Soft x-ray absorption spectroscopy in the M-edge region is explored as a tool for studying 4f instabilities. The quasiatomic MIV,V spectra, recorded by total-electron yield under UHV conditions, carry information on the initial-state 4f occupancy. For compounds of the heavy rare earths, even at high dilution (e.g., Tm0.05Y0.95Se), mean valences can be derived, which agree with those from lattice-constant systematics (e.g., for TmSe and Sm0.3Y0.7S). A moderate surface sensitivity allows the observation of surface-induced valence changes on EuPd3, TmS, and SmAl2. The MIV,V spectra studied for 10 Ce compounds fall in two categories: pure 3d94f1 final-state multiplets for γ-like compounds, and additional peaks at about 5 eV higher energies for α-like and intermediate systems. With increasing intensity of the satellites, which are assigned to 3d94f1 final states, the 3d94f2 multiplet structure gets washed out. An interpretation of the spectra with the recent Anderson-impurity theory of Gunnarsson and Schönhammer leads to 4f occupancies greater than 0.7, even in extreme α-like systems. In addition, MIII-edge spectra exhibit one peak in γ-like and two peaks in α-like and intermediate Ce systems, which are assigned to 3p54f1 and 3p54f0 final corestates. With increasing α-like character, the 4f1 peak is found to move to higher energies presumably due to a decrease in 4f screening with increasing 4f hybridization.
A systematic x-ray photoemission study of Eu 3d and Eu 4d core levels in the Eu -transitionmetal compounds EuRh5, EuNi5, EuPd5, EuPt5, EuCu5 and EuAg& is presented. The spectra in the divalent and trivalent compounds exhibit splittings into two components, caused by the occurrence of 4f and 4f' configurations. For the divalent compounds EuCus and EuAg5, this effect is due to final-state shakeup. For the trivalent compounds, the presence of both configurations is discussed in terms of both initial-and final-state effects. An estimate of the 4f-configurational stability at the surface of the Eu + intermetallics indicates surface valence transition of the Eu ions to the lower valent state in these compounds. Experimental evidence for this behavior is given.
We report the measurement of the dynamic magnetic susceptibility of an intermediate-valence Eu compound in inelastic neutron scattering experiments, namely, on a sample of EuPd2Si2 with enriched '"Eu. The most remarkable feature is an increase of the quasielastic linewidth with decreasing temperature, which is apparently a consequence of the well-known temperature-driven valence change. Moreover, at low temperatures a strong inelastic line is observed, which is interpreted as a multiplet transition within the Eu + configuration (J =0~J = 1). To check the quality of the sample, additional Mossbauer and magnetization measurements were performed. From these we conclude a fraction of approximately 6%%uo of the so-called "satellite" phase in our sample, which is ferromagnetic at low temperatures and whose effects can be separated consistently out of all of our data.
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