Ferromagnetic / Antiferromagnetic Bilayers, The Coercivity And Perpendicular Magnetic Anisotropy In Magnetic Multilayers

According to statistics, 92% of the new information was stored on magnetic media, mostly in hard disk drivers. Thus, hard disk driver storage technology is one of the most important technologies for information processing.Exchange bias is one of essential physical mechanisms in the basic structures of hard disk driver head and magnetic random access memory. Although it has been extensively studied during the last several decades, some aspects remain unclear. On the other hand, materials with high magnetic anisotropy and high coercivity are demanded for next generation hard disk driver storage technology. Both of the above two fields were closely involved in this thesis, which is divided into three parts.In first part, exchange biasing in Co₂₀Ni₈₀/FeMn and Co₈₀Cr₂₀/FeMn bilayers has been investigated where the CoCr layers are of granular structure and the CoNi layers are in the form of single phase. In above two series of bilayers, the exchange field is proportional to 1/tFM (tFM=ferromagnetic layer thickness). For CoNi/FeMn bilayers, the coercivity and the uniaxial anisotropic field decrease with increasing tFM with a linear scale of 1/tFM- Since they are equal to each other, the magnetization reversal process can be described by magnetization coherent rotation and the coercivity enhancement can be explained in terms of a uniaxial anisotropy model. For CoCr/FeMn bilayers, however, the coercivity displays an unusual behavior. First, in comparison with that of single CoCr layer films, the coercivity is reduced instead enhanced. Secondly, it increases with increasing tpM- Finally, the coercivity of the bilayers is not equal to the uniaxial anisotropic field. A multidomain form is proposed to occur during magnetization reversal process. The different characteristics of the coercivity and magnetization reversal mechanisms in the two series of bilayers result from the different microstructures in the CoNi and CoCr layers. The present work might be helpful to clarify the mechanism for the coercivity enhancement in ferromagnet/antiferromagnet bilayers.In second part, magnetic properties of TbFeCO/NiO multilayers are studied, in which Tb_x(Fe₉₀Co₁₀)₁₀₀-x(40 nm) single layer films and Tb_x(Fe₉₀Co₁₀)₁₀₀-x(40 nm) /NiO(3.3 nm) multilayers with x = 17 and 19 were prepared. Perpendicular anisotropy and out-of plane coercivity of multilayers are enhanced, in comparison with those of TbFeCo single layer films. The enhancement of perpendicular magnetic anisotropy is suggested to originate from perpendicular exchange coupling. The perpendicular...