National Institute Of Materials Physics - Romania

A critical discussion on the presently available models for the relaxation time of magnetic nanoparticles approaching the superparamagnetic regime in the presence of interparticle dipolar interactions is considered. The direct implications of such interactions for magnetic fluid hyperthermia therapy through susceptibility loss mechanisms give rise to an indirect method for their study via specific absorption rate measurements performed on ferrofluids of different volume fractions. The theoretical support for the specific evolution of the relaxation time constant and the anisotropy energy barrier versus the interparticle interactions in a perturbation approach of the simple Neel expression for the relaxation time is provided via static and time-dependent micromagnetic simulations.

Severe plastic deformation (SPD) is widely considered to be the most efficient process in obtaining ultrafine-grained bulk materials. The aim of this study is to examine the effects of the SPD process on Ni-Fe-Ga ferromagnetic shape memory alloys (FSMA). High-speed high-pressure torsion (HSHPT) was applied in the as-cast state. The exerted key parameters of deformation are described. Microstructural changes, including morphology that were the result of processing, were investigated by optical and scanning electron microscopy. Energy-dispersive X-ray spectroscopy was used to study the two-phase microstructure of the alloys. The influence of deformation on microstructural features, such as martensitic plates, intragranular gamma phase precipitates, and grain boundaries' dependence of the extent of deformation is disclosed by transmission electron microscopy. Moreover, the work brings to light the influence of deformation on the characteristics of martensitic transformation (MT). Vickers hardness measurements were carried out on disks obtained by SPD so as to correlate the hardness with the microstructure. The method represents a feasible alternative to obtain ultrafine-grained bulk Ni-Fe-Ga alloys.

Among the rare-earth-free systems that are currently investigated in search for novel permanent magnet solutions for various applications, with special emphasis on the magnets required to operate in extreme conditions, the FePt binary system, where the tetragonal hard magnetic L1(0) phase can be formed by suitable microstructure processing via annealing, has been extensively studied. A variation of this system, the ternary FeMnPt system, has been also recently shown to exhibit good permanent magnet behavior due to the suitable formation of the L1(0) phase. In addition to be likely to form the L1(0) phase as its parent binary system, the ternary FeMnPt benefits from the reduced costs due to the reduced amount of Pt and may exhibit particular magnetic structure due to the influence of the antiferromagnetic Mn. In the present work, we have employed a mixed sputtering technique, based on the use of both elemental and compound target for developing L1(0) FeMnPt thin films with specific structural features that triggers better magnetic performances in terms of coercivity and maximum energy products. The as-obtained films have been thermally annealed and characterized by means of transmission electron microscopy, X-ray diffraction, Mossbauer spectroscopy, magneto-optic Kerr effect (MORE) and SQUID magnetometry. The aim is to correlate the Mn induced microstructural and lattice changes with the magnetic properties and to optimize the microstructure for an early formation of the ordered L1(0) phase and increased coercivity compared to the as-prepared, structurally disordered, face centred cubic initial state of the films.

Polycrystalline In2S3 thin films have been grown on SnO2/glass substrates by chemical bath deposition technique and irradiated at different high gamma doses 3, 7, 15 and 40 kGy. X-ray diffraction, Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS), Spectrophotometer, Photoluminescence and Thermoluminescence were used to investigate physical properties of In2S3 thin films induced by gamma irradiation. After being irradiated, structural properties of In2S3 thin films have shown that preferred orientation has been moved from (4 0 0) plan at 2 theta(1)=33.42 degrees to a new created orientation at 2 theta(2)=38.06 degrees for 40 kGy gamma dose. EDS analysis has shown that atomic percentage (S/In) has been strongly varied for 40 kGy which indicate significant changes in stoichiometry. Thermoluminescence of irradiated In2S3 thin films has revealed a good sensitivity toward absorbed gamma dose. After irradiation, optical transmittance of In2S3 thin films has been increased from 50% to a maximum value of 70% in the visible range for 15 kGy dose. Band gap energy E-g has been slightly decreased. Other optical parameters such absorption and extinction coefficients, refractive index and permittivity have been determined. These experimental results show that gamma radiations can be used for tuning physical properties of In2S3 thin films for photovoltaic applications.

We report studies of quasi-remanent polarization states in Pb0.99Nb0.02[(Zr0.57Sn0.43)(0.94)Ti-0.06](0.98)O-3 (PNZST) anti-ferroelectric ceramics and investigation of their relaxation effects using unique in-situ electrically activated time-resolved Synchrotron X-ray powder diffraction (SXPD) and Sn-119 Mossbauer Spectroscopy (MS). The SXPD patterns are consistent with a phase transition from quasi-tetragonal perovskite in 0 V relaxed anti-ferroelectric state to rhombohedral distortion in ferroelectric state under saturating applied voltages of +/- 2 kV. The observed quasi-remanent polarization relaxation processes are due to the fact that tetragonal to rhombohedral distortion does not occur at the applied voltage required to access the quasiremanent polarization states, and the tetragonal symmetry restored after the removal of the applied electric field is preserved. Since these quasi-remanent polarization states were seen as possibly suitable for memory applications, the implications of this study are that anti-ferroelectrics are more feasible for multi-state dynamic random access memories (DRAM), while their application to non-volatile memories requires development of more sophisticated "read-out" protocols, possibly involving dc electrical biasing.

