Hard Magnetic Colloidal Nanoplates with Tunable Size for Magneto-Optical Applications

Introduction

Traditional ferrofluids are suspensions of nanosized superparamagnetic particles (mainly magnetite) in carrier liquids. They have a wide range of industrial applications related to the ability to position them and control their flow by the external magnetic field. Also, colloidal nanoparticles have advanced biomedical applications, including cancer therapy, drug delivery, magnetic labels, and imaging.

Superparamagnetic particles do not have their own permanent magnetic moment, and in a zero magnetic field, they behave like ordinary colloidal particles. However, the presence of a constant magnetic moment rigidly bound to the crystallographic axes of the particle leads to new attractive features.

Such ferrofluids can be made of hard magnetic nanoparticles of M-type hexaferrites SrFe$_{12}$O$_{19}$ or BaFe$_{12}$O$_{19}$. The large magnetic moment of the particles and resulting dipole–dipole interactions induce internal structural self-organization, resulting in the formation of liquid crystal-like structures even in a zero magnetic field. In a non-zero magnetic field, the particle’s magnetic moment tends to line up along the magnetic field lines, making it possible to control the particle’s orientation and give it rotational motion.

Since hexaferrite particles usually have a plate-like shape, they can develop a large mechanical moment and thus affect micro-objects, such as cancer cells, which makes them useful for low-frequency magneto-mechanical therapy. The anisotropic shape of the particles also induces a magneto-optical effect, where the light transmission of the colloid depends on the magnitude and direction of the external magnetic field. This phenomenon can be harnessed for high-frequency light modulation and small magnetic field detection.

The Challenge

Stabilizing magnetically hard hexaferrite particles in the colloidal state is quite challenging:

1. High temperatures are usually required to form the hexaferrite phase, at which strong sintering and intensive particle growth occur
2. Strong magnetic attraction between the particles in the colloid requires each particle to be individually stabilized sterically or electrostatically
3. Controlled production of nanoparticles with the required size and shape has not been thoroughly studied

One of the most successful approaches for producing stable aqueous colloids of hard-magnetic strontium hexaferrite nanoparticles is the crystallization of borate glasses. Among the synthesis methods, the glass-ceramic technique stands out for its ability to produce high-quality, well-dispersed hexaferrite nanoparticles, which can also be doped by various metals for better performance.

Solution: Glass-Ceramic Synthesis with Tunable Size

Researchers have synthesized highly anisotropic plate-like nanoparticles of aluminum-substituted strontium hexaferrite via the crystallization of 4Na$_2$O · 9SrO · 5.5Fe$_2$O$_3$ · 4.5Al$_2$O$_3$ · 4B$_2$O$_3$ glass, achieving tunable sizes by adjusting the annealing temperature (650–750 °C).

Materials and Methods

Glass Preparation

The glass of the initial composition 4Na$_2$O · 9SrO · 5.5Fe$_2$O$_3$ · 4.5Al$_2$O$_3$ · 4B$_2$O$_3$ was prepared by rapid melt quenching:

1. Stoichiometric amounts of starting reagents (NaHCO$_3$, SrCO$_3$, Fe$_2$O$_3$, Al$_2$O$_3$, and H$_3$BO$_3$) were mixed into a 5 g batch using an agate mortar
2. Heated at 10 °C/min to 700 °C and annealed for 2 h
3. The obtained batch was ground and melted in a platinum crucible at 1250 °C
4. After 1 h exposure, the melt was quenched between two rotating steel rollers to form glassy flakes

Glass-Ceramic Formation

The glass-ceramics were obtained by isothermal heat treatment of the glass at annealing temperatures ($T_{ann}$) of 600–900 °C for 2 h followed by air-quenching.

Particle Extraction

The samples were treated with 3 wt% aqueous hydrochloric acid solution to dissolve the borate matrix and leach the magnetic particles. The formers were separated from the solution by centrifugation, thoroughly washed with distilled water, and dried.

Ferrofluid Preparation

Hexaferrite ferrofluids were prepared directly during the extraction of the nanoparticles. After centrifugation, water was added to the wet precipitate. Sonication led to the formation of particle suspensions. To obtain stable colloids, the final pH should be about 3. Large particles and aggregates were separated with a NdFeB magnet or by low-speed centrifugation.

Key Findings

Particle Morphology and Size Control

The XRD analysis showed that the obtained glass contained no crystalline phases and was paramagnetic, confirming the absence of ferromagnetic phases. The DTA curve revealed:

• Glass transition temperature: 535 °C
• Three exothermal effects corresponding to crystallization at 599, 673, and 751 °C
• Melting begins at about 860 °C

Particle size tuning by annealing temperature:

At annealing temperatures $T_{ann}$ = 650–700 °C, the mean platelet diameter slightly rises from 35 to 45 nm while the ratio h/d ≈ 1/10 remains nearly constant. This temperature range matches the primary crystallization of the hexaferrite phase.

At higher annealing temperatures, more intense particle growth occurs involving secondary grain growth by recrystallization. This is accompanied by an increase in the h/d ratio; that is, the particles become thicker.

Chemical Composition and Aluminum Substitution

The unit cell parameters are reduced relative to commonly reported ones of SrFe$_{12}$O$_{19}$ (a = 5.885 and c = 23.05 Å). This indicates that iron ions are partially substituted by aluminum because the ionic radius of Al$^{3+}$ (r$^{VI}$ = 0.535 Å) is significantly smaller than that of Fe$^{3+}$ (r$^{VI}$ = 0.645 Å for a high-spin state).

Chemical analysis results:

The Rietveld refinement shows that aluminum ions preferably occupy the positions 2a and 12k in the hexaferrite structure (approximately the same amount of Al in each position), while they are not found in other sites. This is consistent with most studies of aluminum-substituted hexaferrites.

