Magnetic ordering in EuxSr1−xS, a diluted Heisenberg system with competing interactions (invited)

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Abstract
Presence of additional antiferromagnet bonds in a ferromagnet changes magnetic ordering in a dilution series drastically. An ideal system to study these effects is the insulating EuxSr1−xS, based on EUS, the well‐known model substance for a Heisenberg ferromagnet. The competition between ferromagnetic nearest neighbor exchange J1 and antiferromagnetic next‐nearest neighbor exchange interaction J2 with J2/J1 = −0.5 is shown to cause a breakdown of ferromagnetism in EuxSr1−xS at x = 0.51, far above the percolation threshold xp = 0.13. In the intermediate concentration regime, 0.13⩽x⩽0.51, spin‐glass behavior is observed very similar to the features in metallic spin glasses. Results on (Eu, Sr)S provide clear evidence for a cooperative phenomenon at the spin‐glass transition, as distinguished from ordinary thermal blocking of superparamagnetic clusters. By increasing the Eu concentration just above xc, long‐range ferromagnetic order is found to coexist with a high amount of wrong aligned spins, which again is caused by the competing exchange interactions. Neutron (Bragg and small‐angle) scattering demonstrates an interesting change in magnetic order by lowering the temperature. The unique properties of (EU,Sr)S can be compared with realistic numerical model calculations.