Surface Roughness and Magnetic Properties of Ni and ${\rm Ni}_{78}{\rm Fe}_{22}$ Thin Films on Polyethylene Naphthalate Organic Substrates

We have studied structural, electrical, and magnetic properties of Ni and ${\rm Ni}_{78}{\rm Fe}_{22}$ thin films evaporated on polyethylene naphtalate (PEN) organic substrates towards the fabrication of spin quantum cross (SQC) devices. As we have investigated the scaling properties on the surface roughness, the surface roughness of Ni (16 nm)/PEN is 0.34 nm, corresponding to 2 or 3 atomic layers, in the scanning scale of 16 nm, and the surface roughness of ${\rm Ni}_{78}{\rm Fe}_{22}$ (14 nm)/PEN is also as small as 0.25 nm, corresponding to less than 2 atomic layers, in the scanning scale of 14 nm. These facts denote that Ni/PEN and ${\rm Ni}_{78}{\rm Fe}_{22}/{\rm PEN}$ are suitable for magnetic electrodes on organic substrates used for SQC devices from the viewpoint of the surface morphology. Then, we have investigated magnetic hysteresis curve and magnetoresistance effects for Ni/PEN and ${\rm Ni}_{78}{\rm Fe}_{22}/{\rm PEN}$. The squareness of the hysteresis loop is as small as 0.24 for Ni (25 nm)/PEN, where there is no observation of the anisotropy magnetoresistance (AMR) effect. In contrast, the squareness of the hysteresis loop is as large as 0.86 for ${\rm Ni}_{78}{\rm Fe}_{22}$ (26 nm)/PEN, where the AMR effect has been successfully obtained. These experimental results indicate that ${\rm Ni}_{78}{\rm Fe}_{22}/{\rm PEN}$ is a promising material for use in SQC devices from the viewpoint of not only the surface morphologies but also magnetic properties.