PVP is a conventional surfactant that can be used to activate certain crystal plane, which may lead to newly reduced Co atoms adsorbing on the plane of existed Co particles and finally the stings are formed. In addition, N₂H₄·H₂O only exhibits strong reduction ability under alkaline condition, that’s why the growth of stings is slow under low NaOH content. While increasing NaOH addition may raise the number of reduced Co atoms and promote the rise of stings’ number and length and finally induce the Co flower rudiment growth. Once NaOH reaches 0.8 g, newly reduced Co atoms will be much more and enough to accelerate stings’ growth and the well organized Co flower is shaped.

Under the preparation conditions of well grown Co flower, chain-like Co flower can be obtained without any external assistant magnetic field if CoCl₂·6H₂O addition is increased from 0.01 g to 0.02 g. This may be due to each Co flower generates its own magnetostatic field and the increase of CoCl₂·6H₂O will result in large number of Co flowers, which enhance the magnetostatic field in the reaction system. Hence, these flowers are arranged linearly along magnetic force line to form chain-like Co flower. The following discussions are based on the measure data of these chain-like Co flower.

Fig. 2 shows an XRD pattern of obtained chain-like Co flower (upper) and the pattern from data bank entry (lower). The characteristic peaks arise at 2θ=41.61°, 44.42°, 47.29°, 75.89° can be indexed to (100), (002), (101), (110) planes, respectively, of hexagonal-phase cobalt (JCPDS: 05-0727). No characteristic peaks arising from impurities of cobalt oxides or hydroxides are detected, which may be due to the floating reduction product N₂ and NH₃ can exclude air to inhibit oxidation of newly reduced Co. It is found that the peaks relative intensities corresponding to (002)/(100) and (002)/(101) planes are significantly higher than that of standard values, indicating the preferred growth orientation is along [001] direction, which is very likely to the reported dendritic cobalt [10]. In fact, [001] direction is the easy magnetic axis of Co [11], while magnetostatic energy will be the lowest and reaction system can maintain steady if Co atoms are stacked along easy magnetic axis. Therefore, the growth rate of [001] direction is faster than others.

As shown in Fig. 3, the saturation magnetization (M_s), remanent magnetization (M_r), coercivity (H_c) of chain-like Co flower are 181.38 emu/g, 2.56 emu/g, 33.74 Oe, respectively. It should be noted that the M_s of chain-like Co flower is a little higher than that of bulk Co [12], which may arise from its special chain-like microstructure or tiny experimental error. In addition, the high M_s can induce high initial permeability which is beneficial for microwave absorption [6].

The real parts of relative complex permittivity and permeability symbolize storage ability to electromagnetic energy, while the imaginary parts represent loss ability. The low permittivity imaginary part (ε′′) as shown in Fig. 4 demonstrates dielectric loss is weak. It is worthy to be noted that ε′′ is a little below zero under some bands while there is not a convincing interpretation for the negative value, Deng et al. explain the phenomenon as the magnetic energy being radiated out [2], Chiu et al. point out it is meaningless and might be due to noise [13], while Chen et al. point out it does not arise from test error and needs further study [14]. The real part of relative complex permeability (μ′) decreases as the increase of frequency displays slight frequency-scattering effect. The imaginary part of permeability (μ′′) weak peak around 6 GHz exhibits the existence of natural resonance just as reported dendritic Co [3].

Microwave absorption ability can be preliminarily judged by dielectric loss factor (tanδ_e=ε′′/ε′) and magnetic loss factor (tanδ_m=μ′′/μ′), which are shown in Fig. 5. The larger tanδ_m than tanδ_e demonstrates that main loss mechanism is magnetic loss rather than dielectric loss. There are three dielectric loss factor peaks around 4, 10, and 14.5 GHz which arise from relative high ε′′ and low ε′ at above frequencies. The tanδ_m rapidly increases in 2–6 GHz and undulately increases during 6–16.4 GHz, then dramatically decreases. Due to magnetic loss is main loss mechanism and tanδ_m is large around 16.4 GHz, the microwave absorption peak may emerge at 16.4 GHz.

According to the measured data of permittivity and permeability, reflection loss (RL) usually can be calculated by following equation according to transmission line theory and assuming the absorber is attached on high reflective metal plates [15]–[17]:where Z₀ is the impedance of free space, and Z_in is the input characteristic impedance, which can be expressed as:

where c is the velocity of light and d is the thickness of an absorber. The reflection loss (microwave absorption) ability of chain-like Co flower is shown in Fig. 6.

Conventional measure frequency band of microwave absorbing materials is 2–18 GHz. It is reported that Co nanochains can reach reflection loss peak about −11 dB at 17.2 GHz at the thickness of 2.5 mm, and Co nanoparticles can only reach the maximum reflection loss −5 dB in 2–5 GHz at the thickness of 3.16 mm [4], [5]. While chain-like Co flower at a thinner thickness of 2 mm as shown in Fig. 6 exhibits stronger reflection loss (−11.5 dB at 14.8 GHz) than the above reported Co materials, which may be illustrated from its special microstructure. There are many sharp petals that can be tuned with the incident microwave for the point discharge effect just as the action mechanism of lightning rod to lightning, and electromagnetic energy will be induced into dissipative current. Then, the chain-like structure is beneficial for transmission of dissipative current and finally leads to energy attenuation by transforming to heat.

It should be noted that chain-like Co flower reaches the loss peak at 14.8 GHz rather than the maximum tanδ_m frequency of 16.4 GHz. Loss peak frequency is determined by loss mechanism or absorber thickness that can be characterized as times of λ_m/4 [6]. Although Co is a kind of magnetic loss microwave absorbing material, free electronic will be polarized due to its electronic conductive essentiality and dielectric loss can be induced under external alternated electromagnetic field. Therefore, dielectric loss is also one of the loss mechanisms even though it is not strong. The tanδ_e at 16.4 GHz in Fig. 5 demonstrates dielectric loss is weak, so microwave absorption can not be the strongest even though tanδ_m is the largest at this frequency. However, there is a tanδ_e peak around 14.8 GHz and tanδ_m is also very large at that frequency, so chain-like Co flower reaches the strongest microwave absorption. After 14.8 GHz, the variation tendencies of tanδ_e and tanδ_m are just opposite and result in microwave absorption becoming weaker and weaker in total. The above analysis adequately demonstrates the loss nature is coming from synergistic effect of magnetic loss and dielectric loss, while magnetic loss is the main loss mechanism.