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Electron-spin-resonance ͑ESR͒ experiments on Gd 3ϩ in the intermediate-valence phase (TϽT v) of YbInCu 4 and in its reference compound YInCu 4 are interpreted in terms of an enhanced density of states at the Fermi level for the Yb-based compound. The Korringa rate and g shift measured in ESR and the susceptibility data allowed us to extract the electron-electron exchange enhancement factor ␣ for the Yb-based compound. The exchange interaction between the Gd 3ϩ local moments and the conduction electrons (c-e) is c-e wavevector dependent in both compounds. ͓S0163-1829͑97͒00701-7͔

Magnetic susceptibility measurements have shown anisotropic Van Vleck paramagnetism in Eu~Cu04 single crystals. This behavior is associated with the singlet ground state (Fo) of Eu'+ ions, and the measured anisotropy is related to a crystal-field splitting of the excited multiplets (F&). From the experimental data at low temperatures (T~50 K) a crystal-field parameter A2(r) =-93(5) cm ' and a spin-orbit coupling constant /=303(15) cm ' have been estimated. The temperature dependence of the magnetic susceptibility is predicted in terms of the Boltzmann population of the excited multiplets, and a comparison with experimental data up to 350 K is made. The possibility of a magnetic contribution arising from the Cu ions is discussed in connection with some discrepancies observed between the experimental and calculated magnetic susceptibilities.

We have measured the electron paramagnetic resonance (EPR) of Gd + in single crystals of La2 Sr CuO&+&, as a function of temperature T, magnetic-field angle, oxygen content, microwave frequency, and Sr concentration (0~x~0.024). For temperatures larger than the antiferromagnetic ordering temperature of the Cu lattice, Tz, we have identified four different sites of Gd'+ as expected for twinned crystals and we are able to fit the data for all directions of the magnetic field. Samples annealed under vacuum or oxygen show only small changes on their crystal-field parameters and g values. For temperatures smaller than T&, the EPR lines are further split due to the internal magnetic field acting at the. Gd site associated with the antiferromagnetic ordering of the Cu lattice. The temperature dependence of the splitting of the resonance lines allow us to calculate the moment per Cu ion. A value of-0. 6p, z per Cu ion at T =0 K is inferred, with the Cu moments lying in the bc plane about 5' from the c axis. The data only can be fit for all the directions of the applied magnetic field if other mechanisms such as a Heisenberg type of exchange between the Cu and Gd moments or a distortion of the lattice below T& are included. ''S. Oseroff et al. (unpublished).

We report low-temperature electron spin resonance (ESR) measurements in YBipt, doped with 0.1% Er substituting for Y. The results show that Er is located at a cubic lattice site and has a I~as ground state and an overall crystal-field splitting of-85(20) K. From the ESR spectra we inferred the existence of lattice distortions at the rare-earth site. These results may help in understanding the heavy-fermion system YbBiPt, which has the same structure as YBiPt.

We have measured the EPR spectrum of dilute Gd + ions substituting for Eu'+ in Eu2Cu04. Each one of the resonance lines shows a splitting into two or more weaker lines below T& =215 K. The simultaneous appearance of a low-field microwave-absorption signal is attributed to the onset of long-range antiferromagnetic ordering with a weak ferromagnetic component. The splitting of the EPR spectrum is associated with different Gd sites in the magnetically ordered structure. A reduction of the local symmetry of the rare-earth sites below Tz is described in terms of an internal magnetic field lying in the ab plane and additional crystal-field terms related to local crystallographic distortions. Dipolar and exchange contributions to the internal field are discussed. Possible random static displacements of the oxygen atoms in the Cu02 planes and their role in the development of the weak ferromagnetism and the presence of internal fields at the Gd sites are also analyzed. A strong dependence of the Gd EPR spectrum and the microwave absorption on the magnetic history of the samples has been observed, suggesting the formation of magnetic domains or a glassy state.

We report the results of an extensive investigation of the magnetic properties of a large series of undoped R2Cu04 single crystals with R = -Pr, Nd, Sm, Eu, and Gd (which are the host compounds for the newly discovered series of electron cuprate superconductors) and mixture versions of the form A, B"Cu04, with A = -Pr, Nd, Sm, Eu, or Gd, and B =Gd, Tb, or Dy. We have measured dc and ac magnetization, microwave magnetoabsorption, EPR, and specific heat. These measurements reveal two characteristic transition temperatures associated with a novel complex magnetic behavior, including weak ferromagnetism, two sharp peaks in the low-field dc magnetization, an unusual anisotropy in the EPR resonance field for R =Gd, and two additional anisotropic microwave absorption modes. The higher characteristic transition temeperature at -270 K is associated with antiferromagnetic ordering of the Cu moments which are strongly coupled within the CuOz layers.The lower, at 20 K, cannot be attributed to antiferromagnetic ordering of the R moments and is tentatively attributed to a spontaneous canted spin reorientation. An understanding of this magnetic behavior is important in order to ascertain its relationship to possible mechanisms of hightemperature superconductivity.

Electron spin resonance (ESR) experiments at 4.15 K of Er 3 ÷ in YBiPt show that Er 3 ÷ is in a site of cubic local symmetry, with a Ft83) ground state and an overall crystal field splitting of ~ 85(10) K. We inferred from the spectra the existence of lattice distortions at the rare-earth (RE) site. These results may help in understanding the heavy-fermion system YbBiPt, which has the same structure as YBiPt.