Low-temperature ESR and magnetic behavior of CrCl3- and NiCl2-based intercalation and bi-intercalation compounds.

Abstract

Electron-spin-resonance (ESR) investigations have been carried out on ${\mathrm{CrCl}}_{3}$-based graphite intercalated (GIC) and bi-intercalated (GBIC) compounds as well as on a stage-2 ${\mathrm{NiCl}}_{2}$ graphite intercalation compound. The measurements have been performed at the X-band frequency and over the temperature range 4.2--294 K. In each case, the ESR spectrum displays a single line down to a temperature ${\mathit{T}}_{\mathit{s}}$\ensuremath{\sim}${\mathit{T}}_{\mathrm{pr}}$, the magnetic ordering temperature of the relevant intercalant pristine form. Below ${\mathit{T}}_{\mathit{s}}$, the ESR spectrum becomes complex and consists of several resonance lines. The evolution of the ESR spectrum with changing temperature and angular orientation of the applied field is presented. In the single-line regime, with decreasing temperature, the resonance line always narrows at first, and then broadens rapidly as T approaches ${\mathit{T}}_{\mathit{s}}$. The resonance field (${\mathit{H}}_{\mathit{r}}$) exhibits an anisotropic shift; however, its mean value (Hrm=[(${\mathit{Hr}}_{\mathrm{?}}$)(${\mathit{Hr}}_{\mathrm{\ensuremath{\perp}}}$${)}^{2}$${]}^{1/3}$) does not remain temperature independent throughout the temperature range, as has been predicted for uniaxially anisotropic low-dimensional systems in the paramagnetic phase, but shows a downward trend as T gets closer to ${\mathit{T}}_{\mathit{s}}$. The observed ESR characteristics, and hence the magnetic behavior, are discussed in connection to the well-established islandic nature of the intercalate layer for this class of transition-metal chloride GIC's and GBIC's. The role of intraisland magnetic short-range-order and of interisland frustrated magnetic interactions is stressed.