It has generally been believed that, within the context of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, the conduction electrons in a metal cannot be both ferromagnetically ordered and superconducting. Even when the superconductivity has been interpreted as arising from magnetic mediation of the paired electrons, it was thought that the superconducting state occurs in the paramagnetic phase. Here we report the observation of superconductivity in the ferromagnetically ordered phase of the d-electron compound ZrZn2. The specific heat anomaly associated with the superconducting transition in this material appears to be absent, and the superconducting state is very sensitive to defects, occurring only in very pure samples. Under hydrostatic pressure superconductivity and ferromagnetism disappear at the same pressure, so the ferromagnetic state appears to be a prerequisite for superconductivity. When combined with the recent observation of superconductivity in UGe2 (ref. 4), our results suggest that metallic ferromagnets may universally become superconducting when the magnetization is small.

We report the magnetic field dependence of the low temperature specific heat of single crystals of the first Pr-based heavy fermion superconductor PrOs4Sb12. The low temperature specific heat and the magnetic phase diagram inferred from specific heat, resistivity and magnetisation provide compelling evidence of a doublet ground state and hence superconductivity mediated by quadrupolar fluctuations. This establishes PrOs4Sb12 as a very strong contender of superconductive pairing that is neither electron-phonon nor magnetically mediated.PACS numbers: 74.70. Tx, 7127,+a, 75.30.Mb Superconductivity mediated by a pairing potential other than a conventional electron-phonon interaction has been the subject of a very large number of theoretical and experimental investigations over the decades. In recent years, finally, intermetallic compounds have been discovered which, together with the high-T c cuprates, represent prime candidates for magnetically mediated pairing. Surprisingly, however, magnetically mediated pairing thus far has been considered the only serious alternative to electron-phonon mediated pairing, while excitonic and polaronic mechanisms have also been proposed.We have recently reported the discovery of the first Pr-based heavy fermion superconductor PrOs 4 Sb 12 [1], for which the nonmagnetic ground state appeared best described as a crystalline electric field (CEF) doublet. This in turn suggested that the heavy electron liquid is of quadrupolar origin and that consequently quadrupolar fluctuations mediate the superconductive pairing. PrOs 4 Sb 12 hence is a candidate for being the first material in which neither electron-phonon nor magnetic interactions mediate the pairing. However, a magnetic origin could not be completely ruled out [1] and recent low temperature specific heat measurements are reported to be consistent with a singlet CEF ground state hence questioning the hypothesis of quadrupolar pairing [2].In order to settle the question of the ground state and thus the possibility of the first example of quadrupolar mediated superconductive pairing demands definitive establishement of (i) the CEF level scheme, (ii) the unconventional nature of the superconductivity, and (iii) coupling of the CEF excitations to the conduction electrons. Here we report specific heat measurements of high quality single crystals of PrOs 4 Sb 12 at low T and high magnetic field. We show that the zero field data of the single crystals and the magnetic phase diagram as established from the specific heat, resistivity and magnetisation, as well as the observation of a novel high field ground state, provide compelling evidence of a nonmagnetic doublet CEF ground state intimately linked to the conduction electrons. This unambiguously establishes quadrupolar fluctuations as the most likely pairing mechanism. Moreover, we observe two superconducting transitions giving evidence of two distinct superconducting phases.Previous experiments on pressed pellets of tiny single crystals of PrOs 4 Sb 12 [1] revealed superconductivity ...

The realm of high energy, large wave vector spin waves in ultrathin films and at surfaces is unexplored because a suitable method was not available up to now. We present experimental data for an 8 ML thick Co film deposited on Cu(001) which show that spin-polarized electron energy loss spectroscopy can be used to measure spin-wave dispersion curves of ultrathin ferromagnetic films up to the surface Brillouin zone boundary.

By magnetization induced second harmonic generation (MSHG) the surface magnetization of fcc Fe films on Cu(100) is studied. The tetragonally distorted phase below 4 monolayers (ML) and the relaxed fcc phase above 4 ML can unambigously be distinguished by different average MSHG signals. Nearly constant asymmetry of the second harmonic intensities between 1.3 and 10 ML proves, to a good approximation, thickness independence of the surface magnetization irrespective of the phase transition at 4 ML. The nonlinear Kerr angle amounts to 4 ± . [S0031-9007(96)

No abstract