Fig. 2 (a) ~ Fig. 2 (b) exhibits the X-ray diffraction (XRD) patterns and Scanning electron microscope (SEM) photographs of magnetic ceramic layer, respectively. In Fig. 2 (a), it is observed that the magnetic ceramic layer contains two phases of Al2O3 and LaSrCoO3. From Fig. 2 (b), it is observed that the magnetic ceramic has higher density and a few pores.

Fig. 3 shows the measured electromagnetic parameters of magnetic ceramic in the frequency range of 8.2 ~ 12.4 GHz under the tempurate of 25 ℃ to 500 ℃. Fig. 3. (a) ~ (d) are the real parts and imaginary parts of the complex permittivity and the complex permeability, respectively. It is observed that both the complex permittivity and complex permeability of the ceramic increase with the increase of temperature. The complex permittivity and the complex permeability will directly affect the electromagnetic wave loss of the materials. The bigger of the complex permittivity and complex permeability, the greater loss [33-35]. Thus, the changes of dielectric constant and magnetic permeability may affect the electromagnetic absorption properties of materials.

Fig. 4 shows the reflection loss of the RAM versus frequency for different *C.* The influences of period length C of the FSS on reflection loss(RL) of the RAM are calculated based on electromagnetic parameters of magnetic ceramic at 500℃, while the other parameters remain unchanged. The RL is an effective evaluation standard of the microwave absorbance capacity of metal backed slabs of material, and low RL corresponds to high absorption. From the results in Fig. 4, an absorption peak is observed between the frequency of the 8.2 GHz and 12.4 GHz (X-band). The position of absorption peak shifts to high frequency and the intensity of absorption peaks decreased with the increase of *C*. When the period length *C* is equal to 10 mm, the reflection loss less than -10 dB can be obtained in the frequency range from 8.5 ~ 10.4 GHz and the minimum value of the reflection loss is - 15 dB. Therefore, the optimal period length may be about 10 mm.

Fig. 5 shows the influences of radius *a* of the FSS on the reflection loss of absorber while the other parameters are unchanged. As shown in Fig. 5, an resonance absorption peak appears at different frequencies with variable *a*. The results show that the minimum reflection loss of the designed RAM first decreases and then increases with the increase of radius *a*, while the reflection loss increases gradually in the frequency range from 15 GHz to 18 GHz. When the radius length *a* is equal to 2 mm, the minimum reflection loss is - 16 dB, the absorption band-width with reflection loss less than - 10 dB is the largest. Therefore, the optimal radius of the FSS patch may be about 2 mm.

Fig. 6 shows the effects of ceramic layer thickness *t* on the absorbing properties of RAM while the other parameters remain unchanged. The results show that the resonance absorption peak increased firstly and then decreased with the increase of thickness *t*. The frequency of the absorption peak moves to the low-frequency region and the absorption bandwidth below - 5 dB also increases gradually. Therefore, the absorption of all peaks of absorbing materials are enhanced with increasing thickness *t*. When the ceramic thickness *t* is equal to 2 mm, the absorption bandwidth with reflection loss below - 10 dB is the largest in the X-band. When the thickness *t* is equal to 1.6 mm, the minimum value of the reflection loss is - 19 dB at about 12.5 GHz. Therefore, the optimal thickness may be about 1.6 mm.

In order to make the absorbing performance of RAM the best, the genetic algorithm is employed to solve the optimization problem. According to the above research results, the initial values are redesigned as follows: *C* = 10 mm, *a* = 2 mm, *t* = 1.6 mm. Fig. 7 shows the optimized reflection loss results of RAM under the temperature from 25 ℃ to 500 ℃. From Fig. 7, it is observed that the reflection loss of the RAM below - 5 dB can be obtained in Ku-band under the temperature from 25 ℃ to 500 ℃. As the temperature increases from 25 ℃ to 500 ℃, the reflection loss of the RAM gradually decreases, the absorption peak gradually moves to the low frequency. When the temperature is 500 ℃, the reflection loss below - 10 dB can be obtained in the whole X-band. The reflection loss curve of RAM shows a double absorption peak structure in the frequency range of 8.2 ~18 GHz, the absorption frequency peaks are located at 9 GHz and 13.2 GHz, respectively. The minimum value of reflection loss is - 39 dB at 13.2 GHz. Thus, a broad bandwidth is obtained with a reflectance lower than - 10 dB in the frequency range from 8 ~16 GHz which cover the entire X-band. More than 90% microwave energy can be consumed in the RAM which can fill the requirement of practical application. The optimal structural parameters are as follows: *C* = 10.31 mm, *a* = 1.59 mm and *t* = 1.50 mm.

In order to further study the absorption mechanism with different temperature of the designed RAM, the volume power loss density distribution of RAM are given in Fig. 8 under different temperatures. From Fig. 8, it is observed that the power loss density of the RAM increases as the temperature increases from 25 ℃ to 500 ℃. Power loss density is directly related to the absorption efficiency of the RAM. The bigger of the loss, the greater absorption efficiency [36, 37]. Thus, this result is consistent with the results in Fig. 3. and Fig. 7. When the temperature was raised between 25 ℃ and 200 ℃, the loss is mainly caused by the FSS layer, as shown in Fig. 8 (a) ~ (c). As the temperature was raised between 300 ℃ and 500 ℃, the complex permittivity and the complex permeability of ceramic layer increase gradually, the loss is mainly caused by the FSS and ceramic layer, as shown in Fig. 8 (d) ~ (f). Thus, two absorption peaks are very strong when the temperatures are 300℃, 400 ℃ and 500 ℃, as show in Fig. 7. So, the concept of resonant coupling is realized using the FSS layer and magnetic ceramic for wide-band absorption.