Preparation And Study Of Ferromagnetic / Ferrimagnetic Bilayers

The fast growth of data storage has become the key character of the information age, and magnetic recording is the most important technology of data storage.According to forecast,the total volume of digital information in the whole world will reach 1 trillion Gbit in 2010 hopefully,and it improves the fast growth of storage technology.In order to keep the leading position of Hard Disk Drive(HDD) industry,the researchers in HDD industry work hard to improve the recording density of HDD.As a result,in Aug 2008 Hitachi Ltd announced that it has demonstrated the technical feasibility of magnetic recording at 610 Gbit/in~2,and the size of single recording unit has been reduced to the scale of several nanometers.Under this situation,superparamagnetic effect has become the primary factor which limits the improvement of recording density.In order to overcome this limit,the materials with high K_u has gained universal interest in research field today,but it also triggers another problem of writing limit.Many methods are proposed to solve these problems,such as Heat-Assisted Magnetic Recording(HAMR),Exchange Spring Media, Bit-Patterned Media(BPM),Exchanged-Coupled Composite(ECC) media,and so on. HAMR is believed to be one of the most promising techniques,and the writing-in ability will be further enhanced by adopting bilayer structure.Under this background,we studied the fabrication and magnetic characteristics of ferromagnetic/ ferrimagnetic exchange coupled bilayer thin films.In this thesis,the research work is mainly divided into 2 parts:In the first part,we improved a measurement system of longitudinal surface magneto-optic Kerr effect in large magnetic field,considering magneto-optic Kerr technique has been a powerful tool to deeply investigate the surface magnetics.By increasing the incidence angle of laser beam with a convex lens,improving the intensity of Kerr signal,as well as decreasing the spacing between magnetic poles,we are able to measure the samples with large coercivity in plane.Meanwhile,the incident laser beam is divided into 2 beams in order to solve the instability problem of laser.In the second part,the fabrication and magneitc characteristics of ferromagnetic material with high K_u(FePt),ferrimagnetic material(TbFeCo) and ferromagnetic/ ferrimagnetic exchange coupled bilayer thin films(FePt/TbFeCo) was studied.Firstly,we studied how the ordering temperature of FePt was decreased by doping Cu into FePt alloy; secondly we studied the change of magnetic properties of armophous TbFeCo thin films in different fabrication conditions(changing sputtering power,Ar pressure and annealing time),and observed the separation of magnetic reversal phase by VSM measuring in different applied magnetic field,which implied the existence of crystallites with easy axis of magnetization in plane;thirdly we fabricated the FePt/TbFeCo bilayer thin films,and also observed the separation of magnetic reversal phase in the exchange coupled films by MOKE and VSM.An explanation has been proposed,which attribute this phenomenon to the existence of FeCo-rich crystallites.We hope that this work will help the understanding of how micro-crystallites affact the magnetic properties of amorphous ferrimagnetic films.