Structures And Magnetic Properties Of L1₀-FePt/B2-FeRh Composite Bilayer Films

With the development of information technology, amount of data is exploding at an unprecedented speed. In order to increase the storage capacity of magnetic recording, it is essential to improve the areal storage density. As a kind of important information storage media, hard disk stores data in magnetic material. Improving storage density within a certain level of signal noise ratio (SNR) means that the volume of each magnetic grain should be decreased. In order to overcome the superparamagnetic effect, materials with high uniaxial magnetocrystalline anisotropy energy such as L10-FePt are needed. However, the switch field of grain is normally too large to reverse its magnetization. This brings about the problem of rewriting. The so called "trilemma" of writing ability, SNR and thermal stability restraints the further development of magnetic storage technology. In order to stabilize the saved data and be able to rewrite new data conveniently, a composite media combining B2 phase FeRh alloy showinga special thermal antiferromagnetic(AFM)-ferromagnetic(FM) transition with L10 phase FePt alloy can be fabricated. By using heat assisted magnetic recording, the interlayer coupling switches reversibly between exchange spring and exchange bias. With a laser beam, the neaded writing field can be lowered efficiently by heating the media to the magnetic phase transition temperature of FeRh. At room temperature, antiferromagnetic FeRh layer pins the magnetization of FePt layer.In this thesis, MgO(001) single crystals were selected as substrates and FePt/FeRh bilayer films were magnetron sputtered on them. Changing the degree of A1â†'L10 transition by annealing at different temperatures, the coercive force of FePt layer could be controled. And the AFM-FM transition temperatures of FePt/FeRh could also be raised more away from room temperature. The main research results are summarized as follows:(1) The effect of substrate temperature on growth orientation of FePt was investigated. The results indicate that FePt(25 nm) was not able to grow epitaxially at 50℃. Annealed at 700℃ for 6 h, no obvious texture was observed. By heating substrate to 400 ℃, the film showed good (001) texture with continuous morphology. A1 phase could transit into L10 phase after annealing. Deposited at 600 ℃, L10-FePt was abtained without anealling. The film was morphologically continuous even the thickness was reduced to 15 nm.(2) In order to analyze the influence of annealing on structure, FePt(25 nm) films were deposited at 400℃, and subsequently annealed at dofferent temperatures (Ta) between 400-700 ℃ for 6 h.. At Ta= 450℃, the film was a mixture of A1 phase and L10 phase. Applying field along the direction of hard axis (in plane), the coercive force was as high as 5.6 kOe. However, applying field along the direction of easy axis (vertical to plane),, the coercive force was as small as 1.9 kOe. This suggests the existence of significant exchange coupling between A1 and L10 phases. At Ta=600℃, the in-plane coercive force increased to 6.5 kOe due to enhanced,A1-L10 coupling. On the other hand, the out-of-plane coercive force decreased to 1.8 kOe. This was the result of grain growth in continuous film. The reversal mechanism was dominantly contributed by exchange spring. At Ta =700 ℃, FePt transited into L10 phase thoroughly. The film was composed of separated islands. At the separations, Pt precipitated during ordering was detected. The magnetization was reversed mainly by nucleation.(3) FePt/FeRh composite bilayer films were obtained by covering FeRh(50 nm) at 450 ℃ on annealed FePt(25 nm), followed by an annealing at also 450 ℃ for 24 h. The results showed that the precipitated Pt diffused into FeRh layer. The diffusion threshold was determined by the A1â†'L10 transition degree of FePt layer.. Helped by residual A1-FePt, Pt diffused into FeRh more easily at a lower A1 â†'L10 transition degree. And helped by Pt diffusion, the AFM-FM transition temperature of FeRh in FePt/FeRh composite bilayer film could increase from 100℃ to 200℃ at Ta≤ 600℃. This is useful to improve the thermal stability of heat assisted perpendicular magnetic recording composite media. At a temperature over AFM-FM transition temperature of FeRh, the magnetization of reversal curve jumped at 2 critical fields, suggesting the formation of hard/soft exchange spring. The coercive force could be a half of that measured at room temperature.