Magnetic correlations in subsystems of the misfit ${\left[{\mathrm{Ca}}_2\mathrm{Co}{\mathrm{O}}_3\right]}_{0.62}\left[\mathrm{Co}{\mathrm{O}}_2\right]$ cobaltate

${\left[{\mathrm{Ca}}_2{\mathrm{CoO}}_3\right]}_{0.62}\left[{\mathrm{CoO}}_2\right]$, a two dimensional misfit metallic compound, is famous for its rich phases accessed by temperature, i.e., high temperature spin-state transition, metal-insulator transition (MIT) at intermediate temperature ($\sim 100\mathrm{K}$), and low temperature spin density wave (SDW). It enters into a SDW phase below $T_{\text{MIT}}$ which becomes long range at 27 K. Information on the independent role of misfit layers (rocksalt/${\mathrm{Ca}}_2{\mathrm{CoO}}_3$ and triangular/${\mathrm{CoO}}_2$) in these phases is scarce. By combining a set of complementary macroscopic (DC magnetization and resistivity) and microscopic (neutron diffraction and x-ray absorption fine structure spectroscopy) measurements on pure (CCO) and Tb substituted in the rocksalt layer of CCO (CCO1), magnetic correlations in both subsystems of this misfit compound are unraveled. CCO is found to exhibit glassiness, as well as exchange bias (EB) effects, while CCO1 does not exhibit glassiness, albeit it shows weaker EB effect. By combining local structure investigations from extended x-ray absorption fine structure (EXAFS) spectroscopy and neutron diffraction results on CCO, we confirm that the SDW arises in the ${\mathrm{CoO}}_2$ layer. Our results show that the magnetocrystalline anisotropy associated with the rocksalt layer acts as a source of pinning, which is responsible for EB effect. Ferromagnetic clusters in the ${\mathrm{Ca}}_2{\mathrm{CoO}}_3$ layer affects the SDW in ${\mathrm{CoO}}_2$ and ultimately glassiness arises.

Figure 1

Field cooled susceptibility as a function of temperature, measured at different magnetic fields (a) for CCO and (c) for CCO1. Insets show the 3D plot ($\chi \text{−}H\text{−}T$) showing the bifurcation between FC and ZFC. ZFC relaxation curves measured at 5 and 30 K under the same magnetic field (50 Oe), (b) for CCO and (d) for CCO1. (e) Dependence of $T_{\text{Irr}}$ on the magnetic field for CCO. Blue solid line shows the fitting to data (see text).

Figure 2

For CCO, (a) $M\left(H\right)$ isotherm loops measured at 5 K in ZFC and FC ($+70$ and $-70$ kOe), showing the hysteresis and EB (in field cooled cases). (b) $M\left(H\right)$ loop at 5 K measured after cooling under various fields. Inset shows the successive shifting of loop on the field axis. (c) $M\left(H\right)$ loops measured at different temperatures in $+70$ kOe field cooled condition. (d) Temperature dependence of $H_{\text{EB}}$ and $H_c$. (e) Cooling field dependence of $H_{\text{EB}}$ and $H_c$.

Figure 3

For CCO1, (a) $M\left(H\right)$ isotherm loop measured at 5 K in ZFC and FC ($+70$ and $-70$ kOe), showing the hysteresis (see inset) and EB (in field cooled cases). (b) $M\left(H\right)$ loop at 5 K measured after cooling under various fields. Inset shows the successive shifting of loop on the field axis. (c) $M\left(H\right)$ loops measured at different temperatures in $+70$ kOe field cooled condition. (d) Temperature dependence of $H_{\text{EB}}$ and $H_c$. (e) Cooling field dependence of $H_{\text{EB}}$ and $H_c$.

Figure 4

(a)–(i) Comparison between ${\chi }_{\text{FC}}$ (black symbols), ${\chi }_{\text{ZFC}}$ (filled symbols), and calculated ${\chi }_{\text{FC}}^{\prime }$ (empty circles) for the CCO (see text).

Figure 5

Schematic showing the AFM SDW in the ${\mathrm{CoO}}_2$ layer, having sinusoidal behavior (red arrows), resulting in a linear $M\left(H\right)$ behavior. In the rocksalt layer the green clusters show short range ferromagnetic arrangement of spins with strong spin-orbit coupling (SOC), which restricts the magnetism in the crystal $c$ axis, resulting in a hysteretic $M\left(H\right)$ loop. The resultant of these two gives rise to overall ferrimagnetism (orange arrows), resulting in a nonsaturating hysteretic $M\left(H\right)$. The layer of Ca in between these two magnetic layers controls the coupling.

Figure 6

Neutron diffraction pattern of BANK 3 of the GEM diffractometer, measured at 17 K, profile fitted using three phases (see text). Magnetic peaks are indicated by the vertical arrows. Inset show the temperature dependence of magnetic peak intensities, which become diffusive below $\sim 16$ K.

Figure 7

Temperature dependence of selected reflections from neutron diffraction patterns of BANK 3 (a) and BANK 2 (b). At low temperature the broadness of magnetic peaks shows the short or glassy magnetic correlations, shown in the yellow block.

Figure 8

(a) MSRD of Co-Co pair in ${\mathrm{CoO}}_2$ layer, i.e., ${\sigma }_{\text{Co-Co}}^2$, showing an anomaly at the susceptibility upturn temperature. Black solid lines are guides to the eye. (b) Resistivity as a function of temperature for CCO and CCO1, showing a shift in $T_{\text{MIT}}$ with Tb doping and magnitude change hints towards the change in SDW gap.