X-ray Magnetic Circular Dichroism (XMCD)

Several techniques exist to measure the magnetic properties of materials. Most of them are sensitive to the total magnetization of the measured system and can not discern between the contributions of different atoms in an alloy or multilayer, or between their orbital and spin moments. Moreover, the small quantity of material present in many technologically interesting samples, like magnetic nanostructures, necessitates a very sensitive measuring method. One of the most powerful techniques combining these different properties is X-ray Magnetic Circular Dichroism (XMCD).

XMCD is the difference, for a magnetic material, between the absorption of left and right circularly polarized X-rays. In X-ray absorption, the atom absorbs a photon, giving rise to the transition of a core electron to an empty state above the Fermi level. The absorption cross-sections are large, especially in the soft X-ray range (500-2000 eV). The absorption edges have energies which are characteristic for each element and, due to the dipole selection rules, final states with different symmetries can be probed by choosing the initial state.

XMCD of Co

 History
In 1975, Erskine and Stern [1] predicted that X-ray absorption performed with circularly polarized light can supply magnetic information about the initial state of the absorption process. In 1985, Thole et al. [2] show that the lineshape of the M4,5 absorption edges of a rare earth ion whose ground state is split by a magnetic field, depends on the relative orientation between the magnetization direction and the polarization vector of the x-rays. The effect is related to the Magneto Optical Kerr Effect (MOKE), where an electron makes a transition from an occupied to an empty state inside the valence band. Its interpretation is therefore more difficult than for x-ray dichroism, where the excited electron comes from a core state with a well-defined energy and symmetry.

The experimental proof of X-ray Linear Dichroism (XLD) is given by Van der Laan et al. [3] in 1986, while the first experiments with circularly polarized x-rays are performed in the high energy range by Schütz et al. [4]. In 1990, Chen et al. measure XMCD in the L2,3 edges (2p → 3d transitions) of Ni [5] and of Co and Fe [6] and find effects as large as 20% of the total absorption. For these edges the transitions take place directly to the empty 3d states, which are strongly polarized. A real explosion of the use of XMCD has followed the development of sum rules by Thole and coworkers [7,8]. These sum rules, applied to the total absorption and XMCD spectra, allow to obtain direct values for the orbital and spin moment of the probed atom.

The contrast provided by XMCD can also be used to perform element-selective magnetometry or for magnetic domain imaging when combined with Photoemission Electron Microscopy (PEEM).

References:
[1] J.L.Erskine et E.A.Stern, Phys.Rev.B 12, 5016 (1975).
[2] B.T.Thole, G.van der Laan, et G.A.Sawatzky, Phys.Rev.Lett. 55, 2086 (1985).
[3] G.van der Laan, B.T.Thole, G.A.Sawatzky, J.B.Goedkoop, J.C.Fuggle, J.-M.Esteva, R.Karnatak, J.P.Remeika, et H.A.Dabkowska, Phys.Rev.B 34, 6529 (1986).
[4] G.Schütz, W.Wagner, W.Wilhelm, P.Kienle, R.Zeller, R.Frahm, et G.Materlik, Phys.Rev.Lett. 58, 737 (1987).
[5] C.T.Chen, F.Sette, Y.Ma, et S.Modesti, Phys.Rev.B 42, 7262 (1990).
[6] F.Sette, C.T.Chen, Y.Ma, S.Modesti, et N.V.Smith, dans X-Ray Absorption Fine Structure, ed. S.S.Hasnain (Ellis Horwood, Chichester, United Kingdom, 1991), p.96.
[7] B.T.Thole, P.Carra, F.Sette, et G.van der Laan, Phys.Rev.Lett. 68, 1943 (1992).
[8] P.Carra, B.T.Thole, M.Altarelli, and X.Wang, Phys.Rev.Lett. 70, 694 (1993).

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