Room-temperature ferromagnetism of Ni-dope organic semiconductor

Po Hsiang Fang^1*, Fu Hsuan Chang¹, Wei Yang Chou¹

¹Department of Photonics, National Cheng Kung University, Tainan, Taiwan

* Presenter:Po Hsiang Fang, email:l78011130@gs.ncku.edu.tw

Organic semiconductor materials are mostly used in organic field-effect transistors as the active layers, in which they have the advantages of low cost, flexibility, large-area manufacturing, and easy acquisition. If magnetic atoms are doped into organic semiconductor devices, we can not only control the transmission of carriers by using the electric field but also enhance the transmission and injection of spin electrons by applying an external magnetic field to reduce the dynamic power consumption of the device. In this study, the influences of substrate temperature during growth (substrate temperatures of 25 °C, 50 °C, 75 °C and 100 °C) on the magnetic properties of Ni-doped pentacene thin films are discussed.
From the analysis of atomic force microscope (AFM), it can be observed that the grains of the pentacene films increase with the increase of the growth temperature. When the substrate temperature increases, the lateral diffusion of pentacene molecules causes the crystal grains to become larger. For the Ni-doped magnetic pentacene films, as the growth temperature of the organic semiconductor increases, the intermittent grains are like bursting and the distance between the crystal grains becomes larger. The higher the growth temperature, the greater the substrate stress on the film results in a phenomenon like crystal grains being exploded on the surface of the film.
To study the magnetism of Ni-doped pentacene film, the magnetic force microscope (MFM) was used to measure the magnetic signals on the film surface. The phase diagram of MFM reveals that no magnetic signal can be observed on the native pentacene films. However, the signal of phase change becomes more and more obvious with the increase of substrate temperature for the Ni-doped pentacene films. The magnetic clusters on the MFM image just coincide with the grains on the AFM image, indicating that the spin coupling occurs between Ni atoms and pentacene molecules. The magnetic hysteresis measured by superconducting quantum interference device (SQUID) supports the operation of Ni-doped pentacene samples at room temperature. We found that as the growth temperature of magnetic organic films increases, the coercivity of the magnetic organic films increases from 71.9 Oe to 257.6 Oe, and the saturation magnetization also decreases from 5.81 emu/cm³ to 0.79 emu/cm³. We infer that the electron spin coupling between the π electron of the organic semiconductor pentacene and the d-orbital electron of Ni atom may increase and the size of the magnetic domain of the magnetic organic thin film will also become larger as the growth temperature rises.

Keywords: room-temperature ferromagnetism, ferromagnetic semiconductor, Ni-doped