Tuning of exchange by band filling in low‐carrier‐density magnet Eu(Gd)B6

We report the study of spin relaxation in the Eu1−xGdxB6 (0 ≤ x ≤ 0.039) single crystals with the help of 60 GHz electron spin resonance (ESR) technique. A drastic change in the linear slopes of the temperature dependences of the ESR linewidth is discovered in the paramagnetic phase of Eu1−xGdxB6. The corresponding crossover temperature T0 is shown to decrease from T0(x = 0) ∼ 60 K down to T0(x = 0.039) ∼ 15 K under rising of Gd content. A non‐bottlenecked Korringa relaxation is discussed as the main factor that governs spin dynamics in the unordered state of Eu1−xGdxB6 below T0. Using of the band parameters extracted from static magnetic and transport data allows to estimate on‐site exchange constant between localized spins and itinerant electrons, which is effectively tuned from 110 meV for x = 0 down to 43 meV for x = 0.039 under gradual filling of the Eu1−xGdxB6 conduction band.