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.

Two series of experiments on wellcharacterized systems were performed to examine the probing depth of soft x-ray absorption spectroscopy (XAS) measured in total-electron-yield (TEY) mode. First we measured the Ni 2p,,, absorption spectra of Ni(100) covered with Tb as a function of the overlayer thickness. Secondly we recorded the 0 Is absorption spectra of Ta,O, films produced by controlled anodic oxidation of Ta foils as a function of the oxide thickness. The mean probing depth (MPD) was found to be much shorter than previously assumed (for 0 Is, only 1.9 nm). The relative importance of those cascade mechanisms that lead to the electron current measured in TEY is discussed.

We have studied the quasibinary ferromagnetic semiconductor (Ga0.98Mn0.02)As by magnetic circular dichroism in x-ray absorption. We find a richly structured Mn absorption spectrum typical for localized 3d electrons. An analysis of the magnetization-averaged and dichroism line shapes shows a local Mn moment of 4.6μB, which is close to the Hund’s rule moment for the half-filled 3d shell. The magnitude of the dichroism reveals that only about 1/7 of the Mn atoms participate in the ferromagnetic order. Our experiment does not show a distinction between the ferro- and paramagnetic Mn atoms.

Magnetic circular dichroism has been used to study the orbital and spin moments in supported nanoscale Fe clusters deposited in situ from a gas aggregation source onto highly oriented pyrolitic graphite in ultrahigh vacuum. Mass-filtered ͑2.4 nm, 610 atoms͒ and unfiltered ͑1-5 nm, 40-5000 atoms͒ clusters at low coverage have an orbital magnetic moment about twice that of bulk Fe. With increasing coverage the orbital moment of the unfiltered clusters converges to the bulk value. There is no detectable change in the spin moment as a function of coverage. Mass-filtered clusters show an increase in the magnetic dipole moment which we ascribe to distortion resulting from their higher impact energy. An increasing magnetic remanence with coverage is found. ͓S0163-1829͑99͒02225-0͔

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We investigate the magnetic properties of Mn adsorbates on Fe͑100͒ in the regime up to a few monolayers.Magnetic circular dichroism in absorption shows long-range ferromagnetic order for the Mn adsorbate, with antiferromagnetic alignment with respect to the Fe substrate. Element-specific magnetic domain imaging and hysteresis measurements show that the macroscopic magnetic behavior of the Mn adlayer is fully determined by the Fe substrate. For coverages below 0.5 ML the Mn absorption spectra show rich structures that are typical for localized d states. From this the Mn ground state is identified as a mixture of atomiclike d 5 and d 6 states, with a local spin moment of 4.5 B . However, the circular dichroism is 2.4 times smaller than expected for this ground state, suggesting disorder within the Mn adsorbate with an ordered moment of 1.9 B at 120 K. The magnetic signal vanishes near 1 ML coverage, consistent with the theoretically predicted c(2ϫ2) antiferromagnetic ground state of the monolayer.