Alexey S. Mikheykin

Controlling ferroelectric phase symmetry through strain engineering is crucial for tailoring polarization topology and functional properties in oxide-based devices. Here, we show that a thin conductive buffer layer can over-ride substrate-induced strain in BaTiO3 (BTO) films on MgO. Although the lattice parameter of MgO(001) would be expected to induce tensile misfit strain, in 500 nm BaTiO3 films the thermal-expansion mismatch dominates,resulting in effective compressive stress upon cooling. Inserting a 50-nm La0.67Sr0.33MnO3 (LSMO) buffer layer reverses the effective strain state from compressive to tensile. Using temperature-dependent polarized Raman spectroscopy, X-ray diffraction, and piezoresponse force microscopy, we reveal an unconventional phase sequence upon cooling: paraelectric → orthorhombic (aa-phase, ∼407 K) → monoclinic (r -phase, ∼300–345 K), characterized in-plane and tilted polarization components. The LSMO layer not only serves as a bottom electrode but also modifies the strain state, while interfacial charge redistribution induces a built-in electric field, evidenced by self-polarization and asymmetric PFM hysteresis. These findings highlight the critical role of conductive buffer layers in controlling ferroelectric symmetry and enabling oxide heterostructures with tailored polarization textures for low-power spin- tronics, tunable microwave devices, and neuromorphic computing.

The widespread use of ferroelectric materials in tunable electronic devices is often limited by energy dissipation at domain walls in the ferroelectric state. Low dielectric losses and high tunability can be achieved by operating the material in the paraelectric phase under an external electric field. Under these conditions, a domain-free, field-stabilized ferroelectric-like state can be induced, particularly near phase boundaries or multicritical points. We investigate the electric-field control of dielectric and piezoelectric properties in paraelectric Pb _x Sr _{1− x} TiO ₃ (PST) thin films epitaxially constrained on cubic substrates. Using a Landau–Ginzburg–Devonshire thermodynamic potential that incorporates both polarization and octahedral tilting as coupled order parameters, we construct the room-temperature (300 K) phase diagram in the misfit strain–composition ( u _m – x ) plane. Within the paraelectric region, we analyze the system’s responseunder a perpendicular electric field ( E _z = 100 kV cm ⁻¹ ), which induces a uniaxial, domain-free ferroelectric-like state, often referred to as a ‘forced’ ferroelectric. We calculate the field-dependent permittivities ( ε 33 , ε 11 ) and induced piezoelectric coefficients ( d i j ,   e i j ) along a path within the paraelectric phase near the adjacent ferroelectric phase boundaries. Our results show a pronounced enhancement of dielectric tunability t ( ε ) and non-zero piezoelectric response in regions near the multicritical point ( u _m ≈ 0, x ≈ 0.138), where the paraelectric phase borders the c -phase (tetragonal, P 4 m m ), r -phase (rhombohedral-like monoclinic, C m ), and aa -phase (orthorhombic, A m m 2 ). The most pronounced effects occur near the boundary with the c -phase, where the out-of-plane polarization mode undergoes softening. These findings indicate that operating PST thin films in the paraelectric phase under an external bias enables low-loss, highly tunable behavior, making them promising candidates for reconfigurable microwave and electronic devices.

The Ni‐PANI@GO composite electrode was fabricated via cost effective electrodeposition technique. According to the XRD, FTIR, Raman, SEM, and XPS analyses revealed that the nickel doped PANI@GO composite has been fabricated on the surface of the nickel foam. Addition of nickel significantly enhanced interaction between graphene with PANI leading to higher degree of polyaniline doping though imine groups. Electrochemical investigation revelated the significant performance of the Ni‐PANI@GO composite electrode, boosting an impressive capacitance of 4480 F/g at 40 A/g, surpassing previous Ni‐foam‐based binder‐free electrodes. Notably, Ni‐PANI@GO electrode displayed excellent catalytic activity in both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), generating a considerable volume of the gas bubbles at relatively modest overpotentials of 279 mV and 244 mV respectively. This event allows for the achievement of 20 mA cm⁻² current density. Furthermore, in the laboratory‐scale water electrolyzer, a low cell voltage of 1.72 V was achieved, facilitating a water‐splitting current density of 20 mA cm⁻². This study underscores the premising potential for the real‐world device's application of the versatile Ni‐PANI@GO composite electrode.

Decreasing the scale of vanadium dioxide (VO₂) structures is one of the ways to enhance the switching speed of the material. We study the properties of VO₂ films of altered thicknesses in the range of 20–170[ ]nm prepared on c-sapphire substrates with a TiO₂ sublayer by pulsed laser deposition (PLD) method. The synthesis regime to design a TiO₂ film was preliminarily optimized based on XRD data. XRD patterns reveal an epitaxial growth of the VO₂ films with distortion of the monoclinic cell to hexagonal symmetry. The positions of the lattice vibration modes in Raman spectra are similar to those in bulk VO₂ when the film thickness is greater than [ ][ ]nm. For VO₂ films thicker that [ ][ ]nm, a lattice strain results in the modes’ positions and intensity change. However, the electrically triggered transition in a [ ][ ]nm thick VO₂ film reveals forward and reverse switching times as short as 20[ ]ns and 400[ ]ns, correspondingly.