (magnetic moments, magneto-crystalline anisotropy, Curie temperatures) of the Fe_5-x-yCo_yW_xSiB₂ alloys have been calculated using the spin-polarized relativistic Korringa-Kohn-Rostoker (SPRKKR) band structure method. Our calculations show that for several compounds with x ≥0.5, the magnetocrystalline anisotropy energy (MAE) became axial. Also, theoretical calculations for Fe₄WSiB₂ compound found a magnetization decrease (with about 20%), a Curie temperature decrease of about 30% but an increased axial magnetocrystalline anisotropy compared with the corresponding values for Fe₅SiB_2. Several Fe_5-xW_xSiB₂alloys (x = 0, 0.1, 0.2, 0.5, 1) have been prepared by arc melting of corresponding high purity elements in Ar controlled atmosphere. Composition analysis of the Fe_5-xW_xSiB₂alloys found an impurity phase along with the tetragonal I4/mcm phase of pure Fe₅SiB₂. The magnetic measurements found the decrease of the magnetization any significant increase of the coercivity due to W doping.

CONDUCTANCE OF AN ONE AND DOUBLE-LEVEL QUANTUM DOT

2023-05-10T07:22:23+00:00

We analytically study the expression of the electric charge current through a two-terminal quantum dot in the linear response approximation. We analyse the one-level and the two-level quantum dot scaled conductance taking into account the relevant parameters. We also present the results for the conductance in the Sommerfeld approximation.

STRUCTURE AND MORPHOLOGY OF ZnO FILMS CO-DOPED WITH AL AND RARE EARTH DEPOSITED BY SPRAY COATING TECHNIQUE

2023-05-10T07:39:15+00:00

In this work, the influence of co-doping with Al and rare earth ions (RE= Er, Gd, Nd) on the structure and morphology of zinc oxide (ZnO) films is presented. Spray coated thin films were obtained using a mixture of nitrate substances diluted in equal quantities of distilled water and ethanol. The coated films were deposited on quartz glass and Si(100) substrates using a constant concentration of 0.15M, a temperature of 210ºC, pressure of 2 bar and 10 minutes time of deposition. After this step, the coated films were annealed at 3 different temperatures (600ºC, 800ºC and 1000ºC) for 5 minutes each. The influence of dopant and annealing treatment upon crystallographic structure of the films was analyzed by X-ray diffraction (XRD). Microstructural and surface analysis from scanning electron microscopy (SEM) and AFM measurements evidenced that the type of doping and the annealing treatment modify the surface morphology of the films.

SUSPENSION BASED ON A MIXTURE OF TITANIA-SILICA-FUNCTIONALIZED GRAPHENE OXIDE FOR SURFACE CONSOLIDATION OF HISTORICAL ANDESITE STONE AND MORTAR

2023-05-10T07:50:24+00:00

Three-components suspension, based on a mixture of titania (TiO₂)-silica (SiO₂)-functionalized graphene oxide (GO) is prepared and used for surface consolidation of historical andesite stone and mortar. For comparison, mono- and bi-component suspensions are also involved in this study. In order to increase the attachment capacity of GO to substrate with silicon, GO functionalized with (3-aminopropyl) triethoxysilane (GO-APTES) was involved. Unmodified and modified by painting with prepared suspensions andesite and mortar samples were investigated by microscopy (Optic and SEM-EDX), XRD, FT-IR and Raman spectra before and after 6 months of exposure to ambient conditions. After investigation, even the amount of used consolidant suspension was low, the stability of the modified samples was found that remains constant and in a long time it is possibly to reach even a higher stability level. This approach provides the idea that the prepared suspension could be an interesting option to be involved in stone and mortar consolidation-restoration field.

ENHANCING THE PHOTOLUMINESCENCE OF POLYFLUORENE-BASED THIN FILMS VIA ILLUMINATION

2023-05-10T08:20:40+00:00

We show in this work that exposing thin films of conjugated poly[9,9-bis(2-ethylhexyl)-9H-fluorene-2,7-diyl] to light under nitrogen atmosphere led to an increase of their emission up to 2.2 folds. This enhancement was due to the appearance of structural changes in the glassy and β-phase conformations induced upon thin film illumination, as revealed by the Franck-Condon analysis of the photoluminescence spectra. Interestingly, the photoluminescence of thin films remained at the enhanced value for 75 days after stopping the illumination, most probably due to the permanent structural changes induced upon illumination.

RASHBA FIELD CONTRIBUTION AND ELECTRIC FIELD CONTROL OF THE MAGNETIC ANISOTROPY

2023-05-10T08:55:27+00:00

The anatomy of the Perpendicular Magnetic Anisotropy (PMA) in magnetic multilayered thin film heterostructures and the possibility to efficiently manipulate it by external electric fields represent major issues for technological applications in magnetic data storage devices. Solving a standard quantum model based on a Stoner-Rashba Hamiltonian, we illustrate that the magnetic properties in ultrathin magnetic films arise from the competing components identified in the magnetic energy: the Rashba correction to the Stoner splitting, a pseudo-dipolar contribution to the anisotropy energy proportional to the electric field at the interface that would favor in-plane magnetization configuration and a uniaxial-like perpendicular anisotropy term. This last term is responsible on the perpendicular magnetization configuration in ultrathin films and depends on the square of the electric field at the surface of the film. Investigating the time evolution of the magnetic system, we described the macrospin magnetization dynamics in terms of a Rashba field induced magnetization precession. Despite its simplicity, the quantum approach underlines the basic issues related to the physical origin and the mechanisms of the perpendicular magnetization in ultrathin magnetic films and illustrates the capability of manipulation by external gating electric field, in experiments similarly to Nuclear Magnetic Resonance.

VIBRATIONAL FEATURES OF DIPHENYLHYDANTOIN

2023-05-10T09:04:55+00:00

The molecular vibrations of diphenylhydantoin (DPH) were investigated in crystalline sample at room temperature, by using Fourier transform infrared (FT-IR), FT- and conventional Raman spectroscopies. Furthermore, density functional theory (DFT) calculations were utilized to confirm and clarify the experimental data. Two methods were evaluated for accurate prediction of the vibrational spectra: i) vibrational anharmonic calculations on DPH monomer based on the second-order perturbation theory; ii) harmonic calculations on a cluster of five DPH molecules.