Microwave Absorption and Magnetic Studies of Strontium Hexaferrites Nanoparticles Synthesized by Modified Flux Method

Abstract

M-type strontium hexaferrite nanocrystals of radar absorbing material (NRAM) i.e. SrFe12O19 were synthesized by modified flux method that combine the controlled chemical co-precipitation process for nucleation and complete uniform growth during annealing with NaCl flux. Uniform structural morphological transformation of nanocrystals from needle to hexagonal prism faces were noticed after annealing with increasing of heat treatment (HT) temperature from 800 to 1200°C for 4h. X-ray diffraction (XRD) results show the formation of various phases with increase in annealing temperature. The crystallinity and crystallite size are found to increase with increase in heat treatment temperature (15-40nm). The superparamagnetic behavior of strontium hexaferrite is confirmed by vibrating sample magnetometer (VSM) wherein it was noticed that magnetic field (10000 gauss max) did not have any effect on these materials. The transformation of magnetic properties i.e. from superparamagnetic to ferromagnetic behaviour after heating at various HT temperatures have been revealed by hysteresis loops under VSM study. The increase in saturation magnetization from 2.44 to 75.03 emu/gm is observed. Formation of ultrafine particles has been confirmed through field emission scanning electron microscope (FESEM). About 5 to10% of the needle like crystals in the ‘as synthesized’ condition were transformed to hexagonal pyramidal structure and most of the crystals are found to have plate like hexagonal structures with increase in heat treatment temperatures. The effect of such systematic morphological transformation of nanocrystals was observed in reflection loss properties in X band (8-12 GHz). The maximum reflection loss of -20.05, -24.31, -24.16, -25.22, -25.12, -24.01 dB at 8.1, 8.6, 9.2, 9.6, 10.7, and 12 GHz respectively are observed for the material heat treated at 1200°C. A significant increment from - 6.5 to -25.22 dB at 9.6 GHz in reflection loss (RL) is noticed due to symmetric morphological growth of RAM nanocrystal during annealing.

Synthesis and Characterization of SrFe11.2Zn0.8O19 Nanoparticles for Enhanced Microwave Absorption

“…This makes the process fast, and hexaferrite particles crystallize out completely with hexagonal pyramidal or plate-like shape after cooling in the furnace. [21][22][23][24] Similar morphologies were obtained by Sharma et al for barium hexaferrite and are reported elsewhere. 23,24 In view of the discussion above, the present investigation deals with synthesis of zinc-substituted strontium hexaferrite nanoparticles by the modified flux method.…”

“…9). This increase in saturation magnetization with the HT temperature can be attributed to increased formation of strontium hexaferrites and zinc ferrite, 20,22 as confirmed by x-ray study of the powder heat-treated at 1200°C (Fig. 4).…”

“…The strongest reflection loss and the widest bandwidth value (for RL > 10 dB) are given in Table IV. These values were found to be higher and obtained for wider bandwidth than those of currently used hexaferrites (BaFe 12 [5][6][7]22,[28][29][30][31][32][33] The improvement of reflection loss originated from the formation of hard and soft ferrite, which can be explained on the basis of the exchange coupling interaction between the hard magnetic (SrFe 12 O 19 ) and soft magnetic (ZnFe 2 O 4 ) phases, which changes the relative complex permeability of the materials. In this study, the nanocomposite powders were synthesized by a modified flux method, so two kinds of grains can be combined.…”

“…It is reported that barium and strontium hexaferrite nanoparticles with hexagonal pyramidal and hexagonal plate-like morphology are potential materials for radarabsorption applications. [22][23][24] The magnetic measurements of SrFe 12 O 19 / ZnFe 2 O 4 nanoparticles showed almost negligible coercivity and remanence values (Fig. 9) in the as-synthesized condition, showing the superparamagnetic behavior of the material.…”

“…Thus, the synthesis of spinel ZnFe 2 O 4 along with hexagonal SrFe 12 O 19 particles in the nanosize range resulted in higher saturation magnetization and lower coercivity than those reported for single-phase strontium hexaferrite ferrite. 22,28 Hexaferrite nanoparticles having low coercivity and high saturation magnetization find applications in magnetic recording technologies such as hard disks, floppy disks, video tapes, etc. 29 Complex permittivity and permeability values represent the dielectric and magnetic properties of magnetic materials.…”