synthetic surface and spm specific features

docs/nodes/MFM Domain Generation.md

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+# MFM Domain Generation
+
+Generate the stray field from a periodic pattern of parallel magnetic stripe domains with alternating up/down magnetization. Equivalent to Gwyddion's mfm_parallel.c.
+
+## Inputs
+
+| Name | Type | Required | Description |
+|------|------|----------|-------------|
+| field | DATA_FIELD | Yes | Topography used for grid dimensions |
+
+## Outputs
+
+| Name | Type | Description |
+|------|------|-------------|
+| hz | DATA_FIELD | z-component of the stray field (A/m) |
+| dhz_dz | DATA_FIELD | Vertical gradient of the stray field (A/m²) |
+
+## Controls
+
+| Name | Type | Default | Description |
+|------|------|---------|-------------|
+| height | FLOAT | 50e-9 | Tip-sample separation in metres |
+| thickness | FLOAT | 20e-9 | Magnetic film thickness in metres |
+| magnetization | FLOAT | 1e6 | Saturation magnetization in A/m |
+| stripe_width_a | FLOAT | 200e-9 | Width of the A (+M) domain in metres |
+| stripe_width_b | FLOAT | 200e-9 | Width of the B (-M) domain in metres |
+| gap | FLOAT | 0.0 | Domain wall gap between stripes in metres |
+
+## Notes
+
+- Domains alternate between A (+M, magnetization up) and B (-M, magnetization down) with an optional gap of zero magnetization between them.
+- The stray field is computed via FFT of the magnetization profile using an exponential decay transfer function.
+- The field is uniform along y; stripes run parallel to the y-axis.
+- dHz/dz is proportional to the MFM phase signal (force gradient), making it the quantity most directly comparable to raw MFM images.
+- Useful for simulating MFM calibration samples and testing MFM analysis workflows.
+- Typical stripe widths range from 100 nm to several microns.