Qingfang Liu

Current-induced magnetic skyrmions oscillator

Spin transfer nano-oscillators (STNOs) are nanoscale devices which are promising candidates for on-chip microwave signal sources. For application purposes, they are expected to be nano-sized, to have broad working frequency, narrow spectral linewidth, high output power and low power consumption. In this paper, we demonstrate by micromagnetic simulation that magnetic skyrmions, topologically stable nanoscale magnetization configurations, can be excited into oscillation by a spin-polarized current. Thus, we propose a new kind of STNO using magnetic skyrmions. It is found that the working frequency of this oscillator can range from nearly 0 Hz to gigahertz. The linewidth can be smaller than 1 MHz. Furthermore, this device can work at a current density magnitude as small as 108 A m−2, and it is also expected to improve the output power. Our studies may contribute to the development of skyrmion-based microwave generators.

An induction method to calculate the complex permeability of soft magnetic films without a reference sample

A new analytical method has been proposed by utilizing an electromagnetic induction principle with a short-circuited microstrip line jig and the complex permeability spectra can be calculated without a known reference sample. The new method using the short-circuited microstrip line can exhibit higher sensitivity and a wider frequency band than coplanar waveguide and pick-up coil. Two magnetic thin films having a good in-plane uniaxial anisotropy are measured by using the induction method. The results show typical complex permeability spectra in good agreement with the theoretical analytical results. The measured permeability values are verified by comparing with the initial susceptibility derived from the sweeping field results. The difference of measured permeability values is less than 5%.

Width-controlled M-type hexagonal strontium ferrite (SrFe12O19) nanoribbons with high saturation magnetization and superior coercivity synthesized by electrospinning.

Width-controlled M-type hexagonal SrFe12O19 nanoribbons were synthesized for the first time via polyvinylpyrrolidone (PVP) sol assisted electrospinning followed by heat treatment in air, and their chemical composition, microstructure and magnetic performance were investigated. Results demonstrated that as-obtained SrFe12O19 nanoribbons were well-crystallized with high purity. Each nanoribbon was self-assembled by abundant single-domain SrFe12O19 nanoparticles and was consecutive on structure and uniform on width. PVP in the spinning solution played a significant influence on the microstructure features of SrFe12O19 nanoribbons. With PVP concentration increasing, the ribbon-width was increased but the particle-size was reduced, which distributed on a same ribbon were more intensive, and then the ribbon-surface became flat. The room temperature magnetic performance investigation revealed that considerable large saturation magnetization (Ms) and coercivity (Hc) were obtained for all SrFe12O19 nanoribbons, and they increased with the ribbon-width broadening. The highest Ms of 67.9 emu·g−1 and Hc of 7.31 kOe were concurrently acquired for SrFe12O19 nanoribbons with the maximum ribbon-width. Finally, the Stoner-Wohlfarth curling model was suggested to dominate the magnetization reverse of SrFe12O19 nanoribbons. It is deeply expected that this work is capable of opening up a new insights into the architectural design of 1D magnetic materials and their further utilization.

High saturation magnetization of γ-Fe2O3 nano-particles by a facile one-step synthesis approach.

We have demonstrated the synthesis of γ-Fe2O3 nano-particles through a facile and novel calcination process in the air. There is no pH regulation, gas atmosphere, additive, centrifugation or other complicated procedures during the preparing process. A detailed formation process of the nano-particles is proposed, and DMF as a polar solvent may slower the reaction process of calcination. The structures, morphologies, and magnetic properties of γ-Fe2O3 nano-particles were investigated systematically, and the pure γ-Fe2O3 nano-particles obtained at 200 °C display uniform morphology good magnetic property. The saturation magnetization of obtained pure γ-Fe2O3 is about 74 emu/g, which is comparable with bulk material (76 emu/g) and larger than other results. In addition, the photocatalytic activity for degradation of methylene blue is also studied, which shows proper photocatalytic activity.

Abnormal coercivity dependence on the diameter of Co nanowires in anodic aluminium oxide templates

Co nanowire arrays with diameters of 20 and 50?nm are fabricated into anodic aluminium oxide templates by the ac electrodeposition technique. The coercivity of nanowire arrays with diameter of 50?nm is 1990?Oe, whereas that of nanowire arrays with diameter of 20?nm is only 925?Oe. This is quite different from earlier experimental results of magnetic metal nanowire arrays, which exhibit larger coercivity at a diameter of 20?nm. X-ray diffraction patterns suggest that the anomalous increase in coercivity is related to the appearance of the (0?0?2) and (1?0?1) reflections. Micromagnetism simulation reveals the mechanism of how the (0?0?2) texture of nanowires influences the magnetic properties of Co nanowire arrays.

