Josephson current-assisted reversal of a single-domain nanoscale ferromagnet driven by cosine chirp pulse.

We study the microwave-induced magnetization reversal in two systems, the microwave-driven nanomagnet (NM) and the NM coupled to a Josephson junction (JJ) under the microwave field (NM-JJ-MW). The frequency of the applied cosine chirp pulse changes nonlinearly with time to match the magnetization precession frequency. The coupling between the NM and JJ reduces the magnetization switching time as well as the optimal amplitude of the microwave field as a result of manipulating the magnetization via Josephson-to-magnetic energy ratioG. The reversal effect in NM-JJ-MW is sufficiently robust against changes in pulse amplitude and duration. In this system, the increase ofGdecreases the possibility of the non-reversing magnetic response as the Gilbert damping increases without further increase in the external microwave field. We also discuss the magnetic response of the NM driven by the ac field of two JJs in which the time-dependent frequency is controlled by the voltage across the junctions. Our results provide a controllable scheme of magnetization reversal that might help to realize fast memory devices.

A method for measuring macroscopic cross-sections for thermal neutrons.

A method was proposed for measuring macroscopic absorption and scattering cross-sections for thermal neutrons. It is based on a Pu-Be neutron source and He-3 neutron detectors assembly. A beam of neutrons was obtained from the source imbedded in a water tank. The He-3 detectors oriented inside the sample and at 180° and 0° with respect to the incident neutron beam were used to register neutrons after interaction with the samples. Neutron count rates (detectors responses) were obtained for large (5.5l) as well as small (1.3l) volumes of standard samples. Sensitivities of the results obtained for the large and small samples were compared. A semi-empirical model was proposed to fit the results. It describes the relative detector responses in terms of a dimensionless variable which depends on the geometrical parameters and cross section of the standard samples used. The model successfully fits the results obtained. Advantages and limitations of the method were discussed.