Observation of x-ray magnetic circular dichroism at the $\mathrm{Ru}K$ edge in $\mathrm{Co}\text{−}\mathrm{Ru}$ alloys

X-ray magnetic circular dichroism spectra have been measured from dilute Ru alloys of Co at the $\mathrm{Ru}K$ edge, evidencing the induced magnetization on Ru in contact with ferromagnetic Co. This was made possible using circularly polarized probing beams of approximately $22\mathrm{keV}$ in energy obtained from a germanium Laue-reflection phase retarder at a synchrotron source. The Ru dichroism is quite small with a main peak of $-3\times {10}^{-4}$ in flipping ratio at about $6\mathrm{eV}$ above the $K$ absorption edge. The spectrum has a very similar profile to the one of the $\mathrm{Co}K$-edge dichroism. Ab initio calculations indicate that the Ru dichroism is generated by the $p$-orbital magnetic moment on the core-hole site, which is induced through the $p\text{−}d$ hybridization between Co and Ru.

Figure 1

Experimental setup on the APS 4-ID-D beam line. The 4-ID-D monochromator and the focusing optics are not shown. The active (220) and $\left(2\overline{2}0\right)$ planes in the germanium phase retarder are inclined by $45°$ to the horizontal synchrotron plane. In this setup, circularly polarized x-ray beams of $+$ and $-$ helicities are produced by the $\left(2\overline{2}0\right)$ and (220) reflections, respectively. Rotating the retarder around the horizontal $\omega$ axis scans the x-ray energy. The APS storage ring was filled with 324 electron bunches on the orbit at the time of experiment.

Figure 2

Magnetization curves for the ${\mathrm{Co}}_{0.95}{\mathrm{Ru}}_{0.05}$ and ${\mathrm{Co}}_{0.9}{\mathrm{Ru}}_{0.1}$ samples. Measured with a vibrating-sample magnetometer applying an in-plane field. Openings of the hysteresis loops near zero fields are not visible on the scale of the diagram.

Figure 3

Nonmagnetic (top panel) and magnetic (bottom panel) x-ray absorption spectra measured from the ${\mathrm{Co}}_{0.95}{\mathrm{Ru}}_{0.05}$ (a) and ${\mathrm{Co}}_{0.9}{\mathrm{Ru}}_{0.1}$ (b) samples near the $\mathrm{Ru}K$ edge. In the bottom panel, open circles show raw data obtained using the circularly polarized beams of $+$ and $-$ helicities, whereas filled circles show the differences divided by 2, giving the background-free dichroic spectra. X-ray count time per energy point is $320\mathrm{s}$ (a) and $128\mathrm{s}$ (b). Data collection time: $20\mathrm{h}$ (a) and $12\mathrm{h}$ (b).

Figure 4

Comparing the dichroic spectra from the ${\mathrm{Co}}_{0.95}{\mathrm{Ru}}_{0.05}$ and ${\mathrm{Co}}_{0.9}{\mathrm{Ru}}_{0.1}$ samples at the $\mathrm{Ru}K$ edge with the one from a $\mathrm{Co}∕\mathrm{Cu}$ multilayers at the $\mathrm{Co}K$ edge. $E_0$: absorption-edge energy. Note that the dichroism from induced magnetizations on Ru is one order of magnitude smaller than the one from ferromagnetic Co.

Figure 5

Experimental (circles) and calculated (lines) XAS (top panel) and XMCD (bottom panel) spectra near the $\mathrm{Ru}K$ edge $(E_0=22.117\mathrm{keV})$.

Figure 6

Local density of states (LDOS) including small $s$ and $p$ contributions for ${\mathrm{Co}}_{0.95}{\mathrm{Ru}}_{0.05}$ alloy.