Tuning Microstructure and Magnetic Properties of Highly Al-Substituted Sr/Ca Hexaferrites

Introduction

Hexagonal ferrites attract a great deal of attention due to their current and prospective industrial applications as well as strong correlation between their crystal structure and physical properties. They are chemically and thermally stable compounds, possessing appropriate magnetic properties and low cost.

From standpoint of fundamental physics, hexaferrites show intriguing physical phenomena like spin-glass behavior, quantum paraelectricity, ferroelectricity, quantum electric dipole liquid state, frustrated antiferroelectricity, high thermal spin ordering, helicoidal magnetic structure, magnetoelectric coupling, and new magnetic transitions have been found recently in the Kagome-motif-lattice hexaferrites.

This class of materials includes so-called M-type hexaferrites BaFe$_{12}$O$_{19}$ and SrFe$_{12}$O$_{19}$, which are the only known magnetically hard hexaferrites. The ferrimagnetism of M-type hexaferrites is caused by Fe$^{3+}$ ions network, and hexagonal symmetry of the crystal structure induces very high single-axis magnetic anisotropy.

Hard hexaferrites are used for decades as materials for permanent magnets, microwave devices, magnetic recording media, and they are anticipated for a number of promising applications such as hard magnetic tips for magnetic force microscopy, microwave absorbers, filters and circulators for sixth (6G) and following generations of wireless technologies, ferrofluids, nanocomposites, ceramics, etc.

The Challenge

It is well-known that the coercivity strongly depends on material microstructure. In terms of potential applications in magnetic devices, single-domain particles with maximum diameters of the order of several hundred nanometers are preferred for their improved magnetic hardness.

On the other hand, the magnetic properties are strongly related to the features of the hexaferrite crystal structure, for example, on substituting cations distribution, which, in turn, can be determined by the synthesis conditions. For different applications, hexaferrite particles with a variety of coercive force and morphology are needed.

Thus, it is an important task to develop a technique which allows to simultaneously control both the microstructure of M-type hexaferrites and their chemical composition. However, an effective synthesis of such single-domain hexaferrite particles of complex chemical compositions is still a great challenge.

Solution: Citrate-Nitrate Auto-Combustion with Controlled Annealing

Researchers utilized a citrate-nitrate auto-combustion method to prepare fine particles of strontium hexaferrite highly substituted by calcium and aluminum Sr$_{1-x/12}$Ca$_{x/12}$Fe$_{12-x}$Al$_x$O$_{19}$ (x = 4–6) and established the effect of the annealing temperature on the microstructure, chemical composition, crystal structure, and magnetostatic and magnetodynamic properties.

Materials and Methods

Synthesis Process

The citrate-nitrate auto-combustion method involves two main stages:

1. Precursor formation: The citrate-nitrate aqueous solution containing metal ions decomposes during water evaporation, forming a highly porous amorphous precursor
2. High-temperature annealing: The precursor is annealed at temperatures ranging from 900 to 1400 °C, leading to the formation of weakly sintered hexaferrite particles

Composition Range

The study focused on Sr$_{1-x/12}$Ca$_{x/12}$Fe$_{12-x}$Al$_x$O$_{19}$ with x = 4, 4.5, 5, 5.5, and 6, covering a wide range of aluminum substitution.

Characterization

• Synchrotron X-ray diffraction at Swiss Light Source (SLS) for crystal structure analysis
• Rietveld refinement for aluminum distribution among iron sites
• Scanning and transmission electron microscopy for particle size and morphology
• SQUID magnetometry for magnetic hysteresis loops
• Terahertz time-domain spectroscopy for natural ferromagnetic resonance (NFMR) measurements
• Thermogravimetric analysis in magnetic field for Curie temperature determination

Key Findings

Microstructure Evolution with Annealing Temperature

At low annealing temperatures (900–1000 °C):

• Hexaferrite nanoparticles form with 90% of nominal Al content
• Wide chemical composition distribution
• Particle sizes in the nanoscale range

With increasing annealing temperature:

• The chemical distribution significantly narrows
• Chemical composition becomes close to the nominal one
• Particle size transitions from submicron to micron range

Particle size vs. annealing temperature (for x = 5):

Aluminum Distribution in Crystal Structure

The Rietveld refinement revealed that aluminum ions predominantly occupy specific iron sites:

• 2a and 12k sites: Primary occupation (approximately equal amounts)
• 4f1, 4f2, and 2b sites: Much less affected

Importantly, the aluminum distribution over iron sites is independent of the annealing temperature. This means the site preference is determined during the initial combustion reaction and remains stable throughout the temperature range studied.