Ruthenium-doped copper ferrite(Ru - CuFe2O4) nanoparticles (NPs) have been synthesized using a simple and cost-effective wet chemical co-precipitation deposition method. The crystallographic scanning electron microscopy images confirm cubic crystal structure and agglomerated-type surface appearance. The crystallite sizes are 6-24 nm in the range. Dielectric measurement analysis estimates the dielectric constant and loss of Ru - CuFe2O4 NPs. In this connection, dielectric constant and loss are reduced virtue of air annealing for various temperatures. Also, the dielectric loss confirms the relaxation peak. From magnetic measurement results, the coercivity decreases whereas saturation and remanence magnetization are increased. These features have approved the soft magnetic nature in the Ru - CuFe2O4 NPs.

Thin films of (117) Bi2Sr2Ca2CuO8+ (Bi-2212) were grown by Molecular Organic Chemical Vapor Deposition (MOCVD) on (110) SrTiO3 and (110) LaAlO3 substrates. Substrates were vicinal with off angles up to 20 degrees. Films are 3D epitaxial and X-ray diffraction phi- scans demonstrate that, while the films grown on a flat substrate are composed of twinned grains, the films on vicinal substrate are twin-free. A higher quality is obtained if growth is performed at two temperatures: Growth starts at 550-600 degrees C and continues at 700-750 degrees C. The twin-free film grown by the two-temperature method shows a zero-resistance critical temperature of 37 and 32 K when the measuring current is applied in-plane parallel and perpendicular to [001] direction of the substrate. Twin-free non c-axis thin films are promising for fabrication of novel planar THz devices.

Fe-14Cr-0.4Ti-0.25Y(2)O(3) ferritic steels were produced by varying the amount of residual process control agent, PCA (ethanol), in the ball-milled powders and changing the spark-plasma-sintering, SPS, temperature. Near the-oretical density (99.3%), high Vickers hardness (501-920 HV, measured by applying a load of 100 g for 5 s) and fine grain size (26-36 nm), very stable against heating, can be achieved on ODS ferritic steels, consolidated from powders with a low amount of PCA and processing temperature in the range of 1000 degrees C-1100 degrees C. Additional grain refinement occurs near alpha -> gamma transition which is generated by the reaction of the traces of PCA with the ferritic matrix upon heating. High local temperatures and the evolved thermally activated processes, at the contact points between particles/at the particle surfaces during SPS-consolidation, were demonstrated to be the main factors responsible for improved densities and hardness. The role of PCA in the sintering, thermal and microstructure particularities and its impact on the quality of the final steel was thoroughly analysed throughout the work. (C) 2019 Elsevier B.V. All rights reserved.

Polycrystalline ruthenium-doped lithium-copper-ferrite (Ru - LiCuFe2O4)nanoparticles (NPs) are synthesized using a simple and cost-effective chemical co-precipitation method and annealed at different temperatures for increasing the crystallinity. The transmission and scanning electron microscopy images have confirmed the presence of soft agglomerations and cuboids for the samples annealed at 1100 degrees C. X-ray photoelectron results along with Raman spectra have collectively demonstrated the presence of Ru in the structure of Ru - LiCuFe2O4 NPs. The dielectric properties of as-synthesized Ru - LiCuFe2O4 NPs are investigated using LCR meter where the smaller NPs demonstrates a higher dielectric constant. Also, the results of magnetic measurements of annealed Ru - LiCuFe2O4 NPs have corroborated a soft magnetic nature due to the pinning sites that endow lower coercivity, remanence and saturation magnetization than that of the pristine one. The variation of permittivity and electrical resistivity with respect to frequency under humidity conditions suggested that this material has a potential to use as capacitive and resistive humidity sensor. The results of this study open the doors for utilization of metal-doped magnetic ferrites for humidity sensing applications.

We report on a combined experimental and modelling approach towards the design and fabrication of efficient bulk shields for low-frequency magnetic fields. To this aim, MgB2 is a promising material when its growing technique allows the fabrication of suitably shaped products and a realistic numerical modelling can be exploited to guide the shield design. Here, we report the shielding properties of an MgB2 tube grown by a novel technique that produces fully machinable bulks, which can match specific shape requirements. Despite a height/radius aspect ratio of only 1.75, shielding factors higher than 175 and 55 were measured at temperature T = 20 K and in axially-applied magnetic fields mu H-0(appl) = 0.1 and 1.0 T, respectively, by means of cryogenic Hall probes placed on the tube's axis. The magnetic behaviour of the superconductor was then modelled as follows: first we used a two-step procedure to reconstruct the macroscopic critical current density dependence on magnetic field, J(c)(B), at different temperatures from the local magnetic induction cycles measured by the Hall probes. Next, using these J(c)(B) characteristics, by means of finite-element calculations we reproduced the experimental cycles remarkably well at all the investigated temperatures and positions along the tube's axis. Finally, this validated model was exploited to study the influence both of the tube's wall thickness and of a cap addition on the shield performance. In the latter case, assuming the working temperature of 25 K, shielding factors of 10(5) and 10(4) are predicted in axial applied fields it mu H-0(appl) = 0.1 and 1.0 T, respectively.