X-ray Detected Magnetic Resonance (XDMR) is a new spectroscopic technique in which X-ray Magnetic Circular Dichroism (XMCD) is used to probe the resonant precession of the magnetisation when a strong microwave pump field hp is applied perpendicularly to the static bias field h0. Like XMCD, XDMR is element- and edge-selective and is expected to become a unique tool to investigate how precessional dynamics can affect the spin and orbital magnetisation densities of states (DOS) locally with p- or d- like symmetry [1, 2]. To illustrate this point, we have used XDMR to determine the precession angle at the iron K-edge and at the yttrium LII,III edges in yttrium-iron-garnet (YIG: Y3Fe5O12) thin films.

The first sample that was investigated was a high quality YIG film grown by liquid phase epitaxy on a (111) gadolinium gallium garnet substrate; the normal to the film was tilted by 6° with respect to the direction of H0. XDMR measurements were performed in the longitudinal geometry [3] with an incident microwave power of typically 28 dBm corresponding to a pump field of ca. 0.75 Oe. Resonant pumping occurred in the non-linear foldover regime of magnetic resonance in which the lineshapes are heavily distorted [1]. This led us to tune systematically the bias field near the critical instability field HC2 where the largest precession angle C2 can be obtained [1]. Since the microwave power was amplitude modulated at a low triggering frequency (i.e. fMW = 3.5504298 kHz at the Fe K-edge or fMW = 7.1008596 kHz at the Y L-edges), the XDMR signatures appear as modulation side-bands with respect to the macrobunch repetition frequency of the ESRF ring (F2 = 710.0859611 kHz in the 2*1/3 filling mode). The spectacular dynamic range of our detection system is illustrated by Figure 115 in which the XDMR signal peaks 34 dB (17 dB) above the noise floor at the Fe K-edge (Y L-edges) [3]. To further improve the sensitivity of our measurements, cross-correlated spectral densities of the two side-bands were exploited. Note that the experiments at the Y L-edges were challenging because the fluorescence intensity was typically 27 dB lower than at the Fe K-edge and the circular polarisation rates were also lower than at the Fe K-edge. For brevity, we have shown only a few typical XDMR spectra in Figure 115: these spectra were obtained on tuning the X-ray excitation energy to the first extremum of the XMCD spectrum.

Fig. 115: XDMR spectra measured with a Vector Spectrum Analyser (VSA) at the Fe K-edge and Y LII,III edges. Cross correlated spectra of the low frequency (-) and high (+) frequency side-bands are shown. The strong signal at F = 0, that is proportional to the X-ray excited fluorescence intensity, is used for data renormalisation purposes.

The most significant finding in these experiments is that the precession angles C2 measured at the Fe K-edge and Y L-edges were nearly identical: C2 (Fe K-edge) = 5.55°; C2 (Y LIII edge) = 5.63°; C2 (Y LII edge) = 5.59°. What makes this result particularly remarkable is that XDMR measurements at a K-edge describe the precession dynamics of magnetisation components that are exclusively of orbital nature [1,2]. Furthermore, it is worth emphasising that in YIG, and other rare earth doped films (e.g. Y1.3La0.47Lu1.3 Fe4.84O12), the Y (La, Lu) atoms are most often considered as diamagnetic atoms and, therefore, they are not supposed to contribute to standard FMR spectra. However, this presumption is incorrect because rather intense XMCD signatures were measured at the L-edges of Y (La, Lu) and the respective contributions of the relevant orbital and spin moments can be disentangled by using the XMCD sum rules. As illustrated by Figure 116, the XMCD spectra at the Y L-edges are dominated by induced spin moments, especially at the energy of the first XMCD peak. Moreover, from the sign of the XMCD signatures, it appears that these induced spin moments are parallel to H0. Consequently, they should be ferromagnetically coupled to the total Fe spins. The most probable explanation is super-exchange interactions mediated by the oxygen lattice. Our XDMR experiment thus produces clear evidence that the precession of the orbital component measured at the iron atom is strongly coupled to the precession of the spin components at both the Fe and Y sites, the same as for acoustic modes.

Fig. 116: Orbital and Spin magnetisation of d-projected DOS at the Y site in rare earth doped YIG film. Strictly the same generic spectrum was obtained for the undoped YIG film.



[1] J. Goulon, A. Rogalev, F. Wilhelm, N. Jaouen, C. Goulon-Ginet and Ch. Brouder, Eur. Phys. J. B 53, 169-184 (2006).
[2] J. Goulon, A. Rogalev, F. Wilhelm, N. Jaouen, C. Goulon-Ginet, G. Goujon, J. Ben Youssef and M.V. Indenbom, JETP Lett. 82, 791-796 (2005).
[3] J. Goulon, A. Rogalev, F. Wilhelm, N. Jaouen, C. Goulon-Ginet, G. Goujon, J. Ben Youssef and M.V. Indenbom, J. Electron Spectr. & Relat. Phenom. (2006) in press (doi:10.10.1016/j.elspec.2006.11.047) .

Principal Publication and Authors

J. Goulon (a), A. Rogalev (a), F. Wilhelm (a), N. Jaouen (a), C. Goulon-Ginet (a), G. Goujon (a), J. Ben Youssef (b) and M.V. Indenbom (b), JMMM (2006), submitted.
(a) ESRF
(b) Laboratoire de Magnétisme de Bretagne, CNRS FRE 2697, UFR Sciences & Techniques, Brest (France)