Superconductivity, Weak Itinerant Ferromagnetism and Electronic Band Structure of Y₉Co₇

Abstract

The paper presents experimental results for the intermetallic compound Y9Co7 which is the first very weak itinerant ferromagnetic superconductor. The measurements of electrical resistivity, susceptibility, magnetization, specific heat, nuclear magnetic resonance, and other properties show the magnetic transition at T C ≈ 4.5 K and the onset of superconductivity at about TS ≈ 2.5 K, revealing the coexistence state within a temperature interval below T S. A few of the results are shortly reviewed and discussed. We focus attention on our recent ultraviolet photoemission spectroscopy and their comparison with our band structure calculations. Interpretations are considered which take into account the characteristic structure of the compound and different possible types of magnetic ordering.

Electronic structure, magnetism, and spin fluctuations in the superconducting weak ferromagnetY4Co3

“…In 80-ties, after revealing the coexistence of ferromagnetism and superconductivity in Y 4 Co 3 , the Ho 4 Co 3type unit cell appeared to be only an approximation for the real structure of the system. There was a debate whether this system should be called Y 9 Co 7 (instead of Y 4 Co 3 ), since it was found, 6,7,15 that Y 9 Co 7 (richer in cobalt) exhibited better superconducting properties (∼ 0.5 K higher T s ) than Y 4 Co 3 . It was even suggested, that Y 4 Co 3 as a single phase compound might not exist.…”

“…external pressure suppresses magnetism and enhance superconductivity 19,28 ), but their coexistence is possible due to some spatial 'separation' of atom sublattices responsible for different phenomena. 6,7 However, this separation cannot be treated in the strict sense, since the Ginzburg-Landau coherence length was estimated to be as large as 300 Å (roughly thirty times larger than the unit cell size). 29 One can tentatively imagine this uncommon state as a 'superconducting sea' with the Co(3) 'magnetic islands' embedded.…”

“…5). Moreover, Y 4 Co 3 was likely the first example, where superconductivity occurred below a transition to ferromagnetism, with both phenomena coexisted at least in the range between 1 K and 2.5 K. 6,7 This was another difference to previously known magnetic superconductors e.g. ErRh 4 B 4 or HoMo 6 S 8 (see, Refs.…”