Magnetic layers

A layer can be given a magnetisation, so that the two neutron spin states see different scattering length densities. This page covers the Model page controls; loading and fitting measured spin channels is described in polarised data.

Note

Magnetic layers can only be calculated by refl1d. refnx has no magnetism, so the app asks to switch the project’s calculation engine the first time a layer is made magnetic.

The Magnetism group

The Magnetism group is in the basic controls of the Model page, below the layer editor. It is titled after the assembly currently selected in the Layer editor, for example Magnetism: Multi-layer, and shows one row per layer of that assembly.

Column	Meaning
No.	Position of the layer in the assembly.
Layer	Layer name, as in the Layer editor.
ρM/10⁻⁶Å⁻²	Magnetic scattering length density of the layer.
θM/°	In-plane angle of the magnetic moment.
Magn.	Makes the layer magnetic. Unticking it removes the magnetisation.

ρM and θM are only editable once Magn. is ticked.

Note

θM = 270° aligns the moment with the guide field, which produces no spin-flip scattering. This is the value to start from for a simple saturated film.

Switching the calculation engine

Ticking Magn. while the project uses an engine that cannot model magnetism opens the Switch calculation engine? dialog. Accepting it makes the layer magnetic and switches the project to refl1d in one step - the page does not change under you.

Switching recalculates the reflectivity and makes any existing fit result stale; the sample and the loaded data are untouched. The engine can be switched back once no layer is magnetic any more.

The engine itself lives in the Calculation engine group of the advanced controls on the Model page (and in Analysis › Advanced, see simple fitting). Selecting an engine that cannot model magnetism while the sample still has magnetic layers is refused, with a message on both pages.

Fitting ρM and θM

ρM and θM appear in the Analysis parameter table like any other layer parameter, named after their assembly and model - for example Model Fe rho_m. They come with default limits, a fit checkbox and can be used in constraints. The parameter name filter accepts magnetic as a keyword to show only the magnetic parameters.

Magnetic depth profiles

Once at least one layer is magnetic, the Magnetic profile group appears in the basic controls, below Magnetism. The same switches are repeated in Analysis › Advanced › Plot control, and the two share one selection.

• Show ρ↑ and ρ↓ - adds the spin-up and spin-down potentials ρ ± ρM·cos(θM − A) for each magnetic model to the SLD chart, dashed in the model’s colour. For non-magnetic layers the two curves collapse onto the nuclear SLD.

• Show ρM - the magnetic SLD profile on its own.

• Show θM - the in-plane moment angle, on its own right-hand axis. θM is only defined where there is a moment, so the curve is drawn in pieces rather than joined across the gaps.

• Show R↑↑ and R↓↓ - splits each magnetic model’s reflectivity into its two non-spin-flip cross-sections on the Model page reflectivity chart, dashed in the model’s colour with their own legend rows. Off by default.

The y-range of the SLD chart covers every visible curve and grows when a curve is switched on, so ρ + ρM is never clipped. If no model is magnetic, the chart, its legend and the sidebar are unchanged.

Note

For a magnetic sample the plain model curve is not an unpolarised average - the calculator returns the ↑↑ cross-section - so R↑↑ is drawn on top of it. The sidebar says so as well.

The Analysis reflectivity chart is unaffected by this switch: it already draws one calculated curve per measured spin channel when the experiment is polarised.