# X-ray Magnetic Circular Dichroism

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Forschungszentrum Jülich | Daniel Wilson Figure 1: The image indicates the absorption edges of Nickel, Cobalt and Iron. In blue/ red the spektrum of gas discharges of oxygen and nitrogen are shown. The dashed line shows the reflectivity of the used multilayer mirror.

Generation of circularly polarized light in the extreme ultraviolet (EUV/XUV) spectral region (about 25 eV-250 eV) is highly desirable for applications in spectroscopy and microscopy but very challenging to achieve in a small-scale laboratory. In this spectral range (see figure 1), the 3p absorption edges of Fe (54 eV), Co (60 eV) and Ni (67 eV) offer a high magnetic contrast often employed for magneto-optical and electron spectroscopy as well as for magnetic imaging [1-5]. We simulated and designed an instrument for generation of linearly and circularly polarized EUV radiation and performed polarimetric measurements of the degree of linear and circular polarization. Furthermore, we demonstrate first measurements of the X-ray magnetic circular dichroism (XMCD) at the Co 3p absorption edge with a plasma-based EUV light source.

Another application of polarized EUV and soft X-ray radiation, magneto-optical polarization spectroscopy, provides valuable information about magneto-optical constants and enables studies of element- and layer-selective magnetization.

For magneto-optical spectroscopy, both linearly and circularly polarized light is required. A straightforward concept for conversion of linear to circular EUV polarization is to exploit the phase shift between the s – and p-components of light upon reflection from a flat surface. For that purpose, a phase shift of ±90° between the s- and p-components and identical reflectivity for the s- and p-components are required.13,14,16,17 Laboratory-based instruments for generation of circularly polarized EUV light employ up to four mirrors. Due to the low overall reflectivity of a few percent in the EUV spectral range a sufficiently intense EUV light source is required to obtain a reasonable photon flux after the conversion. To linearly polarize the initially unpolarized EUV light and simultaneously select emission lines around the 3p absorption edge of Co (60 eV), we designed a multilayer Bragg mirror linear polarizer operating close to the Brewster angle. Behind the linear polarizer, we placed a broadband triple-reflection circular polarizer, which covers the 3p absorption edges of Fe, Co and Ni between 50 eV and 70 eV.

## Results of the Measurements and Future Steps

Forschungszentrum Jülich | Daniel Wilson Figure 2: XMCD Difference signal as recorded by the CCD camera for 70° and 110° polarizer angle. The inset shows the magnetization curve of [Co (0.8 nm) /Pt (1.4 nm)] multilayer by polar magneto-optical Kerr effect (P-MOKE) with visible light.

For a rotation angle of 70° of the circular polarizer we efficiently convert linearly to circularly polarized light at 60.5 eV as expected from simulations and derived a maximum polarization of $$p_c=0.81 \pm 0.15$$.

In our measurements regarding the XMCD effect, we first set the circular polarizer to maximum polarization, alternately applied $$\pm 320$$ mT magnetic field and then recorded the transmitted signal on the CCD camera for both magnetic fields. The difference of the transmitted intensity averaged over 50 measurements for each magnetic field (10 s or 200 pulses per measurement), is shown in Fig. 2. For the background-corrected XMCD asymmetry $$A_{XMCD}$$calculated according to

$$A_{XMCD}=\frac{I_+(+320 mT)-I_-(-320 mT)}{I_+(+320 mT)+I_-(-320 mT)}$$

We obtained $$A_{XMCD}=+(2.7\pm 0.1)\%$$.After changing the rotation angle of the circular light, as expected the difference signals changes the signs but keeps the same magnitude with $$A_{XMCD}=-(2.8\pm 0.1)\%$$. Detail description is published elsewhere [6].

## References

[1] H.-Ch. Mertins, S. Valencia, A. Gaupp, W. Gudat, P. M. Oppeneer, and C.M. Schneider, Appl. Phys. A 80, 1011 (2005).
[2] M. F. Tesch, M. C. Gilbert, H.-Ch. Mertins, D. E. Bürgler, U. Berges, and C. M. Schneider, Appl. Opt. 52(18), 4294 (2013).
[3] S. Valencia, A. Gaupp, W. Gudat, H.-C. Mertins, P. M. Oppeneer, D. Abramsohn, and C. M. Schneider, New J. Phys. 8, 254 (2006).
[4] L. Baumgarten, C.M. Schneider, H. Petersen, F. Schäfers, and J. Kirschner, Phys. Rev. Lett. 65, 492 (1990).
[5] F. U. Hillebrecht, Ch. Roth, H. B. Rose, M. Finazzi, and L. Braicovich, Phys. Rev. B 51, 9333 (1995).
[6] Wilson, D. et al.: ‘Generation of circularly polarized radiation from a compact plasma-based extreme ultraviolet light source for tabletop X-ray magnetic circular dichroism studies’, Rev. Sci. Inst., 85,103110, 2014