Guanghua Yu

Magnetic properties and microstructure of FePt/Au multilayers with high perpendicular magnetocrystalline anisotropy

FePt/Au multilayers were deposited on preheated 100 °C MgO (001) single crystal substrates by magnetron sputtering. Magnetic properties and microstructure of the films were studied. Results show that the film possesses high coercivity HC, excellent perpendicular magnetic anisotropy (PMA), and weak interparticle exchange coupling (IEC) after annealing. FePt and Au can grow epitaxially on MgO substrates along the [001] direction, leading to excellent PMA. Au atoms partly diffuse to the boundaries of FePt phases and function as grain refiners and particle isolators, which markedly decrease the FePt grain size and IEC. Moreover, the additional FePt/Au interface energy, the stress energy due to the mismatch between FePt and Au lattices, and the diffusion of Au atoms all promote the ordering process of FePt film. This leads to the decrease in ordering temperature by 150 °C and a considerable increase in HC.

Low-temperature ordering and enhanced coercivity of L10-FePt thin film promoted by a Bi underlayer

We have studied L10-FePt thin films with a Bi underlayer. Experimental results show that the ordering temperature of the L10-FePt film can be lowered down to 300–350°C and the in-plane coercivity HC of the film is obviously enhanced by introduction of a Bi underlayer. After annealing at 400°C for 20min, a Bi∕FePt film realizes a HC of 13.5kOe and the ratio of the remnant magnetization Mr to the saturation one MS of as high as 0.93. The reduction of the ordering temperature and the enhanced magnetic properties of L10-FePt films can be understood by considering the large diffusivities of Fe and Pt associated with Bi diffusion.

Interfacial oxygen migration and its effect on the magnetic anisotropy in Pt/Co/MgO/Pt films

This paper reports the interfacial oxygen migration effect and its induced magnetic anisotropy evolution in Pt/Co/MgO/Pt films. During depositing the MgO layer, oxygen atoms from the MgO combine with the neighboring Co atoms, leading to the formation of CoO at the Co/MgO interface. Meanwhile, the films show in-plane magnetic anisotropy (IMA). After annealing, most of the oxygen atoms in CoO migrate back to the MgO layer, resulting in obvious improvement of Co/MgO interface and the enhancement of effective Co-O orbital hybridization. These favor the evolution of magnetic anisotropy from IMA to perpendicular magnetic anisotropy (PMA). The oxygen migration effect is achieved by the redox reaction at the Co/MgO interface. On the contrary, the transfer from IMA to PMA cannot be observed in Pt/Co/Pt films due to the lack of interfacial oxygen migration.

Magnetic properties and microstructure of FePt∕BN nanocomposite films with perpendicular magnetic anisotropy

We have prepared L10-FePt∕BN nanocomposite films by depositing the FePt∕BN mutilayers on the MgO(100) substrates using magnetron sputtering followed by a postannealing process. We found the excellent perpendicular magnetic anisotropy of [FePt(2nm)∕BN(0.5nm)]10 film after annealing at 700°C for 1h x-ray diffraction patterns also indicate an excellent (001) texture of the film. This film shows a perpendicular coercivity of 7.48kOe and the ratio of the remnant magnetization Mr to the saturation one MS of as high as 0.93. We also found the good epitaxial growth of FePt grains on the MgO(100) substrate by cross-section high resolution transmitting microscopy.

Improvement of magnetic property of L10‐FePt film by FePt∕Au multilayer structure

We have prepared L10‐FePt films by sputtering FePt∕Au mutilayers on glass substrates followed by a vacuum annealing process. The magnetic measurement and x-ray diffraction indicate that the ordering temperature of the FePt films is reduced to 350°C and the coercivity (HC) is greatly enhanced by using the FePt∕Au multilayer structure. The interface energy and the stress energy due to the misfit between FePt and Au crystal lattices provide extra driving force to the ordering process of FePt films. Additionally, partial Au atoms are supposed to diffuse to the FePt grain boundaries during annealing, which consequently pins the magnetic domain wall and leads to the high HC.We have prepared L10‐FePt films by sputtering FePt∕Au mutilayers on glass substrates followed by a vacuum annealing process. The magnetic measurement and x-ray diffraction indicate that the ordering temperature of the FePt films is reduced to 350°C and the coercivity (HC) is greatly enhanced by using the FePt∕Au multilayer structure. The interface energy and the stress energy due to the misfit between FePt and Au crystal lattices provide extra driving force to the ordering process of FePt films. Additionally, partial Au atoms are supposed to diffuse to the FePt grain boundaries during annealing, which consequently pins the magnetic domain wall and leads to the high HC.

