Opening the Blood Brain Barrier with an Electropermanent Magnet System.

Opening the blood brain barrier (BBB) under imaging guidance may be useful for the treatment of many brain disorders. Rapidly applied magnetic fields have the potential to generate electric fields in brain tissue that, if properly timed, may enable safe and effective BBB opening. By tuning magnetic pulses generated by a novel electropermanent magnet (EPM) array, we demonstrate the opening of tight junctions in a BBB model culture in vitro, and show that induced monophasic electrical pulses are more effective than biphasic ones. We confirmed, with in vivo contrast-enhanced MRI, that the BBB can be opened with monophasic pulses. As electropermanent magnets have demonstrated efficacy at tuning B0 fields for magnetic resonance imaging studies, our results suggest the possibility of implementing an EPM-based hybrid theragnostic device that could both image the brain and enhance drug transport across the BBB in a single sitting.

Tuning Exchange Coupling in a New Family of Nanocrystal-Based Granular Multiferroics Using an Applied Electric Field.

In this work, we demonstrate an experimental realization of a granular multiferroic composite, where the magnetic state of a nanocrystal array is modified by tuning the interparticle exchange coupling using an applied electric field. Previous theoretical models of a granular multiferroic composite predicted a unique magnetoelectric coupling mechanism, in which the magnetic spins of the ensemble are governed by interparticle exchange. The extent of these exchange interactions can be controlled by varying the local dielectric environment between grains. We specifically utilize the strong dielectric dependence of ferroelectric materials to modify the interparticle coupling of closely spaced magnetic nanoparticles using either a change in temperature or an electric field. This coupling modifies the ensemble magnetic coercivity and thus the superparamagnetic-to-ferromagnetic phase transition temperature. Through the use of two different ferroelectrics, our results suggest that this magnetoelectric coupling mechanism could be generalized as a new class of multiferroic material, applicable to a broad range of ferroelectric/magnetic nanocrystal composites.

Artificial Dense Lattices of Magnetic Skyrmions.

Multilayer Co/Pt films with perpendicular magnetic anisotropy are irradiated by focused a He+ ion beam to locally reduce the anisotropy value. The irradiated spots with the diameters of 100 and 200 nm are arranged in square lattices with the periods of 200 and 300 nm. The formation of nonuniform magnetic states within the spots was observed by magnetic force microscopy methods. We use the concentric distribution of the irradiation fluence within the spot to obtain the radial modulation of the anisotropy constant. This allows us to induce magnetic skyrmions during magnetization reversal of the system. The skyrmions remained stable at zero external magnetic field at room temperature. Magnetization hysteresis loops of the samples were investigated by magnetooptical methods and the results are in good agreement with micromagnetic simulations.