Britt Rosendahl Hansen

Consequences of the magnetocaloric effect on magnetometry measurements

Magnetization curves recorded at high sweep-rates on magnetic materials near a phase transition temperature can be affected by temperature changes in the material due to the magnetocaloric effect. This change in the sample temperature is a result of the quasiadiabatic conditions that can occur under such conditions and we demonstrate its effects on magnetization curves of two magnetocaloric materials, La(Fe0.945Co0.055)11.9Si1.1 and Gd5Si2Ge2. We show how a quantity calculated from isothermal magnetization curves, the magnetic entropy change, ΔSM, is affected by the erroneous data. As ΔSM is a measure of the magnetocaloric effect, the discrepancies demonstrated here are more severe close to a peak in ΔSM, which is precisely the quantity that is of interest and reported on in the literature from possibly erroneous magnetization data. We also demonstrate how, through simple measurements and without a direct measurement of the sample temperature, one can determine an appropriate sweep-rate of the magnetic field.

Simulation of a suite of generic long-pulse neutron instruments to optimize the time structure of the European Spallation Source.

We here describe the result of simulations of 15 generic neutron instruments for the long-pulsed European Spallation Source. All instruments have been simulated for 20 different settings of the source time structure, corresponding to pulse lengths between 1 ms and 2 ms; and repetition frequencies between 10 Hz and 25 Hz. The relative change in performance with time structure is given for each instrument, and an unweighted average is calculated. The performance of the instrument suite is proportional to (a) the peak flux and (b) the duty cycle to a power of approximately 0.3. This information is an important input to determining the best accelerator parameters. In addition, we find that in our simple guide systems, most neutrons reaching the sample originate from the central 3-5 cm of the moderator. This result can be used as an input in later optimization of the moderator design. We discuss the relevance and validity of defining a single figure-of-merit for a full facility and compare with evaluations of the individual instrument classes.

Neutron study of the magnetism in NiCl2·4SC(NH2)2.

We study the strongly anisotropic quasi-one-dimensional S = 1 quantum magnet NiCl2·4SC(NH2)2 using elastic and inelastic neutron scattering. We demonstrate that a magnetic field splits the excited doublet state and drives the lower doublet state to zero energy at a critical field Hc1. For Hc1 < H < Hc2, where Hc2 indicates the transition to a fully magnetized state, three-dimensional magnetic order is established with the AF moment perpendicular to the magnetic field. We mapped the temperature/magnetic field phase diagram, and we find that the total ordered magnetic moment reaches m(tot) = 2.1 μB at the field μ(0)H = 6 T and is thus close to the saturation value of the fully ordered moment. We study the magnetic spin dynamics in the fully magnetized state for H > Hc2, and we demonstrate the presence of an AF interaction between Ni(2+) on the two interpenetrating sublattices. In the antiferromagnetically ordered phase, the spin-waves that develop from the lower-energy doublet are split into two modes. This is most likely the result of the presence of the AF interaction between the interpenetrating lattices.