| Recently, the information explosion stresses the lack of data storage year by year, which obliges the storage capability to be raised urgently. However, the areal density of hard disk drive (HDD), as the leading technology for data storage, is approaching to the superparamagnetic limit of1Tb/in2. Ll0-FePt, whose perpendicular magnetocrystalline anisotropy constant Ku is as high as7×107erg/cc and smallest thermal stable grain size reaches2.4nm, is one of the most candidates to realize10Tb/in2ultra-high areal recording density. In order to make the high Ku media such as Ll0-FePt writable, exchange coupled composite (ECC) media consisting of a hard storage layer and a soft switching-assisted layer has been proposed. Ll0-FePt based ECC films has become a hot research point in the past several years. Fe and other in-plane Ku soft materials were initially used as the soft region and decreased the switching field of FePt effectively. However it is disadvantage for thermal stability of ECC structure while a non-perpendicular anisotropy soft layer covered on the top of FePt layer, revealed by micromagnetism simulation. The recording properties are supposed to be well improved for perpendicular ECC structure with both perpendicular soft and hard layers. Under the background all above, we take a lot of experimental work on fabrication and property study of Ll0-FePt based perpendicular ECC films.Firstly, Co/Pt and Co/Ni multilayer with perpendicular anisotropy are used to replace Fe as the soft region coupled with Ll0-FePt to prepare perpendicular ECC structure. Well island-like grains morphology, small switching field, high tolerance of switching field angular-dependence and high thermal stability are observed in Ll0-FePt/[Co/Pt]N and Ll0-FePt/[Co/Ni]N composite films.Secondly, the magnetic reversal mechanism of composites changes from consistent rotation model to incoherent rotation model, and finnaly changes to domain-wall assisted reversal model, while the thickness of soft Co/X (X=Pt or Ni) multilayer varies from thin to thick. When the soft is thin, decreasing the strength of exchange coupling (Jex) mildly promots the rotaion of soft and benefits the reduction of coercivty of FePt. On the contrary, decreasing Jex, leading to domain-wall pinned on the interface much more easily, makes the switching of FePt more hard while the soft is thick.At last, in order to overcome the ledge-type continuous soft structure observed in conventional FePt-ECC films while the soft region is thin, which is disadvantage of signal-to-noise ratio (SNR) and thermal stability of recording media, we prepare a series of [FePt-Ag]/[Co/Ni]N films with a Ag interlayer deposited in Ll0-FePt. Well distributed isolated-grain structure is observed for all [FePt-Ag]/[Co/Ni]N films with N various from3to30. And the average grain size is keeps constant of46nm. Meanwhile the thermal stability represented by KeffV/KBT of [FePt-Ag]/[Co/Ni]N is always over300. |
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