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

• Received 4 February 2011

DOI:https://doi.org/10.1103/PhysRevB.84.224415