Tailoring Magnetic Properties of Al-substituted M-Type Strontium Hexaferrites

M-type Strontium Hexaferrites have gained considerable attention for their applications in the permanent magnets, microwave devices and magnetic recording media due to the large uniaxial magnetocrystalline anisotropy, excellent remanent magnetization, and corrosion resistivity and low price [1]. Any further enhancement to the magnetic properties of M-type Sr-hexaferrite is of relevance for technological innovation. Aiming at this target, many works have been done to investigate the influence of main compositions, fabricating methods, and sintering processes on the magnetic properties. Y. Yang et al. [2] reported Al substituted M-type Ca-Sr-hexaferrites (Ca 0.6 Sr 0.1 La 0.3 Fe $_{12- {x}}$ Al x O 19 , $x= 0 \sim 1.4)$, and the results indicated that the saturation magnetization $( M_{s})$ linearly decreases while the coercivity $( H_{c})$ increases with the increase of Al contents. F. Rhein et al. [3] prepared SrFe $_{12- {x}}$ Al x O 19 powders via mechanochemical activation, and afterwards the powders were milled and annealed at $1000 ^{circ}\mathrm {C}$ in NaCl to produce ultrafine nano-particles. It is concluded that the furthermore milling and annealing treatment allow to improve both the coercivity $H_{c}$ and saturation magnetization $M_{s}$ of Al substituted Sr-hexaferrites $( x=0 ,1)$ as compared to the conventional ceramic process. Z. Chen et al. [4] studied Sr $_{0.61- {x}}$ La 0.39 Ca x Fe 11.7 Co 0.3 O 19 ferrites with various calcium concentrations $( x=0 ,0.1,0.2,0.3)$ through microwave calcining. The largest saturation magnetization of 68emu/g and coercivity of 5320Oe were observed for a composition of Sr 0.41 La 0.39 Ca 0.2 Fe 11.7 Co 0.3 O 19 calcined at $1140 ^{circ}\mathrm {C}$ for 30min. In this work, the M-type hexaferrite Sr 0.7 La 0.3 Fe $_{12- {x}}$ Al $_{x} \mathrm {O}_{19} ( x= 1 \sim 1.4)$ have been synthesized by the standard solid-state method. The influence of Al $^{3+}$ substitutions on the microstructure and magnetic properties of M-type Sr hexaferrites have been investigated in detail. In order to probe into the effect of Al $^{3+}$ substitutions on the magnetocrystalline anisotropy, the law of approach to saturation was utilized to calculate the magnetocrystalline anisotropy constant $K_{1}$, which could be written as Eq. $(1) \sim (3)$ [5]: $M=M_{s}(1- e/ H- f/ H^{2} - {\dots }) + \chi _{p}\mathrm {H}(1) f=H_{a}^{2}/ 15 (2) H_{a}= 2 K_{1}/ \mu _{0}M_{s}(3)$ where $e$ and $f$ represent the resistance of technical magnetization, $M_{s}$ the saturation magnetization, $H$ the applied magnetic field, $ \chi _{p}$ the paramagnetic susceptibility. $H_{a}$ can be derived accordingly. The X-ray diffraction patterns of the hexaferrite Sr 0.7 La 0.3 Fe $_{12- {x}}$ Al x O 19 with $x= 0 \sim 1.4$ are shown in Fig.1. It can be observed that the XRD patterns of all the samples are single magnetoplumbite phase patterns. The result indicates that Al $^{3+}$ ions are all indexed to the standard powder diffraction pattern of M-type hexaferrite. Fig.2 illustrates the magnetic hysteresis loops of the as-sintered samples. Note that the coercivity $H_{c}$ of Al substituted hexaferrites increase significantly with increasing Al substitutions. Concomitantly, the relatively high remanence ratio $( M_{r}/ M_{s})$ of 80% is observed in Al substituted hexaferrites.