Fabrication And Properties Of Vertically Oriented Hard/soft Magnetic FePt Exchange Springs

The hard/soft exchange coupling is an important problem in Magnetic research,and it is particular interested in the development of ultra-high magnetic energy product permanent magnet materials and high density magnetic recording materials.The L10-FePt(Face Centered Tetragonal,fct)alloy,with its high uniaxial magnetocrystalline anisotropy Ku,ultra-small superparamagnetic limit size,high coercive force Hc,high saturation magnetization Ms and excellent chemical stability,so that it is a promising candicate for next generation ultra-high density magnetic storage media.The coercive force is too high to overwrite the data.Exchange coupling is an effective way to solve the problem.By inserting a layer of soft material that low magnetocrystalline anisotropy Ku and low coercive force Hc,the exchange spring at hard/soft interface can adjust the writing magnetic field,and ensure the high magnetocrystalline anisotropy and stability of the hard layer.By using the magnetron sputtering,FePt(001)and L10-FePt(001)/A1-FePt and L10-FePt(001)/MgO/A1-FePt textured bilayer exchange springs were fabricated on MgO(001)single-crystal substrates.By changing the annealed temperature(Ta)for hard layer and the thickness of soft layer of L10-FePt/A1-FePt bilayer films and changing the thickness of MgO middle layer of L10-FePt/MgO/A1-FePt Multilayer films,the properties of films that structure and Morphology and magnetic behavior were investigated.The results are as follows:(1)Soft A1-FePt(30 nm)films were deposited on MgO(001)single-crystal substrates at 400?,and subsequently annealed at temperatures(Ta)in range of [400?,700?] for 6 h in order to turn them into hard magnets with different degrees of A1?L10 transformation.At Ta ? 600?,the coverage of hard layer was 100%,though the grain size increased with Ta.At Ta = 500? and Ta ? 600?,A1-FePt and L10-FePt coexisted in the hard layer with an ordering degree of 0.61 and 0.84,respectively.It formed an exchange coupling complex,the exchange interaction between the two phase resulted in a vertical magnetocrystalline anisotropy.And the easy magnetization curves showed rectangle-like shapes,and a moderate coercive force of ~ 5 kOe was obtained at Ta = 500?.In consideration of the ordering degree of film and the continuity of surface morphology,the preparation of hard magnetic/soft magnetic spring,The hard layer of Ta = 500? and Ta ? 600? can be used to prepare hard/soft magnetic exchange spring.(2)Soft A1-FePt(x nm)films were deposited on the hard layer that using films of Ta = 500? and 600? in order to obtain L10-FePt(30 nm,Ta)/A1-FePt(x nm)textured bilayer exchange springs.L10-FePt/A1-FePt bilayer films with 20 nm thick soft layers kept rigid behaviors,and the easy magnetization curves showed rectangle-like shapes,Soft layer and hard layer together with the reversal moments.Two reversal steps for respective soft layer and hard layer appeared in magnetization curves if the thickness of soft layer was 30 nm.This indicates that the hard/soft interlayer exchange length was in range of 20 nm < lex < 30 nm,at least twice of ideal L10/A1-FePt exchange spring.The reason is due to that the coexistence of A1-FePt and L10-FePt in hard lowered the efficient uniaxial magnetocrystalline anisotropy.By changing the degrees of A1?L10 transformation,we can control the coercive force and the exchange length.This is benefit to design the properties of magnetic springs for different applications.(3)When a MgO middle layer inserted into L10-FePt(001)(30 nm,Ta)/A1-FePt(30 nm)bilayers,the middle layer can prevent the mutual diffusion between the two phase layer in order to ensure the complete partition of the two phases at the interface is in accordance with the ideal exchange coupling model.The change of the thickness of the MgO middle layer affected the exchange behavior of the exchange coupling system,which indicated that the addition of the MgO middle layer can effectively adjust the coupling strength.It is possible to change the magnitude of the reverse magnetic field of the hard magnetic layer by adding the thickness of the intermediate layer while maintaining the thermal stability of the hard magnetic layer.