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Polly Mitchell

Publications and source records attributed to Polly Mitchell.

2 recordsLinked to original sources

Physics-informed X-ray tomography of magnetization and material parameters

Three-dimensional spin textures are stabilized by microscopic interactions within their host materials. X-ray vector tomography can image these nanoscale structures, but weak magnetic contrast and a limited range of viewing angles often leave parts of the magnetization poorly determined. Here we show that a reconstruction constrained by the micromagnetic energy recovers the three-dimensional magnetization from such data and gains sensitivity to the underlying material interactions. Applied to the chiral magnet Co$_8$Zn$_9$Mn$_3$, the method resolves the three-dimensional helical texture even from a small fraction of the dual-axis projections typically required for a successful tomographic reconstruction, or from a single tilt axis alone. By testing which ratio of exchange to Dzyaloshinskii-Moriya interaction in the reconstruction best reproduces the measured projections, we obtain a value for the sample that differs by about 10 % from the literature value derived from the helical winding period. Taken together, our findings suggest a way to reduce the time needed for vector tomography and to extend it to determining fundamental magnetic interactions in a material.

cond-mat.mes-hall↗

Polarized Neutrons at ISIS: Recent Developments And Highlights

We present two recent projects which aim to improve the performance of polarized neutron scattering experiments using hyperpolarized $^{3}He$ spin filters at ISIS. The first is the optimization of a new compact magnetostatic cavity ("Magic Box") to house the $^{3}He$ spin filters based on an existing design. With a length of only 380 mm, it provides a field gradient relaxation time for the $^{3}He$ cell of 421 h in ambient conditions. It also contains a radiofrequency coil for adiabatic fast passage flipping. The second project is dedicated to the improvement of the $^{3}He$ relaxation time inside the spin filter cell. We have developed a chamber which allows for the deposition of alkali metal coatings on the surface of substrates. This emulates the spin filter cell walls, as well as subsequent heat treatment, thus mimicking the preparation of a new spin filter cell. The chamber is air-tight and has transparent windows, so that the structure resulting from the deposition of alkali metal on the surface of the wafer can be studied by X-ray or neutron reflectometry. We plan to continue this work by performing a systematic study at various conditions, which should help to shed light on the long-standing mystery of how alkali metal coatings help to improve relaxation time of $^{3}He$ cells. The first results are discussed in the text.

physics.ins-det↗