Fabrication of ferromagnetic rolled-up microtubes for magnetic sensors on fluids

Ferromagnetic microtubes are fabricated by the deterministic release of thin magnetic films from a photoresist sacrificial layer. Crucial steps towards the fabrication of well positioned and uniform single rolled-up tubes are presented, where the final and main approach consists of releasing angled deposited magnetic films by dissolving an underlying patterned photoresist layer. Microtubes from soft magnetic Ni80Fe20 films and Co/Pt multilayer stacks with perpendicular magnetic anisotropy are obtained with predefined lengths and diameters down to 1.5 µm. The nonlinear rotational dynamics of the tubes in viscous fluids is investigated. Sensitivities to viscosity variations for tubes with different lengths are observed and suggest their potential application as single sensing probes in a wide range of viscosities. Moreover, the variation in the characteristic critical frequency of the rotational motion for chains of magnetically coupled microtubes agrees with the expected changes in the theoretical shape factor of cylindrical particles. Evidence of tube remagnetization during the rotation opens the way to investigate properties such as the switching fields of individual tubes.