Tungsten substituted BaFe₁₂O₁₉ single crystal growth and characterization

Highlights

• Growth of large W-substituted crystals BaFe_12−xW_xO₁₉.

• W-content controllable by flux composition.

• W substitution influence on the unit cell parameters.

• Magnetic characterization depending on W-content.

Abstract

Tungsten-substituted barium hexaferrite single crystals were grown by flux method at 900–1260 °C. Crystals of BaFe_12–xW_xO₁₉ with x up to 0.06 were obtained. The W content in the crystals can be controlled via the W content in the flux. Upon increase of the tungsten concentration the cell parameters and magnetic properties change. W doping results in an increase of coercive force from 331 to 489 Oe, which makes these materials prospective for permanent magnets. The substitution results in a small reduction of the Curie temperature from 455 to 453.1 °C and in a decreasing saturation magnetization at room temperature from 71 to 64 emu/g for powder samples.

Introduction

Barium hexaferrite is one of the most important commercial hard magnetic materials, representing the class of hexagonal M-type ferrites [1], [2], [3], [4], [5]. The large magnetocrystalline anisotropy, high saturation magnetization and coercivity, high electrical resistivity and corrosion resistance of these materials make them useful as components in high frequency devices, permanent magnets, high density recording media and microwave devices [6], [7]. However, it is important to modify some of these properties in order to meet the various demands of specific purposes [8], [9], [10], [42], [43]. Doping by different metals can improve the properties of barium hexaferrites and thus make the material suitable for application [11].

Various techniques for ferrite material synthesis are known, most popular work is devoted to the following methods: chemical co-precipitation [6], [12], [13], [14], [15], hydrothermal reaction [16], [17], [18], sol–gel process [19], [20], [21], [22], solution combustion technique [23], topotactic reactive diffusion process [24], ceramic method [25], reverse microemulsion technique [26], and solid state reaction route [27], [28], [29]. The aim of the present work is to grow W-doped barium hexaferrite single crystals at the mm-length scale using flux growth technique and to study the influence of W-concentration on the magnetic properties. Small amounts of tungsten in this respect should tune the magnetic properties in a desired direction without changing the electronic properties drastically.

Using carbonate flux technique we were able to grow high-quality W-substituted Ba-ferrite crystals of composition BaFe_12−xW_xO₁₉ with x up to 0.06. The flux technique reduces the crystal growth temperature to below 1300 °C, compared with temperatures well above 1500 °C necessary in own-melt growth methods [30], [31]. In this process oxygen or an alternative gas was used to minimize evaporation of the melt.