Monitoring antiferromagnetism via angle-resolved Auger photoelectron coincidence spectroscopy: The case of NiO/Ag(001)

Spin selectivity in angle-resolved Auger photoelectron coincidence spectroscopy (AR-APECS) is used to probe electronic structure in antiferromagnetic thin films. In particular, exploiting the AR-APECS capability to discriminate Auger electron emission events characterized by a different spin of the ion in its final state, a sharp multiplet structure in the Ni $\mathit{MVV}$ Auger line shape of NiO/Ag(001) thin films is measured below the critical Néel temperature. The assignment of multiplet terms follows from a close comparison of the experimental AR-APECS line shapes with the predictions based on semiempirical calculations on a cluster model and an open-band extension of the Cini-Sawatzky approach. In analogy to CoO, also in NiO, above the Néel temperature a more featureless Auger spectrum appears and AR-APECS does not disentangle anymore high-spin and low-spin contributions to the total Auger intensity. Such a behavior, which seems to be a general result for metal oxide antiferromagnetic systems, is discussed.

Figure 1

Ni-L${}_2$ absorption edge at three different temperatures for a 15-$\AA$-thick film after a linear background subtraction. Normalization to unity for the low-energy peak is performed to show changes in relative intensity of the two main components of the edge.

Figure 2

Intensity ratio of the two main components of Ni-L${}_2$ absorption edge acquired at different sample temperatures (for the geometry, see text). The lines are a guide for the eyes.

Figure 3

AR-APECS spectra of NiO/Ag in its AFM state at room temperature (upper panel) and its paramagnetic state at 420 K (lower panel). Green triangles and red circles with error bars show data collected in the AN and NN geometry, respectively. The solid lines joining coincidence data are spline fit acting as guides to the eyes.

Figure 4

Comparison of experimental and calculated valence-band XPS profiles. Experimental spectrum from Ref. 33 (dashed line) and calculated ($0.8\mathrm{Ni}+0.2\mathrm{O}$)-removal spectrum (solid line).

Figure 5

Comparison between experimental DEAR-APECS spectra of NiO (upper panel), for the AN (green triangles) and NN (red circles) geometries, and calculated unrelaxed spectrum (lower panel), resolved in spin for $S=2$ (green, marked Q), $S=1$ (blue, marked T), and $S=0$ (red, marked S) components.