Effects of surface oxidation on the exchange-bias properties of the single-crystal antiferromagnetic/ferromagnetic junction Mn/Co/Cu(001)

The effects of a chemical alteration of the antiferromagnet surface in the exchange-biased ultrathin epitaxial antiferromagnetic/ferromagnetic bilayer Mn/Co/Cu(001) have been investigated. The formation of a nanometer-thick paramagnetic oxide at the surface of the Mn layer was induced through the controlled exposure to oxygen. Although the buried Mn/Co interface was not affected by the oxidation, strong changes of the exchange-bias properties of the system, which cannot be accounted for by the mere reduction of the antiferromagnetic Mn thickness, were observed. We propose that the produced surface modification affects the magnetic configuration of the near-surface region and the domain structure in the whole antiferromagnet layer which, in turn, influences the pinned uncompensated spin density responsible for the exchange bias.

Figure 1

Top panel: XAS spectrum at the $\text{Mn-}{L}_{2,3}$ edges of as-deposited Mn/Co/Cu(001), collected for $t_{\mathrm{Mn}}\approx 18$ ML. Inset: scheme of the sample geometry. Bottom panel: Mn-thickness dependent XAS spectra at the $\text{Mn-}{L}_{2,3}$ edges of Mn/Co/Cu(001) after exposure to 20 L of ${\mathrm{O}}_2$. Spectra corresponding to 5, 10, 14, and 18 ML are shown. The spectra are displaced along the vertical axis for clarity. Inset: sample geometry following the oxidation (see text for details).

Figure 2

(a) Experimental (black circles) and fit (solid gray line) XAS spectra at the $\text{Mn-}{L}_{2,3}$ edges for the 18-ML-thick oxidized Mn overlayer on Co/Cu(001). The blue (orange) area depicts the oxide (metallic) spectral contribution. Inset: fractions of the metallic (orange circles) and oxide (blue squares) contribution to the total XAS spectra (see text for details). (b) Experimental (markers) and calculated (solid line) XAS spectra at the $\text{Mn-}{L}_{2,3}$ edges (see text for details).

Figure 3

Hysteresis loops of exchange-biased Mn/Co/Cu(001) for a selected set of Mn thicknesses in the pristine state [(a)–(d)] and upon oxidation [(e)–(h)]. Comparison of $H_c$ (i) and $H_{\mathrm{EB}}$ (j) of Mn/Co/Cu(001) vs Mn thickness before (full squares) and after oxidation (open circles). Dashed lines mark the correspondence between the onset of $H_{\mathrm{EB}}$ and the $H_c$ maximum.

Figure 4

XPS spectra of the as-deposited (top panel) and oxidized (bottom panel) Mn/Co/Cu(001). The spectra refer to a thickness of the Mn overlayer of about 18 ML. The insets show a HR-XPS spectra closeup of the Mn $2p$ doublet. XPS spectra were taken with a photon energy of 750 eV and an angle of incidence of 45°. Photoemitted electrons were collected at normal incidence.