Semiconducting ferromagnetic states in ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_{1+x}\mathrm{Co}{\mathrm{O}}_4$

Electrical, magnetic, and structural properties have been investigated for single-layered (${\mathrm{K}}_2\mathrm{Ni}{\mathrm{F}}_4$-type structure) compounds of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_{1+x}\mathrm{Co}{\mathrm{O}}_4$ $(0⩽x⩽0.5)$ as the hole-doped system derived from the Mott insulator $(x=0)$. The doping variations of the collective Jahn-Teller distortion and the effective magnetic moment indicate that the doped holes mostly enter the $t_{2g}$ orbital states with keeping the intermediate spin configuration. Anomalous Hall effect and intergrain tunneling magnetoresistance are observed as hallmarks of (semi-)conducting ferromagnets. Detailed comparison of temperature dependence of high- and low-field magnetizations suggests the presence of ferromagnetic cluster state (or Griffiths-type phase) as a consequence of the competition between the ferromagnetic and antiferromagnetic interaction under random potential.

Figure 1

Temperature dependence of (a) magnetization $M$ and (b) resistivity $\rho$ for polycrystalline samples of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_{1+x}\mathrm{Co}{\mathrm{O}}_4$.

Figure 2

(a) Typical powder x-ray diffraction profile for ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_{1+x}\mathrm{Co}{\mathrm{O}}_4$ $(x=0.3)$. Dots and a solid line represent the observed and calculated profiles, respectively. The difference plot is drawn below the profile, and vertical bars represent the allowed reflections. Variation of (b) the lattice parameters $a$ and $c$; (c) the ratio of the out-of-plane Co–O bond length $d_{\mathit{Co}–\mathit{O}\left(\mathit{ap}\right)}$ to the in-plane Co–O bond length $d_{\mathit{Co}-\mathrm{O}\left(\mathit{eq}\right)}$, and (d) the bond valence sum (BVS) with doping $x$.

Figure 3

Magnetic field $\left(H\right)$ dependence of (a) magnetization $M$ and (b) normalized magnetoresistivity $\rho \left(H\right)∕\rho (H=0)$ for ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}\mathrm{Co}{\mathrm{O}}_4$ $(x=0.5)$.

Figure 4

${\rho }_H∕H$ vs $M∕H$ plot for ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}\mathrm{Co}{\mathrm{O}}_4$. The solid lines represent the result of least-square fitting with the relation that ${\rho }_H∕H=R_o+R_s(M∕H)$. Intersection represents the ordinary Hall coefficient $R_o$. The inset shows temperature dependence of anomalous Hall coefficient $R_s$.

Figure 5

The temperature dependence of inverse magnetization $H∕M$ at $H=100\mathrm{Oe}$ (open circles) and $H=5000\mathrm{Oe}$ (closed circles) for ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_{1+x}\mathrm{Co}{\mathrm{O}}_4$. Typical spin-glass behavior observed in ac susceptibility at low temperatures below $10\mathrm{K}$ for $\mathrm{La}\mathrm{Sr}\mathrm{Co}{\mathrm{O}}_4$ $(x=0)$ as displayed in the inset.

Figure 6

(a) Schematic representation of density of states for up-spin and down-spin Co $t_{2g}$ and $e_g$ bands in conducting ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_{1+x}\mathrm{Co}{\mathrm{O}}_4$ (e.g., $x=0.5$). (b) Effective moment ${\mu }_{\mathit{eff}}$ per Co site for ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_{1+x}\mathrm{Co}{\mathrm{O}}_4$. Broken lines are calculated for the case that all doped holes go into $t_{2g}$ or $e_g$ state, respectively. (c) Magnetic phase diagram for ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_{1+x}\mathrm{Co}{\mathrm{O}}_4$. Filled circles, open circles, filled triangles, and open triangles represent the inset temperature for the low-field magnetization anomaly (as anticipated to be Griffiths temperature) $T_G$, Weiss temperature $\theta$, spin-glass transition temperature $T_{SG}$, and ferromagnetic Curie temperature $T_C$, respectively. Solid and dotted lines are merely to guide the eyes.