SIZE-EFFECT IN POINT-CONTACT SPECTROSCOPY OF MAGNETIC-IMPURITIES IN METALS

Point contact spectra dV/dI(V) and dR/dV(V) were determined for variou s noble metals (Cu; Au) doped with magnetic impurities (Mn) by utilizi ng mechanically controllable break-junctions. The contact diameter d c ould be varied continuously from congruent-to 2 to 100 nm. We observed an anomalous d-dependence of the width, amplitude and field dependenc e of the Kondo-effect zero bias peak, revealing an increase in the eff ective Kondo scattering in point contacts. The zero bias peak in dV/dI due to Kondo scattering exhibits a large broadening and increase of r elative (compared to phonons) amplitude with decreasing d. By using th e theoretical expression for the logarithmic slope of dV/dI vs. V depe ndence we found a significant enhancement in the exchange coupling par ameter J, which leads to an enormous increase in the effective Kondo t emperature T(K) for small contact diameters. For alloys with higher im purity concentration, the position of the spin-glass resistance peak o n the voltage scale was found to decrease with d in accordance with La rsen's [24] predictions for the maximum of bulk resistivity on the tem perature dependence for large values of J. Thus, both the Kondo scatte ring and the RKKY interaction were strongly modified by the contact si ze and could be attributed to an increase in the effective exchange in teraction between the magnetic impurity and the conduction electrons i n point contacts.