Tuning the Morphology and Magnetic Properties of Single-Domain SrFe8Al4O19 Particles Prepared by Citrate Auto-Combustion Method

Problem

M-type hexaferrites ($\mathrm{BaFe}_{12}\mathrm{O}_{19}$ and $\mathrm{SrFe}_{12}\mathrm{O}_{19}$) are well-studied compounds with a unique set of finely tunable functional properties. Large magnetocrystalline anisotropy and high coercivity enable their wide applications as permanent magnets, while high thermal and chemical stability makes them suitable for nanomagnets in magnetic recording media, fast-response magnetoactive colloids, and hard magnetic cores of exchange-coupled nanocomposites. Moreover, hexaferrites display specific millimeter-wave (sub-THz) absorption due to ferromagnetic resonance (FMR), which is essential for modern wireless communication technologies.

The properties of hexaferrites are extremely sensitive to particle morphology and ionic substitutions in their crystal structure. Recently, a method was presented for manufacturing highly aluminum-substituted strontium hexaferrite ($\mathrm{SrFe}_8\mathrm{Al}_4\mathrm{O}_{19}$) with giant coercivity up to 40 kOe and record-high natural FMR frequencies of 160–250 GHz. However, the possibility to modify the morphology and functional properties of particles by varying heat treatment parameters needs further investigation.

Methods/Ideas

The authors studied the influence of annealing time at 1200 °C on the morphology, magnetic properties, and millimeter-wave absorption of $\mathrm{SrFe}_8\mathrm{Al}_4\mathrm{O}_{19}$ particles.

Synthesis method:

• Citrate auto-combustion method: strontium, iron, and aluminum nitrates mixed with citric acid (molar ratio 1:3 between metal and citrate ions)
• Solution neutralized with $\mathrm{NH}_3$(aq.) and dehydrated by heating
• Product spontaneously combusted to form highly porous precursor powder
• Powder heated to 1200 °C at 10 K/min and exposed for 0, 0.5, 2, 8, 14, and 24 h

Characterization:

• XRD (Rigaku D-Max 2500, CuK$\alpha$ radiation) for phase analysis and lattice parameters
• SEM (Carl Zeiss NVision40) for particle morphology
• VSM (Quantum Design PPMS, fields up to 6 T) for magnetic hysteresis loops
• Terahertz time-domain spectroscopy (Teraview TPS 3000) for FMR spectra at room temperature without external magnetic field

Results

Phase Composition and Structure

XRD Analysis:

• All samples contain single crystalline M-type hexaferrite phase
• 0 h sample: slightly larger lattice parameters, incomplete Al substitution ($\mathrm{SrFe}_{8.15}\mathrm{Al}_{3.85}\mathrm{O}_{19}$)
• 0.5 h and longer: lattice parameters agree with $\mathrm{SrFe}_8\mathrm{Al}_4\mathrm{O}_{19}$ phase

Unit Cell Parameters:

Particle Morphology

SEM Analysis:

• Particles have thick-plate shape with wide diameter distributions
• Mean particle diameter increases with exposure time

• Single-domain limit of $\mathrm{SrFe}_8\mathrm{Al}_4\mathrm{O}_{19}$ estimated at ~800 nm
• All particles occur mainly in single-domain state

Magnetic Properties

Hysteresis Loop Characteristics:

• Shapes typical of randomly oriented single-domain Stoner–Wohlfarth particles ($M_r/M_S \approx 0.5$)

Key observations:

• $M_S$ close to reported values for $\mathrm{SrFe}_8\mathrm{Al}_4\mathrm{O}_{19}$ (0.5–24 h samples)
• 0 h sample has higher $M_S$ indicating lower Al substitution
• $H_C$ sharply increases from 0 to 2 h due to higher Al substitution
• For 2–24 h, $H_C$ gradually rises from 15.6 to 18.4 kOe due to particle enlargement

Mechanism:

• $\mathrm{Al}^{3+}$ substitutes for $\mathrm{Fe}^{3+}$ in octahedral 12k and 2a sites
• Al incorporation slightly decreases magnetocrystalline anisotropy constant $K_1$ but considerably reduces $M_S$
• According to Stoner–Wohlfarth model: $H_C \propto K_1/M_S$, leading to significant $H_C$ increase
• Coercivity much higher than unsubstituted hexaferrites (not higher than 7 kOe)
• Observed $H_C$ increase due to continuous recrystallization and particle enlargement

Ferromagnetic Resonance

FMR Frequencies:

Key findings:

• $f_r$ sensitive only to hexaferrite composition, not particle size
• Magnetic anisotropy constant and saturation magnetization do not depend on particle size in sub-micron range (100–1000 nm)
• Particle shape has no effect on $f_r$ due to low magnetization and negligible demagnetization field
• According to Kittel formula: $f_r \propto H_a = 2K_1/M_S$, affected only by Al content

Difference between $H_C$ and $f_r$ behavior:

• Different relationships to thermal fluctuation of particle magnetization
• Probability of spontaneous demagnetization increases with decreasing particle size
• This leads to decreased coercivity for smaller particles but does not affect average $H_a$ and $f_r$

Conclusions

The study demonstrates a promising method for preparing highly substituted hexaferrite particles with tunable sizes and high magnetic and millimeter-wave absorption properties:

1. Single-phase $\mathrm{SrFe}_8\mathrm{Al}_4\mathrm{O}_{19}$ hexaferrite obtained by annealing porous auto-combustion products at 1200 °C

2. Tunable particle size: Mean diameter from 100 to 460 nm by varying exposure time

3. High coercivity: 14.5–18.4 kOe (highest reported for nanosized hexaferrite particles)

   • 100 nm particles: 14.5 kOe
   • 460 nm particles: 18.4 kOe

4. Stable FMR frequency: 149–164 GHz

   • For 230–460 nm particles: constant 164 GHz
   • Particle size does not affect $f_r$ in single-domain region

5. Applications: The developed hexaferrite materials are promising for:

   • Spintronics
   • Electromagnetic shielding
   • Durable magnetic recording
   • Next generation wireless technologies (5G/6G)
   • Fine-grained ceramics, hard magnetic films, coatings, and composites

The method provides hexaferrite powders with high phase purity and tunable particle size within the single-domain region, essential for various advanced applications.