LuGd: A system exhibiting positive exchange coupling between conduction electrons and localised magnetic moments

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Abstract
We have measured the magnetization and the low temperature resistivity and magnetoresistance of a series of alloys of Lu containing up to 1.6 at.% Gd. The resistivity was found to vary A+D lnT, with A and D scaling linearly with Gd concentration showing that this is a single impurity effect and thus establishing LuGd as the first unambiguous example of a positive exchange Kondo system. Furthermore, the value of the exchange constant deduced from the resistivity data is in excellent agreement with that estimated from e.p.r. measurements. The low field susceptibility data (J of comparable magnitude; this effect is attributed to the competing field dependences of the second and third order terms in the usual expression for the magnetoresistance when J is positive, in contrast to the negative J situation.