Dangwei Guo

In situ fabrication of Co90Nb10 soft magnetic thin films with adjustable resonance frequency from 1.3to4.9GHz

In this work, we realized in situ fabricated Co90Nb10 soft magnetic thin films, with variable in-plane uniaxial magnetic anisotropy, without using the field induced method or postfabrication treatment. In situ control over the in-plane uniaxial magnetic anisotropy field, which varied from 1.6to22.7kAm−1, was achieved by adjusting the deposition oblique angle from 0° to 38°. As a consequence, the resonance frequencies of the films were continuously increased from 1.3to4.9GHz.

Adjust the resonance frequency of (Co90Nb10/Ta)(n) multilayers from 1.4 to 6.5 GHz by controlling the thickness of Ta interlayers

In this work, the static and high frequency magnetic properties of (Co90Nb10/Ta)(n) multilayers have been investigated. The results show that the in-plane uniaxial magnetic anisotropy fields can be adjusted from 12 to 520 Oe only by decreasing the thickness of Ta interlayers from 8.0 to 1.8 nm. As a consequence, the resonance frequencies of the multilayers continuously increased from 1.4 to 6.5 GHz. It was found that the changes in the in-plane uniaxial anisotropy field are ascribed to the interlayer interactions among the magnetic layers by investigating the delta M(H) curves.

Extending the Snoek’s limit of single layer film in (Co96Zr4∕Cu)n multilayers

The present work investigates the high frequency characteristics of (Co96Zr4∕Cu)n multilayers. The results reveal that the Snoek’s limit of (Co96Zr4∕Cu)n multilayers (revised Acher’s limit) are much larger (as a fact 1.76 for some samples) than that of single layer {Acher’s limit [O. Acher and A. L. Adenot, Phys. Rev. B 62, 11324 (2000)]} by comparing their static and dynamic magnetism. It is found that the differences of Acher’s limit between multilayers and single layers are caused by magnetic interface anisotropy. This work might facilitate search for new materials with high permeability at high frequency.

Adjustable resonance frequency and linewidth by Zr doping in Co thin films

Co100−xZrx (40 nm) thin films with different Zr compositions were grown on silicon substrates by radio frequency magnetron sputtering. The coercivity decreased with an increase in the Zr composition. A uniaxial anisotropy existed in the Co100−xZrx films, and the anisotropy field of the films decreased from 55 to 40 Oe with the increase of Zr composition. The resonance frequency and linewidth were decreased with the increase of the Zr composition during the permeability measurements. For samples with x=0 and 9, the magnetic anisotropy effective field and saturated field were obtained by fitting the external magnetic field dependent resonance frequency with Landau–Lifschitz–Gilbert equation. Therefore, it is an effective way to get the adjustable anisotropy field and linewidth, which is desirable for obtaining the high resonance and high permeability ferromagnetic film materials for high frequency application.

Piezoelectric control of magnetic anisotropy in the Ni0.46Zn0.54Fe2O4/Pb(Mg1/3Nb2/3)O3-PbTiO3 composite

A gate-controllable in-plane magnetic anisotropy with C2v symmetry was observed in a Ni0.46Zn0.54Fe2O4/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure. Detailed amplitude analysis reveals a linearly electric modulation in anisotropy energy that arises from a strain-mediated magnetoelectric coupling across the interface. In particular, an electrically-driven rotational in-plane magnetic easy axis and anisotropic-isotropic transition in NiZn ferrite film, respectively, enable possibilities for magnetization control in multiferroic devices.

High-frequency magnetic properties of Zn ferrite films deposited by magnetron sputtering

The effect of thermal annealing on structural and magnetic properties has been investigated for Zn ferrite films deposited on Si (111) substrates using radio frequency magnetron sputtering. The saturation magnetization at room temperature was enhanced upto 303 emu/cm3 by annealing at relatively low temperature of 200 °C and decreased at higher temperatures. The complex permeability μ=μ′-iμ″ values of the ferrite films as-deposited and annealed at 200 and 400 °C were measured at frequency upto 5 GHz. These films exhibited better high-frequency properties, especially, the film annealed at 200 °C had a large μ′ of 19.5 and high resonance frequency fr of 1.61 GHz. And the reason was investigated preliminarily based on the bianisotropy model.

