Electronic structure and magnetic property of Ba1-xSrx- CoFe11O19 hexaferrites

Abstract

Ferrite materials play an important role in promoting the development of modern electrical and electronic devices. In the oxide form, two ferrite systems are usually interested to be cubic spinel ferrites (MeFe 2 O 4 , where Me is a divalent cation) and hexagonal ferrites (hexaferrites). Though the hexaferrites were discovered since 1950s, there has been an exponentially increasing interest until now [1]. Among the hexaferrites, M-type barium hexaferrite (BaFe 12 O 19 ) has attracted a special interest because this material can be fabricated by simple techniques and has many intriguing properties. For example, with the large coercive force $( H_{c})$, high Curie temperature ($T_{C}$, above 700 K [2]), corrosion resistance, and high chemical stability, BaFe 12 O 19 is usually used to fabricate permanent magnets. One has also found its promising applications in high-density recording media, microwave-related devices, and energy conversion and storage devices [1, 3]. Because the magnetic property of BaFe 12 O 19 can easily tuned as expected upon changing fabrication conditions, crystal sizes, and dopants, its application range is remarkably widened. It has been found that the changes in fabrication conditions and dopants would create different intrinsic defects (such as crystal defects, impurities, and grain morphology), which influences the magnitude of $H_{c}$ and the saturation magnetization $(M_{s})$. For the doping case, Ba and Fe in BaFe 12 O 19 can be substituted with alkaline-earth (or rare-earth) and 3d transition metals, respectively [2–5]. To explain the magnetic property of these compounds, it is necessary to know the crystal and electronic structures. Such works are usually based on X-ray diffraction, X-ray photoelectron spectroscopy, and Mossbauer technique [2, 4]. To the best of our knowledge, there is no work on X-ray absorption spectroscopy (XAS) to find out the relation between the electronic structure and magnetic property of doped BaFe 12 O 19 . Taking into this problem, we have prepared Ba 1-x S-r x CoFe 11 O 19 samples. Their crystalline/electronic structures and magnetic property were then studied. Ba 1-x Sr x CoFe 11 O 19 $(x = 0-1)$ nanoparticles (NPs) were prepared by co-precipitation method, using Ba(NO 3 ) 2 , Sr(NO 3 ) 2 , Co(NO 3 ) 2 .6H 2 O, and Fe(NO 3 ) 3 .9H 2 O as precursors. These chemicals with stoichiometric ratios were dissolved in 100 mL DI water at 90 °C. After that, 50 mL NaOH solution (1 M) was added to the mixtures and continuously stirred for 2 h. The products after chemical reactions were filtered, washed by ethanol and DI water, and annealed at 900 °C for 3 h in air. Finally, the products were checked the grain morphology by a scanning electron microscope (SEM, JSM-5410LV). Their crystal and electronic structures were studied by an X-ray diffractometer (Rigaku, MiniFlex) and XAS (the 8C nano-XAFS beamline, Pohang Light Source). The magnetic property was studied by a vibrating sample magnetometer. All investigations were carried out at room temperature. Results revealed that fabricated Ba 1-x Sr x CoFe 11 O 19 NPs with average particle sizes of 100~300 nm crystallized in a hexagonal structure (P6 3 /mmc space group). When Sr content (x) increases, the shape of crystals modifies very much while the unit-cell parameters decrease due to the replacement of Sr2+ (1.18 A) for Ba2+ (1.35 A). Detailed analyses for the Fe and Co K-edge XAS spectra prove oxidation numbers of Fe and Co to be 3+ and 2+, respectively, which are stable versus an x change in Ba 1-x Sr x CoFe 11 O 19 , Fig. 1. Our study has also found a small change of the Fe-O bond length in between 1.89 and 1.91 A. Though both the hexagonal and electronic structures are almost unchanged by Sr doping (excepting for the lattice parameters), our study on the magnetic property has indicated the x dependence of magnetic parameters, as seen in Fig. 2. While $M_{s}$ decreases from 46.1 emu/g for $x=0$ to 34.2 emu/g for $x=1$, $H_{c}$ tends to increase from 1630 Oe for $x=0$ to $\sim 2200$ Oe for $x=0.5$, but slightly decreases to 2040 Oe as $x=1$. We think that such $M_{s}$ changes are related to in the exchange interactions between Fe3+ and/or Co2+ ions located at the octahedral, tetrahedral, and bipyramid sites, which are sensitive to the changes related to local structures, such as Fe/Co-O bond lengths and Fe/Co-O-Fe/Co bond angles. Meanwhile, the $H_{c}$ change is mainly due to different grain sizes, morphology, and shapes. Our work also reviews recent reports on M-hexaferrites, and discuss in detail their magnetic property, particularly BaFe 12 O 19 related compounds.