Mechanism of magnetoresistance ratio enhancement in MgO/NiFe/MgO heterostructure by rapid thermal annealing

To reveal thermal effects on the film quality/microstructure evolution and the resulted magnetoresistance (MR) ratio in MgO/NiFe/MgO heterostructures, positron annihilation spectroscopy studies have been performed. It is found that the ionic interstitials in the MgO layers recombine with the nearby vacancies at lower annealing temperatures (200-300 degrees C) and lead to a slow increase in sample MR. Meanwhile, vacancy defects agglomeration/removal and ordering acceleration in MgO will occur at higher annealing temperatures (450-550 degrees C) and the improved MgO and MgO/NiFe interfaces microstructure are responsible for the observed significant MR enhancement

Enhancement of the magnetic field sensitivity in Al2O3 encapsulated NiFe films with anisotropic magnetoresistance

The anisotropic magnetoresistance value (ΔR/R) and magnetic field sensitivity (Sv) of NiFe films can be remarkably enhanced by using Al2O3 encapsulation, and the Sv is comparable to that of a spin valve. For an ultrathin NiFe film with the structure of Ta/Al2O3/NiFe/Al2O3/Ta, the Al2O3 layers suppress the formation of the magnetic dead layers and the interdiffusions between the NiFe layer and Ta layers, and decrease the current shunting of the Ta layers. More importantly, the flatter Al2O3/NiFe and NiFe/Al2O3 interfaces can significantly enhance the specular reflection of conduction electrons and lead to a higher ΔR/R. In addition, the formations of the NiFe (111) texture and the columnar grains by annealing can also increase the ΔR/R.

Iron cobalt/polypyrrole nanoplates with tunable broadband electromagnetic wave absorption

Magnetic nanoparticles (NPs)/conducting polymer nanoplates are great potential candidates for advanced electromagnetic wave absorbents due to their unique physical properties such as strong absorption, low density and good anti-oxidation. In this work, iron cobalt nanoparticles (FeCo NPs) were prepared by a modified chemical reduction method and FeCo/polypyrrole (PPy) nanoplates were synthesized by in situ chemical oxidative polymerization. The nanoplates exhibited an excellent electromagnetic wave absorbing performance. The maximum reflection loss (RL) reaches −52.30 dB at 13.40 GHz with an absorber thickness of 2.15 mm and the effective absorbing width (RL < −10 dB) is located at approximately 11.31–16.37 GHz. The dipole polarization, interface polarization and space-charge polarization of the nanocomposites contribute to the dielectric loss of the electromagnetic wave. Moreover, the natural resonance, exchange resonance and eddy current loss of the nanocomposites contribute to the magnetic loss of the electromagnetic wave. The synergetic effect of both the dielectric and magnetic losses of the nanocomposites endows the FeCo/PPy nanoplates with excellent broadband electromagnetic wave absorption.

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.

Anomalous Hall effect engineering via interface modification in Co/Pt multilayers

An enhanced anomalous Hall effect (AHE) is observed via interface modification in MgO/[Co/Pt]2/Co/MgO multilayers, due to the insertion of a Hf metal layer at the Co/MgO interface. It is shown that the saturation anomalous Hall resistivity is 215% larger than that in the multilayers without Hf insertion. More importantly, thermally stable AHE and perpendicular magnetic anisotropy features are obtained in MgO/[Co/Pt]2/Co/Hf/MgO multilayers. The AHE is improved for sample MgO/[Co/Pt]2/Co/Hf/MgO by annealing, which is attributed to the enhancement of the side jump contribution.