Tuning the Metal-Insulator Transition in Manganite Films through Surface Exchange Coupling with Magnetic Nanodots

In strongly correlated electronic systems, the global transport behavior depends sensitively on spin ordering. We show that spin ordering in manganites can be controlled by depositing isolated ferromagnetic nanodots at the surface. The exchange field at the interface is tunable with nanodot density and makes it possible to overcome dimensionality and strain effects in frustrated systems to greatly increasing the metal-insulator transition and magnetoresistance. These findings indicate that electronic phase separation can be controlled by the presence of magnetic nanodots.

Figure 1

Transport properties of ultrathin LCMO film before and after application of nanodots. (a) Resistivity behavior for 20 nm ultrathin film of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ shows insulating behavior and no clear metal-insulator transition. (b) Resistivity data of the same film after applying Fe nanodots to surface shows a recovery to bulklike behavior with a MIT temperature of 255 K at 0 T (note the change in scale). (c) Ferromagnetic Fe nanodots drive a huge change in the film resistivity compared to the diamagnetic Cu nanodots. (Insets) Atomic force microscope images of typical nanodot coverages for Cu and Fe systems on LCMO films. (d) Magnetoresistive behavior shows a much higher magnetic response in the spin coupled system.

Figure 2

Magnetization at 150 K. (a) As-grown LCMO film shows a weak out-of-plane easy axis. (Inset) Zoom showing a small relative Fe signal on a bare LAO substrate. (b) After application of Fe nanodots, the easy axis shifts to a stronger in-plane easy axis. (c) The relative percent change in magnetization before and after application of Fe nanodots shows that there is a clear increase in the in-plane component with a correspondingly strong decrease in the out-of-plane component, demonstrating that there has been an overall increase in the underlying magnetic cohesion of the system.

Figure 3

Schematic diagrams of local spin state cross sections corresponding to the global film state. (a) The cross section shown above as-grown LCMO film exhibits weak spin orientation which leads to only small pockets of the FMM phase at low temperature. (b) Ferromagnetic Fe nanodots at the surface act to align spins in the LCMO film and allow for the FMM phase to form and open conduction paths at a greatly increased $T_C$.

Figure 4

Transport properties with reduced Fe nanodot density. The resistive data for an ultrathin LCMO film after application of low density Fe nanodots show recovery of the metal-insulator transition but with a much lower transition temperature than that seen at higher nanodot densities. (Inset) Atomic force microscope image of typical low density coverage on a LCMO film.