Stable colloidal solutions of strontium hexaferrite hard magnetic nanoparticles

Abstract

Herein we demonstrate an approach to prepare a colloidal solution of strontium hexaferrite via a glass-ceramic route. The as obtained colloids are stable and resistive to aggregation or sedimentation. They reveal outstanding magnetic and magneto-optical properties because of their platelet-like anisotropic shape and high permanent magnetic moment.

Glass-Ceramic Synthesis of Cr-Substituted Strontium Hexaferrite Nanoparticles with Enhanced Coercivity

“…The coercivity of the nanoparticles is also higher than that obtained by most of the other methods [ 15 , 24 , 37 , 40 ]. As compared with Al-substituted hexaferrites prepared by glass crystallization [ 7 , 29 ], Cr-substituted powders possess very similar properties for larger particles; however, they allow for the production of much smaller nanoparticles with enhanced coercivity. Although the smallest ferrite nanomagnet made of ε-Fe 2 O 3 reveals higher coercive forces (e.g., 270 kA m −1 and 660 kA m −1 for spherical 8.2-nm and 10.5-nm particles, respectively) [ 41 ], Cr-substituted hexaferrite has a magnetization that is at least two times higher and features a plate-like particle shape, which may attract additional attention, for example due to the unique magneto-optical properties [ 9 ] or the ability to self-organize [ 10 ].…”

“…Thus, the hexaferrite nanoparticles are a reasonable material when nanomagnets are required for diverse purposes. For example, hexaferrites are attractive for high-density magnetic recording tape media [ 1 , 4 , 5 ] and low-frequency magnetic hyperthermia [ 6 ], as well as for magnetic colloids of hard magnetic particles, which are hugely different from traditional magnetite-based ferrofluids and demonstrate several unique properties [ 7 , 8 , 9 ]. Certainly, the colloidal hexafeFrrite nanoparticles are building blocks for the creation of magnetic nanostructures [ 10 , 11 ], nanocomposites [ 12 , 13 ], and various nanomaterials [ 14 , 15 , 16 , 17 , 18 ].…”

“…The particles smaller than 10 nm become superparamagnetic and lose their coercivity. Typical reported colloidal hexaferrite nanoparticles with diameters of 10-100 nm display coercivity values between zero and 360 kA m −1 (4500 Oe) for the largest particles [7,15,[22][23][24]. A common way to increase the coercivity of the hexaferrites is the partial substitution of iron ions with aluminum [1,3,21,[25][26][27], e.g., this can result in a coercivity of submicron single-domain particles of up to 3180 kA m −1 (40 kOe) [25].…”

“…The crystallization of glasses in the system SrO-Fe 2 O 3 -Al 2 O 3 -B 2 O 3 could be a solution [28,29]; however, the high substitution degrees are still not achieved, probably due to the high glass-forming ability of aluminum oxide in the presence of strontium oxide [30]. Nevertheless, this approach allowed one to produce nonsintered submicron hexaferrite particles with coercivities over 800 kA m −1 (10 kOe) [29] and colloidal nanoparticles with a coercivity of up to 445 kA m −1 (5600 Oe) [7]. Chromium substitution should have a similar coercivity increase effect [1,3,21,31]; however, the preparation of nonsintered submicron or nanosized particles of Cr-substituted hexaferrite has not been described so far.…”

Self-replication with magnetic dipolar colloids

“…We begin with a random distribution of monomers and dimers in solution and allow them to evolve in a diffusive regime. An external field H mitigates magnetic sedimentation [15] and accidental dimer formation by forcing monomers to aggregate only in mutually repulsive parallel strings [16], as illustrated in Fig. 1c.…”