Polarised data

Polarised data#

A polarised measurement records up to four spin channels of the same sample. The app loads them as a single experiment, draws them separately, and fits them together against one model.

Channel

Meaning

pp ↑↑

non-spin-flip, both incoming and outgoing spin up

mm ↓↓

non-spin-flip, both incoming and outgoing spin down

pm ↑↓

spin-flip

mp ↓↑

spin-flip

Loading one file per channel#

The Experimental data group on the Experiment page has a second button, Load polarized experiment (file per channel), below the ordinary Load experiment(s) from file(s). Select one file per spin channel - the file picker allows multiple selection - and the Assign spin channels dialog opens.

Each file gets a channel drop-down, pre-assigned from the ORSO polarization header or, failing that, from the file name. Adjust anything that came out wrong. Any number of channels may be assigned, a single one included, and a file that should be ignored is set to not used.

The dialog refuses to continue and explains why if a channel is assigned to more than one file, if a file is missing, or if nothing at all is assigned.

Note

One resolution function is used for the whole polarised experiment, taken from the first assigned channel. Differing per-channel resolution metadata in the other files is ignored; the dialog says so.

If the project’s calculation engine cannot model magnetism, the dialog also notes that the channels will be loaded and displayed but not modelled until the sample has magnetic layers and the engine is switched - see magnetic layers.

Seeing the channels#

  • The experiment chart draws one measured series, with its error bounds, per visible channel, in a fixed palette (↑↑ pp, ↑↓ pm, ↓↑ mp, ↓↓ mm), with a per-channel legend.

  • The Polarization channels group in the advanced controls of the Experiment page toggles which channels are drawn. At least one measured channel always stays visible.

  • Experiment lists mark a polarised experiment with a ⇅N badge, N being the number of measured spin channels.

  • With several experiments selected, each polarised experiment contributes one series per visible channel, using the experiment colour as the hue base.

Spin asymmetry#

The spin asymmetry

SA(q) = (R↑↑ − R↓↓) / (R↑↑ + R↓↓)

is available as a Spin asymmetry tab next to Reflectivity on the Experiment page (measured data only) and as a third tab of the lower panel on the Analysis page (measured data plus the model). Neither tab is shown unless the experiment measured both non-spin-flip channels; without one, the Experiment page has no tab strip at all.

Error bars are propagated from the channel uncertainties. Two kinds of point are dropped, with a note of how many:

  • points where R↑↑ + R↓↓ is not significantly above zero - the background-dominated tail, which would otherwise wreck the axis;

  • points where the two channels do not share the same q.

The axis shows the full [−1, 1] window and expands only if background-subtracted data go outside it.

Fitting a polarised experiment#

Fitting a polarised experiment fits all its measured spin channels at once against the shared model. Thickness, roughness, nuclear SLD, scale and background are common to every channel; ρM and θM are constrained by all of them. Polarised and ordinary experiments can be fitted together in the same refinement.

The analysis and residual charts draw one measured/calculated pair per visible channel, each with that channel’s cross-section. A channel the model cannot calculate - a spin-flip channel on a non-magnetic sample - shows its measured points only and contributes no residuals.

In the Summary report a polarised experiment contributes one row per measured spin channel, named after the channel, and the report figures plot each channel in its channel colour.

Limitations#

  • One resolution function per polarised experiment, taken from the first assigned channel.

  • Bayesian sampling of polarised experiments is not supported yet; see Bayesian analysis.

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