Bimagnetic core/shell microwires with asymmetric external shell

A novel family of core/shell bimagnetic microwires is here introduced consisting of a cylindrical magnetic nucleus covered by a concentric but asymmetric magnetic external shell. The two magnetic phases are isolated by an intermediate glassy microtube, and they are designed to show soft/soft or soft/hard bimagnetic behavior. The nucleus is an amorphous FeSiB soft magnetic alloy, while the shell is either magnetically soft NiFe or harder Co polycrystalline alloy. Particularly, the shell asymmetry was achieved by a planar sputtering onto the precursor glass-coated amorphous microwire fabricated by rapid solidification. A first objective has been to correlate the morphology, geometry and surface roughness of these bimagnetic microwires with their magnetic properties and especially with the role played by the asymmetric shell. For that, bulk and surface hysteresis loops were respectively measured by vibrating sample magnetometry, VSM, and by magneto-optical Kerr effect, MOKE. Particularly, the asymmetry of the shell is correlated to the asymmetric magnetic response evidenced in the angular dependence of MOKE surface loops along the azimuthal. Since bimagnetic microwires with asymmetric shell exhibit a spontaneous bending curvature, a consequence of the complex mechanical stresses originating in the fabrication processes, a second objective has been to determine the action of a DC magnetic field to compensate that bending. The increasing DC field is experimentally correlated to the resulting increasing radius of curvature which is interpreted to be a consequence of the direct magnetostrictive effect. As a final conclusion, the presence of the asymmetric shell confers outstanding technical magnetic properties to these bimagnetic microwires very suitable for sensing applications.