Ferromagnetic order in epitaxially strained $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ thin films

$\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ films grown epitaxially on ⟨001⟩ oriented ${\left(\mathrm{La}\mathrm{Al}{\mathrm{O}}_3\right)}_{0.3}{\left({\mathrm{Sr}}_2\mathrm{Al}\mathrm{Ta}{\mathrm{O}}_6\right)}_{0.7}$ substrates by pulsed laser deposition exhibit ferromagnetic ordering below a critical temperature, $T_c$, of $85\mathrm{K}$. Polycrystalline films of $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ prepared in the same way did not show ferromagnetic order down to $T\approx 5\mathrm{K}$, and their temperature dependent susceptibility was identical to that of bulk $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$. The ferromagnetism in epitaxial films is not simply a property of the surface region, rather it extends over the complete film thickness, as shown by the linear increase of the saturated magnetic moment with increasing film thickness. We discuss this surprising result in terms of epitaxial tensile strain via the properly chosen substrate inducing ferromagnetic order.

Figure 1

(a) $\theta ∕2\theta$ scan of a polycrystalline LCO film deposited on a (001) oriented LSAT substrate precovered with a $20\mathrm{nm}$ thick polycrystalline $\mathrm{Ce}{\mathrm{O}}_2$ film. The reflections from LSAT and $\mathrm{Ce}{\mathrm{O}}_2$ are marked by stars and crosses, respectively. The remaining reflections can all be attributed to LCO. (b) $\theta ∕2\theta$ scan of an epitaxial LCO film deposited directly on LSAT.

Figure 2

Reciprocal space map of the 113 LCO film and LSAT substrate reflection. The contour plot is on a logarithmic scale and shows the scattered intensity as a function of the scattering vector $q$. Its components are expressed in noninteger Miller indices $h$ and $l$ of the LSAT substrate reflection, referring to the azimuth reference [110] and the surface normal [001]. The difference between neighboring contour lines is $\Delta \log =0.5$.

Figure 3

Energy dispersive x-ray analysis of an epitaxial LCO film. The x-ray fluorescence intensity coming from the ${\mathrm{La}}_L$ and ${\mathrm{Co}}_K$ shell and the intensity ratio of both is plotted versus the position in the film plane.

Figure 4

Field cooled magnetization of polycrystalline (엯) and epitaxial (●) $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ films $(d=200\mathrm{nm})$ measured in an applied field ${\mu }_{0}H=20\mathrm{mT}$. The inset shows the magnetic moment of epitaxial $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ at $T=5\mathrm{K}$ as a function of the applied magnetic field.

Figure 5

(a) The magnetic moment at $T=5\mathrm{K}$ as a function of the applied magnetic field (virgin curves) for epitaxial films with different film thickness. The remaining unsaturated moment above $2\mathrm{T}$ was subtracted from the measurements. (b) The saturated ferromagnetic moment, $M_{\mathrm{sat}}$, as a function of the film thickness of epitaxial $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ films. $M_{\mathrm{sat}}$ was obtained from the measurements in Fig. 3a at ${\mu }_{0}H=5\mathrm{T}$. The line is a guide to the eye.