Spin-Orbital Superstructure in Strained Ferrimagnetic Perovskite Cobalt Oxide

We have investigated the Co-$3d$ spin-orbital state in a thin film of perovskite ${\mathrm{LaCoO}}_3$ to clarify the origin of strain induced spontaneous magnetization ($T_C=94\mathrm{K}$) by means of x-ray diffraction, optical spectroscopy, and magnetization measurements. A lattice distortion with the propagation vector ($1/4$ $-\text{}\text{}1/4$ $1/4$) and an anomalous activation of optical phonons coupled to Co-$3d$ orbital are observed below 126 K. Combined with the azimuthal angle analysis of superlattice reflection, we propose that the ordering of Co-$3d$ orbital promoted by an epitaxial strain produces a unique ferrimagnetic structure.

Figure 1

(a) Schematic view of the spin states of trivalent Co ion (${\mathrm{Co}}^{3+}$); the low spin-, intermediate spin-, and high spin states. The arrows denote the electron spins. (b) Electronic phase diagrams of the tensile strained ${\mathrm{LaCoO}}_3$ and the bulk $R{\mathrm{CoO}}_3$ ($R$ is rare earth element). The circle indicates the typical spin state crossover temperature from nonmagnetic to paramagnetic state as determined by the linear thermal expansion [6]. The square and triangle denote the transition temperature of orbital ordering (OO) and that of the ferrimagnetic (FerriM) ordering, respectively. The inset shows the schematic view of the tensile strained ${\mathrm{LaCoO}}_3$ film grown on LSAT(110) substrate. The lattice constant ($a$) is defined as the cubic root of the unit cell volume in the pseudocubic setting. (c) Temperature dependence of in-plane magnetization. The inset shows the in-plane magnetization ($M$) curve at 10 K.

Figure 2

Temperature dependence of (a) inverse susceptibility (${\chi }^{-1}$), (b) $d_{110}$ (green), $\beta$ (blue), and $\gamma$ (red), (c) integrated intensity of superlattice reflection measured at 7.73 keV, (d) oscillator strength (square) and damping constant (circle) of the Co-O stretching optical phonon. (e) Profiles of superlattice reflection at ($5/4$, $3/4$, $1/4$). Here the reciprocal lattice units are represented in the basis of the LSAT substrate. r.l.u. means the reciprocal lattice unit. (f) Schematic view of the lattice distortion projected on the (001) plane. The dotted line indicates the unit cell of monoclinic phase with $I2/a$ symmetry (see Supplemental Material [20]).

Figure 3

(a) Energy spectra of the scattered intensity in $\sigma \to {\pi }^{\prime }$ geometry. Inset is the energy spectra of the intensity of ($5/4$ $3/4$ $1/4$) diffraction (without polarization analysis, circles) in comparison with the x-ray absorption spectra (XAS) at Co $K$ edge measured in the fluorescence yield method (triangles). (b) Azimuthal angle dependence of the resonant scattering intensity measured at 10 K ($<T_S$). The solid lines are the results of simulation of scattering intensity. (c) Energy spectra of imaginary part of the dielectric constant (${ϵ}_2$) around 0.067 eV. The peak structures marked by closed triangle, closed circle, open circle, and open triangle are assigned to the Co-O stretching mode at 0.065, 0.067, 0.070, and 0.072 eV, respectively.

Figure 4

The proposed models of the spin and orbital ordered structure: (a) High spin (HS)/low spin (LS)-state ordered state with the rocksalt type configuration. (b) Intermediate spin (IS)/HS-state ordered state. (c) HS/LS-state ordered state with $3/1$ ratio. (d) homogeneous IS state. The lobes denote the $e_g$ orbital of IS state and the active $t_{2g}$ orbital of HS state. The $t_{2g}$ orbital of IS state as well as the fully occupied, Hund’s-rule coupled $t_{2g}$ orbital is omitted for clarity. The dashed line indicates the unit cell of this spin-orbital superstructure.