Muon spin relaxation and susceptibility measurements of an itinerant-electron system Sr _1-xCa _xRuO ₃: Quantum evolution from ferromagnet to paramagnet

Muon spin relaxation (μSR) and magnetic susceptibility measurements have been performed in the itinerant-electron magnet Sr _1-xCa _xRuO ₃, with x= 0.0, 0.3, 0.5, 0.65, 0.7, 0.75, 0.8, 0.9, and 1.0. SrRuO ₃ is a ferromagnet with the critical temperature T _c∼160 K. Upon (Sr, Ca) substitution, T _c decreases monotonically with increasing Ca concentration x and the ferromagnetic order disappears around x= 0.7. Very weak static magnetism is observed in the x= 0.75 and 0.8 systems, while the x= 0.9 and 1.0 systems remain paramagnetic in their full volume. Phase separation between volumes with and without static magnetism was observed in the x= 0.65, 0.7, 0.75, and 0.8 systems, near the magnetic crossover around x= 0.7. In this concentration region, μSR measurements revealed discontinuous evolution of magnetic properties in contrast to magnetization measurements, which exhibit seemingly continuous evolution. Unlike the volume-integrated magnetization measurements, μSR can separate the effects of the ordered moment size and the volume fraction of magnetically ordered regions. The muon spin relaxation rate 1/T ₁ exhibits critical slowing down of spin fluctuations near T _c in the ferromagnetic systems with x= 0.0-0.65, consistent with the behavior expected in the self-consistent renormalization theory of itinerant electron ferromagnets. The lack of maximum of 1/T ₁ in the x= 0.7 system indicates the disappearance of critical slowing down. These results demonstrate a first-order quantum evolution in the ferromagnet to paramagnet crossover near x= 0.7.