Nonlinear magnetic susceptibility and aging phenomena in reentrant ferromagnet: Cu₀.₂Co₀.₈Cl₂-FeCl₃ graphite bi-intercalation compound

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Linear and nonlinear dynamic properties of a reentrant ferromagnet Cu₀.₂Co₀.₈Cl₂-FeCl₃ graphite bi-intercalation compound are studied using AC and DC magnetic susceptibility. This compound undergoes successive phase transitions at the transition temperatures Tₕ (= 16 K), Tc (= 9.7 K), and TRSG (= 3.5 K). The static and dynamic behaviors of the reentrant spin glass phase below TRSG are characterized by those of normal spin glass phase with critical exponent β = 0.57 ± 0.10, a dynamic critical exponent x = 8.5 ± 1.8, and an exponent p (= 1.55 ± 0.13) for the de Almeida -Thouless line. A prominent nonlinear susceptibility is observed between TRSG and Tc and around Tₕ, suggesting a chaotic nature of the ferromagnetic phase (TRSG ≤ T ≤ Tc) and the helical spin ordered phase (Tc ≤ T ≤ Tₕ). The aging phenomena are observed both in the RSG and FM phases, with the same qualitative features as in normal spin glasses. The aging of zero-field cooled magnetization indicates a drastic change of relaxation mechanism below and above TRSG. The time dependence of the absorption χ′ ′ is described by a power law form (≈ t⁻ᵇ′ ′) in the ferromagnetic phase, where b′ ′ ≈ 0.074 ± 0.016 at f = 0.05 Hz and T = 7 K. No ωt-scaling law for χ′ ′ [≈ (ωt)⁻ᵇ′ ′] is observed.