Synthesis And Exchange Bias Of CoO/γ-fe₂O₃Nanocomposite Particles

Exchange bias effect plays an important role both in the fundamental research and the development of the application of spintronics. In the early research, the bulk of exchange bias research has focused mainly on thin film systems, due to the application on the spin-valve and magnetic tunneling devices. In the recent years, the development of new routes for the synthesis of magnetic nanoparticles has inspired complex composite ferromagnetic (FM)/antiferromagnetic (AFM) nanostructures from alternative materials other than partially oxidized metal nanoparticles, which offers a wonderful base to study exchange bias in nanoparticles systems. As discovering the application of exchange bias in the area of high-performance permanent magnet and high-density magnetic recording, the enthusiasm for the study of exchange bias in nanoparticles has been further raised. In this paper, the CoO/y-Fe2O3nanocomposite was obtained through two-step high temperature decomposition of acetylacetonate salt, and the structure, morphology, composition and magnetic properties of the materials were characterized, the exchange bias effect was further studied in this system.The main research contents and results include the following aspects:1. At first, CoO/y-Fe2O3nanocomposite was successfully synthesized through two-step high temperature decomposition, the effect of synthesis condition (e.g. concentration, reaction temperature and time) on the morphology, size, structure and dispersibility of the synthesized sample was studied.2. Well-disperse and uniform CoO/y-Fe2O3nanoparticles with size from7nm to24nm were obtained through changing the precursor concentrations. A large exchange bias field about8000Oe was obtained for the sample with size of21nm. With the increasing of the size, exchange bias field displayed a monotonic tendency, first increased and then decreased, which indicated that one could control the sample size through changing the synthesized condition, and further control the value of exchange bias field.3. Accompanied with the negative shift along the horizonal direction (or field axis), hysteresis loop displayed a positive shift along the vertical direction (or magnetization axis), indicating the existence of uncompensated pinned spins in this system. And the linear relation of the amount of horizonal shift (exchange bias field) and vertical shift was found, which indicated that the uncompensated pinned spins played an important role on the exchange bias effect.4. Compared with other reports, in this system the exchange bias field displays a remarkable thermostability with a higher blocking temperature about290K, which was seldom reported in granular systems.5. The dependence of exchange bias on cooling field was studied at two representive temperature of5K and250K. Exchange bias field showed a strong dependence on the cooling field, and increased with the increasing cooling field until saturated for large cooling field. At5K, the coercivity displayed a monotonic variation with the cooling field, which was closely related to the uncompensated pinned spins changed with cooling field in the system.6. Exchange bias showed a obvious training effect, which could be affected by the cooling field and temperature, the training effect became strong with the increasing temperature and cooling field. And the data was successfully fitted by Binek formula.