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We have used x-ray-absorption spectroscopy to study a series of compounds in which Cu assumes a formal valence between 0 and 3. We find that the shape, the threshold energy, and the intensity of the Cu L3 absorption edge is strongly inAuenced by the chemical state of the Cu atoms. We use the Cu 2p x-ray-absorption spectra of a large number of Cu compounds, including sulfides, oxides, LaSr-Cu-0 compounds, a phthalocyanine complex, and various minerals to show that the presence of a strong 2p-3d excitonic transition is a fingerprint of the Cu(d } contribution to the ground state. A simple ionic picture is generally inadequate to describe these compounds.

%hat we believe to be the first experimental results have been obtained on strong magnetic xray dichroism in the M4, 5 absorption spectra of magnetically ordered rare-earth materials, in accordance with recent predictions.The feasibility of using x-rays to determine the magnetic structure of magnetically ordered materials by magnetic dichroism has recently been predicted theoretically. ' Strong magnetic x-ray dichroism (MXD) is expected in the Mq, q absorption edge structure of rare-earth-metal compounds. Polarized synchrotron radiation can therefore be used to reveal information on the local rare-earth-metal magnetic moments in solids, thin films, and surfaces. In this Brief Report we will give what we believe is the first experimental proof of this effect.The M4, q absorption in rare-earth-metal compounds shows good agreement with the atomic Hartree-Fock calculations for the transitions from the 4f"(J) Hund's rule ground state to the manifold of 3d 4f"+'(J') final states.Although in x-ray absorption spectroscopy (XAS) hundreds of excited levels may be involved, one can distinguish between three different types of excitations, namely for J' -J -1, 0, and 1.In the presence of a magnetic field the 2J+1 degenerate ground state 4f"(J) splits into sublevels MJ -J, . . . , +J. The relative population of these sublevels depends on the temperature. That the polarization vector of the x rays has a drastic effect on the spectrum can be seen from the simple case where only M -J is populated (T 0 K). With the polarization direction parallel to the magnetization only the hM 0 transitions are allowed.The transitions J' -J -1 will then vanish, because the M' -Jsublevel is not present in the J' J -l state.Here, we will illustrate in more detail the use of MXD on terbium iron garnet (TbIG), which has a rather complicated magnetic structure. The rhombohedral (or trigonal distorted cubic) cell of ferrimagnetic rare-earth-metal iron garnets (R3Fe50t2) contains eight formula units. 5 The Fe + ions occupy the 24 d (tetrahedron) and the 16 a (octahedron) positions. The larger R + ions occupy the 6 c (dodecahedral) positions. Below the Weel temperature the spins of the 24 d and 16 a irons are ordered antiparallel along the [111]axis. The rare-earth-metal moments couple antiferromagnetically to the net iron moment. By symmetry the 6 c sites can be divided into c~, c2, c3 and cI c2 c3 where C2 (c2 ) and C3 (c3 ) are obtained from the c~(ct ) by rotation around the trigonal [111]axis. From neutron diffraction at 4.2 K the Tb moments in TbIG are known to form a double umbrella structure. 7s The magnetic moments on c~and cI are both in the (011) plane, having angles P 30. 79' and P' -28. 07' with the trigonal axis, and absolute values m 8.18ptt and m' 8.90ptt, respectively. Above 4.2 K the values of P, P', m and rn' are expected to decrease. In our experiment, a single crystal of TbIG was mounted on a rotatable helium-flow cryostat in an ultrahigh vacuum of -10 ' Torr. The temperature at the surface of the sample was 55+'5 K. A CosSm pe...

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