Effects of Au nanoparticles on the magnetic and transport properties of ${\text{La}}_{0.67}{\text{Sr}}_{0.33}{\text{MnO}}_3$ ultrathin layers

The effect of the proximity of Au nanoparticles on the transport and magnetic properties of ultrathin ${\text{La}}_{2/3}{\text{Sr}}_{1/3}{\text{MnO}}_3$ (LSMO) films has been investigated. We find a huge increase in the resistivity of the manganite (by four orders of magnitude for a Au nominal thickness of 2 nm), which is accompanied by a strong decrease in the Curie temperature. A combined scanning transmission electron microscopy and electron energy-loss spectroscopy analysis shows that interfaces are coherent and atomically sharp, and that the structural quality is very high. On the other hand, a strong reduction in the Mn oxidation state is seen upon Au capping. NMR data show a strong attenuation of the double exchange signal upon formation of Au nanoparticles. Ab initio calculations indicate a negligible influence of Au on LSMO at an ideal interface, with the LSMO surface magnetic and electronic properties essentially unchanged upon creation of the Au/LSMO interface. In view of these calculations, the experimental results cannot be explained in terms of purely electrostatic effects induced by the proximity of a noble metal. Here we propose that the main driving force underlying the observed change in physical properties is the high reactivity of Au nanoparticles, which can locally pump oxygen from the manganite, thus favoring a phase separation ensuing from O inhomogeneity which deteriorates the transport and electrical properties.

Figure 1

Panel (a): ADF STEM image of Au nanoparticles on LSMO(6 nm)/STO(001). Panel (b): ADF STEM image of Au nanoparticles on STO(2 nm)/LSMO(6 nm)/STO(001).

Figure 2

(Color online) Normalized magnetization vs temperature measured at 100 Oe in different heterostructures. The thickness both of the gold layer and of the STO buffer layer is 2 nm.

Figure 3

(Color online) Resistivity as a function of temperature for 4 nm LSMO (squares), 1 nm Au/4 nm LSMO (circles) and 2 nm Au/4nm LSMO (triangles).

Figure 4

(Color online) Zero-field ${}^{55}\text{M}\text{n}$ NMR in various heterostructures: circles—LSMO/STO, squares—Au/LSMO/STO, and triangles—Au/STO/LSMO/STO. The thickness of both the STO buffer layer and Au layer is fixed to be 2 nm while the thickness of the LSMO layer is $t_{\text{LSMO}}=4$, 6, 8, and 12 nm. Solid lines demonstrate the best fit to the experimental data.

Figure 5

(Color online) Thickness dependence of the NMR integrated intensity $I_{\text{DE}}$ measured in the samples with (squares) and without (circles) gold overlayer.

Figure 6

(Color online) Panel (a): ADF STEM image of a Au nanoparticle on LSMO(6 nm)/STO(001). Panel (b): EELS profiles of the different elements along the line scan indicated in panel (a). $\text{La}{M}_{4,5}$ (black squares), $\text{Mn}{L}_{2,3}$ (red circles), $\text{Ti}{L}_{2,3}$ (green triangles), $\text{O}K$ (blue downward triangles), $\text{Au}{M}_{4,5}$ (magenta stars), and $\text{Sr}{L}_{2,3}$ (cyan dots). Panel (c): profile of the Mn oxidation state along the line scan (integrated over 50 nm in direction perpendicular to the line scan) derived from the separation of the prepeak and of the main peak of the $\text{O}K$ EELS edge. (See text for details).

Figure 7

(Color online) Panel (a): ADF STEM image of a Au nanoparticle on STO(2 nm)/LSMO(6 nm)/STO(001). Panel (b): EELS profiles of the different elements along the line scan indicated in panel (a) $\text{La}{M}_{4,5}$ (black squares), $\text{Mn}{L}_{2,3}$ (red dots), $\text{Ti}{L}_{2,3}$ (green triangles), and $\text{O}K$ (blue downward triangles). Panel (c): profile of the Mn oxidation state along the line scan derived from the separation of the prepeak and of the main peak of the $\text{O}K$ EELS edge. (See text for details).

Figure 8

(Color online) Calculated density of states for Mn placed in the uppermost layer of the LSMO film free and in contact with Au in comparison with bulk DOS of LSMO. The Fermi energy is taken as reference for the energy scale. The structure used in the simulation, corresponding to the [100] axis of Au parallel to the [100] axis of LSMO, is shown on the right.