Victor Hugo Etgens

The kinetics of magnetization reversal in a rare-earth-free Mn–Al–C permanent magnet is reported. The L10-phase material was prepared using a combination of the melt spinning and spark plasma sintering techniques and characterized with a vibrating sample magnetometer. Magnetic relaxation and susceptibility experiments were performed. Magnetic viscosity, fluctuation field and activation volume were measured in a wide temperature range. These data were used to analyze the magnetization reversal and temperature dependence of the coercive field. The thermally activated mechanism of the reversed nucleus formation was confirmed for the intermediate temperatures. The role of wall thickness and crystallite size as additional characteristic lengths in magnetic domain nucleation is discussed.

Structural and magnetic characterization of Mn-Al-C permanent magnets obtained by spark plasma sintering (SPS) is reported. The transformation from the parent E-phase to the ferromagnetic τ-phase occurs simultaneously with the process of sintering. The use of a tungsten carbide crucible enabled us to decrease the sintering temperature, which hampered the precipitation of secondary phases. As a result, sintered samples show higher coercivity as compared with annealed powder. However, the finely milled powder, ensuring better densification, turned out to be more prone to phase decomposition during sintering. The phase constitution of the samples was determined by X-ray diffraction, and magnetic hysteresis loops were recorded using a vibrating sample magnetometer. Scanning electron microscopy and electron backscatter diffraction were employed to study the microstructure and orientation distribution of grains after SPS. The dependence of coercivity and magnetization on the preparation conditions and sintering temperature is analyzed. To combine good magnetic properties with proper density, further optimization of the production parameters is necessary.

The near-equiatomic Mn-Al alloys are considered for possible applications as rare-earth-free permanent magnets due to existence of a ferromagnetic -phase with L10 crystal structure and other attractive physical properties. However, this tetragonal phase is metastable and can only be obtained from a hexagonal -phase enriched in manganese. The partial atomic disorder can results in antiferromagnetic coupling of the Mn atoms occupying different sublattices. Moreover, the carbon addition, used to improve phase stability and coercivity, leads to deformation of the unit cell and can affect the Mn-Mn coupling. To clarify this question, a series of magnetic measurements and neutron diffraction experiments in a wide temperature range were carried out on the Mn55Al45 alloys doped with a small quantity of C. The materials were prepared by melt-spinning and subsequent annealing. Different methods of carbon introduction were tried, the best results were obtained by melting Mn and Al with Mn23C6. The structural and magnetic properties of the ferromagnetic and paramagnetic -phase as well as the precursor -phase were determined. The average magnetic moments on two sublattices were analysed using Weiss plot of the magnetometric data and Rietveld refinement of the neutron diffraction data. The dependence of the magnetic moments on the C content and temperature is reported. Ferrimagnetic behaviour of the Mn-Al alloys with excess of manganese, caused by antiferromagnetic coupling and responsible for the decrease of magnetisation with increase of Mn content, was confirmed.

Most studies on MnAs material in its bulk form have been focused on its temperature-dependent structural phase transition accompanied by a magnetic one. Magnetostructural phase transition parameters in thin MnAs films grown on substrates present however some differences from the bulk behavior, and local studies become mandatory for a deeper understanding of the mechanisms involved within the transition. Up to now, only surface techniques have been carried out, while the transition is a three-dimensional phenomenon. We therefore developed an original nanometer scale methodology using electron holography to investigate the phase transition in an epitaxial MnAs thin film on GaAs(001) from the cross-section view. Using quantitative magnetic maps recorded at the nanometer scale as a function of the temperature, our work provides a direct in situ observation of the inhomogeneous spatial distribution of the transition in the layer depth and brings new insights on the fundamental transition mechanisms.

There is a converging interest of society, academia, and industry about intelligent vehicles. Meanwhile, it is necessary to consider some legal aspects that could help these vehicles to arrive safely on public roads. The difficulty lies in the fact that a complex cybernetic system will replace a human-driver in a complete or partial manner, depending on the level of the delegation to the machine. The data-recording device appears mandatory to support regulatory standards for these future vehicles, providing important information helping to determine what is in the origin of incidents and failure causes. Nevertheless, there is no standard data recording device developed specifically for intelligent vehicles. This article presents a short survey about the existing recording devices and discusses the importance of developing a specific one to intelligent vehicles. The legal aspects involved in accident analyses are also presented, showing the importance and applicability of such device.

Investigations of the complex behavior of the magnetization of manganese arsenide thin films due to defects induced by irradiation of slow heavy ions are presented. In addition to the thermal hysteresis suppression already highlighted in Trassinelli et al (2014 Appl. Phys. Lett. 104 081906), we report here on new local magnetic features recorded by a magnetic force microscope at different temperatures close to the characteristic sample phase transition. Complementary measurements of the global magnetization in different conditions (applied magnetic field and temperatures) enable the film characterization to be completed. The obtained results suggest that the ion bombardment produces regions where the local mechanical constraints are significantly different from the average, promoting the local presence of magneto-structural phases far from the equilibrium. These regions could be responsible for the thermal hysteresis suppression previously reported, irradiation-induced defects acting as seeds in the phase transition.

The magnetic properties of an iron nitride thin film obtained by ion implantation have been investigated. N[ ] ions were implanted in a pristine iron layer epitaxially grown on ZnSe/GaAs(001). X-ray diffraction measurements revealed the formation of body-centered tetragonal N-martensite whose [ ]-axis is perpendicular to the thin film plane and [ ]-parameter is close to that of [ ]-Fe₈N. Magnetic measurements disclosed a weak perpendicular magnetic anisotropy (PMA) whose energy density [ ] was assessed to about 10⁵[ ]J/m³. A sharp decline of the in-plane magnetocrystalline anisotropy (MCA) was also observed, in comparison with the body-centered cubic iron. The origin of the PMA is attributed to the MCA of N-martensite and/or stress-induced anisotropy. As a result of the PMA, weak magnetic stripe domains with a period of about 130[ ]nm aligned along the last saturating magnetic field direction were observed at remanence by magnetic force microscopy. The application of an increasing in-plane magnetic field transverse to the stripes [ ] highlighted a threshold value ([ ][ ]T) above which these magnetic domains irreversibly rotated. Interestingly, below this threshold, the stripes do not rotate, leading to a zero remanent magnetization along the direction of the applied field. The interest of this system for magnetization dynamics is discussed.