Investigation of Kondo Alloys and Compound by the Superconductive Proximity Effect

Abstract

The proximity effect between a superconductor and a normal metal is sensitive to presence, amount, and state of order of the impurity spin in the normal metal. The present experiments were performed with ${\mathrm{Cu}}_{0.99}$ ${\mathrm{Fe}}_{0.01}$, ${\mathrm{Cu}}_{1-x}{\mathrm{Mn}}_x$ ($x=0.\mathrm{}0025, 0.05$), ${\mathrm{Cu}}_{0.9975}$ ${\mathrm{Co}}_{0.0025}$ Kondo alloys, and with the Kondo compound Ce${\mathrm{Al}}_2$. The alloys were sputtered first at temperatures ranging from about — 100 to 530 °C followed by Pb sputtering at 77 °K. Electron diffraction showed that the Cu alloy films prepared at — 100 °C were quasiamorphous and did not have the structure of bulk Cu. In such a state, regardless of the Kondo temperature of the bulk alloys, the magnetic impurities in the dilute alloys behave as random free spins; the spin depairing in the super-conductive proximity effect is almost independent of the type of host metal (Cu or Mo) or the type of impurity (Fe, Mn, Co) and is proportional to the amount of impurity present. The ${\mathrm{Cu}}_{0.95}$ ${\mathrm{Mn}}_{0.05}$ alloy, when deposited at — 100 °C, becomes ferromagnetic at 5.5 °K and the magnetic transition is clearly exhibited by the proximity effect. Alloys prepared at and above room temperature have the bulk Cu structure. In the temperature range of the measurements (1-7. 2 °K) which is well below the Kondo temperature of ${\mathrm{Cu}}_{0.99}$ ${\mathrm{Fe}}_{0.01}$ ($\simeq 13$ °K as determined by resistivity), the ${\mathrm{Cu}}_{0.99}$ ${\mathrm{Fe}}_{0.01}$ alloy films with the bulk Cu structure behave as an almost spin-less system. The Cu-Mn alloys with the bulk Cu structure are antiferromagnetic at low temperature and give rise to a spin-depairing factor about 10 times smaller than the one corresponding to free random spins. The same qualitative result was obtained with Cr films, where the free-spin state can be achieved by sputtering at 77 °K. The proximity-effect experiments were also performed with the Kondo compound Ce${\mathrm{Al}}_2$; above 3.5 °K Ce${\mathrm{Al}}_2$ behaves as a spin-less system which implies that Ce${\mathrm{Al}}_2$ has a Kondo temperature above 7.2 °K. The antiferro-magnetic transition at 3.5 °K is clearly shown by the proximity effect.