Yakov Kopelevich

This work outlines the assembly of a cost-effective and straightforward apparatus designed to automate contact switching for electrical transport measurements. We report example of applications for this custom device and showcase its versatility by employing it for electrical resistivity and Hall effect measurements following the van der Pauw method on oxygen-deficient strontium titanate single crystals, measurements on compressed bismuth using the Montgomery method, and measurements on fresh cleaved Highly Oriented Pyrolytic Graphite using a multi-line electrode configuration.

Magnetoelectric nanoparticles (NPs) present an important class of nanomaterials with a wide interest in piezocatalytic and biomedical applications. Herein, the results of magnetoelectric and magnetization measurements performed on core–shell NPs having magnetic core (MnFe₂O₄, MFO) and ferroelectric shell (Ba_0.85Ca_0.15Ti_0.5Zr_0.5O₃, BCZT) synthesized by the microwave hydrothermal method are reported. Magnetic results are compared with the measurements on reference MFO NPs prepared under identical conditions. Detailed SQUID magnetometer measurements of the magnetization hysteresis loops M(H) down to 2 K reveal the existence of a clear exchange bias effect in pure MFO NPs attributed to the coexistence of ferromagnetic and antiferromagnetic short‐range interactions. When the magnetic core is covered by the thin ferroelectric BCZT shell, it is observed that 1) the shell suppresses the apparent bias effect and 2) induces an “extra” ferromagnetic magnetization at T < 20 K. The results indicate that this “extra” ferromagnetism has a 2D character and it is most likely related to the interface interactions between the MFO core and BCZT shell. Ferroelectric properties and strong magnetoelectric effect in core–shell NPs are revealed via piezoresponse force microscopy under magnetic field. The mechanisms of the observed effects are discussed.

Room temperature superconductivity under normal conditions has been a major challenge of physics and material science since its very discovery. Here we report the global room-temperature superconductivity observed in cleaved highly oriented pyrolytic graphite carrying dense arrays of nearly parallel surface line defects. The multiterminal measurements performed at the ambient pressure in the temperature interval 4.5 K < T < 300 K and at magnetic fields 0 < B < 9 T applied perpendicular to the basal graphitic planes reveal that the superconducting critical current I_c(T,B) is governed by the normal state resistance RN(T, B) so that I_c(T,B) is proportional to 1/R_N(T, B). Magnetization M(T, B) measurements of superconducting screening and hysteresis loops together with the critical current oscillations with temperature that are characteristic for superconductor-ferromagnet-superconductor Josephson chains, provide strong support for occurrence of superconductivity at T > 300 K. We develop a theory of global superconductivity emerging in the array of linear structural defects which well describes the experimental findings and demonstrate that global superconductivity arises as a global phase coherence of superconducting granules in linear defects promoted by the stabilizing effect of underlying Bernal graphite via tunneling coupling to the three dimensional (3D) material.

Ferroelectric materials based on lead zirconate titanate (PZT) are widely used as sensors and actuators because of their strong piezoelectric activity. However, their application is limited because of the high processing temperature, brittleness, lack of conformal deposition, and a limited possibility to be integrated with the microelectromechanical systems (MEMS). Recent studies on the piezoelectricity in the 2-D materials have demonstrated their potential in these applications, essentially due to their flexibility and integrability with the MEMS. In this work, we deposited a few layer graphene (FLG) on the amorphous oxidized Si ₃ N ₄ membranes and studied their piezoelectric response by sensitive laser interferometry and rigorous finite-element modeling (FEM) analysis. Modal analysis by FEM and comparison with the experimental results show that the driving force for the piezoelectric-like response can be a polar interface layer formed between the residual oxygen in Si ₃ N ₄ and the FLG. The response was about 14 nm/V at resonance and could be further enhanced by adjusting the geometry of the device. These phenomena are fully consistent with the earlier piezoresponse force microscopy (PFM) observations of the piezoelectricity of the graphene on SiO ₂ and open up an avenue for using graphene-coated structures in the MEMS.

Single crystals of a new silver and-cobalt based vanadate AgCo2V3O10 were grown from a melted mixture. The crystal structure determination reveals that this new vanadate crystallizes in triclinic system with space group P-1. The structure of the titled compound is constructed from CoO6octahedra and VO4 tetrahedra sharing edges and vertices leading to an open three-dimensional framework delimiting tunnels along [001], where the Ag cations are located. The bands observed in Raman spectrum were assigned to corresponding vibrations of the VO4 groups. DC and AC magnetization (susceptibility) measurements revealed the spin-glass (SG) -type transition below a frequency-independent temperature Tf(H) that is nearly independent on the applied magnetic field. Magnetic field H> 10 kOe induces the spin glass antiferromagnet. These observations brings AgCo2V3O10 into the class of geometrically frustrated magnetic systems.

High performance lead-free multiferroic composites are desired to replace the lead-based ceramics in multifunctional devices applications. Laminated compounds were prepared from ferroelectric and ferromagnetic materials. In this work, we present laminated ceramics compound by considering the ferromagnetic La0.5Ca0.5MnO3 (LCMO) and the ferroelectric BaTi0.8Sn0.2O3 (BTSO) in two different proportions. Compounds (1-x) LCMO-(x) BTSO with x=1 and 0 (pure materials) were synthesized by the sol gel method and x=0.7 and 0.5 (laminated) compounds were elaborated by welding appropriate mass ratios of each pure material by using the silver paste technique. Structural, dielectric, ferroelectric, microstructure and magnetic characterization were conducted on these samples. X-ray scattering results showed pure perovskite phases confirming the successful formation of both LCMO and BTSO. SEM images evidenced the laminated structure and good quality of the interfaces. The laminated composite materials have demonstrated a multiferroic behavior characterized by the ferroelectric and the ferromagnetic hysteresis loops. Furthermore, the enhancement of the dielectric constant in the laminated composite samples is mainly attributed to the Maxwell-Wagner polarization.