Spin Freezing and Magnetic Inhomogeneities in Bilayer Manganites

We have performed a muon spin rotation study on polycrystalline samples of electron-doped layered manganites, ${\mathrm{L}\mathrm{a}}_{2-2x}{\mathrm{S}\mathrm{r}}_{1+2x}\mathrm{M}{\mathrm{n}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{7}}$ ($0.4\le x<1$), in order to investigate the local magnetic structure and spin dynamics. Our results provide evidence for phase separation into $A$-type antiferromagnetic and charge-ordered phases for $x=0.52$ and spin freezing at low temperatures ($T<100\mathrm{K}$) for $0.52\le x<0.75$. A new phase diagram which includes this spin-freezing region is proposed.

Figure 1

Muon spin relaxation data at $T=1.8\mathrm{K}$ for ${\mathrm{L}\mathrm{a}}_{2-2x}{\mathrm{S}\mathrm{r}}_{1+2x}\mathrm{M}{\mathrm{n}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{7}}$ with (a) $x=0.40$ (FM), (b) $x=0.52$ ($A$-type AFM), (c) $x=0.68$ (NLRO), (d) $x=0.82$ ($C/C^{*}$-type AFM), and (e) $x=0.98$ ($G$-type AFM). Two oscillation frequencies can be observed for $x=0.98$ and a very strongly damped oscillation for $x=0.68$. The solid lines are fits to the data using Eq. (1).

Figure 2

Temperature dependence of the zero-field muon precession frequency, ${\nu }_{\mu }$, and internal field, $B_{\mu }$, for (a) $x=0.52$ and (b) $x=0.98$. The inset shows the temperature dependence of the relaxing asymmetry, $A_{rlx}$ for the two compositions.

Figure 3

Temperature dependence of different fitting parameters for $x=0.68$; (a) the zero-field cooled (open circles) and field-cooled (closed circles) magnetization in 10 mT; (b) the static amplitude ($A_{osc}$ for $T<T_f$ and $A_s$ for $T>T_f$) and the dynamic amplitude ($A_{rlx}$ for $T<T_f$ and $A_f$ for $T>T_f$); inset shows ${\nu }_{\mu }(T)$ and ${\Delta }_{\mathrm{K}\mathrm{T}}/(2\pi )$; (c) longitudinal relaxation rate, $1/T_1$ (the inset shows the transverse relaxation rate, $1/T_2$, and slow component, ${\lambda }_s$). The freezing temperature $T_f$ (vertical dotted line) separates the static and the dynamic regions. The vertical dashed line shows $T_{\mathrm{C}\mathrm{O}}$ defined as the maximum in the magnetization data, in agreement with electron diffraction studies [15]. The solid lines are a guide to the eye.

Figure 4

(a) Phase diagram of ${\mathrm{L}\mathrm{a}}_{2-2x}{\mathrm{S}\mathrm{r}}_{1+2x}\mathrm{M}{\mathrm{n}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{7}}$ ($0.5<x<1$). The charge ordering temperatures were obtained from magnetization data [12]. Solid and dashed lines are guides to the eye. (b) Degree of magnetic inhomogeneity at 1.8 K expressed by $1/(2\pi {\nu }_{\mu }{T}_2)$.