Nonmetal sulfur-doped coral-like cobalt ferrite nanoparticles with enhanced magnetic properties

We have successfully prepared a series of sulfur-doped cobalt ferrite nanoparticles via a facile and novel calcining process in air. The technique requires neither pH regulation nor a gas atmosphere nor centrifugation nor any other supplementary reagents during the preparation process. Our various characterizations confirmed that the nonmetal sulfur element was successfully doped in the cobalt ferrite nanoparticles. As the sulfur concentration was increased, the shapes of the sulfur-doped cobalt ferrite nanoparticles changed, from compact microspheres to sparse coral-like nanoparticles. Significantly, the saturation magnetization of sulfur-doped cobalt ferrites was measured to be as high as 81 emu g−1, which is particularly different from the decreased saturation magnetization of previously reported metal element doping.

Top-down control of dynamic anisotropy in permalloy thin films with stripe domains

Permalloy films with different thickness were prepared for comparison, and the dynamic properties of the stripe domains were investigated by using a vector network analyzer ferromagnetic resonance (VNA-FMR) technique. A top-down approach was used to control the thickness of the films. In the measurement of permeability spectra, an applied magnetic field H app was employed parallel and perpendicular to the stripe domains, respectively. Dynamic pseudo-uniaxial anisotropy fields are derived. In order to explain the anisotropy variation of permalloy films, a theoretical model has been proposed, and the result shows that the pseudo-uniaxial anisotropy can be attributed to the contributions of saturation magnetization, exchange interaction, perpendicular anisotropy, and film thickness. In addition, with the increase of H app, the stripe domains gradually rotate towards the direction of the applied field, which can be clearly observed by magnetic force microscopy. Moreover, the dynamic permeability spectra exhibit a rhythmic variation dependent on the rotation of the stripe domains, i.e. the relationship between the rotational stripe domains and resonance frequency.

Static magnetic and microwave absorption properties of FeCo/Al2O3 composites synthesized by high-energy ball milling method

FeCo/Al2O3 composites were prepared by mechanochemical synthesis route using a mixture of Fe2O3, Co3O4 and Al as the starting materials. FeCo alloy can be obtained in a short time in contrast to the traditional method by means of milling Fe and Co powders directly. Most FeCo/Al2O3 composites can be formed when ball milling time has achieved 15 min. As the milling time increases, the saturation magnetization (Ms: magnetic moment per unit mass) and coercive force (Hc) of FeCo/Al2O3 composites increase gradually. The FeCo/Al2O3 composites exhibit a value of Ms of 98.5 emu g−1 and Hc of 350 Oe finally. The products with ball milling time 15 min and 30 min provided better microwave absorption performances in the frequency ranges of 8.6–18 GHz and 2.5–6.5 GHz for an absorber thickness of 1.2–2.2 mm and 2.1–4.9 mm, respectively. An optimal reflection loss of −43.1 dB can be obtained at 4.1 GHz with an absorber thickness of 3.1 mm. The good microwave absorption performances result from an appropriate impedance matching due to the insulator Al2O3 dispersed around FeCo alloy.

Annealing influence on the exchange stiffness constant of Permalloy films with stripe domains

An investigation of the annealing influence on the stripe domains structure in Permalloy films has been performed by comparing the static and dynamic magnetic properties. An increasing exchange stiffness constant A ex is found when the annealing temperature rises. The magnetic force microscopy images show that the width of stripe domain increases with the increase of annealing temperature, and the increasing exchange stiffness constant is considered to be the most prominent reason. A perpendicular standing spin-wave model is used to deduce the A ex from the evolution of resonance frequencies, and the A ex increases nearly twice from 7.2 × 10−7 ergs cm−1 for the as-deposited film to 12.9 × 10−7 ergs cm−1 for the film annealing at 400 °C. Micromagnetic calculation is also used to compute the magnetization configuration, and the results agree well with the experimental conclusions.

Electrodeposition of FeCoCd films with in-plane uniaxial magnetic anisotropy for microwave applications

FeCoCd thin films with 500 nm thickness are directly prepared through electrodeposition in the sulphate bath in which glycine and citric acid were added as complex agents. The composition, structure, and magnetic of FeCoCd films were investigated as a function of Cd2+ concentration, cathode current density, and deposition temperature. A wonderful soft magnetic FeCoCd film was prepared and its coercivity of easy axis and hard axis are 5 Oe and 4 Oe, respectively. The natural resonance frequency is about 3.0 GHz, which imply that the FeCoCd film is potential candidate for high frequency applications.