Experimental Station for Hard X-ray Magnetic Circular Dichroism on Ultra-dilute Samples

Research on magnetism has undergone a renaissance over the last few decades, following the discovery of a variety of interesting phenomena in artificially-fabricated layered magnetic structures such as the enhancement of magnetic moments in ultrathin films and at surfaces, perpendicular magnetic anisotropy and giant magnetoresistance. Some of these discoveries are of great importance for technological applications, however, the fundamental mechanisms governing the magnetic properties of these structures are not fully understood. One of the important mechanisms is the oscillatory interlayer exchange coupling of the magnetic layers via the 'nonmagnetic' ones. Surprisingly, the induced magnetism in the 'nonmagnetic' layer is often neglected when the magnetic properties of the whole structure are discussed. These induced magnetic moments determine the magneto-optic response, the magneto-transport properties and the magnetic anisotropy of the systems.

A new experimental station mainly dedicated to X-ray magnetic circular dichroism (XMCD) measurements on ultra-dilute magnetic samples has been recently developed at the beamline ID12. The key element is a 35-channel silicon drift diode (SDD) detector array [1] developed in collaboration with Eurisys-Mesures (now Canberra Eurisys). The detector shown in Figure 124 consists of an array of 7 x 5 cylindrical Si drift-diodes with an active area of 10 mm² for each diode. The detector is cryogenically cooled to the optimum temperature (T ~143K). The anode diameter (200 µm) can accommodate a new type of external J-FET (EuriFET) featuring a very low input capacitance (0.9 pF). This results in a very small readout noise: the FWHM energy resolution of the individual diodes measured with a ⁵⁵Fe source is as good as 129 eV using a standard pulse processing time of 12 µs, whereas the peak-to-background ratio is in excess of 1000. Under normal operating conditions, the peaking time can be reduced to 0.5 µs in order to maximise the counting rate (10⁵ cps), however with some deterioration of the energy resolution. A complementary aspect of the project concerned the development of low-cost multichannel digital pulse processing electronics for energy-resolved spectroscopy (XDS boards). Each compact board, designed in the VXI c-size, can accommodate 4 channels and is fully controllable by software. XDS offers nearly the same energy resolution as a standard analog pulse processing system.

Fig. 124: The 35-element Si drift-diode array installed on ID12.

For routine XMCD measurements, a UHV compatible compact sample chamber is inserted between the poles of a 0.7 T electromagnet, while the detector is housed in a special chamber separated from the sample chamber by a gate valve. Special care has been taken to maximise the solid angle of collection of the fluorescence photons. The detector is systematically operated windowless: this allowed us to extend the operation of the detector down to the soft X-ray range where scattering is a major problem.

The XMCD spectra in Figure 125 illustrates the excellent performances of the 35-element Si drift diode detector. It was recorded at the Pd L_II,III-absorption edges from only 0.25 atomic layers of Pd sandwiched between 30 atomic layers of Fe deposited on an MgO substrate. The emission spectrum is strongly dominated by the Mg K emission line (Figure 125 insert), while not resolved Cr L, Fe L and O K lines are of the same intensity as Pd L,ß lines. This shows that the SDD are working perfectly well at low photon energies down to 500 eV. Recall that the circular polarisation rates of the monochromatic beam at the Pd L_II,III-edges are only 12% and 19%, respectively: this clearly indicates that the corresponding XMCD spectra were recorded under particularly unfavourable experimental conditions. XMCD spectra were also recorded at the Pt L-edges on highly dispersed Fe₇₀Pt₃₀ nanoparticles deposited on a Si wafer: this indicates that the performances of the detector are also excellent in the hard X-ray range. The counting rates in both the Pd L and Pt L lines were ca. 2·10⁴ cps per channel with a peaking time of 0.5 µs. Even though the X-ray beam was impinging on the sample with an angle of incidence of ca. 15, the beam footprint was quite small: 300 x 30 µm². This is because we had to close down the slits in order to avoid saturation of the detector by the intense soft X-ray fluorescence signal from the substrates. Typical energy resolution of the emission spectra recorded with one single SDD channel (data acquisition time: 60 s; peaking time: 0.5 µs) was of the order of 132 eV for the unresolved Si Kß, 159 eV for the Fe K line and 202 eV for the Pt La1 line. These results establish that high quality XMCD spectra can be measured on submonolayers and ultra-dilute systems in the hard X-ray range.

Fig. 125: XANES and XMCD spectra of Fe30ML/Pd0.25ML/Fe30ML trilayer at the Pd LII,III-edges. Measurements were at room temperature and under applied magnetic field of 0.3T. Insert: X-ray emission spectrum of the sample with excitation at the Pd LII-edge.

References
[1] J. Goulon et al., Advanced Detection Systems for X-ray Fluorescence Excitation Spectroscopy, J. Synch. Rad. (2004), in press.

Authors
A. Rogalev, J. Goulon, F. Wilhelm, N. Jaouen, S. Feite, G. Goujon.
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