Shape-anisotropic enhanced damping in CoZr periodic arrays of nanohill structure
© Wang et al.; licensee Springer. 2013
Received: 3 April 2013
Accepted: 7 June 2013
Published: 12 June 2013
The preparation of CoZr nanostructure films to replicate the order of anodized aluminum oxide template with barrier layer was described. Coercivity and in-plane magnetic anisotropy were increased with the increase of oblique sputtering angle. Resonance frequency and damping factor had the same tendency also. Note that larger damping factor in nanostructure films was observed compared with that of continuous films in Si substrate, which was induced by magnetic anisotropy distribution particularly with a significant out-of-plane contribution due to the competition of shape anisotropy.
75.75.-c, 75.70.-i, 07.57.Pt
KeywordsAnodized aluminum oxide Thin films Nanohill structure
where Ms represents saturation magnetization, Heff is the anisotropy effective field, γ is the gyromagnetic factor, and α is the damping constant. From Equations 1 and 2, it can be seen that magnetic anisotropy and saturation magnetization are the two key material parameters which determine the magnetic properties of the magnetic film. The resonance frequency can be regulated through magnetic anisotropy. Generally, magnetic anisotropy is affected by many factors, such as demagnetization energy from the sample’s shape or microstructure , magneto-crystalline energy from the material’s crystal symmetry , magneto-elastic interactions from the stress state of the sample , single-ion anisotropy or pair order from chemical short-range order effect , exchange anisotropy from the ferromagnetic-antiferromagnetic coupling , etc. For thin films, in-plane uniaxial anisotropy determines microwave magnetic properties. Usually, uniaxial magnetic anisotropy is induced by many methods, for example, controlling the sputtering angle [12, 13], changing the target-substrate distance , controlling the stress [9, 15], using nanowire arrays , etc.
Ordered magnetic nanostructures, composed of arrays of different kinds of magnetic elements arranged in a periodic fashion, have attracted increasing attention in recent years [17, 18]. Shape anisotropy was introduced with spatial dependence on a very small length scale when a periodic nanostructure is defined in a continuous magnetic thin film. The rapid advance in the fabrication of nanostructures, with controlled submicron size and shape offered by modern lithography techniques like ion or electron beam lithography, has triggered increased research on magnetic nanostructures (dots, stripe, or antidots) with a variety of shapes [19–21]. Anodized aluminum oxide (AAO) template with a high areal density [22, 23] (up to 1,011 pores/cm2) and narrow size distribution over a large area has received much attention because of its simple and inexpensive control of structural parameters and excellent thermal and mechanical stability.
Various routes have been proposed to replicate the ordering of AAO where the final replicated nanostructures consist of highly ordered glassy antidots, nanowire, etc. In these nanostructured materials, large coercivity is induced due to strong shape anisotropy, which have attracted a great deal of interest owing to their potential applications as optoelectronics, data storage materials, surface modifiers with specific wetting behavior, etc. . However, in order to apply magneto-electronic devices in the gigahertz region, a soft magnetic film with low coercivity and in-plane uniaxial anisotropy is developed. Therefore, in the present work, we use an AAO nanostructure with barrier layer as a substrate. CoZr nanohill structured magnetic film (approximately 25 nm) has been sputtered onto a barrier layer of AAO by oblique sputtering. Oblique sputtering would induce in-plane uniaxial anisotropy  and increase shape anisotropy. We investigated static and dynamic magnetic properties of CoZr nanostructured films with various oblique sputtering angles and obtained adjustable resonance frequency and linewidth.
The annealed aluminum foil (99.95%) was used to prepare the single anodic alumina template (AAO). Two-step oxidation was used to obtain the anodic alumina template. At the first step, Al was anodized in 0.3 M oxalic acid at 40 V for 1 h. Then the alumina from the first step was etched away by an alumina etchant (chromic acid and phosphoric acid) at 60°C for 30 min. At the second step, the oxidation was similar to the first step, but the oxidation time was 8 h.
