Anomalous Local Magnetism in the 4f-Localized Ferromagnets CeRu2X2B (X = Al, Ga) Revealed by Using ZF-μSR

We report zero-field muon spin relaxation/rotation (ZF-μSR) measurements on the 4f-localized ferromagnets CeRu2X2B (X = Al, Ga) with Ising-type anisotropy. Our ZF-μSR spectra of CeRu2Ga2B with a Curie temperature at TC = 16.3 K show a well-defined muon procession at temperatures below TC . The extracted critical exponent of β = 0.533 is close to the mean-field value of β = 0.5. On the other hand, the isomorphic CeRu2Al2B exhibits an intriguing evolution of the ZF-μSR spectrum through a spin density wave/ferromagnetic transition at TC = 12.8 K with a subsequent magnetic anomaly at T ∗ = 6 K. Strikingly, the muon frequency decreases upon cooling to temperatures below TC and can be no longer resolved at temperatures below T ∗. At the same time, the muon relaxation rate increases continuously at temperatures below TC . These give an indication of the development of an inhomogeneous and dynamic ferromagnetic state. The contrasting ferromagnetism between CeRu2Ga2B and CeRu2Al2B evidences that a certain type of intrinsic/extrinsic perturbation, for example, exchange frustration and defects/impurities, can exert a drastic impact on the local magnetic properties in a family of 4f-localized magnetic materials with frustration.