Frustration-induced quantum criticality in Ni-doped CePdAl as revealed by the μSR technique

In CePdAl, the 4f moments of cerium arrange to form a geometrically frustrated kagome lattice. Due to frustration, in addition to Kondo- and Ruderman-Kittel-Kasuya-Yosida interactions, this metallic system shows a long-range magnetic order (LRO) with a TN of only 2.7\,K. Upon Ni doping at the Pd sites, TN is further suppressed, to reach zero at a critical concentration xc ≈ 0.15. Here, by using muon-spin relaxation and rotation (μSR), we investigate CePd1-xNiₓAl at a local level for five different Ni-concentrations, both above and below xc. Like the parent CePdAl compound, for $x = 0.05$, we observe an incommensurate LRO, which turns into a quasi-static magnetic order for $x = 0.1$ and 0.14. More interestingly, away from xc, for $x = 0.16$ and 0.18, we still observe a non-Fermi liquid regime, evidenced by a power-law divergence of the longitudinal relaxation at low temperatures. In this case, longitudinal field measurements exhibit a time-field scaling, indicative of a cooperative spin dynamics that persists for x > xc. Furthermore, like the externally applied pressure, the chemical pressure induced by Ni doping suppresses the region below T^*, characterized by a spin-liquid like dynamical behavior. Our results suggest that the magnetic properties of CePdAl are similarly affected by the hydrostatic- and the chemical pressure. We also confirm that the unusual non-Fermi liquid regime (compared with conventional quantum critical systems) is due to the presence of frustration that persists up to the highest Ni concentrations.