Interplay of structural and magnetic nanoscale phase separation in layered cobaltites

We report on the structural, electronic, and magnetic phases of a previously unexplored region in the phase diagram of $\mathrm{GdBaC}{\mathrm{o}}_2{{\mathrm{O}}_5}_{+\delta }$ ($\delta =0.57$ and 0.63). Despite a homogenous average structure displayed by both the samples, the orthorhombic highly oxygenated $\mathrm{GdBaC}{\mathrm{o}}_2{\mathrm{O}}_{5.63}$ shows clear signatures of structural nanoscale phase separation. By combining a pair distribution function with photoluminescence and electron spin resonance techniques, we found that the nanoscale phase separation is induced by an inhomogeneous distribution of ferromagnetic $\mathrm{C}{\mathrm{o}}^{3+}-\mathrm{C}{\mathrm{o}}^{4+}$ clusters embedded in an antiferromagnetic $\mathrm{C}{\mathrm{o}}^{3+}$-rich matrix. In addition, we uncovered a phase evolution involving the collapse of the orthorhombic strain below room temperature. The origin of this noncanonical transition seems to be associated with the interplay of the observed nanoscale phase separation and a new magnetic phase transition occurring below $T\sim 180\mathrm{K}$.

Figure 1

(a) Temperature evolution of (040), (200) diffraction peaks for the $\delta =0.63$ sample. (b) Magnified view of patterns collected at $T=300\mathrm{K}$ (empty circles) and $T=150\mathrm{K}$ (full circles). (c) $\eta (\%)$ as a function of temperature. Inset to (c) reports the temperature dependence of photoluminescence intensity ratio. Full circles and diamonds in panels are data related to $\delta =0.54, \delta =0.63$ samples.

Figure 2

Temperature dependence of photoluminescence spectra for samples (a) $\delta =0.54$ and (b) $\delta =0.63$. The spectra are normalized to the maximum intensity of the peak at 1.77 eV for an easier comparison. Star marks the spurious peak due to the Nd:YAG laser pump at 1.53 eV.

Figure 3

(a) Example of box-car refinement. Vertical dashed lines indicate the boundaries between selected $r$ ranges. (b) PDF refined orthorhombic strain $\eta (\%)$ as a function of centroid mean values of selected $r$ range $\left(r_{\mathrm{mean}}\right)$. Dashed lines are the values of $\eta$ obtained by the Rietveld method and the solid line is the fit to the envelope function. (c) Pictorial view of the proposed $\delta =0.63$ structure containing ${\mathrm{Co}}_{\mathrm{Co}}^{•}$- rich regions (full circles) embedded in a LS/IS $\mathrm{C}{\mathrm{o}}^{3+}$-rich matrix (dotted area). (d) $r$ dependence of $U_{\mathrm{m}}$ parameter as defined in the text. In panels (b) and (d), circles and diamonds are data related to $\delta =0.54$ and $\delta =0.63$ samples, respectively.

Figure 4

Temperature evolution of ESR spectra for samples (a) $\delta =0.54$ and (b) $\delta =0.63$. Inset to (b) shows a magnified view of the low-field part of the spectra. Circular markers highlight the emergence of FMR below room temperature.

Figure 5

Temperature evolution of ESR spectra at low temperature for samples (a) $\delta =0.54$ and (b) $\delta =0.63$.

Figure 6

(a) Temperature evolution of $\mathrm{\Delta }{H}_{\mathrm{pp}}$ for $\delta =0.54$ (circles), $\delta =0.63$ (diamonds). Inset to (a) shows the log-log plot of $\mathrm{\Delta }{H}_{\mathrm{pp}}$ for $\delta =0.54$. (b) Magnetic susceptibility of $\delta =0.54$ (circles), $\delta =0.63$ (diamonds) samples, as a function of temperature collected at applied field $H=20\mathrm{Oe}$. Inset to (b) shows the crystal structure of GBCO with the oxygen site $(0,0,1/2)$ (squares) fully unoccupied. The alternation of $\mathrm{ISC}{\mathrm{o}}^{3+}$ and $\mathrm{LSC}{\mathrm{o}}^{3+}$ located at $\mathrm{Co}{\mathrm{O}}_5$ and $\mathrm{Co}{\mathrm{O}}_6$ units is shown. Ba and Gd are omitted for clarity.

Figure 7

Phase diagram proposed for $\mathrm{GdBaC}{\mathrm{o}}_2{\mathrm{O}}_{5+\delta }$ in the $0.5\le \delta \le 0.77$ region. Empty symbols are data taken from Refs. [6, 11, 12]. Insets at $\delta =0.54, \delta =0.63$ show portions of powder diffraction patterns at $T=300\mathrm{K}$ and $T=150\mathrm{K}$. For $\delta =0.63$ the change of the spectrum due to the dramatic decrease of the orthorhombic strain going from the PM to AFM2 phases on cooling is clearly shown. Solid line indicates the boundary of MIT.