Guozhi Chai

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.

Observation of rotatable stripe domain in permalloy films with oblique sputtering

Stripe domain (SD) in obliquely sputtered permalloy films were investigated by comparing with normally sputtered ones. The critical thickness for SD formation of obliquely sputtered films was about 100 nm thinner than that of normally sputtered films. The hysteresis loops of obliquely sputtered films showed a peculiar shape. A rotation of SD towards easy axis was observed in the obliquely sputtered films, which was confirmed by permeability spectra under a bias field. The origin of the rotation could result from in-plane uniaxial anisotropy, which is induced by the shape effect of the oblique columnar growth of permalloy grains.

Exchange coupling driven omnidirectional rotatable anisotropy in ferrite doped CoFe thin film.

Isotropic magnetic materials with high resonant frequencies are useful for applications in microwave devices. Undoped CoFe thin films, as common soft magnetic materials with high saturation magnetization, show isotropic characteristics but no high frequency response. Here, we use ferrite doped CoFe thin film to realize a resonant frequency higher than 4.5 GHz at all orientations. The exchange coupling between ferrimagnet and ferromagnet is assumed to play a key role on the omnidirectional rotatable anisotropy.

High thermal stability of zero-field ferromagnetic resonance above 5 GHz in ferrite-doped CoFe thin films

Ferrite doped CoFe films with stripe domains are demonstrated to possess zero-field ferromagnetic resonance frequency above 5 GHz. The high resonance frequency is driven by the rotatable magnetic anisotropy propagated from the stripe domain structure and exchange coupling between rotatable ferrimagnetic spins and the ferromagnetic grains. The high temperature results show that these films have excellent thermal stability, which may have a great implication for microwave applications.

Temperature-dependent dynamic magnetization of FeCoHf thin films fabricated by oblique deposition

Temperature-dependent dynamic magnetization of FeCoHf thin films grown by oblique deposition technique was investigated systematically through the measurement of permeability spectra in a temperature range from 300 K to 420 K. With the change of oblique angle from 27° to 45°, the resonance frequency is increased from 2.4 GHz to 3.8 GHz at 300 K. The thermal stability for the samples with high oblique angle is not as good as that of the ones with lower oblique angle. In addition, the variation of the effective Gilbert damping factor, frequency linewidth, and static and dynamic permeabilities with temperature for different oblique angles are presented and discussed in detail.

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.

Tunable zero-field ferromagnetic resonance frequency from S to X band in oblique deposited CoFeB thin films.

Tunable zero-field ferromagnetic resonance frequency in wide range is very useful for the application of microwave devices. We performed an investigation of the static and high frequency magnetic properties for oblique sputtered CoFeB thin films. The static magnetic results revealed that oblique sputtered CoFeB thin films possess well defined in-plane uniaxial magnetic anisotropy, which increases monotonically from 50.1 to 608.8 Oe with the increasing of deposition angle from 10° to 70°. Continuous modification of the resonance frequency of CoFeB thin films in a range of 2.83–9.71 GHz (covers three microwave bands including S, C and X bands) has been achieved. This behavior can be explained as the result of the microstructure due to the self-shadowing effect mainly. These CoFeB thin films with tunable magnetic properties may be good candidates for usage in microwave devices.

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.

Enhancing exchange bias and tailoring microwave properties of FeCo/MnIr multilayers by oblique deposition

A systematic study of the static and dynamic magnetic properties with regards to temperature for FeCo/MnIr multilayered thin films fabricated by oblique sputtering technique was carried out. Compared with the film produced by conventional non-oblique sputtering method, those films grown by oblique deposition show a significant increment of exchange bias. The magnetic anisotropy field and the ferromagnetic resonance frequency can also be enhanced and tailored by changing oblique deposition angle. In addition, thermal stability of the dynamic characteristics of the films was presented and discussed from application-oriented perspective.