Nonlinear magnetic susceptibility and aging phenomena in reentrant ferromagnet: Cu${}_{0.2}$Co${}_{0.8}$Cl${}_2$-FeCl${}_3$ graphite bi-intercalation compound

Abstract

Linear and nonlinear dynamic properties of a reentrant ferromagnet Cu${}_{0.2}$Co${}_{0.8}$Cl${}_2$-FeCl${}_3$ graphite bi-intercalation compound are studied using AC and DC magnetic susceptibility. This compound undergoes successive phase transitions at the transition temperatures $T_h$ (= 16 K), $T_c$ (= 9.7 K), and $T_{RSG}$ (= 3.5 K). The static and dynamic behaviors of the reentrant spin glass phase below $T_{RSG}$ are characterized by those of normal spin glass phase with critical exponent $\beta$ = 0.57 $\pm$ 0.10, a dynamic critical exponent $x$ = 8.5 $\pm$ 1.8, and an exponent $p$ (= 1.55 $\pm$ 0.13) for the de Almeida -Thouless line. A prominent nonlinear susceptibility is observed between $T_{RSG}$ and $T_c$ and around $T_h$, suggesting a chaotic nature of the ferromagnetic phase ($T_{RSG}\le T\le T_c$) and the helical spin ordered phase ($T_c\le T\le T_h$). 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 $T_{RSG}$. The time dependence of the absorption ${\chi }^{\prime \prime }$ is described by a power law form ($\approx t^{-b^{\prime \prime }}$) in the ferromagnetic phase, where $b^{\prime \prime }\approx 0.074\pm 0.016$ at $f$ = 0.05 Hz and $T$ = 7 K. No $\omega t$-scaling law for ${\chi }^{\prime \prime }$ [$\approx (\omega t{)}^{-b^{\prime \prime }}$] is observed.

14 pages, 16 figures, and 2 tables