Enhanced exchange bias and improved ferromagnetic properties in Permalloy-BiFe0.95Co0.05O3 core-shell nanostructures

Exchange Bias in Nanostructures: An Update

Exchange bias (EB) is a unidirectional anisotropy occurring in exchange-coupled ferromagnetic/antiferromagnetic systems, such as thin films, core-shell particles, or nanostructures. In addition to a horizontal shift of the hysteresis loop, defining the exchange bias, asymmetric loops and even vertical shifts can often be found. While the effect is used in hard disk read heads and several spintronics applications, its origin is still not fully understood. Especially in nanostructures with their additional shape anisotropies, interesting and often unexpected effects can occur. Here, we provide an overview of the most recent experimental findings and theoretical models of exchange bias in nanostructures from different materials.

Magnetoelectric dissociation of Alzheimer's β-amyloid aggregates

The abnormal self-assembly of β-amyloid (Aβ) peptides and their deposition in the brain is a major pathological feature of Alzheimer's disease (AD), the most prevalent chronic neurodegenerative disease affecting nearly 50 million people worldwide. Here, we report a newly discovered function of magnetoelectric nanomaterials for the dissociation of highly stable Aβ aggregates under low-frequency magnetic field. We synthesized magnetoelectric BiFeO₃-coated CoFe₂O₄ (BCFO) nanoparticles, which emit excited charge carriers in response to low-frequency magnetic field without generating heat. We demonstrated that the magnetoelectric coupling effect of BCFO nanoparticles successfully dissociates Aβ aggregates via water and dissolved oxygen molecules. Our cytotoxicity evaluation confirmed the alleviating effect of magnetoelectrically excited BCFO nanoparticles on Aβ-associated toxicity. We found high efficacy of BCFO nanoparticles for the clearance of microsized Aβ plaques in ex vivo brain tissues of an AD mouse model. This study shows the potential of magnetoelectric materials for future AD treatment using magnetic field.

Biosensing platform on ferrite magnetic nanoparticles: Synthesis, functionalization, mechanism and applications

Ferrite magnetic nanoparticles (FMNPs) are gaining popularity to design biosensors for high-performance clinical diagnosis. The fusion of information shows that FMNPs based biosensors require well-tuned FMNPs as detection probes to produce large and specific biological signals with minimal non-specific binding. Nevertheless, there is a noticeable lacuna of information to solve the issues related to suitable synthesis route, particle size reduction, functionalization, sensitivity towards targeted intercellular biological tiny particles, and lower signal-to-noise ratio. Therefore it allows exploring unique characteristics of FMNPs to design a suitable sensing device for intracellular measurements and diseases detection. This review focuses on the extensively used synthesis routes, their advantages and limitations, crystalline structure, functionalization, along with recent applications of FMNPs in biosensors, taking into consideration their analytical figures of merit and range of linearity. This work also addresses the current progress, key factors for sensitivity, selectivity and productivity improvement along with the challenges, future trends and perspectives of FMNPs based biosensors.

Exchange Bias in Thin Films—An Update

The exchange bias (EB) is an effect occurring in coupled ferromagnetic/antiferromagnetic materials of diverse shapes, from core–shell nanoparticles to stacked nanostructures and thin films. The interface coupling typically results in a horizontal—often also vertical—shift of the hysteresis loop, combined with an increased coercivity, as compared to the pure ferromagnet, and the possibility of asymmetric hysteresis loops. Several models have been developed since its discovery in 1956 which still have some drawbacks and some unexplained points, while exchange bias systems are at the same time being used in hard drive read heads and are part of highly important elements for spintronics applications. Here, we give an update of new theoretical models and experimental findings regarding exchange bias phenomena in thin films during the last years, including new material combinations in which an exchange bias was found.

TiO2-Templated BaTiO3 Nanorod as a Piezocatalyst for Generating Wireless Cellular Stress

Although the piezoelectric property of a BaTiO₃ nanoparticle is routinely used in energy harvesting application, it can also be exploited for wireless cell stimulation and cell therapy. However, such biomedical application is rare due to limited availability of colloidal BaTiO₃ nanoparticles of <100 nm hydrodynamic size with good piezocatalytic property and efficient biolabeling performance. Here, we report a colloidal form of a piezocatalytic BaTiO₃-based nanorod of <100 nm hydrodynamic size that can offer wireless cell stimulation. The nanorod is prepared using a TiO₂ nanorod as the template, and the resultant TiO₂-BaTiO₃-based composite nanorod is coated with a hydrophilic polymer shell. These nanorods can label cells and, under the ultrasound exposure, produce reactive oxygen species inside cells via piezocatalysis, leading to cell death. These nanorods can be used for wireless modulation of intracellular processes.

