STUDY FOR MECHANISM OF GIANT EXCHANGE ANISOTROPY AND DEVELOPMENT OF HIGH PERFORMANCE Mn BASED ANTIFERROMAGNETIC MATERIALS

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The exchange anisotropy is a phenomenon to fix the spins in ferromagnet and utilized in spintronics devices such as magnetic random access memories and spin valve type reproducing head element in ultrahigh density magnetic recording system. The purpose of the present study is to understand the mechanism of exchange anisotropy and to develop the high performance antiferromagnetic materials, through the study of inducing mechanism of giant exchange anisotropy (J_K=1.3 erg/cm^2) that has been discovered by the present authors group. In this research project, following terms have been investigated. 1. Correlation between the microstructure and exchange anisotropy of the exchange biased bilayers. 2. Spin structure and magnetization process of antiferromagnetic layer by soft x-ray magnetic circular dichroism (XMCD). 3. Development of microscopic model for exchange biased bilayers and theoretical calculations. 4. Fabrication and estimation of theoretically designed practical exchange biased bilayers. Main results obtained are summarized as follows. Uncompensated antiferromagnetic spin induced at the interface was quantitatively investigated by XMCD technique. The uncompensated antiferromagnetic spins change their sign and magnitude, as the ferromagnetic Co-Fe layer composition is changed. While, the exchange biasing strength is also changed as a function of the Co-Fe composition, close correlation has been suggested between them. Standing on the above finding, ultra-thin Mn layer insertion at the interface has been designed to enhance the exchange anisotropy and experimentally confirmed the doubling strength of exchange anisotropy. From a dependence of exchange anisotropy on the antiferromagetic layer composition, it is suggested that the inserted Mn layer has 3Q spin structure differently from the bulk Mn, in order to trace the fcc structure of the underlying Mn-Ir layer.