Spin-State Polarons in Lightly-Hole-Doped ${\mathrm{LaCoO}}_3$

Inelastic neutron scattering (INS), electron spin resonance (ESR), and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly-hole-doped ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$, $x\sim 0.002$. Both INS and ESR low temperature spectra show intense excitations with large effective $g$ factors $\sim 10–18$. NMR data indicate the creation of extended magnetic clusters. From the $Q$ dependence of the INS magnetic intensity, we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions. The present INS, ESR, and NMR data give evidence for two regimes in the lightly-hole-doped samples: (i) $T<35\mathrm{K}$ dominated by spin polarons; (ii) $T>35\mathrm{K}$ dominated by thermally activated magnetic ${\mathrm{Co}}^{3+}$ ions.

Figure 1

Temperature ($B=0$) and magnetic field ($T=1.5\mathrm{K}$) evolutions of the INS spectra of ${\mathrm{La}}_{0.998}{\mathrm{Sr}}_{0.002}{\mathrm{CoO}}_3$ measured on FOCUS (data from Ref. 6) and NEAT, respectively. Solid black squares correspond to data taken from nonmagnetic ${\mathrm{LaAlO}}_3$ at $T=50\mathrm{K}$, black lines refer to least-squares fits of Gaussian functions, and red lines are guides to the eye.

Figure 2

(a) Frequency vs magnetic field dependence (branches) of the ESR modes of the low-$T$ spectrum. Straight lines through data points are linear fits (see text). Open squares denote a small presumably impurity peak visible below $\sim 200\mathrm{GHz}$. (b) $T$ dependence of the ESR spectrum at 384 GHz. $A$ and $B$ label ESR modes due to thermally activated ${\mathrm{Co}}^{3+}$ HS state ions. (c),(d) Low-$T$ ${}^{59}\mathrm{Co}$ NMR spectra and $T$ dependences of the nuclear relaxation rate $T_1^{-1}$, respectively, for ${\mathrm{LaCoO}}_3$ and ${\mathrm{La}}_{0.998}{\mathrm{Sr}}_{0.002}{\mathrm{CoO}}_3$ single crystals (open and closed circles). Lines connecting the data points are guides for the eye.

Figure 3

(a) Excitation INS spectrum collected on FOCUS from ${\mathrm{La}}_{0.998}{\mathrm{Sr}}_{0.002}{\mathrm{CoO}}_3$ at $T=1.5\mathrm{K}$. (b) Circles: Experimental $Q$ dependence of the intensity of the peak observed at 0.75 meV. Lines: Calculated $Q$ dependence of the neutron cross section [Eq. (1)] for different Co multimers (visualized in the figure) in the cubic perovskite lattice of ${\mathrm{LaCoO}}_3$ and for $|S⟩\Rightarrow |S⟩$ transitions. The nearest-neighbor Co-Co distance was fixed at $R_{\mathrm{Co}\mathrm{\text{−}}\mathrm{Co}}=3.9\AA$ determined for ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$ from the Co-O pair density function [2].