Anomalous thickness-dependent strain states and strain-tunable magnetization in Zn-doped ferrite epitaxial films

Abstract

A series of ZnxFe3−xO4 (ZFO, x = 0.4) thin films were epitaxially deposited on single-crystal (001)-SrTiO3 (STO) substrates by radio frequency magnetron sputtering. The anomalous thickness-dependent strain states of ZFO films were found, i.e., a tensile in-plane strain exists in the thinner ZFO film and which monotonously turns into compressive in the thicker films. Considering the lattice constant of bulk ZFO is bigger than that of STO, this strain state cannot be explained in the conventional framework of lattice-mismatch-induced strain in the hetero-epitaxial system. This unusual phenomenon is proposed to be closely related to the Volmer-Weber film growth mode in the thinner films and incorporation of the interstitial atoms into the island's boundaries during subsequent epitaxial growth of the thicker films. The ZFO/STO epitaxial film is found in the nature of magnetic semiconductor by transport measurements. The in-plane magnetization of the ZFO/STO films is found to increase as the in-plane compressive strain develops, which is further proved in the (001)-ZFO/PMN-PT film where the film strain state can be in situ controlled with applied electric field. This compressive-strain-enhanced magnetization can be attributed to the strain-mediated electric-field-induced in-plane magnetic anisotropy field enhancement. The above results indicate that strain engineering on magnetic oxide semiconductor ZFO films is promising for novel oxide-electronic devices.

Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

“…Both atomic force microscopy and polarized optical microscopy confirmed the smooth surface (root mean square: ~0.4 nm) and uniformity of VO 2 thin films (not shown here). We thus can calculate the thickness ( h ) with the formula 28 :…”

Interfacial Modulation on Co0.2Fe2.8O4 Epitaxial Thin Films for Anomalous Hall Sensor Applications

“…All of the (001) lattice spacings of films calculated from the θ–2θ scan are larger than that of the bulk (∼8.39 Å), indicating a strong compressive strain from the substrate. Moreover, a larger lattice strain was found in thicker films, which contradicts with the strain–relax mechanism for normal heteroepitaxial system . Considering the higher strain in CoFeO/G/MAO systems, there should be other strain origination in our epitaxial systems.…”

“…The nanopillar-like structure of the film observed by the STEM image of our CoFeO/G/MAO heterostructure might result from the strain or dislocation and the “bridge structure” during the initial growth of the film. We argue that the growth mechanism of nanopillars should also contribute to the stronger in-plane strain in CoFeO/G/MAO heterostructures, which is similar to the Volmer–Weber mode in the ZFO/STO thin film system . At the initial stage, the CoFeO particle discretely nucleates and forms islands due to the inhomogeneous surface energy of the graphene-covered MAO.…”

“…Moreover, a larger lattice strain was found in thicker films, which contradicts with the strain−relax mechanism for normal heteroepitaxial system. 29 Considering the higher strain in CoFeO/G/MAO systems, there should be other strain origination in our epitaxial systems. To further understand the interface strain of films with various thicknesses, reciprocal space maps were performed around the (206) reflections of the films and MAO substrates (Figure S2), and the results are summarized in Figure 2b.…”

“…We argue that the growth mechanism of nanopillars should also contribute to the stronger in-plane strain in CoFeO/G/MAO heterostructures, which is similar to the Volmer−Weber mode in the ZFO/STO thin film system. 29 At the initial stage, the CoFeO particle discretely nucleates and forms islands due to the inhomogeneous surface energy of the graphene-covered MAO. As the thickness of the CoFeO film increases, the islands begin to grow and merge, which may induce in-plane tensile strain and out-of-plane compressive strain to balance the surface energy of the neighbor islands and the free energy of the boundaries.…”