Effects of magnetic and structural phase transitions on the normal and anomalous Hall effects in Ni-Mn-In-B Heusler alloys

Magnetization, electrical resistivity, magnetoresistance, and Hall resistivity of Ni50Mn35In14.25B0.75 and Ni50Mn35In14.5B0.5 Heusler alloys were studied in a temperature range T=80-400K in magnetic fields up to 20 kOe. Both alloys exhibit a martensitic transformation from a higherature ferromagnetic austenite phase to a lowerature, low-magnetization martensitic phase. The electrical resistivity nearly doubles as a result of the martensitic transformation, reaching 180 and 100 μ cm in the martensitic states of Ni50Mn35In14.25B0.75 and Ni50Mn35In14.5B0.5, respectively. The temperature dependence of the electrical resistivity does not corresponded with the Mooij correlation. The magnetoresistance is negative with a narrow negative peak at the martensitic transition. Normal and anomalous Hall effect coefficients were determined by fitting the field dependences of the Hall resistivity using magnetization data. The coefficients of the normal Hall effect for both compositions were found to decrease with temperature from positive values in the austenite to negative values in the martensite phase. None of the known correlations between the anomalous Hall effect coefficient and resistivity were satisfied. Significant changes in the values of the anomalous Hall coefficients during the martensitic transformation are explained by the difference in spin-up and spin-down state occupations in the martensite and austenite phases. First-principles calculations of the electronic structures confirm this explanation

Development of a powerful UCN source at PNPI's WWR-M reactor

The WWR-M reactor at PNPI is planned to be equipped with a high-flux source for ultracold neutrons (UCNs). The method of UCN production is based on neutron conversion in superfluid helium, exploiting the particular qualities of that quantum liquid. As a result of optimizing the source parameters, we expect a temperature of superfluid helium of 1.2 K and a UCN density of 1.3 × 104 cm−3 in a neutron electric dipole moment (EDM) spectrometer. The expected flux densities of cold neutrons (with wavelengths in the range 2–20 Å) and very cold neutrons (50–100 Å) at the output of a neutron guide with a cross section of 30 × 200 mm2 are 9.7 × 107 cm−2s−1 and 8.3 × 105 cm−2s−1, respectively. The capability of maintaining a temperature of 1.37 K at a thermal load of 60 W was shown experimentally, while the theoretical load is expected to be 37 W. Calculations show that it is possible to decrease the helium temperature down to 1.2 K at similar heat load. The project includes the development of experimental stations at UCN beams, such as for a neutron EDM search, measurements of the neutron lifetime, and for a search for neutron-to-mirror-neutron transitions. In addition, three beams of cold and very cold neutrons are foreseen. At present, the vacuum container of the UCN source has been manufactured and the production of the low-temperature deuterium and helium parts of the source has been started

Development of a powerful UCN source at PNPI's WWR-M reactor

The WWR-M reactor at PNPI is planned to be equipped with a high-flux source for ultracold neutrons (UCNs). The method of UCN production is based on neutron conversion in superfluid helium, exploiting the particular qualities of that quantum liquid. As a result of optimizing the source parameters, we expect a temperature of superfluid helium of 1.2 K and a UCN density of 1.3 × 104 cm−3 in a neutron electric dipole moment (EDM) spectrometer. The expected flux densities of cold neutrons (with wavelengths in the range 2–20 Å) and very cold neutrons (50–100 Å) at the output of a neutron guide with a cross section of 30 × 200 mm2 are 9.7 × 107 cm−2s−1 and 8.3 × 105 cm−2s−1, respectively. The capability of maintaining a temperature of 1.37 K at a thermal load of 60 W was shown experimentally, while the theoretical load is expected to be 37 W. Calculations show that it is possible to decrease the helium temperature down to 1.2 K at similar heat load. The project includes the development of experimental stations at UCN beams, such as for a neutron EDM search, measurements of the neutron lifetime, and for a search for neutron-to-mirror-neutron transitions. In addition, three beams of cold and very cold neutrons are foreseen. At present, the vacuum container of the UCN source has been manufactured and the production of the low-temperature deuterium and helium parts of the source has been started