Meiyin Yang

The effect of strain induced by Ag underlayer on saturation magnetization of partially ordered Fe16N2 thin films

Partially ordered Fe-N thin films were grown by a facing target sputtering process on the surface of a (001) Ag underlayer on MgO substrates. It was confirmed by x-ray diffraction that the Ag layer enlarged the in-plane lattice of the Fe-N thin films. Domains of the ordered α″-Fe16N2 phase within an epitaxial (001) α′-FexN phase were identified by electron diffraction and high-resolution aberration-corrected scanning transmission electron microscopy (STEM) methods. STEM dark-field and bright-field images showed the fully ordered structure of the α″-Fe16N2 at the atomic column level. High saturation magnetization(Ms) of 1890 emu/cc was obtained for α″-Fe16N2 on the Ag underlayer, while only 1500 emu/cc was measured for Fe-N on the Fe underlayer. The results are likely due to a tensile strain induced in the α″-Fe16N2 phase by the Ag structure at the interface.

Thermal stability of partially ordered Fe16N2 film on non-magnetic Ag under layer

Partially ordered Fe16N2 thin film with (001) texture is successfully grown on a Ag under layer using a facing target sputtering system. Fe16N2 phase is formed after post-annealing, which is detected by X-ray diffraction (XRD). High saturation magnetization (Ms) of Fe16N2 thin films is observed by vibrating sample magnetometry. It is found that Fe16N2 phase can be stable up to 225 °C, which is demonstrated by the Fe16N2 finger print peak (002) in XRD. After heating to 250 °C, the Fe16N2 phase decomposes, which leads to low Ms and soft magnetic behavior. To further study Fe16N2 decomposition, X-ray photoelectron spectroscopy is performed to detect the binding energy of nitrogen atoms. Differences of binding energy corresponding to before and after heat treatment show the variation of nitrogen atom in electronic state with surrounding Fe atoms, indicating nitrogen atomic migration during heat treatment.

Electromigration induced fast L10 ordering phase transition in perpendicular FePt films

Realizing fast L10 ordering phase transition (LOPT) in L10 structured magnetic materials without heat treatment is crucial for their applications in spintronic devices. This article reports on the electromigration controlled momentum transfer and rapid ordering of Fe and Pt atoms in the as-deposited FePt films. Lattice defects in the films provide enough diffusion pathways and allow the Fe and Pt atoms rearranging. Through the current driven atomic motion and rearrangement, fast LOPT can result in the establishment of perpendicular magnetic anisotropy of the FePt films at room temperature. This effect is expected to work with other L10 typed magnetic materials for spintronic devices development.

Tuning perpendicular magnetic anisotropy and coercivity of L10-FePt nanocomposite film by interfacial manipulation

Based on the interfacial anisotropy manipulation of multilayer structure and the orientation manipulation of surfactant Au atoms, Fe/Pt/Au multilayers were designed to achieve tunable perpendicular magnetic anisotropy (PMA) of L10-FePt nanocomposite films. In the mean time, the ordering degree of the Fe/Pt/Au multilayers was modified by adjusting the defect concentration in the film, which can be controlled by using different multilayer structures and by the diffusion of Au atoms. This makes the coercivity (HC) of L10-FePt nanocomposite films able to be tailored. Thus a L10-FePt nanocomposite film with high PMA and tunable HC was constructed that provides important experimental data for preparing writable FePt perpendicular magnetic recording media.

Dynamical mechanism for coercivity tunability in the electrically controlled FePt perpendicular films with small grain size

This article reports property manipulations and related dynamical evolution in electromigration controlled FePt perpendicular films. Through altering voltage and treatment time of the power supply applied on the films, electronic momentum was fleetly controlled to manipulate the kinetic energy of Fe and Pt atoms based on momentum exchanges. The electromigration control behavior was proven to cause steerable ordering degree and grain growth in the films without thermal treatment. Processed FePt films with small grain size, high magnetocrystalline anisotropy, and controllable coercivity can be easily obtained. The results provide a novel method for tuning magnetic properties of other L10 structured films.

Synthesis of L10-FePt perpendicular films with controllable coercivity and intergranular exchange coupling by interfacial microstructure control

A series of FePtBi/Au multilayers were fabricated by magnetron sputtering. The interfacial microstructure control of Bi and Au atoms and its effect on comprehensive properties of L10-FePt perpendicular films were carefully studied. Results show that: perpendicular magnetic anisotropy of the L10-FePt film can be remarkably enhanced with the epitaxial inducement of Au atoms. On the other hand, intergranular exchange coupling (IEC) of the film is greatly decreased due to the isolation of FePt particles by nonmagnetic Au particles. Moreover, the controllable coercivity of the film can be realized by adjusting ordering degree of the film through diffusion of Bi atoms. Thus, an L10-FePt perpendicular film with controllable coercivity and no IEC is realized with the interfacial microstructure control of surfactant Bi and Au atoms.

Current-Induced Fast-Ordering of L1

Current-induced fast-ordering method (CIFOM) was designed to achieve an ordered L10 -FePt film with small grain size under low energy consumption by applying direct current (DC) in the FePt film. The grain size (~ 8 nm) of the L10-FePt film prepared by CIFOM method is about one-third of that produced by the post-annealing method (PAM). The fast ordering of the L10-FePt film by CIFOM is probably attributed to the momentum exchange between the FePt atoms and the conductive electrons. The short ordering time by CIFOM contributes to the smaller grain size of FePt film.

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(001) textured Fe16N2thin film with Ag under layer is successfully grown on GaAs substrate using a facing target sputtering (FTS) system. After post annealing, chemically ordered Fe16N2 phase is formed and detected by X-ray diffraction(XRD). High saturation magnetization (Ms) is measured by a vibrating sample magnetometer (VSM). In comparison with Fe16N2 with Ag under layer on MgO substrate and Fe16N2 with Fe under layer on GaAs substrate, the current layer structure shows a higher Ms value, with a magnetically softer feature in contrast to the above cases. In addition, X-ray photoelectron spectroscopy(XPS) is performed to characterize the binding energy of N atoms. To verify the role of strain that the FeN layer experiences in the above three structures,Grazing Incidence X-ray Diffraction (GIXRD) is conducted to reveal a large in-plane lattice constant due to the in-plane biaxial tensile strain.

-FePt Films With Small Grain Size

A micromagnetic simulation analysis is systematically carried out to explore the magnetization reversal mechanism, residual stress and exchange interaction in L10-FePt/Au nanocomposite films with perpendicular magnetic anisotropy. Results show that: (1) the domain-wall pinning mode is the main mechanism responsible for magnetization reversal in L10-FePt/Au films; (2) considering the magnetoelastic energy produced by lattice mismatches between Au and FePt, the simulated out-of-plane loop matches the experimental loop very well. The residual tensile stress in the films is quantitatively described by both experimental calculations and micromagnetic simulations; (3) the exchange interaction within FePt grains of the films is strong, which allows for the coherent switching of the magnetization moments of an FePt grain.