Tuning the magnetic properties of $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ thin films by epitaxial strain

Ferromagnetic order can be induced in $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ (LCO) thin films by epitaxial strain. Here, we show that the magnetic properties can be “tuned” by epitaxial strain imposed on LCO thin films by the epitaxial growth on various substrate materials, i.e., (001) oriented $\mathrm{Sr}\mathrm{La}\mathrm{Al}{\mathrm{O}}_4$, $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$, $\mathrm{Sr}\mathrm{La}\mathrm{Ga}{\mathrm{O}}_4$, ${\left(\mathrm{La}\mathrm{Al}{\mathrm{O}}_3\right)}_{0.3}{\left({\mathrm{Sr}}_2\mathrm{Al}\mathrm{Ta}{\mathrm{O}}_6\right)}_{0.7}$, and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$. The lattice mismatch at room temperature of the in-plane lattice parameters between the substrate, $a_s$, and bulk LCO, $a_b$, ranges from $-1.31\%$ to $+2.63\%$. Single-phase, ⟨001⟩ oriented LCO thin films were grown by pulsed laser deposition on all these substrates. Due to the difference of the thermal-expansion coefficients between LCO and the substrates, the films experience an additional tensile strain of about $+0.3\%$ during the cooling process after the deposition at $T_s=650°\mathrm{C}$. The film lattice parameters display an elastic behavior, i.e., an increase of the in-plane film lattice parameter with increasing $a_s$. From the ratio between the out-of-plane and in-plane strain, we obtain a Poisson ratio of $\nu \approx 1∕3$. All films show a ferromagnetic transition as determined from magnetization measurements. The magnetization increases strongly with increasing tensile strain, whereas the transition temperature $T_C$ after a rapid initial rise appears to saturate at $T_C\approx 85\mathrm{K}$ above $a=3.86\mathrm{\AA }$. The effective magnetic moment ${\mu }_{\mathrm{eff}}$ in the paramagnetic state increases almost linearly as a function of the mean lattice parameter $⟨a⟩$, indicating an enhanced population of higher spin states, i.e., intermediate- or high-spin states. The experimental results are discussed in terms of a decrease of the octahedral-site rotation with increasing tensile strain.

Figure 1

Lattice parameters $a$ (triangles) and $c$ (inverted triangles) and the cube root of the unit-cell volume $V^{1∕3}$ (circles) of the LCO films as a function of the lattice parameter $a_s$ of the corresponding substrate material at room temperature. The dashed lines are guides to the eyes.

Figure 2

Out-of-plane lattice strain ${\epsilon }_{zz}$ versus the in-plane lattice strain ${\epsilon }_{xx}$. The dashed line is a linear fit to the data points and reflects ${\epsilon }_{zz}\approx -{\epsilon }_{xx}$ which results in a Poisson ratio of $\nu =1∕3$.

Figure 3

Field-cooled magnetization of LCO films on various substrate materials. The magnetic field strength of ${\mu }_{0}H=20\mathrm{mT}$ was applied parallel to the film surface.

Figure 4

The inverse of the susceptibility $H∕M$ versus $T$ for LCO films on various substrates. Data are offset vertically for clarity.

Figure 5

In-plane and out-of-plane magnetization reversal loops normalized to the saturated magnetization $M_s$ for LCO films on LSAT (top) and LAO (bottom) at $T∕T_C\approx 0.8$. For the in-plane (out-of-plane) magnetization, the field was applied parallel (perpendicular) to the film surface.

Figure 6

The Curie temperature $T_C$ as a function of the in-plane film lattice parameter $a$. The data points are labeled with the corresponding substrates. The data point for bulk LCO is denoted by the circle. Dashed line is a guide to the eyes.

Figure 7

(Color online) The mean Co-O-Co bond angle $⟨\beta ⟩$ calculated from the mean lattice parameter $⟨a⟩=(2∕3a+1∕3c)=2d\cos [(\pi -⟨\beta ⟩)∕2]$ as a function of $⟨a⟩$. The inset shows ${(\pi -⟨\beta ⟩)}^2$ versus $⟨a⟩$. The solid line is a linear fit to the data points and demonstrates the validity of the first order Taylor series approximation. The sketch above the diagram shows the untilting of the oxygen octahedra with increasing $\beta$ if the Co-O bond length $d$ is kept constant. Note that with increasing $\beta$, the unit-cell volume increases.

Figure 8

The effective paramagnetic moment ${\mu }_{\mathrm{eff}}$ of LCO films as a function of the mean lattice parameter $⟨a⟩=(2∕3a+1∕3c)$ (closed symbols). The data points are labeled with the names of the corresponding substrates. The bulk value of LCO is displayed by the open symbol. The dashed line is a linear fit to the data points.