NEUTRON SCATTERING STUDIES OF MAGNETIC SUPERCONDUCTORS J. W. LYNN Department of Physics, University of Maryland*, College Park, Md. 20742 and National Bureau of Standards, Washington, DC 20234, USA and R. N. SHELTON Ames Laboratory-USDOE and Department of Physics, lowa State University**, Ames, 1.4 50011, USA Measurements have been carried out on a series of rare earth Chevrel-phase superconductors. In the selenide materials well defined crystal field transitions have been observed, which can be understood to a first approximation on the basis of a cubic crystal field with a magnetic ground state. In HoMo6S s, on the other hand, no crystal field excitations have been observed over a wide range of energies. Diffraction data show that essentially the full free-ion moment is readily induced in HoMotSs, but that in ErMotSe s less than half the free-ion moment is induced at T = 5 K and H = 70 kOe. The induced-moment data on HoMotS s can be readily interpreted on the basis of one Ho atom per unit cell, whereas for ErMotSe ~ this appears not to be the case. These data also demonstrate that the only significant magnetic impurity phases in these samples are (RE)202Se, and these are typically a few percent or less in volume. The ternary Chevrel-phase superconductors, RMo6X s (R-rare earth; X---Se, S), exhibit a variety of unique and interesting phenomen/t re- lated to the interplay between magnetism and su- perconductivity [1]. In all these materials the mag- netic ordering temperatures are typically 1 K or less, so that we may anticipate that the crystal field splittings of the rare earth ions may dominate the magnetic energies and thus determine the magnetic (or non-magnetic) properties at low temperatures. To investigate these materials we have carried out neutron scattering experiments on ErMo6Ses, TbMo6Ses, HoMo6Se s and HoMo6S s as a function of temperature and magnetic field. The measure- ments were performed on triple-axis spectrometers at the National Bureau of Standards research reac- tor. Fig. 1 shows data on HoMo6Se s at 4 K. An excitation is clearly seen at 4.58 meV, and there is additional scattering at low energies which, under higher resolution, can be identified as another ex- citation at 1.06 meV. This scattering has been uniquely identified as crystal field in origin since: (i) the energy of the scattering is independent of the wavevector transfer K; (ii) the intensity de- creases with increasing temperature, in contrast to Bose excitations whose intensity increases with temperature; and (iii) the intensity as a function of *Work supported by the NSF, DMR 79-00908. **Work supported by the US Department of Energy, contract No. W-7405-Eng 82, Division of Basic Energy Sciences, .43(- 01-02-02-2. K quantitatively follows the magnetic form factor f(K). The observance of crystal-field transitions in HoMo6Se 8 is characteristic of the selenide materi- als, in which crystal field transitions have been found in all the compounds studied to date [2]. The overall crystal-field level scheme can be described to a first approximation by a cubic crystal field; additional splittings due to the perturbations from cubic symmetry which are present may be treated as second-order effects. A cubic crystal field is 1750 1500 1250 o I000 g- o 750 (..) 5OO 25O # 0 I,'" I - 2.0 0.0 HoMo6Se 8 I~l: 14o ~,-' Ef =14.80 meV T=4K ÷ # • o°O .... • ~o° ....... ~*° 2.5 5.0 7.5 E (meV) Fig. 1. Observed crystal field scatter;rag at low temperatures in HoMo~Se 8. Under high resolution the scattering at low energies is identified as a crystal field transition at 1.06 meV. Journal of Magnetism and Magnetic Materials 15-18 (1980) 1577-1578 ©North Holland 1577