E58 · Surface spectromicroscopy

PEEM: Thresholds & Magnetic Domains

Illuminate a synthetic surface across its full field. Compare UV work-function thresholds, absorption spectra and opposite-helicity images. Extract spectra from stored image stacks, then correct incident flux before interpreting magnetic contrast.

Active contrast $\text{—}$

Selected photon energy $\text{—}$

Selected-region recorded counts $\text{—}$

Selected-region helicity asymmetry $\text{—}$

Conditional counting standard error $\text{—}$

Pixels in selected region $\text{—}$

Negative / positive flux monitor $\text{—}$

Total two-helicity stack exposure $\text{—}$

Experiment target $\text{—}$

Predict · select · explain

Can you separate magnetic contrast from unequal illumination?

Contrast mode: 1 UV, 2 absorption, 3 XMCD

3

Stored X-ray photon energy

h\nu

707.8\,\mathrm{eV}

Stored UV photon energy

h\nu_{UV}

4.7\,\mathrm{eV}

Exposure per energy and helicity

t

0.4\,\mathrm{s}

Assumed image FWHM

w

0.18\,\mathrm{\mu m}

Negative / positive incident flux

1.5

Negative-helicity image displacement

\delta x

0\,\mathrm{\mu m}

Use incident monitors: 0 off, 1 on

0

Magnetization angle to photon projection

\alpha

0\,\mathrm{^\circ}

Stored region center: horizontal

x_s

-1.5\,\mathrm{\mu m}

Stored region center: vertical

y_s

1.5\,\mathrm{\mu m}

Stored region radius

r_s

0.6\,\mathrm{\mu m}

Target: use XMCD mode and monitor normalization at the phase-A resonance. Select the upper left domain without crossing an edge. Keep displacement at or below 0.05 micrometre, image width at or below 0.3 micrometre, more than 1,000 region counts and counting error below 0.035. Align the magnetic projection. Flux correction cannot repair drift.

Teaching model of UV-PEEM and secondary-yield XAS/XMCD-PEEM, with an ideal 18 keV cathode image relay. This Lab does not implement energy-filtered core-level XPS. The 8 micrometre square surface has two invented absorption resonances and work functions, plus two opposed magnetic projections. A common collection-yield ridge imitates topographic brightness. UV yield uses a zero-temperature quadratic threshold. X-ray spectra are assumed Gaussian resonances, not computed atomic cross sections. Both helicity stacks contain sampled detector counts after a Gaussian spatial blur; their flux monitors are exact teaching values. Negative-helicity drift causes misregistration. Region selection and normalization reuse stored counts. Gray asymmetry pixels have fewer than five weighted counts. Each image has an independent logarithmic color scale; the asymmetry scale is linear. The 0.2 micrometre pixels limit spatial sampling. No electrostatic field solution, material calibration, damage, space charge or magnetic-vector reconstruction.

Physics tutorial

PEEM: Thresholds & Magnetic Domains

BackgroundIlluminate a synthetic surface across its full field. Compare UV work-function thresholds, absorption spectra and opposite-helicity images. Extract spectra from stored image stacks, then correct incident flux before interpreting magnetic contrast.

Why it mattersTarget: use XMCD mode and monitor normalization at the phase-A resonance. Select the upper left domain without crossing an edge. Keep displacement at or below 0.05 micrometre, image width at or below 0.3 micrometre, more than 1,000 region counts and counting error below 0.035. Align the magnetic projection. Flux correction cannot repair drift.

Start with the essentials

Focus question: Can you separate magnetic contrast from unequal illumination?
One-sentence intuition: Chemical, threshold and magnetic contrast have different experimental dependencies. Correct flux and inspect local spectra before assigning a bright region to a physical cause.

Core mathematical model

UV threshold

Y_{UV}\propto(h\nu-\Phi)_+^2

Assumed zero-temperature near-threshold yield; brightness also depends on collection.

Monitor-normalized intensities

I_\pm=\frac{C_\pm}{F_\pm}

Counts are divided by incident exposure, preserving unequal counting precision.

Magnetic helicity asymmetry

A=\frac{I_+-I_-}{I_++I_-}

The recorded ratio measures beam-projected dichroism under the assumed response, not a full magnetic vector.

Common difficulties

Brightness needs a model

Typical misconceptionA bright region directly reveals its physical cause.

Better mental modelChemical, threshold and magnetic contrast have different experimental dependencies. Correct flux and inspect local spectra before assigning a bright region to a physical cause.

Run the experiment

01
Predict false contrast
Check the unequal-flux preset, then enable monitor normalization.
What to observe: The upper and lower magnetic domains recover opposite signs; illumination imbalance shifts the raw ratio.
02
Separate chemistry and threshold
Compare absorption and UV presets. Move the region across the chemical boundary.
What to observe: Resonance positions and emission thresholds differ. A common bright ridge is not a chemical concentration map.
03
Challenge a magnetic interpretation
Try low counts, drift and transverse magnetization before checking the target.
What to observe: Noise, misregistration and a small beam projection can all defeat the target.