Room temperature magnetoresistance effects in ferroelectric poly(vinylidene fluoride) spin valves

Ferroelectric poly(vinylidene fluoride) (PVDF) nanofilms have been fabricated by the Langmuir-Blodgett technique, possessing mainly a ferroelectric active phase and a controllable film thickness of 2.3 nm per layer. Atomic force microscopy and Fourier transform infrared spectroscopy were utilized to characterize the film properties. Importantly, the PVDF films could act as barrier layers to prepare spin transport devices using Fe₃O₄ and Co as bottom and top ferromagnetic electrodes, respectively. Spin-dependent electron transport behaviors were systemically studied in these devices by varying the PVDF film thickness from 3 layers (7 nm) to 13 layers (30 nm). With increasing PVDF layer numbers, the magnetoresistance (MR) response decreases likely due to the change in spin transport from tunneling to hopping transport. We further investigated the MR dependence on operation temperatures (150 K, 200 K, 250 K and 300 K). It is noteworthy that the MR effect was observed even at 300 K with an MR ratio exceeding 2.5%, which is achieved for the first time in such organic devices. The device performance could be further improved at lower operation temperatures. The MR ratios, device resistances and electron transport mechanisms in the present devices were also discussed to analyze the spin transport behaviors. The results indicate that the ferroelectric PVDF nanofilms are promising candidates for spin devices operated at room temperature, thereby shedding light on the design of organic ferroelectric spintronics with a higher performance.