Site occupancy and magnetic properties of Al-substituted M-type strontium hexaferrite

Abstract

We use first-principles total-energy calculations based on density functional theory to study the site occupancy and magnetic properties of Al-substituted M -type strontium hexaferrite SrFe12−xAlxO19 with x = 0.5 and x = 1.0. We find that the non-magnetic Al 3+ ions preferentially replace Fe 3+ ions at two of the majority spin sites, 2a and 12k, eliminating their positive contribution to the total magnetization causing the saturation magnetization Ms to be reduced as Al concentration x is increased. Our formation probability analysis further provides the explanation for increased magnetic anisotropy field when the fraction of Al is increased. Although Al 3+ ions preferentially occupy the 2a sites at a low temperature, the occupation probability of the 12k site increases with the rise of the temperature. At a typical annealing temperature (> 700 • C) Al 3+ ions are much more likely to occupy the 12k site than the 2a site. Although this causes the magnetocrystalline anisotropy K1 to be reduced slightly, the reduction in Ms is much more significant. Their combined effect causes the anisotropy field Ha to increase as the fraction of Al is increased, consistent with recent experimental measurements.[4] decreased coercivity. However, the coercivity of the M-type hexaferrites is not increased significantly by these cation substitutions, and is still much smaller than that of Nd-Fe-B magnet [15].Al substitution in the M-type hexaferrite has been more effective in enhancing coercivity [16][17][18][19][20]. Particularly, Wang et al synthesized Al-doped SFO SrFe 12−x Al x O 19 (SFAO) with Al content of x = 0 − 4 using glycinnitrate method and subsequent annealing in a temperature over 700 • C obtaining the largest coercivity of 17.570 kOe, which is much larger than that of SFO (5.356 kOe) and exceeds even the coercivity of the Nd 2 Fe 17 B (15.072 kOe) [1]. Wang and co-workers also observed that the coercivity of the SFAO increases with increasing Al concentration at a fixed annealing temperature. These results call for a systematic understanding, from first principles, of why certain combinations of dopants lead to particular results. This theoretical understanding will be essential in systematically tailoring the properties of SFO.

Calculation of exchange integrals and Curie temperature for La-substituted barium hexaferrites

“…As shown in Fig. 1, 24 Fe 3+ ions of M-type hexaferrites are distributed in five different sublattices: 3 parallel sites (12 k , 2 a and 2 b ) and 2 antiparallel sites (4 f 1 and 4 f 2 )17. La-based substitutions could contribute to some Fe 3+ transforming into Fe 2+ at the 2 a and 4 f 2 sites18.…”

Synthesis and characterization of aluminium substituted calcium hexaferrite

“…The result is that resultant magnetic moment and saturation magnetisation increases. For higher Al 3þ ion concentration (above x ¼ 0.6), Al 3þ ions enter 12K site [20]. 12K site has spin up electronic configuration.…”

“…From the magnetization curve the squareness ratio (M r /M s ) is less than 0.5. Such a material is good for recording medium [20]. For longitudinal magnetic recording medium high coercivity up to 600 Oe is required.…”

Direct imaging of dopant sites in rare-earth element-doped permanent magnet and correlated magnetism origin

“…It has been reported that the magneto-crystalline anisotropy and coercive field of M-type strontium hexaferrite can be increased or reduced according to the concentrations of the substituted La 3+ cations. [20][21][22][23] Fe or Sr lattice sites are argued to be the strong preference for the La 3+ substitutions, which are thought to be responsible for the intrinsic physical mechanism of mag-netism modulation. Many efforts using the abovementioned techniques have been devoted to confirming the site preference of the substituted ions in M-type strontium hexaferrite.…”