31P NMR study on Eu(Ni1-xFex)2P2: from valence fluctuations to ferromagnetic order

Among unstable f shell sytems the magnetic phase diagram of Eu systems differs from that of Ce, Yb or U systems. In contrast to these systems the Eu valence mostly could not be tuned (by substitutions or external pressure) continuously between the magnetic (Eu+2, J=7/2) and the non magnetic (Eu+3, J=0) state. Instead of that, first order valence transitions (like in EuNi2(Si/Ge)2) or an unstable, fluctuating valence (VF) at high temperatures (like in EuCu2Si2) are rather common. For EuCu2Si2 we have shown that upon Ge substitution the VF are suppressed and a small Kondo regime exist in the phase diagram before entering the antiferromagnetic ordered state on the Ge rich side. For the valence fluctuator EuNi2P2 the question arises whether a Kondo regime exists upon Fe substitution. In contrast to the EuCu2(Si/Ge)2 series the end point of the study here, EuFe2P2, is ferromagnetically ordered. This brings up the question of competition and/or coexistence of ferromagnetic and Kondo type of interactions in this new series. 31P NMR measurements are carried out on Eu(Ni1-xFex)2P2 samples with x=0, 0.15, 0.5 and 1.