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Growth and characterization of metal‐oxide thin films foster successful development of oxide‐material‐integrated thin‐film devices represented by metal‐oxide‐semiconductor field‐effect transistors (MOSFET), drawing enormous technological and scientific interest for several decades. In recent years, functional oxide heterostructures have demonstrated remarkable achievements in modern technologies and provided deeper insights into condensed‐matter physics and materials science owing to their versatile tunability and selective amplification of the functionalities. One of the most critical aspects of their physical properties is the polar perturbation stemming from the ionic framework of an oxide. By engineering and exploiting the structural, electrical, magnetic, and optical characteristics through various routes, numerous perceptive studies have clearly shown how polar perturbations advance functionalities or drive exotic physical phenomena in complex oxide heterostructures. In this review, both intrinsic (engraved by thin‐film heteroepitaxy) and extrinsic (reversibly controllable defect‐mediated disorder and polar adsorbates) elements of polar perturbations, highlighting their abilities for the development of highly tunable functional properties are summarized. Scientifically, the recent approaches of polar perturbations render one to consolidate a prospect of atomic‐level manipulation of polar order in epitaxial oxide thin films. Technologically, this review also offers useful guidelines for rational design to heterogeneously integrated oxide‐based multi‐functional devices with high performances. [ABSTRACT FROM AUTHOR]

Understanding the magnetotransport properties of SrRuO3 thin films and heterostructures is essential for both fundamental research and practical applications. While the anomalous Hall effect has been studied extensively, the longitudinal magnetotransport (i.e. magnetoresistance) is not well understood and requires further investigation. Herein, SrRuO3/SrTiO3(001) epitaxial film is used as a model system to systematically investigate the correlation between magnetoresistance and structure symmetry. As the thickness of the SrRuO3 film increases, a structural symmetry transition from tetragonal to orthorhombic occurs. At the same time, a clear sign reversal of magnetoresistance from negative to positive, accompanied by the appearance of strong anisotropy in magnetoresistance measured along the two perpendicular in‐plane axes, is observed. These findings further suggest that the correlation between structural symmetry and magnetoresistance is mediated by the modulations in magnetic anisotropy. These results clarified the relationships between structure symmetry, magnetoresistance, and magnetic anisotropy, which can pave a feasible way for harnessing the magnetotransport properties of SrRuO3 films and using this material in oxide‐based spintronic devices. [ABSTRACT FROM AUTHOR]

One of the keys to the construction of metal oxide heterostructures is the short characteristic length scale, which requires controlled growth and interface engineering on an atomic level. At present, the growth mode of SrRuO3 (SRO) thin films grown on TiO2‐terminated (001) SrTiO3 (STO) substrates usually transitions from 2D layer‐by‐layer to step‐flow at the first few growth periods, which is not conducive to the construction of superlattices or multilayer films. In this paper, persistent layer‐by‐layer growth of SRO thin films is demonstrated by regulating the growth conditions. As a result, the thickness can be precisely controlled down to a single unit cell (u.c.) and achieve precise regulation of superlattices. Interestingly, the physical properties are well comparable to that of traditional step‐flow mode and the critical thickness (tc) for the dead layer is only 3 u.c. Through constructing SRO/STO superlattices, the SRO layers become more conductive and a quasi‐2D ferromagnetic monolayer film is achieved. The results pave a path toward SRO heterostructure synthesis and properties control at an atomic scale. [ABSTRACT FROM AUTHOR]

Oxygen vacancy in oxide ferroelectrics can be strongly coupled to the polar order via local strain and electric fields, thus holding the capability of producing and stabilizing exotic polarization patterns. However, despite intense theoretical studies, an explicit microscopic picture to correlate the polarization pattern and the distribution of oxygen vacancies remains absent in experiments. Here we show that in a high-quality, uniaxial ferroelectric system, i.e., compressively strained BaTiO3 ultrathin films (below 10 nm), nanoscale polarization structures can be created by intentionally introducing oxygen vacancies in the film while maintaining structure integrity (namely no extended lattice defects). Using scanning transmission electron microscopy, we reveal that the nanodomain is composed of swirling electric dipoles in the vicinity of clustered oxygen vacancies. This finding opens a new path toward the creation and understanding of the long-sought topological polar objects such as vortices and skyrmions. [ABSTRACT FROM AUTHOR]