Anomalous positive exchange bias in nanostructured FeMn/Co/FeMn networks

FeMn/Co/FeMn multilayers are sputtered onto porous alumina templates and silicon, respectively. The FeMn/Co/FeMn multilayer on the porous alumina templates forms an interconnected network nanostructure, while the FeMn/Co/FeMn multilayer on the silicon substrate forms a continuous film. The SQUID testing results show that the exchange bias (HE) and coercivity (Hc) of the FeMn/Co/FeMn multilayer on the porous alumina templates strongly depend on the temperature. A positive exchange bias loops shift is observed at 250 K under field-cooled conditions. However, this is not found in the FeMn/Co/FeMn multilayer on silicon for the same layer thickness. We attribute the positive exchange bias loops shift of the network nanostructured FeMn/Co/FeMn multilayer to the decreased exchange coupling due to the existence of the holes in the interconnected nanostructure.

Magnetic properties of (Co0.65Fe0.35)1-x(Ni0.5Zn0.5Fe2O4)x bi-magnetic composite granular films for high frequency application

A series of (Co0.65Fe0.35)1?x(Ni0.5Zn0.5Fe2O4)x bi-magnetic composite granular films with different ferrite atom fraction x are fabricated by magnetron sputtering. Scanning electron micrographs and x-ray diffraction results show that the films consist of bcc Co0.65Fe0.35 particles, uniformly with particle size around a few nanometres. These results reveal that the Ni0.5Zn0.5Fe2O4 in the composite films is electrically insulate for Ni0.5Zn0.5Fe2O4, thus exhibiting promising magnetic properties for suitable proportions of Co0.65Fe0.35. They also reveal that the films show in-plane isotropy, in-plane uniaxial anisotropy and super paramagnetic magnetic properties while x changes from 0 to 0.16. In particular, for the samples with x = 0.085 and x = 0.116, the saturation magnetizations are 1.41?T and 1.19?T, the resistivities reach 677????cm and 1371????cm, the real part of the complex permeability is more than 100 and 150 below 2.0?GHz and the ferromagnetic resonance frequencies reach 3.61?GHz and 2.84?GHz, respectively.

Electric field tuning of non-volatile three-state magnetoelectric memory in FeCo-NiFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures

We demonstrate electric field impulse-induced reversible tristable magnetization switching in FeCo-NiFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) heterostructures at room temperature. The magnetic properties of the FeCo-NiFe2O4 film can be changed reversibly by the strain-mediated magnetoelectric coupling effect. Three piezostrain-mediated reversible and stable electric resistance states were obtained in the FeCo-NiFe2O4 film when different electric field impulses were applied, including large positive and negative fields and an impulse that was smaller than the electric coercive field. Consequently, reversible electric field impulse tuning of the tristable resistance state, which is related to the different magnetization switching properties of the materials, was realized. These results provide a promising approach for low loss multistate magnetoelectric memory devices for information storage applications.

Growth, structure, morphology, and magnetic properties of Ni ferrite films

The morphology, structure, and magnetic properties of nickel ferrite (NiFe2O4) films fabricated by radio frequency magnetron sputtering on Si(111) substrate have been investigated as functions of film thickness. Prepared films that have not undergone post-annealing show the better spinel crystal structure with increasing growth time. Meanwhile, the size of grain also increases, which induces the change of magnetic properties: saturation magnetization increased and coercivity increased at first and then decreased. Note that the sample of 10-nm thickness is the superparamagnetic property. Transmission electron microscopy displays that the film grew with a disorder structure at initial growth, then forms spinel crystal structure as its thickness increases, which is relative to lattice matching between substrate Si and NiFe2O4.