Structural, thermal, magnetic and microwave-absorbing properties of spinel-structured high entropy oxide (MnFeCoNiX)3O4 (X = Zn, Cu, Cr)

Within the present work, a series of spinel-structured (MnFeCoNiX)3O4 (X = Zn, Cu, Cr) high entropy oxides (HEOs) were fabricated by co-precipitation method and the subsequent heat treatment. The phase composition, microstructure, thermal stability, magnetic property and electromagnetic (EM) wave loss ability were comprehensively studied. It is found that spinel-structural (MnFeCoNiX)3O4 HEOs are well-crystallized at above 800 °C with a space group Fd-3m. Morphological analysis reveals that grains of different HEOs contain nano-domains with particle sizes between 100 nm and 500 nm. In addition, Mn, Ni, Co, Fe, Zn, Cr and Cu elements are homogeneously distributed and the chemical states of Mn, Ni, Co and Fe are all +2 and + 3, while Cu only exists in +2 state. Thermal stability analysis shows that (MnFeCoNiCu)3O4 HEO is able to keep stable at 1000 °C. Moreover, the (MnFeCoNiCu)3O4 sample exhibits the best magnetic properties and EM microwave loss ability in comparison with (MnFeCoNiZn)3O4 and (MnFeCoNiCr)3O4. For (MnFeCoNiCu)3O4, the values of saturation magnetization (Ms) and coercivity (Hc) are 38.6 emu/g and 52.6 Oe, respectively. Meanwhile, (MnFeCoNiCu)3O4 displays a minimum reflection loss (RL) of −15.5 dB at 8.48 GHz with a thickness of 4.5 mm and a wide effective absorption bandwidth (EAB) of 8.84 GHz with the sample thickness of 5 mm. It is expected that the excellent high-temperature stability and good EM wave absorbing performance of high entropy (MnFeCoNiCu)3O4 could make it potentially useful in EM wave absorption under elevated temperatures.