Vertically aligned nanocomposite thin films with ordered two phases, grown epitaxially on substrates, have attracted tremendous interest in the past decade. These unique nanostructured composite thin films with large vertical interfacial area, controllable vertical lattice strain, and defects provide an intriguing playground, allowing for the manipulation of a variety of functional properties of the materials via the interplay among strain, defect, and interface. This field has evolved from basic growth and characterization to functionality tuning as well as potential applications in energy conversion and information technology. Here, the remarkable progress achieved in vertically aligned nanocomposite thin films from a perspective of tuning functionalities through control of strain, defect, and interface is summarized.

Metal Oxide Nanocomposites: A Perspective from Strain, Defect, and Interface

Ferrimagnetism and possible double perovskite structure in half Cr-doped YMn0.5Cr0.5O3

The strain dependent magnetocaloric properties of La0.67Sr0.33MnO3 thin films deposited on three different substrates (001) LaAlO3 (LAO), (001) SrTiO3 (STO), and (001) La0.3Sr0.7Al0.65Ta0.35O9 (LSAT) have been investigated under low magnetic fields and around magnetic phase transition temperatures. Compared to bulk samples, we observe a remarkable decrease in the ferromagnetic transition temperature that is close to room temperature, closely matched isothermal magnetic entropy change and relative cooling power values in tensile strained La0.67Sr0.33MnO3 films. The epitaxial strain plays a significant role in tuning the peak position of isothermal magnetic entropy change towards room temperature with improved cooling capacity.

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Strain dependent magnetocaloric effect in La0.67Sr0.33MnO3 thin-films

Interfacial properties of the La0.67Ca0.33MnO3 films grown on SrTiO3 and SrTiO3:Nb, respectively, have been experimentally studied. An interface layer, ∼13 or ∼4.4 nm for the films on SrTiO3 or SrTiO3:Nb, with degenerated magnetic/conductive properties is found in the film. The most remarkable result is the significantly different layer width on different substrates. The built-in electric field yielded by charge exchange may be responsible for the layer shrinkage in La0.67Ca0.33MnO3/SrTiO3:Nb. A depression of this layer by magnetic field is also observed and ascribed to field-induced enhancement of the double exchange between Mn ions.

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Magnetic and conductive dead layer at the La0.67Ca0.33MnO3–SrTiO3:Nb interface

A variety of emergent phenomena have been enabled by interface engineering in complex oxides. The existence of an intrinsic interfacial layer has often been found at oxide heterointerfaces. However, the role of such an interlayerin controlling functionalities is not fully explored. Here, we report the control of the exchange bias (EB) in single-phase manganite thin films with nominallyuniform chemical composition across the interfaces. The sign of EB depends on the magnitude of the cooling field. A pinned layer, confirmed by polarized neutron reflectometry, provides the source of unidirectional anisotropy. The origin of the exchange bias coupling is discussed in terms of magnetic interactions between the interfacial ferromagnetically reduced layer and the bulk ferromagnetic region. The sign of EB is related to the frustration of antiferromagnetic coupling between the ferromagnetic region and the pinned layer. Our results shed new light on using oxide interfaces to design functional spintronic devices.

Hidden Interface Driven Exchange Coupling in Oxide Heterostructures.

We report the electrical transport and magnetic properties of the dead layer of La(0.67)Sr(0.33)MnO(3) grown on LaAlO(3) substrate under the influence of magnetic field and electric voltage. The electrical resistance of the dead layer shows exponential decrease with both magnetic field and electric voltage, leading to colossal magnetoresistance and electroresistance, respectively. However, the sample cannot be driven into a metallic state with the available magnetic field and electric voltage. At low temperatures, the magnetic-field dependence of both magnetization and resistance show remarkable hysteresis. Exchange bias effect was observed in the magnetization vs magnetic-field curves. Magnetic force microscope measurement reveals the coexistence of different magnetic phases in the dead layer. The results were discussed in terms of phase separation in the dead layer. This work demonstrates the presence of phase separation in the manganite dead layer and its tunability by magnetic field and electric voltage.

B. T. Xie

Papers

Electric and magnetic modulation of fully strained dead layers in La(0.67)Sr(0.33)MnO(3) films