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

Problem

Hard magnetic hexaferrites $\mathrm{MFe}_{12}\mathrm{O}_{19}$ (M = Ba, Sr) are widely used in ceramic permanent magnets and are promising for nanotechnology applications. Due to high magnetocrystalline anisotropy, even very small hexaferrite nanoparticles remain hard-magnetic, preserving high coercivity and permanent magnetization. However, most synthesis techniques include a high-temperature treatment step that leads to particle agglomeration and sintering, making the products unsuitable for nanotechnology applications.

Chromium substitution in hexaferrites has been rarely reported and not well investigated due to early studies revealing reduction of the anisotropy field with increasing chromium content. The challenge is to develop a method for producing nonsintered single-domain Cr-substituted hexaferrite nanoparticles with enhanced magnetic properties.

Methods/Ideas

The authors report an approach to prepare nonsintered single-domain nanoparticles of chromium-substituted hexaferrite via crystallization of glass in the $\mathrm{SrO-Fe}_2\mathrm{O}_3-\mathrm{Cr}_2\mathrm{O}_3-\mathrm{B}_2\mathrm{O}_3$ system.

Key aspects of the method:

• Glass-ceramic synthesis where hexaferrite nanoparticles are separated from each other by a nonmagnetic borate matrix
• The borate matrix is easily soluble in weakly acidic solutions, allowing extraction of pure hexaferrite phase
• Annealing temperatures from 650 °C to 900 °C to control particle size
• Chemical analysis by ICP-MS to determine chromium content
• XRD analysis with Rietveld refinement for structural characterization
• TEM and SEM for particle size and morphology analysis
• Magnetic measurements (VSM) for hysteresis loops, saturation magnetization, and coercivity

Results

Structural and Morphological Characterization

XRD Analysis:

• All samples contain the M-type hexaferrite phase ($\mathrm{SrFe}_{12-x}\mathrm{Cr}_x\mathrm{O}_{19}$)
• Unit cell parameters slightly reduced from pure $\mathrm{SrFe}_{12}\mathrm{O}_{19}$ due to smaller ionic radius of $\mathrm{Cr}^{3+}$ (0.615 Å) vs $\mathrm{Fe}^{3+}$ (0.645 Å)
• Strong anisotropy of particles with plate-like shape (smaller dimension along crystallographic c-axis)

Particle Size vs Annealing Temperature:

• Particles remain plate-like with d/h ratio ~5
• All particles are in single-domain state (sizes significantly below critical diameter of 500 nm)

Chemical Composition

ICP-MS Analysis ($\mathrm{SrFe}_{12-x}\mathrm{Cr}_x\mathrm{O}_{19}$):

• Chromium content increases with temperature for nanoparticle samples (650–750 °C)
• Decreases at higher temperatures due to formation of chromium-depleted secondary hexaferrite during recrystallization

Magnetic Properties

Key findings:

• Saturation magnetization ($M_S$): 31.3–42.3 A m² kg⁻¹ (increases with particle size)
• Coercivity ($H_C$): 334–732 kA m⁻¹ (4200–9200 Oe)
• Curie temperature: 622–658 K (lower than pure $\mathrm{SrFe}_{12}\mathrm{O}_{19}$ at 740 K)

Coercivity vs Particle Size:

Comparison with pure hexaferrite:

• Saturation magnetization reduced (Cr prefers 2a and 12k sites with uncompensated spins)
• Coercivity increased by 90% for smallest nanoparticles and 60% for submicron particles
• Glass-ceramics show even higher coercivity (up to 795 kA m⁻¹ / 10,000 Oe) due to particle separation by nonmagnetic matrix

Conclusions

The study demonstrates a successful strategy for producing high-coercivity ferrite nanomagnets:

1. Cr-substituted single-domain hexaferrite particles obtained via glass-ceramic method

2. Record size/coercivity ratio for hexaferrite nanoparticles:

   • 20 × 4 nm particles: 334 kA m⁻¹
   • 25 × 5 nm particles: 509 kA m⁻¹
   • 65 × 11 nm particles: 581 kA m⁻¹
   • 190 × 55 nm particles: 732 kA m⁻¹

3. Chromium substitution enhances coercivity contrary to early expectations, especially important for nanoparticles

4. Nonsintered particles suitable for various applications:

   • Durable magnetic recording media
   • Electromagnetic wave shielding
   • Magnetic force microscopy tips
   • Ferrofluids with magnetically adjustable refractive index
   • Magneto-mechanical microsystems
   • Magnetic self-assembled nanostructures

The glass-ceramic technique provides room for future optimization of particle size, morphology, and chromium content for further improvement of magnetic properties.