Phase Separation in $A$-Site-Ordered Perovskite Manganite ${\mathrm{LaBaMn}}_2{\mathrm{O}}_6$ Probed by ${}^{139}\mathrm{La}$ and ${}^{55}\mathrm{Mn}$ NMR

${}^{139}\mathrm{La}$- and ${}^{55}\mathrm{Mn}$-NMR spectra demonstrate that the ground state of the $A$-site-ordered perovskite manganite ${\mathrm{LaBaMn}}_2{\mathrm{O}}_6$ is a spatial mixture of the ferromagnetic and antiferromagnetic regions, which are assigned to the metallic and the insulating charge ordered state, respectively. This exotic coexisting state appears below 200 K via a first-order-like formation of the antiferromagnetic charge ordered state inside the ferromagnetic metal one. The Mn spin-spin relaxation rate indicates that the ferromagnetic region coexisting with the antiferromagnetic one in ${\mathrm{LaBaMn}}_2{\mathrm{O}}_6$ is identical to the bulk ferromagnetic metal phase of the disordered form ${\mathrm{La}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{MnO}}_3$ in spite of the absence of $A$-site disorder. This suggests a mesoscopic rather than nanoscopic nature of the ferromagnetic region in ${\mathrm{LaBaMn}}_2{\mathrm{O}}_6$.

Figure 1

Field-swept ${}^{139}\mathrm{La}$-NMR spectra of ${\mathrm{LaBaMn}}_2{\mathrm{O}}_6$ at 1.6 K with (a) $f_1=6.016\mathrm{MHz}$ and (b) $f_2=37.600\mathrm{MHz}$.

Figure 2

${}^{55}\mathrm{Mn}$-NMR spectra of (a) ${\mathrm{LaBaMn}}_2{\mathrm{O}}_6$ under $H=0$, 0.7, and 1.35 T and of (b) ${\mathrm{La}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{MnO}}_3$ in zero field at 4.2 K.

Figure 3

${}^{55}\mathrm{Mn}$-NMR spectra of ${\mathrm{LaBaMn}}_2{\mathrm{O}}_6$ in zero field at several temperatures.

Figure 4

Temperature dependences of (a) $H_{\mathrm{int}}^{\mathrm{F}}$ (solid circles) and $H_{\mathrm{int}}^{\mathrm{AF}}$ (open squares), (b) $I_{\mathrm{B}}/(H_{\mathrm{int}}^{\mathrm{F}}{)}^2$ normalized by the value at 220 K, and (c) $1/T_2$’s measured at the peak from the FM region of ${\mathrm{LaBaMn}}_2{\mathrm{O}}_6$ (solid circles) and that of ${\mathrm{La}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{MnO}}_3$ (open circles).