Paper-based spintronics: Magneto-resistivity of permalloy onto paper substrates

Driven by low-cost, resource abundance and distinct material properties, the use of paper in electronics, optics and fluidics is under investigation. In order to realize a dense coverage of sensor networks as part of the roadmap of the internet-of-things, achieving lower manufacturing cost of the aforementioned sensors is required. Considering microelectro-mechanical systems based on magneto-resistance principles (anisotropic, giant, tunnel) that are conventionally manufactured onto inorganic semiconductor materials, we propose the use of paper substrates for cost reduction purposes primarily. In particular, we studied the magneto-resistance sensitivity of permalloy (Py:Ni₈₁Fe₁₉) onto paper substrates utilized in various daily applications. Here, the Py:Ni₈₁Fe₁₉ coating was developed by means of, but not limited to, sputter deposition, and spans an area of 10x10 mm² and a thickness of 70 nm. In this research, we investigated several paper materials covering a range of grammage of [80, 350] g/m², surface roughness of [0.21, 3.462] μm given by the root mean squared R_ims, various impregnations, porosity levels and surface macro-structures. Yet, in this paper, we focus on our findings with clean room paper (80 g/m², R_rms = 2.877 μm, 23% surface porosity, latex impregnation, no embossed macro-structure). Employing a four-point-probe resistivity measurement setup, we investigated the change of electrical resistance of Py:Ni₈₁Fe₁₉ under the presence of an oriented external magnetic field. Compared to the theoretical limit of 2.5% of Py:Ni₈₁Fe₁₉ on smooth surfaces, we have obtained large and positive magneto-resistive changes (2.5 - 14%) for these aforementioned systems. Principally, we analyzed the effect of surface topology of the clean room paper on the magneto-resistance sensitivity of Py:Ni₈₁Fe_{19- /sub>. We concluded that the occurrence of such magnetic behavior is most probably due to tunneling of electron waves through the asperities and porosity of the paper surface and subsequent scattering of electrons at pinned domain walls.}