Gd3+ as a probing and tuning tool of strong electronic correlations in the heavy-fermion Kondo lattice compound YbFe2Zn20

We report on the magnetic, thermodynamic, and electronic properties of the Gd-doped Kondo-lattice compound YbFe2Zn20 (T-K approximate to 32 K) by means of T-dependent magnetization, specific heat, and electron spin resonance (ESR) measurements. As Gd is incorporated in this system (Yb1-xGdx Fe2Zn20), the Yb contribution to the Sommerfeld coefficient remains almost unaltered for x = 0.005 and 0.01 at gamma(eff) approximate to 500 mJ mol(-1) K-2, however, for x = 0.05, it is reduced to gamma(eff) = 450 mJ mol(-1) K-2. As expected for heavy-fermion systems, the Gd3+ ESR experiments show an enhanced Korringa relaxation rate, b = d(Delta H)/(dT), due to the exchange interaction between the Gd3+ localized magnetic moment and the high 4f-conduction electrons (ce) density of states at the Fermi level. Below T-K, the Gd3+ ESR g shift presents a peculiar T dependence. For x less than or similar to 0.01, we associate the g shift to an internal molecular AFM field due to the ce screening of the Yb3+ magnetic moments in the Kondo condensate with a value of lambda = -2.80(1) mol Oe/emu. The negative value reflects a singlet nonmagnetic ground-state formation, consistent with a Kondo-lattice system. Still below T-K, but for x = 0.05, the Gd3+ ESR g shift presents a positive T dependence, which we now associate to an internal molecular FM field due to a Gd3+-Gd3+ superexchangelike interaction via extended Fe 3d ce. A Fermi surface reconstruction process is found to take place in the crossover from the high-T to low-T regimes, such that a momentum transfer dependence of the Gd3+-ce exchange interaction [J(f-ce) (q)] in the former is lost as the Kondo condensate sets in.