Ferromagnetism with zero intrinsic magnetization: μSRμSR on Gd-doped SmAl2SmAl2

A ferromagnetic system which has an external magnetization that is intrinsically zero is an interesting and unusual type of magnet. The rare earth ion Sm3+Sm3+ provides a basis for building such a system, having the property that its spin and orbital moments are both close to 4μB4μB. Antiparallel coupling of these moments by spin–orbit interaction leads to a very small resultant moment that is either parallel or antiparallel to the spin moment of the ion, depending on the exact balance. By doping Gd into the ferromagnet SmAl2(Tc=125K) an experimental realization of a zero magnetization ferromagnet can be produced, with exactly zero magnetization occurring at temperature TcompTcomp. Zero-field muon spin relaxation measurements have been made on single crystal and polycrystalline samples of the Sm1-xGdxAl2Sm1-xGdxAl2 system and for all the compositions studied an extremely sharp transition into the ferromagnetic state was observed. Muon spin precession was seen in the magnetically ordered state of the Sm compounds and anomalies in the relaxation were observed around TcompTcomp, reflecting the unusual nature of this magnetic state.