Oxide heterostructures have great potential for spintronics applications due to their well‐defined heterointerfaces and vast functionalities. To integrate such compelling features into practical spintronics devices, effective control of the magnetic switching behavior is key. Here, continuous control of the magnetic coercive field in SrTiO3/SrRuO3 ultrathin heterostructures is achieved by oxygen vacancy (VO) engineering. Pulsed laser deposition of an oxygen‐deficient SrTiO3 capping layer can trigger VO migration into the SrRuO3 layer while avoiding the formation of Ru vacancies. Moreover, by varying the thickness and growth conditions of the SrTiO3 capping layer, the value of the coercive field (HC) in the ferromagnetic SrRuO3 layer can be continuously tuned. The maximum enhancement of HC at 5 K is 3.2 T. Such a wide‐range tunability of HC may originate from a VO‐induced enhancement of perpendicular magnetic anisotropy and domain wall pinning. This study offers effective approaches for controlling physical properties of oxide heterostructures via VO engineering, which may facilitate the development of oxide‐based functional devices. [ABSTRACT FROM AUTHOR]

Electron-electron (e-e) correlation and spin-orbit coupling (SOC) are two essential control parameters that determine the physical properties of transition-metal-oxide-based thin films and heterostructures. Here, by harnessing these two parameters, we report the systematic control of both ferromagnetism and in-plane magnetic anisotropy in La0.67Ca0.33MnO3/CaIr1-xRuxO3 (LCMO/CIRO) superlattices grown on NdGaO3 substrates. In these all-oxide epitaxial systems, we demonstrate that the e-e correlation and the SOC depend strongly on the chemical composition of CIRO layers. By simply increasing the Ru doping level x, we can significantly suppress the degradation of ferromagnetism in ultrathin LCMO layers and enhance the TC by ∼90 K. Moreover, we observe a clear reorientation of the magnetic easy axis from orthorhombic [010] to [100] as x decreased to 0, which may originate from the enhanced SOC. Our work provides an insight for utilizing the 4d/5d transition-metal-oxides to tune the functionality of magnetic heterostructures. [ABSTRACT FROM AUTHOR]

Systematic transport measurements have been performed on a series of La0.67Ca0.33MnO3 thin films with varying degrees of anisotropic strain. The strain is induced via epitaxial growth on NdGaO3(001) substrates and varied by controlling the thermal annealing time. An antiferromagnetic insulating (AFI) state, possibly associated with charge ordering, emerges upon thermal annealing. The Hall effect in these materials exhibits features that are indicative of a percolative phase transition and correlate closely with the emergence of the AFI state. In the paramagnetic phase, the Hall resistivity takes on two slopes in all samples: a decreasing negative slope with increasing temperature at low fields, which is attributed to the carrier hopping motion, and an almost temperature independent positive slope at high fields due to diffusive transport of holes. Significantly, the crossover fields of the Hall resistivity slope at different temperatures correspond to the same magnetization, which is interpreted as the critical point of a magnetic field-driven percolative phase transition. At lower temperatures near the zero-field metal–insulator transition, pronounced enhancement of the Hall coefficient with the development of the AFI state is observed. The enhancement peaks near the magnetic field-driven percolation; its magnitude correlates with the strength of the AFI state and is suppressed with the melting of the AFI state by an in-plane magnetic field. The observations resemble many features of the enhancement of the Hall coefficient in granular metal films near the composition-driven percolation. [ABSTRACT FROM AUTHOR]