Synthesis and Characterization of Ni-Ti Partially Substituted Hexaferrites for High-Density Magnetic Recording Applications

Abstract

M-type hexaferrite (AFe12O19; A = Ba2+, Sr2+, Pb2+) is an important magnetic oxide exhibiting magnetic properties suitable for a wide range of technological and industrial applications. The magnetic properties of M-type hexaferrite can be tuned for a specific application by adopting suitable synthesis routes and/or using special cationic substitutions for either Fe3+ or A2+ cations. In particular, coercive fields in the range of ~ 1 – 3 kOe and remnant magnetization > 20 emu/g are required for data storage media in high-density magnetic recording applications. Partial substitution of Fe3+ ions by Co2+-Ti4+ ions in BaM (BaFe12O19) or SrM (SrFe12O19) hexaferrite was long recognized as an effective procedure for reducing the coercivity to values appropriate for high- density magnetic recording, without decreasing the remnant magnetization appreciably. Also, the effects of other substitutions were extensively investigated. However, the M-type hexaferrite with Ni2+-Ti4+ substitution was generally ignored, especially when compared with the extensively investigated Co2+-Ti4+ substituted system. This work was motivated by the potential of Ni-Ti substitution to reduce the coercivity of SrM hexaferrite to appropriate levels and maintain the remnant magnetization high enough for high-density magnetic recording applications. A set of SrFe12–2xNixTixO19 hexaferrites was prepared by mixing and ball milling stoichiometric ratios of high-purity starting powders, pelletizing in the form of 4 cm-diameter disks and sintering in air at 1100 C for 2 hours. Rietveld analysis of the X-ray diffraction (XRD) patterns (Fig. 1) revealed that all samples examined in this work (0.0 ≤ x ≤ 0.8) consisted of a single SrFe12O19 (SrM) hexaferrite phase (standard pattern ICDD file: 00-033-1340), with no secondary phases. The refined lattice parameters decreased slightly (≤ 0.1%), but monotonically with the increase of x. Further, the crystallite size in all samples fluctuated in the range of 60 – 70 nm, without any systematic behavior. M-type hexaferrite (AFe12O19; A = Ba2+, Sr2+, Pb2+) is an important magnetic oxide exhibiting magnetic properties suitable for a wide range of technological and industrial applications. The magnetic properties of M-type hexaferrite can be tuned for a specific application by adopting suitable synthesis routes and/or using special cationic substitutions for either Fe3+ or A2+ cations. In particular, coercive fields in the range of ~ 1 – 3 kOe and remnant magnetization > 20 emu/g are required for data storage media in high-density magnetic recording applications. Partial substitution of Fe3+ ions by Co2+-Ti4+ ions in BaM (BaFe12O19) or SrM (SrFe12O19) hexaferrite was long recognized as an effective procedure for reducing the coercivity to values appropriate for high- density magnetic recording, without decreasing the remnant magnetization appreciably. Also, the effects of other substitutions were extensively investigated. However, the M-type hexaferrite with Ni2+-Ti4+ substitution was generally ignored, especially when compared with the extensively investigated Co2+-Ti4+ substituted system. This work was motivated by the potential of Ni-Ti substitution to reduce the coercivity of SrM hexaferrite to appropriate levels and maintain the remnant magnetization high enough for high-density magnetic recording applications. A set of SrFe12–2xNixTixO19 hexaferrites was prepared by mixing and ball milling stoichiometric ratios of high-purity starting powders, pelletizing in the form of 4 cm-diameter disks and sintering in air at 1100 C for 2 hours. Rietveld analysis of the X-ray diffraction (XRD) patterns (Fig. 1) revealed that all samples examined in this work (0.0 ≤ x ≤ 0.8) consisted of a single SrFe12O19 (SrM) hexaferrite phase (standard pattern ICDD file: 00-033-1340), with no secondary phases. The refined lattice parameters decreased slightly (≤ 0.1%), but monotonically with the increase of x. Further, the crystallite size in all samples fluctuated in the range of 60 – 70 nm, without any systematic behavior. Analysis of the magnetic data revealed a slow decrease of the saturation magnetization (from 67.6 emu/g at x = 0.0 to 65.3 emu/g at x = 0.8) and remnant magnetization (from 38.8 emu/g at x = 0.0 to 30.4 emu/g at x = 0.8) with the increase of x. These values, however, remained relatively high for practical applications. The coercivity, on the other hand, exhibited a significant reduction with the increase of x (from 4386 Oe at x = 0.0 to 1150 Oe at x = 0.8)). The remnant magnetization of ~ 30 – 36 emu/g and intermediate coercivity of ~ 1.2 – 3 kOe for the samples with 0.4 ≤ x ≤ 0.8 render these materials suitable for high-density magnetic recording media. The effectiveness of Ni-Ti substitution in reducing the coercivity without appreciably influencing the remnant magnetization is comparable with the reported effectiveness of Co-Ti substitution, thus providing a cheaper alternative by avoiding the use of Co. The switching field distribution (SFD) revealed a progressive reduction of the mean magnetic anisotropy field, Ha, from 10 kOe at x = 0.0 to 2.8 kOe at x = 0.8. Fig. 3 shows representative curves of the reduced isothermal remnant magnetization (mr) and their derivatives representing the SFD, from which the mean anisotropy field was evaluated. The behavior of the SFD and Ha is the main mechanism responsible for the monotonic decrease of the coercivity with the increase of x. The magnetization induced by an applied field of 100 Oe was measured versus temperature for all samples. The results indicated that the Ni-Ti substitution did not lead to a significant reduction of the Curie temperature, rendering the substituted hexaferrites suitable for high-density magnetic recording at relatively high operating temperatures. Keywords: Hexaferrites, Partial substitution, Nickel, Titanium, High-density magnetic recording.