Magnetism and valence mixing in Eu systems with contrasting band structures

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Abstract
Eu compounds at or near the strongly magnetic (J=7/2) Eu2+ valence should enter a mixed valent Eu2+δ (‖δ‖<1) nonmagnetic state at sufficiently high pressures. We have performed high pressure (P≤250 kbars) and low temperature (T≥4.2 or 77 K) resistivity measurements on semiconducting EuO and metallic Eu(AuxPd1−x)2Si2 (0≤x≤1). For EuO our results show a nonlinear gap variation above 130 kbar with a hybridization gap and ferromagnetism (Tc ≥200 K) being maintained out to 250 kbars. In the Eu(Au,Pd)2Si2 system the cooperative valence transition can be traced to higher pressures with increasing Au content. In the pure Au the magnetic order persists out to 250 kbars. A common theme in both systems is the substantial pressure-induced enhancement of the magnetic ordering temperature precursive to the eventual large Eu valence change.