Tuning between the metallic antiferromagnetic and ferromagnetic phases of ${\text{La}}_{1-x}{\text{Sr}}_x{\text{MnO}}_3$ near $x=0.5$ by digital synthesis

We investigated cation-ordered ${\text{La}}_{1-x}{\text{Sr}}_x{\text{MnO}}_3$ about the half-doping level $x\sim 0.5$ in superlattices of alternating, single-unit-cell layers of ${\text{LaMnO}}_3$ and ${\text{SrMnO}}_3$. The effect of La/Sr cation order was addressed by comparing the structural, magnetic and transport properties of these superlattices with random-alloy films of equivalent composition. The samples were synthesized by ozone-assisted molecular-beam epitaxy onto ${\text{SrTiO}}_3$ substrates. The superlattices could be tuned between ferromagnetic and antiferromagnetic metallic states by inserting extra single-unit-cell layers of ${\text{LaMnO}}_3$ and ${\text{SrMnO}}_3$, respectively. For $x<0.5$, a ferromagnetic, metallic phase was observed. For $x=0.50$ and 0.55, $A$-type antiferromagnetic order was confirmed by neutron diffraction, with a Néel temperature of 300 K, significantly higher than bulk values. The enhanced Néel temperature was attributed to lattice strain rather than cation order.

Figure 1

(Color online) Schematic of a digital SL comprised of alternating, single-unit-cell layers of ${\text{LaMnO}}_3$ and ${\text{SrMnO}}_3$, maintaining an epitaxial, perovskite crystalline structure (a). The octahedrons represent the sixfold coordination of oxygen atoms around each centrally located Mn atom. Pink and green spheres represent the La and Sr atoms, respectively. (b) Layer stacking sequence of the SLs, showing a single supercell that is repeated the indicated number of times. The arrows denote the $A$-type AF spin structure.

Figure 2

(Color online) RHEED pattern of the STO substrate surface before film deposition (a) and of the ${\left[{\left({\text{SMO}}_1/{\text{LMO}}_1\right)}_4{\text{LMO}}_1\right]}_9$ superlattice surface at the end of deposition (b), with the incident electron beam parallel to the [100] axis. Oscillations of the specular peak intensity (c) for the deposition of five supercells: pink and yellow regions mark the shutter-open times for LMO and SMO unit-cell layers, respectively, separated by 30 s anneal times.

Figure 3

(Color online) X-ray reflectivity of digitally synthesized SMO/LMO superlattices, with $x=0.50$ (alternating SMO and LMO layers), $x=0.55$ (inserting an additional SMO) and $x=0.44$ (inserting an additional LMO). The black lines for each spectrum are the fit calculated using Parratt’s dynamical formalism. The spectra are offset for clarity.

Figure 4

(Color online) XRD (002) peaks of the SLs, showing a smaller, average $c$-axis parameter as the Sr content increases. The smaller peaks at higher and lower $2\theta$ are thickness fringes. The inset shows the average $c$-axis as a function of Sr content, for both the SLs and alloys.

Figure 5

(Color online) (a) $M\left(T\right)$ of the SLs, zero-field cooled (solid symbols) and cooled in 200 Oe field (open symbols). (b)–(d)$M\left(T\right)$ of the SLs compared with alloy films of equivalent composition. $M\left(T\right)$ was measured while increasing $T$ in 200 Oe field.

Figure 6

(Color online) (a) The structurally forbidden $\left(00\frac{1}{2}\right)$ neutron-diffraction peak (circles) of the $x=0.55\text{SL}$, between structural, x-ray peaks (black line, from Fig. 3), confirming $A$-type AF order. (b) Temperature dependence of the $\left(00\frac{1}{2}\right)$ peak intensity for the $x=0.55\text{SL}$ and random alloy, both revealing $T_N$ significantly greater than that of bulk. (c) Neutron diffraction $\left(00\frac{1}{2}\right)$ peak at the indicated temperatures for the $x=0.50$ SL. The inset is a schematic of the spin order on the ${\text{MnO}}_2$ planes that produces the $\left(00\frac{1}{2}\right)$ peak. (d) Temperature dependence of the peak intensity for the $x=0.50\text{SL}$ and random alloy, both showing $T_N\sim 300\text{K}$.

Figure 7

(Color online) (a) $\rho \left(T\right)$ of the SLs, measured in zero magnetic field (solid symbols) and in 7 T field (open symbols). (b)–(d)$\rho \left(T\right)$ of the SLs compared with the alloy films of equivalent composition.