ZnSnO3–hBN nanocomposite-based piezocatalyst: ultrasound assisted reactive oxygen species generation for degradation of organic pollutants

Ultrasound assisted sustainable degradation of RhB by a lead-free ferroelectric ZnSnO₃–hBN piezocatalyst.

Tunable Ferroelectricity in Ruddlesden-Popper Halide Perovskites

Ruddlesden-Popper (RP) halide perovskites are the new kids on the block for high-performance perovskite photovoltaics with excellent ambient stability. The layered nature of these perovskites offers an exciting possibility of harnessing their ferroelectric property for photovoltaics. Adjacent polar domains in a ferroelectric material allow the spatial separation of electrons and holes. Presently, the structure-function properties governing the ferroelectric behavior of RP perovskites are an open question. Herein, we realize tunable ferroelectricity in 2-phenylethylammonium (PEA) and methylammonium (MA) RP perovskite (PEA)₂(MA) _n̅-1Pb _n̅I_{3 n̅+1}. Second harmonic generation (SHG) confirms the noncentrosymmetric nature of these polycrystalline thin films, whereas piezoresponse force microscopy and polarization-electric field measurements validate the microscopic and macroscopic ferroelectric properties. Temperature-dependent SHG and dielectric constant measurements uncover a phase transition temperature at around 170 °C in these films. Extensive molecular dynamics simulations support the experimental results and identified the correlated reorientation of MA molecules and ion translations as the source of ferroelectricity. Current-voltage characteristics in the dark reveal the persistence of hysteresis in these devices, which has profound implications for light-harvesting and light-emitting applications. Importantly, our findings disclose a viable approach for engineering the ferroelectric properties of RP perovskites that may unlock new functionalities for perovskite optoelectronics.

Short-Range Correlated Magnetic Core-Shell CrO₂/Cr₂O₃ Nanorods: Experimental Observations and Theoretical Considerations

With the evolution of synthesis and the critical characterization of core-shell nanostructures, short-range magnetic correlation is of prime interest in employing their properties to develop novel devices and widespread applications. In this regard, a novel approach of the magnetic core-shell saturated magnetization (CSSM) cylinder model solely based on the contribution of saturated magnetization in one-dimensional CrO₂/Cr₂O₃ core-shell nanorods (NRs) has been developed and applied for the determination of core-diameter and shell-thickness. The nanosized effect leads to a short-range magnetic correlation of ferromagnetic core-CrO₂ extracted from CSSM, which can be explained using finite size scaling method. The outcome of this study is important in terms of utilizing magnetic properties for the critical characterization of core-shell nanomagnetic materials.

Controllable synthesis of ferromagnetic-antiferromagnetic core-shell NWs with tunable magnetic properties

Several nanotechnology applications are based on the promising scheme of highly anisotropic magnetic nanomaterials. Using this idea, we investigated the structure, magnetic properties, and interfacial exchange anisotropy effects of the Ni/Cr₂O₃ and Fe/Cr₂O₃ core-shell nanowires (NWs) geometry. A template-based strategy was developed to synthesize Ni (Fe)-Cr₂O₃ core-shell NWs, which combines a wet-chemical route and electrodeposition within the nanopores of the membranes. Structural determination in correlation with magnetic testing shows that the crystalline Cr₂O₃-nanoshells (NSs) cause an enhanced exchange bias, providing an extra source of anisotropy that leads to their magnetic stability. This core-shell NWs geometry, with enhanced anisotropy, should, therefore, motivate further study related to the applicability of anisotropic nanostructures. Our design opens a new pathway to obtain optimized heterostructured nanomaterials exhibiting tunable magnetic properties.

Diameter-dependent multiferroic functionality in hybrid core/shell NWs

A versatile approach towards nanofabrication of highly reproducible Co/BiCoO3 (Co/BCO) core/shell (CS) nanowires (NWs) with different diameters has been adopted by demonstrating easily available and low cost sol-gel and electrodeposition routes. X-ray diffraction (XRD) analysis confirmed the tetragonal system of the BCO nanoshells (NSs) with the space group P4mm. Scanning electron microscopy (SEM) clearly demonstrates the uniform morphology with well aligned CS NWs. The magnetization reversal processes (MRPs), experimentally and with analytical modelling, have been discussed for CS NWs with θ ranging from 0° (in-plane magnetic easy axis) to 90° (out-of-plane magnetic hard axis) with magnetic hysteresis loops and geometrical parameters. Crossover from the vortex to transverse reversal mode on increasing θ has been observed for all diameters. An exchange bias effect has been observed for smaller CS NWs diameters and it is attributed to the shell thickness of ∼25 nm. Furthermore, the magnetic anisotropy effect has been discussed in some detail.