Magnetism, conductivity, and orbital order in ${\left({\text{LaMnO}}_3\right)}_{2n}/{\left({\text{SrMnO}}_3\right)}_n$ superlattices

The modulation of charge density and spin order in ${\left({\text{LaMnO}}_3\right)}_{2n}/{\left({\text{SrMnO}}_3\right)}_n$ $\left(n=1–4\right)$ superlattices is studied via Monte Carlo simulations of the double-exchange model. $G$-type antiferromagnetic barriers in the ${\text{SrMnO}}_3$ regions with low charge density are found to separate ferromagnetic ${\text{LaMnO}}_3$ layers with high charge density. A metal-insulator transition with increasing $n$ is observed in the direction perpendicular to the interfaces. Our simulations provide insight into how disorder-induced localization may cause the metal-insulator transition occurring at $n=3$ in experiments.

Figure 1

(Color online) (a) Superlattice unit studied here. (b) On-site potential used in our simulation. (c) The $e_g$ charge density, NN spin correlation, and conductivity (see Ref. 17 for its units) of a $4\times 4\times 4$ lattice vs $\mu$ at $T=0.01$. All ${ϵ}_i$ are set to zero to simulate bulk clean-limit LSMO.

Figure 2

(Color online) (a) Charge modulation in the superlattices with different $V$’s (the case of L2S1 with $V=1.2$ cannot be obtained due to phase separation). The two horizontal lines denote 0.5 and 2/3. (b) In-plane spin structure factor for $V=0.9$. In (a) and (b), pink bars denote SrO layers in the ${\left(\text{LMO}\right)}_{2n}/{\left(\text{SMO}\right)}_n$ superlattice, while LaO layers are not highlighted. The lattices are simply repeated along the $z$ direction if their periods are shorter than 12.

Figure 3

(Color online) (a) In-plane conductivity for the superlattices studied here. (b) Perpendicular conductivity. (c) Sketch of the spin order at interfaces for L4S2 (left) and L6S3 (right). (d) Sketch of experimental setup for resistance measurements. Pink bars are SMO regions. Typical conducting paths via the DE process (black curves) connect NN interfaces, but they will be broken when $n\ge 3$.

Figure 4

(Color online) (a) Orbital occupation of the sixth to ninth layers for L6S3 when $V=0.9$ and $T=0.01$. The sixth and ninth are interface layers between the LaO and SrO layers, while the seventh and eighth are within the SMO region. Here the circle’s area is proportional to the local $e_g$ charge density. (b) Sketch of orbital ordering at the interface for $n=2$ and 3.