Results and discussion
Figure 1c,d shows the AFM surface morphology of the barrier layer in the anodic alumina oxide template. From the figure, the barrier layer surface presented smooth mountains with heights of around 10 nm. In the template production process, the process parameters of template projection were oxidation voltage and electrolyte concentration. With the increase of oxidation voltage, the diameter of the projection increases; when electrolyte concentration increases, the current density increases, and there is increase in the diameter of the projection. The reason for the projections formed could be explained by the electric field under the support of the template oxidation process dissolution model . The charge was the most concentrated at the bottom of the holes, and dissolution rate was the fastest. Figure 1e,f shows the SEM micrographs of the 0° and 60° samples. As shown from the figure, the sample of the oblique 0° kept the nanohill shape from replicating the order of an anodized aluminum oxide template with barrier layer; however, this nanostructure disappeared with oblique sputtering, as shown Figure 1f.
The permeability spectrum can be fitted with Equation 3, as shown by the solid lines in Figure 4b. The fitting parameters are plotted in Figure 4c. The resonance frequency (fr) increased from 2.9 to 4.2 GHz with the increase of oblique sputtering angle, which had the same tendency with that of Hk. The damping factor also increased from 0.015 to 0.165, which was larger than that of continuous films at around 0.01 . Intrinsic damping and extrinsic sample inhomogeneities were two dominant contributions to the linewidth. The intrinsic LLG damping was generally a confluent process such as magnon-electron scattering. There was also extrinsic damping via two-magnon processes, such as the result of scattering from grain and grain boundaries, etc. Both the intrinsic and extrinsic processes lead to loss in the system. Besides the above two factors, an additional source of the linewidth was the sample inhomogeneities (not a real loss) which typically resulted in the distribution of material properties, such as the anisotropy, that would increase the linewidth. In order to understand the origin of the enhancement of the linewidth and/or damping factor, FMR was measured as a function of the angle between external magnetic field and in-plane easy axis.
where γ is the gyromagnetic ratio, 4πMs is the saturation magnetization of the film, K⊥ is the perpendicular magnetic anisotropy constant, θH is the angle between the external field and film normal, and θM is the angle between magnetization vector and film normal.
Here the saturation magnetization 4πMs was obtained by static VSM measurement; the perpendicular magnetic anisotropy constant could be acquired by fitting the experimental data with Equation 5. The fitted result showed that K⊥ of 60° was 16.3 × 103 erg/cm3 larger than the 12.9 × 103 erg/cm3 of 0°, which indicated increase with increasing oblique sputtering angle. Generally, the K⊥ of continuous film was almost zero due to strong demagnetization energy. In our case, the decrease of demagnetization energy was caused by shape anisotropy of nanostructure films, which induced the increase of K⊥. Therefore, the increase of K⊥ induced inhomogeneities of magnetic anisotropy, which resulted in the increase of linewidth and/or damping factor.
The static and dynamic magnetic properties of CoZr/AAO films with different oblique sputtering angles have been investigated. All the properties and parameters were found to be dependent on magnetic anisotropy field which was induced by the shape of the AAO template and oblique sputtering. The competition between the two factors resulted in the trend of dependence on anisotropy field Hk and remanence ratio Mr/Ms, with various oblique sputtering angles. The resonance frequency change of CoZr/AAO films was also attributed to the effect of properties and oblique sputtering. Enhanced microwave absorption was confirmed by complex permeability measurement comparing with continuous film on a Si substrate.
Anodized aluminum oxide
Atomic force microscope
Scanning electron microscope
Vibrating sample magnetometer
This work is supported by the National Basic Research Program of China (grant no. 2012CB933101), the National Science Fund for Distinguished Young Scholars (grant no. 50925103), and the National Natural Science Foundation of China (grant no. 11034004 and 50902064).
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