Magnetic order and excitations in Ce$_3$TiBi$_5$ and Ce$_3$ZrBi$_5$

Abstract

The R3MBi5 rare-earth intermetallics (R = rare earth, M = Ti, Zr, Sc) provide a versatile platform to explore how Kondo hybridization, RKKY exchange, magnetic frustration, and broken inversion symmetry may cooperate to generate unusual magnetic behavior. We present a comprehensive neutron scattering investigation of the magnetic structure, crystal electric field (CEF), and low-energy excitations in the locally noncentrosymmetric Kondo-lattice compounds Ce3TiBi5 and Ce3ZrBi5. Powder and single-crystal neutron diffraction reveals incommensurate cycloidal antiferromagnetic order in Ce3TiBi5 with propagation vector k = (0, 0, 0.388) and a reduced ordered moment of m = 0.53(3)$\mu_{B}$. Ce3ZrBi5 exhibits a qualitatively similar magnetic diffraction profile, with k $\simeq$ (0, 0, 0.37). Inelastic neutron scattering measurements resolve two clear, well-separated CEF excitations in both compounds with nearly the same profile, confirming a well-isolated Kramers doublet ground state. At low energies, a broad, quasi-elastic magnetic response is observed at T $\simeq$ TN, whose momentum-dependence is inconsistent with that expected from conventional collective excitations of localized moments. This discrepancy, along with a Kondo temperature estimate TK ~ 3--5 K -- comparable to TN -- indicates sizable Kondo hybridization, which accounts for the moment reduction and the spiral magnetic order that appears to involve the magnetic hard direction. Our results place these compounds in a regime where local inversion symmetry breaking, anisotropic CEF effects, and competing Kondo and RKKY interactions collectively give rise to unconventional magnetic order.