Magnetic Properties

Coercivity

The coercivity shows a strong dependence on both aluminum content and annealing temperature:

Coercivity vs. aluminum content (at optimal 1200 °C):

Coercivity vs. annealing temperature:

• At 900–1000 °C: Coercivities up to 25 kOe (nanoparticles)
• At 1200 °C: Maximum coercivity for all compositions (22.8–36 kOe)
• Above 1300 °C: Coercivity decreases due to transition to polydomain state

The maximum coercivity of 36 kOe was achieved for x = 5.5 sample annealed at 1200 °C. This is a giant value comparable to only hard magnetic oxide nanomaterial – $\epsilon$-Fe$_2$O$_3$.

Saturation Magnetization

The saturation magnetization decreases with increasing aluminum content:

• This is expected due to the diamagnetic nature of Al$^{3+}$ ions replacing magnetic Fe$^{3+}$ ions
• For x = 6, both coercivity and magnetization sharply decrease, likely due to destruction of the magnetic subsystem caused by strong diamagnetic dilution

Natural Ferromagnetic Resonance (NFMR)

High aluminum substitution resulted in sub-terahertz electromagnetic wave absorption. The NFMR frequency depends on the degree of Al substitution:

NFMR frequency vs. composition:

Key observations:

• The NFMR frequency rises with increasing aluminum content
• For samples with x = 5 and x = 5.5, the NFMR frequency drifts slightly with annealing temperature
• The absorption line tends to narrow with increasing annealing temperature, due to narrowing of chemical composition distribution

The record-high NFMR frequency value of 270 GHz was achieved for x = 5.5 sample annealed at 1400 °C. This is today a record value among known powder materials.

Curie Temperature

The Curie temperature decreases with increasing aluminum substitution:

• For x = 4: T$_C$ ≈ 650 K
• For x = 5.5: T$_C$ ≈ 550 K

This is consistent with the dilution of the magnetic sublattice by non-magnetic Al$^{3+}$ ions.

Optimal Conditions

For maximum coercivity (single-domain particles):

• Annealing temperature: 1200 °C
• Particle size: 300–500 nm
• Optimal composition: x = 5.5
• Coercivity: 36 kOe

For maximum NFMR frequency:

• Annealing temperature: 1400 °C
• Composition: x = 5.5
• NFMR frequency: 270 GHz

For nanoparticles with high coercivity:

• Annealing temperature: 900–1000 °C
• Particle size: <200 nm
• Coercivity: >10 kOe (first time achieved for highly Al-substituted hexaferrite nanoparticles)

Applications

The unique properties of these materials make them suitable for various applications:

1. Dense and durable magnetic recording – nanoparticles with coercivity above 10 kOe
2. Terahertz electronics – generation of pure spin current in the absence of external magnetic field
3. 6G and beyond wireless technologies – filters and circulators operating at sub-terahertz frequencies
4. Microwave absorbers – selective absorption at NFMR frequencies
5. Nanoscale applications – particles with controlled size and magnetic properties

Comparison with Competing Materials

Despite the lower magnetization compared to unsubstituted hexaferrites, the Al/Ca-substituted material offers:

• Much higher coercivity (36 kOe vs. ~5 kOe)
• Record NFMR frequency (270 GHz vs. ~50 GHz)
• Simple synthesis via citrate-nitrate auto-combustion

Conclusions

This work demonstrates several important achievements:

1. First nanoparticles of M-type hexaferrite with giant coercivity (>20 kOe) – comparable to $\epsilon$-Fe$_2$O$_3$ but with simpler synthesis

2. Controlled microstructure via annealing temperature – from nanoparticles (900 °C) to submicron (1200 °C) to micron-sized particles (1400 °C)

3. Record coercivity of 36 kOe for x = 5.5 composition at 1200 °C annealing

4. Record NFMR frequency of 270 GHz among powder materials for x = 5.5 at 1400 °C

5. Temperature-independent aluminum site distribution – Al ions preferentially occupy 2a and 12k sites regardless of annealing temperature

6. Narrowing chemical distribution with temperature – leading to sharper NFMR absorption lines at higher annealing temperatures

7. First highly Al-substituted hexaferrite nanoparticles with coercivity above 10 kOe at 900–1000 °C annealing

The precession of the magnetic moment at such high frequencies (270 GHz) is in demand not only for isolating electromagnetic waves but can be used to convert a high-frequency signal into a DC spin current in terahertz devices.

This work represents a significant step towards the creation of a nanomaterial alternative to $\epsilon$-Fe$_2$O$_3$, which has a rather complicated synthesis method.

This article is based on research published in Ceramics International (2023): “Tuning the microstructure, magnetostatic and magnetodynamic properties of highly Al-substituted M-type Sr/Ca hexaferrites prepared by citrate-nitrate auto-combustion method” by Evgeny A. Gorbachev, Vasily A. Lebedev, Ekaterina S. Kozlyakova, Liudmila N. Alyabyeva, Asmaa Ahmed, Antonio Cervellino, and Lev A. Trusov.