Surface-Stabilized Nonferromagnetic Ordering of a Layered Ferromagnetic Manganite

An outstanding question regarding the probing or possible device applications of correlated electronic materials (CEMs) with layered structure is the extent to which their bulk and surface properties differ or not. The broken translational symmetry at the surface can lead to distinct functionality due to the charge, lattice, orbital, and spin coupling. Here we report on the case of bilayered manganites with hole doping levels corresponding to bulk ferromagnetic order. We find that, although the hole doping level is measured to be the same as in the bulk, the surface layer is not ferromagnetic. Further, our low-energy electron diffraction and x-ray measurements show that there is a $c$-axis collapse in the outermost layer. Bulk theoretical calculations reveal that, even at fixed doping level, the relaxation of the Jahn-Teller distortion at the surface is consistent with the stabilization of an $A$-type antiferromagnetic state.

Figure 1

(a) LEED diffraction pattern for the $x=0.36$ sample at 150 eV. (b) LEED: comparison between theoretical and experimental $I(V)$ curves for the $x=0.40$ system at a temperature of 300 K. Red (gray) and black lines correspond, respectively, to the theoretical and experimental intensities. $I(V)$ curves for four distinct diffracted beams [(1,0), (1,1), (2,1), and (4,1)] are presented. (c) Top half of the first bilayer, showing the three distinct manganese-oxygen distances: (1) Mn-O(I), distance between manganese and oxygen in the center of the bilayer; (2) Mn-O(II), distance between manganese and top/bottom oxygen; (3) Mn-O(III), distance between manganese and in-plane oxygen.

Figure 2

(a) Comparison of surface-sensitive x-ray absorption (electron yield) for in versus ex situ cleaving showing identical surface electronic structure for the two different surface preparations. (b) Comparison of surface versus bulk absorption showing similar composition for both regions. X-ray resonant magnetic scattering (XRMS) as a function of angle for (c) in situ and (d) ex situ cleaved samples. The sign reversal is attributed to a non-FM surface bilayer and is clearly seen in both cases, indicating that this is an intrinsic property of the surface layer.

Figure 3

FLAPW for $x=0.4$ (bulk). The total energy difference between AF and FM configurations ($\Delta E=E_{\mathrm{AF}}-E_{\mathrm{FM}}$) is presented as the $\mathrm{Mn}\mathrm{\text{−}}\mathrm{O}(\mathrm{I})/\mathrm{Mn}\mathrm{\text{−}}\mathrm{O}(\mathrm{II})$ ratio varied by changing the Mn-O(II) distance. The associated Jahn-Teller distortion (${\Delta }_{\mathrm{JT}}$) is also presented. The bulk experimental values for $x=0.40$, as well as the surface values for $x=0.36$ and 0.40, are presented in the graph.