Micro-structure Of The Magnetic Properties Of Organic Semiconductor Granular Films, Magnetic And Transport Properties

Recently, organic spintronics has attracted a lot of interest due to the advantage of organic semiconductor and its potential applications. During the past few years, remarkable progress has been achieved in this field. However, the mechanism of these novel spin-related phenomena in organic semiconductors devices is currently not clear. Otherwise, in the past two decades, GMR effect and Hall effect of the inorganic magnetic granular films which consist of single-domain ferromagnetic granules embedded in an immiscible inorganic medium have been extensively studied. It is very interesting that what will happen if the inorganic matrix is replaced by organic semiconductor materials in granular films. Till now, the transport properties of the granular films involving organic semiconductors have not been reported.In this thesis, the microstructures, magnetic and magneto-transport properties in magnetic metal-organic semiconductor granular films were investigated systemically.1. The microstructure and magnetic properties in Co_x(Alq₃)_x granular films with different x have been studied. The HRTEM images show typical characteristics of granular films, roughly spherical dark cobalt granules are embedded in Alq₃ matrix. The average size of Co granules is about 1-5 ran. With the increase of Co concentration, the mount of Co granules increase and the inter-granule space decrease accordingly. In higher x samples, Co granules begin to form network gradually. VSM results show a gradual change from superparamagnetism to ferromagnetism as x increases in these samples.2. The magnetoresistance and transport behavior in Co_x(Alq₃)_x granular film samples have been investigated at different temperature. A transition from negative to positive magnetoresistance effect was observed with the increase of temperature from low temperature to room temperature. Typically, in x=35vol% sample, the positive MR ratio reaches 1.36% at room temperature, and the negative MR ratio reaches -0.6% at 50K. Furthermore, a metal-insulator transition (MIT) was also observed from 300K to 50K. The concentration dependence of MR and resistivity is also investigated. The optimal negative MR is appears in x=44% sample, and the optimal positive MR found in x=0.39 sample. The possible mechanism of transport behavior in the samples is also discussed. At low temperature, the inter-granule tunneling is believed to account for the negative MR effect due to the drastic increasing of the resistance of Alq₃. The origin of positive MR is not very clear now, is perhaps related to the ordinary magnetoresistance in the interface between magnetic metal and organic semiconductor.3. The magnetoresistance in Co/TPD and Fe/Alq₃ granular films has been investigated. It shows that there are the similar physics phenomena in these samples, comparing with the above Co/Alq₃ granular films.