Charge-ordered state in half-doped Bi-based manganites studied by ${}^{17}\mathrm{O}$ and ${}^{209}\mathrm{Bi}$ NMR

We present a ${}^{209}\mathrm{Bi}$ and ${}^{17}\mathrm{O}$ NMR study of the Mn electron spin correlations developed in the charge ordered state of ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$ and ${\mathrm{Bi}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$. The unusually large local magnetic field ${}^{209}H_{\mathit{loc}}$ indicates the dominant $6s^2$ character of the lone electron pair of ${\mathrm{Bi}}^{3+}$ ions in both compounds, probably, responsible for high charge ordering temperature $T_{\mathit{CO}}$. The observed difference in ${}^{209}H_{\mathit{loc}}$ for ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$ to ${\mathrm{Bi}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ is suggested to be due to a decrease in the canting of the staggered magnetic moments of ${\mathrm{Mn}}^{3+}$ ions. The modification of the ${}^{17}\mathrm{O}$ spectra below $T_{\mathit{CO}}$ demonstrates that the line due to the apical oxygen is a unique local tool to study the development of the Mn spin correlations. In the AF state the analysis of the ${}^{17}\mathrm{O}$ spectrum of ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$ points toward a bond-centered model of the CO state in this material.

Figure 1

${\mathrm{CE}}^{\prime }$-type magnetic structure of ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$ (Ref. 10) represented in the frame of a pure ionic model (site-centered order). The 3D structure is represented in (a). The ferromagnetically correlated zigzag of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ ions in the $ab$ planes is shown by a thick broken line. The environment of the Bi cation in the $ac$ plane (b) and in the $bc$ plane (c) is represented with the $\mathrm{mn}\text{−}{t}_{2g}$ orbitals orientation and the spin direction of the four nearest Mn ions, ${\mathrm{Mn}}^{+4}$ and ${\mathrm{Mn}}^{3+}$, respectively. The (O1–O5) sites of oxygen are specified in accordance with the notation in Ref. 12 (see the text). In contrast to the ${\mathrm{CE}}^{\prime }$-type magnetic structure, in the CE structure the magnetic moments of Mn ions $\mu \left(\mathrm{Mn}\right)$ are aligned in the $ab$ plane and ${\mu }_c\left(\mathrm{Mn}\right)=0$.

Figure 2

Zero-field ${}^{209}\mathrm{Bi}$ NMR spectra in ${\mathrm{Bi}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ and ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$ measured by the spin-echo technique at 4.2 K. The echo intensity ${}^{209}\mathrm{Int}$ (solid squares) is plotted versus the frequency $\nu$. The set of narrow peaks shows the calculated transitions with the parameters given in Table . The grey curve is the calculated broadened spectrum, assuming a Gaussian distribution for both the local field ${}^{209}H_{\mathit{loc}}$ and the quadrupole frequency ${\nu }_Q$ (see the text for details).

Figure 3

Zero-field ${}^{17}\mathrm{O}$ NMR spectra measured at 4.2 K in the antiferromagnetic charge-ordered state of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ (a), ${\mathrm{Bi}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ (b), and ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$ (c). The solid lines are a guide for the eye.

Figure 4

${}^{17}\mathrm{O}$ NMR spectra measured in the paramagnetic state of ${\mathrm{Bi}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ (a), ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$ (b), and ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ (c) (Ref. 3). The position of the Larmor frequency $(K=0)$ is shown by a dotted vertical line. The grey narrow stripe indicates $T_{\mathit{CO}}$—critical temperature of the charge ordering.

Figure 5

Evolution of the ${}^{17}\mathrm{O}$ spectrum pattern for ${\mathrm{Bi}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ showing the onset of the splitting into two lines when cooling below $T_{\mathit{CO}}$ from the charge-disordered to the charge-ordered paramagnetic phase. The solid line shows the echo intensity versus frequency measured with the time delay $t=12\mu \mathrm{s}$ between the rf-exciting pulses; open squares indicate the echo intensity measured with $t=100\mu \mathrm{s}$. Solid squares are the normalized high-frequency part of the spectra obtained by reconstructing at $t=0$ the difference between the echo intensities measured with 12 and $100\mu \mathrm{s}$. The open up-triangles are the ${}^{17}T_2^{-1}$ data measured between 53 and 59 MHz at $T=170\mathrm{K}$.

Figure 6

Thermal behavior of ${}^{17}\mathrm{O}$ spin-spin relaxation rate, ${}^{17}T_2^{-1}$, in the charge-ordered, paramagnetic, and antiferromagnetic phases for the two Bi-based manganites. ${}^{17}T_2^{-1}$ of the oxygen in the $ab$ plane (O4-site) measured with $H_{\mathit{ext}}=0-(▵)$ and of the apical oxygen (▪) measured with $H_{\mathit{ext}}=50\mathrm{kOe}$ in ${\mathrm{Bi}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$. For ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3,{}^{17}T_2^{-1}$ of the apical oxygen $(H_{\mathit{ext}}=120\mathrm{kOe})$ is shown by open squares.

Figure 7

${}^{17}\mathrm{O}$ NMR spectrum in the antiferromagnetic charge-ordered phase of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ (a) and ${\mathrm{Bi}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ (b). The spectra were obtained at the field-cooled samples.