Surface antiferromagnetism and incipient metal-insulator transition in strained manganite films

Using first-principles calculations, we show that the (001) surface of the ferromagnet La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$ under an epitaxial compressive strain favors antiferromagnetic (AF) order in the surface layers, coexisting with ferromagnetic (FM) bulk order. Surface antiferromagnetism is accompanied by a very marked surface-related spectral pseudogap, signaling an incomplete metal-insulator transition at the surface. The different relaxation and rumpling of the MnO${}_2$ and LaO surface planes in the two competing magnetic phases cause distinct work-function changes, which are of potential diagnostic use. The AF phase is recognized as an extreme surface-assisted case of the combination of in-plane AF super-exchange and vertical FM double-exchange couplings that rules magnetism in manganites under in-plane compression.

Figure 1

Sketch of the simulation supercell (small red circles: O; large gray circles: La/Sr; intermediate blue circles: Mn). Left: top view (in-plane surface cell outlined). Center: side view (including vertical rumpling of surface layers). Right: 1f and 2f magnetic structures (surface is upper side).

Figure 2

Calculated energies vs strain for the FM (circles) and 1f (squares) structures. Lines are fourth-order polynomial fits; the energy zero is the minimum of the FM fit.

Figure 3

Structural parameters vs position (top surface: left; bottom surface: right). Top: Mn-apical O interlayer distance; center: octahedron rotation arouns $\widehat{s}$; bottom, Mn magnetic moments.

Figure 4

Local potentials and their periodicity-filtering running averages for FM (dashed) and 1f surfaces (solid) between surface (left) and slab center (right). Energy zero is the Fermi energy. The dash-dot line at the center is the difference $\delta$V${}_{\mathrm{ave}}$ of the 1f and FM averages (amplified by a factor 10).

Figure 5

Orbitally resolved DOS for Mn at the surface (top) and in the innermost layer (bottom) in the FM (left) and 1f (right) magnetic order. Majority (minority) spin is positive (negative). Both surfaces are metallic, but the 1f has a much smaller DOS. The bulk DOS is identical in the two cases, demonstrating that the AF surface does not affect bulk magnetism beyond about 1 nm depth.

Figure 6

A closeup view in the Fermi level region of the DOS projected on the spin-up surface Mn and the corresponding apical subsurface O for the 1f structure (see also Fig. 5, top right). The residual metallic character is due to the vertical, nonbonding overlap of Mn $d_{z^2}$ with O${}_a$ $p_z$.