Metal-Insulator Transition and Its Relation to Magnetic Structure in $({\mathrm{LaMnO}}_3{)}_{2n}/({\mathrm{SrMnO}}_3{)}_n$ Superlattices

Superlattices of $({\mathrm{LaMnO}}_3{)}_{2n}/({\mathrm{SrMnO}}_3{)}_n$ ($1\le n\le 5$), composed of the gapped insulators ${\mathrm{LaMnO}}_3$ and ${\mathrm{SrMnO}}_3$, undergo a metal-insulator transition as a function of $n$, being metallic for $n\le 2$ and insulating for $n\ge 3$. Measurements of transport, magnetization, and polarized neutron reflectivity reveal that the ferromagnetism is relatively uniform in the metallic state, and is strongly modulated in the insulating state, being high in ${\mathrm{LaMnO}}_3$ and suppressed in ${\mathrm{SrMnO}}_3$. The modulation is consistent with a Mott transition driven by the proximity between the $({\mathrm{LaMnO}}_3)/({\mathrm{SrMnO}}_3)$ interfaces. The insulating state for $n\ge 3$ obeys variable range hopping at low temperatures. We suggest that this is due to states at the Fermi level that emerge at the $({\mathrm{LaMnO}}_3)/({\mathrm{SrMnO}}_3)$ interfaces and are localized by disorder.

Figure 1

(a) X-ray reflectivity and diffraction for $n=1$ to 5. Simulations of the first superlattice peak for $n=1$ (inset) are shown for roughnesses of 2.3 Å, 3 Å, and 4 Å. (b) STEM $z$-contrast images from an $n=4$ superlattice showing $(\mathrm{LMO}{)}_8/(\mathrm{SMO}{)}_4$ regions, labeled L and S. Rows of atoms are shown on the right as a guide to the eye.

Figure 2

Resistivity of ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ random-alloy film, and corresponding $({\mathrm{SrMnO}}_3{)}_n/({\mathrm{LaMnO}}_3{)}_{2n}$ superlattices, $1\le n\le 5$. The inset shows the resistivity of a pure ${\mathrm{LaMnO}}_3$ thin film with a fit of $\rho ={\rho }_0\exp (E_A/kT)$ with $E_A=125\mathrm{meV}$ to the data, and also the resistivity of a $({\mathrm{SrMnO}}_3{)}_3/({\mathrm{LaMnO}}_3{)}_1$ superlattice for reference.

Figure 3

(a) Average magnetization vs $H$ for $n=1$ to 5 at $T=10\mathrm{K}$. (b) Evolution of the saturation magnetization and coercivity at 10 K with increasing $n$.

Figure 4

(a) Polarized neutron reflectivity measurements of an $n=5$ superlattice at 300 K and (inset) the nuclear scattering potential of the superlattice, with zero magnetization. The gray region is the extent of LMO in one superlattice period, and the green is the SMO. (b) PNR at 10 K in a field of 5.5 kOe ($T_C=180\mathrm{K}$). The inset shows the inferred magnetic structure from the best fit. (c) PNR measurements for the $n=3$ sample, and (inset) the inferred magnetic structure from a calculation that best reproduced the data.

Figure 5

(a) Fit of the low temperature data for the $n=3–5$ superlattices to the form for Mott’s variable range hopping in 3D: $\rho ={\rho }_0\exp (T_0/T{)}^{1/4}$. (b) MR at 10 K. The insulating superlattices ($n\ge 3$) show a higher value of magnetoresistance ($>44.5\%$ at 88 kOe) than the metallic samples ($n\le 2$) ($<6.3\%$ at 88 kOe). The $n=5$ superlattice shows a positive MR at low fields, with a maximum value at 4 kOe.