Growth, structural and magnetic characterization of Zn-substituted barium hexaferrite single crystals
Introduction
The interest in magnetic materials has historically been driven by use to create permanent magnets and high density magnetic recording storage devices. In the last decade, it has undergone significant changes due to the new opportunities for application of the unique properties of magnetic materials.
An example of such material is barium hexaferrite (M-type barium ferrite). The possibility to use hexaferrites in new applications explains the exponentially increasing degree of interest in these materials, which is ongoing today [1]. The structural and magnetic properties of barium hexaferrites were found to be strongly dependent on the fabrication method as well as on the substitution of Fe with ions of different nature and concentration (doping). In literature, results of comprehensive studies on modifying the properties by substitution of iron atoms in the magnetoplumbite-type BaFe12O19 are available [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Such functional magnetic characteristics as coercivity and Curie temperature can be significantly changed by doping with magnetic and non-magnetic metals [29], [30], [31]. The modification of BaFe12O19 by doping makes it possible to obtain unique and previously unknown properties of this material, which leads to the development of new applications. One example of such newly arising applications may be the use of barium hexaferrite as microwave absorbing material [32].
Currently, most commonly used materials for microwave absorbers are ferrite spinels with cubic crystal structure. However, the use of those spinels at frequencies of about 2 GHz is limited due to Snoek's limit [33], [34], [35]. Hexagonal ferrites with magnetoplumbite structure exhibit higher resonance frequency and higher microwave permeability and are thus expected to be more effective in the higher frequency range [36].
So far, the effect of Zn substitution on the properties of barium hexaferrite is poorly studied, although several reports can be found in literature [4], [18], [19], [20], [37], [38], [39], [40], [41]. Different cations or combinations of cations have been used to substitute the Fe3+ ions to reduce the grain size and high magnetic uniaxial anisotropy field of BaFe12O19 without affecting the saturation magnetization Ms for applications in high-density magnetic recording and microwave absorption devices [36]. For example, samples BaFe12–2xZnxTixO19 (0 ≤ x ≤ 0.6) were produced by mechanical milling. Upon increasing substitution of Fe a reduction can be observed in both the intrinsic coercivity, Hci, and the remanent magnetization, Mr, whereas the saturation magnetization, Ms, diminishes gradually, having a maximum at x = 0.3. The Curie temperature was also found to be decreasing with x. The average temperature coefficient of Hc for samples BaFe12−2xCox/2Znx/2SnxO19 (x = 0.0–0.6) grown by co-precipitation/molten salt method decreases with x[21].
Since the information about the influence of solely Zn doping on the structure and functional properties of barium hexaferrites is sparse in literature, especially for case of flux grown crystals, this cation was chosen as an object for a detailed study of the substitution effect on the properties.
Section snippets
Experimental part
Crystals of Zn-substituted barium hexaferrites were grown from flux [42] composed of iron oxide (γ-Fe2O3), zinc oxide (ZnO), barium carbonate (BaCO3) and sodium carbonate (Na2CO3) of 99.5% purity. Compositions of the batches are listed in Table 1.
The initial mixture was ground in an agate mortar, filled into a 30 mL platinum crucible and placed in a resistive furnace. The details can be found elsewhere [43], [44]. Heating was controlled by a type B thermocouple and a precision thermocontroller
Results and discussion
High quality single crystals of BaFe12−xZnxO19 with Zn contents varying in the range 0 ≤ x ≤ 0.065 were successfully grown from flux (see Table 1). We were able to perform the fabrication of samples at a temperature of 1260 °C, which is significantly lower than in the case of other crystal growth methods [45]. Similar to other transition metal substitutions like Ti, Co, Ni, Cu, or W, the doping level of Zn in barium hexaferrite crystals can be varied using different initial concentrations of
Conclusions
Zn-substituted barium ferrite single crystals BaFe12−xZnxO19 (x up to 0.065) with size up to 7 mm were grown from flux composed of iron oxide, zinc oxide and barium and sodium carbonates. This method can be used at temperatures significantly lower than for other fabrication techniques. The influence of Zn concentration on the structural and magnetic properties was studied. The lattice parameters were found to be slightly increasing with concentration of the dopant. Zn substitution significantly
Acknowledgments
The work at SUrSU was supported by the National Research University program. Additionally the work was partially performed using equipment of MIPT Centers of Collective Usage and with financial support from the Ministry of Education and Science of the Russian Federation (Grant No. RFMEFI59414X0009). Financial support of RFBR (No. 13-02-12443) is acknowledged.
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