Abstract
Due to its large magnetocrystalline anisotropy energy and low critical current density in MRAM, L10 ordered FePd thin films with perpendicular magnetic anisotropy have been probed as candidates in applications of dense magnetic recording media, spintronics devices, etc. FePd/IrMn bilayer system was used to investigate its fundamental exchange coupling effect and applications of hard disk and MRAM. First of all, FePd films were deposited on glass substrates to find out the influences of in-situ annealing temperatures and film stoichiometry on L10 phase transformation and magnetic properties of FePd thin films. When annealing temperature was raised from 400 to 800oC, the magnetic behavior changed from magnetically soft to hard for the polycrystalline FePd films with Pd contents of 51 at.%, while the ordering parameter S increased from 0 to near unity. The 800oC-deposited FePd film has two-phase (L10 + A1) structure and island-like morphology, which exhibited a coercivity of 4.1 kOe and saturated magnetization of 814 emu/cm3. In terms of the stoichiometric effect, the results showed disordered structure and soft magnetic properties for the FePd films with Pd concentration ≦ 47 at.%. For the samples with Pd content of 56 at.%, the single L10 phase with magnetically hard properties was obtained. With increasing Pd contents up to 62 at.%, the crystal structure changed from L10 to L12 phase, as accompanied with magnetic softening. Second, FePd films were grown on MgO(001) substrates to expect better perpendicular magnetic anisotropy. With substrate temperature increasing from 300 to 550oC, the magnetic behavior altered from longitudinal to perpendicular anisotropy for epitaxial (001) FePd films with 54 at. % Fe, whereas ordering parameter S increased from 0.33 to 0.84. With regard to the stoichiometric effect, XRD tests with a synchrotron radiation source showed L10 epitaxial structures of the FePd films with Fe concentrations over a wide range 40–54 at. %, unlike the equilibrium phase diagram of FePd bulk. Magnetic results revealed the presence of a perpendicular anisotropy for epitaxial FePd films with Fe contents from 46 to 54 at.%. The measured Hk was correlated with the L10 ordering of the FePd films. The greatest S, 0.84, and Hk, 20 kOe, were obtained for a film with 54 at. % Fe. The results of this work indicated that a Fe-rich FePd film might be preferable for applications that require a large magnetic anisotropy. Finally, two series of samples of single-layer IrMn and IrMn/FePd films were grown on a singlecrystal MgO substrate at different IrMn deposition temperatures (Ts=300–700 oC). XRD characterization using a synchrotron radiation source revealed that the L12 ordered IrMn phases were obtained in the epitaxial (001) IrMn films with Ts≧400oC. As Ts was raised from 400 to 700oC, the ordering parameter of L12 phase increased from 0.17 to 0.81 as surface roughness increased. The exchange bias behavior of the IrMn/FePd bilayer films with an Heb value of 22 Oe was obtained when the FePd layer was deposited on the disordered IrMn layer. Increasing the L12 ordering of IrMn layers gradually reduced the Heb values to 0 Oe. The increased surface roughness of the IrMn layers with compensated surface spins did not enhance the Heb of the FePd. The change of the antiferromagnetic structure of IrMn layers from the A1 phase to the L12 phase was responsible for the evolution of Heb of FePd.