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

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.