Magnetism and mixed valence in europium compounds under pressure

Magnetic order and mixed valence of local moment rare earth systems have been thought to be mutually exclusive due to one of the valence states having a net zero local moment {dollar}mu{dollar}. Only for Tm, which possesses a nonzero J in both valence states, has magnetic order been unambiguously observed in the mixed valent regime. For Ce compounds with a relatively extended Ce 4f shell, Kondo screening quenches any long range magnetic order in weakly mixed valent systems. Eu, however, has a much more localized 4f shell which can lead to more rapid and complete valence changes and which excludes the Kondo effect.; If temperature, pressure, and chemical environment is varied in the right way, a weakly mixed valent Eu compound may still be able to support magnetic order. To this end we have studied two Eu compounds. The first is EuO, which is an integral valent ferromagnetic semiconductor. The second is the series Eu(Au{dollar}sb{lcub}rm x{rcub}{dollar}Pd{dollar}sb{lcub}rm 1-x{rcub}{dollar}){dollar}sb2{dollar}Si{dollar}sb2{dollar}, which is an intermetallic and which exhibits mixed valence or antiferromagnetism depending on the relative amount of Au to Pd. L{dollar}sb{lcub}rm III{rcub}{dollar} absorption and high pressure-temperature dependent resistance measurements (using a new modified two probe method) were made on both compounds.; For EuO, it was found that pressure dramatically enhances the ordering temperature T{dollar}sb{lcub}rm c{rcub}{dollar} below 120 kbar, and above 150 kbar T{dollar}sb{lcub}rm c{rcub}{dollar} saturates at about 200{dollar}spcirc{dollar}K up to 250 kbar. At 140 kbar a transition into a narrow gap regime is observed at room temperature, which confirms recent observations of a transition in that pressure region. The narrow gap confirms Nolting's prediction of a weakly mixed valent hybridization gap at high pressures. That the system still orders magnetically in this regime also confirms Noltings predictions.; For Eu(Au{dollar}sb{lcub}rm x{rcub}{dollar}Pd{dollar}sb{lcub}rm 1-x{rcub})sb2{dollar}Si{dollar}sb2{dollar}, L{dollar}sb{lcub}rm III{rcub}{dollar} absorption at room temperature shows an unstable valence regime when the Au concentration drops below x = 0.4. For x = 0.6, pressure above 40 kbar has completely destroyed magnetism in this normally magnetic compound. The results for x = 0.6 are reminiscent (albeit on a higher temperature scale) of EuPd{dollar}sb2{dollar}Si{dollar}sb2{dollar}. For x = 0.8 we have some hints of a coexistence of mixed valence and magnetic order. (Abstract shortened with permission of author.)...