Acciai inossidabili ferritici per elettrovalvole: effetto del trattamento termico e delle lavorazioni a freddo sulle caratteristiche magnetiche

Il settore applicativo delle elettrovalvole impone limiti molto stringenti sulle proprietà magnetiche (campocoercitivo, curva di isteresi e permeabilità magnetica) dei materiali utilizzati per la realizzazionedei componenti: tali requisiti non vengono soddisfatti dagli acciai inossidabili ferritici ricotti incondizioni “standard”. Questo lavoro descrive il complesso processo di messa a punto, su un acciaionon appartenente alla categoria appositamente creata per questa applicazione, di un trattamentotermico dedicato e di un ciclo di trasformazione delle barre finalizzato alla produzione di proprietà ottimali.In particolare vengono correlati alle caratteristiche magnetiche i diversi parametri del ciclo produttivo,le caratteristiche microstrutturali e le misure di tensioni residue ottenute mediante diffrattometria raggi X

Probing the shape of the Weyl Fermi surface of NbP using transverse electron focusing

The topology of the Fermi surface significantly influences the transport properties of a material. Firstly measured through quantum oscillation experiments, the Fermi surfaces of crystals are now commonly characterized using angle-resolved photoemission spectroscopy (ARPES), given the larger information volume it provides. In the case of Weyl semimetals, ARPES has proven remarkably successful in verifying the existence of the Weyl points and the Fermi arcs, which define a Weyl Fermi surface. However, ARPES is limited in resolution, leading to significant uncertainty when measuring relevant features such as the distance between the Weyl points. While quantum oscillation measurements offer higher resolution, they do not reveal insights into the cross-sectional shape of a Fermi surface. Moreover, both techniques lack critical information about transport, like the carriers mean free path. Here, we report measurements unveiling the distinctive peanut-shaped cross-section of the Fermi surface of Weyl fermions and accurately determine the separation between Weyl points in the Weyl semimetal NbP. To surpass the resolution of ARPES, we combine quantum oscillation measurements with transverse electron focusing (TEF) experiments, conducted on microstructured single-crystals. The TEF spectrum relates to the Fermi surface shape, while the frequency of the quantum oscillations to its area. Together, these techniques offer complementary information, enabling the reconstruction of the distinctive Weyl Fermi surface geometry. Concurrently, we extract the electrical transport properties of the bulk Weyl fermions. Our work showcases the integration of quantum oscillations and transverse electron focusing in a singular experiment, allowing for the measurements of complex Fermi surface geometries in high-mobility quantum materials

Microstructure and magnetic properties of pure iron for cyclotron electromagnets

The microstructural and magnetic properties of pure iron ingots used as cores of cyclotron electromagnets have been investigated upon annealing sequence from 650 degrees C to 820 degrees C. Optical Microscopy, Scanning Electron Microscopy, and X-ray Diffraction were employed in the structural analysis, while magnetization curve and hysteresis loops were obtained in rod and ring samples, machined out of the large (around 4 m diameter) cast yoke, by combination of point-by-point and continuous (frequency f = 0.03 Hz) hysteresisgraph methods. Quite inhomogeneous grain structure was observed, with large and irregularly shaped grains (size of several mm), posing special constraints to the conventional magnetic testing approach (e.g., the IEC 60404-4 standard). Reproducible DC magnetic measurements could be performed using large cross-sectional area samples and suitable spatial averaging of the measured effective magnetic field, the eddy current effects being minimized at the same time. Full magnetic softening was obtained upon the annealing sequence in the absence of grain growth and change of distribution and morphology of the inclusions. The measured increase of permeability and decrease of coercivity are therefore entirely ascribed to the relief of the residual stresses, affecting the as-prepared and preliminarily annealed massive (similar to 80 x 10(3) kg) yoke. By recognizing in this way the role of stresses, we are eventually able to estimate the additional contributions to the material coercivity provided by the grain boundaries and the precipitated impurities, the latter playing a major role

Origin of the extreme and anisotropic magnetoresistance in the Weyl semimetal NbP

The fascination with semimetals, especially Dirac and Weyl semimetals, is given by their surprisingly strong response to magnetic fields. In particular, the extremely large magnetoresistance (XMR), i.e., the change in electrical resistivity as a function of the applied magnetic field, has attracted interest because of its deviation by several orders of magnitude from the behavior of normal metals, and its potential for technological applications. To date, it is unclear if the XMR in topological semimetals is inherently correlated to the very high electron mobility and electron-hole compensation, or to other exotic mechanisms. Here, we show that the relativistic and topological nature of charge carriers of the Weyl semimetal niobium phosphide (NbP) are only indirect causes of the XMR. Instead, the XMR can be explained by the very long mean free path le(4 K) approximate to 8 mu m in combination with the small cyclotron orbits emerging in the presence of a magnetic field rc (9 T) approximate to 20 nm of the NbP's Weyl electrons. More precisely we find MR = c le/rc, where c is a parameter independent of temperature and angle between the magnetic field and the crystal. To demonstrate, we use temperature and angle-dependent magnetoresistance measurements, and extract the mean free path and cyclotron radius from an analysis of the Shubnikov-de Haas oscillation