Muon spin relaxation and susceptibility measurements of an itinerant-electron system Sr1−xCaxRuO3: Quantum evolution from ferromagnet to paramagnet

Abstract

Muon spin relaxation ($\ensuremath{\mu}$SR) and magnetic susceptibility measurements have been performed in the itinerant-electron magnet Sr${}_{1\ensuremath{-}x}$Ca${}_{x}$RuO${}_{3}$, 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 ${T}_{c}\ensuremath{\sim}160$ 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, $\ensuremath{\mu}$SR measurements revealed discontinuous evolution of magnetic properties in contrast to magnetization measurements, which exhibit seemingly continuous evolution. Unlike the volume-integrated magnetization measurements, $\ensuremath{\mu}$SR 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.