Eng. Andrei ALEXANDRU-DINU

The precise control of the magnetic compensation temperature (theta c) in ferrimagnetic garnets is essential for the development of cutting-edge ultrafast customizable spintronic devices. In this work we demonstrate how fine variation in stoichiometry and cation distribution in iron gadolinium garnets significanty influences theta c. Two samples of Gd3Fe5O112 garnets synthesized via a new hydrothermal method and a conventional solid-state reaction, respectively, were considered. The complex study was carried out using a complex approach combining X-ray diffraction, magnetometry, and M & ouml;ssbauer spectroscopy. Atomic-scale analysis revealed with unprecedent accuracy a cationic inversion between Fe3+ ang Gd3+ at octahedral and dodecahedral sites in both samples, and their chemical compositions were determined as Gd2.70Fe4.76O11.9 and Gd2.96Fe4.68O11.5, respectively. These local rearrangements have been shown to have a consistent influence on theta c (290 K and 317 K, respectively) around room temperature, emphasizing the high sensitivity of exchange interactions to internal atomic order. Results clearly illustrate the strong correlation between the processing, atomic configuration and macroscopic magnetic behavior, establishing a new paradigm for the design of garnet-based materials with tunable theta c. The strategy for the accurate determination of cation inversion illustrated in this work exhibits great potential in guiding material innovations for next-generation spintronics.

Fabrication and extensive characterization of hard-soft nanocomposites composed of hard magnetic low-temperature phase LTP-MnBi and amorphous Fe70Si10B20 soft magnetic phase for bulk magnets are reported. Samples with compositions Mn55Bi45 + x center dot(Fe70Si10B20) (x = 0, 3, 5, 10, 20 wt.%) were prepared by spark plasma sintering of powder mixtures. Characterization has been performed by X-ray diffraction, scanning and transmission electron microscopy, magnetometry and Fe-57 Mossbauer spectroscopy. It was shown that samples contain crystallized and nanometric LTP-MnBi phases with various elemental compositions depending on the degree of Bi clustering. Complex correlations between starting compositions, processes during fabrication, and functional magnetic characteristics were observed. Unexpected special situations of the relation between microstructure and magnetic coupling mechanisms are discovered. Exchange spring effects of different strengths occur, being very sensitive to morpho-structural and compositional features, which in turn are controlled by processing conditions. An in-depth analysis of related microscopic characteristics is provided. Results of this work suggest that fabrication by powder metallurgy routes, such as spark plasma sintering of hard and soft magnetic powder mixtures, of MnBi-based composites with exchange spring phenomena have a high potential in designing and optimization of suitable materials with tunable magnetic properties towards rare-earth-free permanent magnet applications.

Romula (today Resca, Dobrosloveni Village, Romania) was the largest urban and economic center of Dacia Inferior (Malvensis), a Roman province located in the north of the Lower Danube region. In this context, the city market included workshops for the production of ceramic, metal, stone, bone, and glass objects. In 2013, 2015, and 2018, during excavations of the former Roman city, two rectangular glass furnaces were discovered. One has only one chamber, the other has two chambers. A melted glass layer was found on the walls of furnace no. 1, as well as in one room of furnace no. 2. Broken fragments of glass were also found in both. The furnaces are located in the central area of the Roman city. The evidence suggests that the furnaces belong to secondary glass workshops. The glass may have arrived in raw form, where it was remelted and processed. The discovery of these furnaces contributes to the growing body of evidence for Roman glass production around the empire.1

Sesquipedalian mud and burnt bricks (second to third century AD) were excavated from the Roman city of Romula located in the Lower Danube Region (Olt county, Romania). Along with local soils, bricks are investigated by petrographic analysis, X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FT-IR), electron microscopy (SEM/EDX), X-ray microtomography (XRT), thermal analysis (DTA-TG), M.ssbauer spectroscopy, magnetometry, colorimetry, and mechanical properties assessment. The results correlate well with each other, being useful for conservation/restoration purposes and as reference data for other ceramic materials. Remarkably, our analysis and comparison with literature data indicate possible control and wise optimization by the ancient brickmakers through the recipe, design (size, shape, and micro/ macrostructure), and technology of the desired physical-chemical-mechanical properties. We discuss the Roman bricks as materials that can adapt to external factors, similar, to some extent, to modern "smart" or "intelligent" materials. These features can explain their outstanding durability to changes of weather/climate and mechanical load.

Pure and Er-doped hematite (alpha-Fe2O3) nanorods were prepared by a two-step method involving hydrothermal synthesis and calcination of pure and Er-doped goethite (alpha-FeOOH) nanorods. Substitution of Fe3+ by Er3+ in the crystal structure of hematite caused morpho-structural changes such as expansion of the unit cell and gradual shortening and rounding of hematite nanorods towards formation of nanoellipsoids. These changes induced modification of magnetic and optical properties suggesting the possibility of a systematic control of physical properties via rare earth substitution. A decrease in the hyperfine magnetic field, coercive field and Morin transition temperature, as well as an increase of the magnetic susceptibility and a narrowing of the optical band gap were observed by substitution. Intimate mechanisms related to the formation of more and more defect-like hematite phases with decreased temperatures for the transition to the low temperature antiferromagnetic phase at increased doping level were evidenced via temperature dependent Mo center dot ssbauer spectroscopy.

Pottery vessels made of kaolin clay from the Roman Period (2nd, 3rd centuries CE) found in Romula (Re?ca village, Olt County, Romania) from Dacia Inferior (Malvensis) were investigated by petrographic, X-ray diffraction, X-ray fluorescence, thermal analysis, electron microscopy, and mechanical tests. Our results are compared with available data on kaolin clays and pottery vessels from other sites located along the lower course of Danube river and near the Black Sea, namely in Moesia Superior, Moesia Inferior, and Thracia. Archeological and geographical contexts are addressed. Results of our analysis suggest a local production of ceramics in Romula, by using raw materials from the north of Lower Danube, in opposition to the idea that kaolin ware was imported from the provinces south of the Danube.