Magnetic Properties

The sample annealed at 600 °C demonstrates a reversible M(H) curve without hysteresis, typical for superparamagnetic particles. Starting from $T_{ann}$ = 650 °C, the samples reveal pronounced hysteresis loops with ratio $M_R/M_S$ = 0.5 characteristic for randomly oriented single-domain Stoner–Wohlfarth particles with uniaxial magnetocrystalline anisotropy.

Key magnetic properties:

• Saturation magnetization increases from 15.3 emu/g at 650 °C to 50 emu/g at 750 °C and above
• Coercivity gradually rises from 2700 Oe at 650 °C to 7700 Oe at 900 °C
• The increase in coercivity is mainly caused by an increase in particle size, and above $T_{ann}$ = 720 °C, the h/d ratio also rises, contributing to improved magnetic hardness

Aluminum substitution effect:

Due to aluminum substitution, the observed coercivity values are significantly higher compared to unsubstituted hexaferrite particles with similar morphology:

• Nanoplates with dimensions of about 40 nm × 5 nm have coercivity of 2600 Oe for unsubstituted and 3800–4500 Oe for Al-substituted hexaferrite
• The highest obtained coercivity of 7700 Oe exceeds the maximum values of 6500–7000 Oe usually reported for unsubstituted strontium hexaferrite

The observed increase in coercivity is explained by a considerable reduction of $M_S$ due to the introduction of Al$^{3+}$ ions into crystallographic sites 2a and 12k, where Fe$^{3+}$ ions provide a positive contribution to the net magnetization. According to the Stoner–Wohlfarth model, the coercivity of single-domain particles is expressed as $H_C \propto K_1 M_S^{-1}$, therefore, the aluminum substitution leads to an improvement of the coercivity.

Colloidal Stability

The nanoparticles form stable aqueous colloids in the pH range of about 2–4. The particles in the colloids are electrostatically stabilized. According to zeta-potential measurements:

• The surface of the particles is positively charged in an acidic solution and negatively charged in an alkaline one
• The isoelectric point corresponds to pH ≈ 7
• In a neutral solution, particles aggregate rapidly and irreversibly due to strong magnetic attraction
• At pH below 2, the particles begin to dissolve

For the sample obtained at $T_{ann}$ = 700 °C, in the pH range from 2 to 4, the zeta-potential values are higher than +30 mV, corresponding to good stabilization by surface charge.

Colloidal particle properties:

Magneto-Optical Properties: The “Jalousie Effect”

Due to the permanent magnetic moment rigidly aligned along the crystallographic axis c, not only can the position of the colloidal hexaferrite particles be controlled using a magnetic field, but so can their orientation. If the particles have a highly anisotropic shape (plate-like), this leads to a remarkable phenomenon – the magnetic field-dependent optical transmission of the colloids, or “jalousie effect”.

How it works:

• When the nanoplates are positioned perpendicular to the light beam, the optical transmission is minimal (closed state)
• When the light is directed along the plates, the transmission is maximum (open state)
• For a typical colloid with particle concentration of 150 mg/L, the transmittance in closed and open states is about 15% and 30%, respectively

The magneto-optical response reaches its maximum in a field of about 100 Oe when all particles line up in the same direction. Changes in optical absorption in magnetic fields of 1 Oe can be detected using a conventional photodiode, and above 10 Oe, the changes are noticeable even to the naked eye.

Size-dependent magneto-optical response:

The difference in transmission between closed and open states rises with increasing nanoplate diameter and anisotropy factor.

High-Frequency Dynamic Response

Another distinctive feature of hexaferrite colloids is the high-speed switching between closed and open states compared to other magnetic colloidal systems. In an aqueous medium at room temperature, the particles rotate following the direction of the magnetic field without delay up to field frequencies of about 100 Hz.

Frequency-dependent behavior:

• As the frequency of the applied field increases, the drag force of the medium increases, so the particles cannot fully follow the direction of the field
• The frequency dependence of the magneto-optical response for smaller particles is shifted to a higher frequency range
• The imaginary component peaks at 600 Hz and 1000 Hz for 90 nm × 7.1 nm and 48 nm × 5.4 nm particles, respectively

Thus, small particles are better suited for high-frequency applications, while a stronger magneto-optical response of large particles is necessary for greater sensitivity.

Applications

The unique properties of these hexaferrite colloids make them suitable for various emerging applications:

1. Microfluidic stirring – magnetically induced rotational motion for micro-scale mixing
2. Mechanical impacting for cancer treatment – low-frequency magneto-mechanical therapy
3. High-frequency light modulation – fast switching between optical states
4. Optical probing of magnetic fields – detection of small magnetic fields (down to 1 Oe)
5. Micrometer-scale viscoelasticity sensing – by detecting the AC phase lag of the optical response

Advantages Over Competing Systems

The proposed hexaferrite colloids have unique advantages:

• High remanence allows manipulation by weak magnetic fields (just a few Oersted)
• Simple electromagnets can generate high-frequency particle rotation in liquids
• Tunable particle size enables optimization for specific applications:
  • Smaller particles: higher relaxation frequencies, better stability, higher mobility
  • Larger particles: better light scattering, higher mechanical momentum

Conclusions

This work demonstrates several breakthrough achievements:

1. First production of highly anisotropic plate-like nanoparticles of Al-substituted strontium hexaferrite with tunable size via glass crystallization
2. Size control from 39 nm × 4.5 nm to 90 nm × 7.1 nm by adjusting annealing temperature (650–750 °C)
3. Significantly enhanced coercivity due to aluminum substitution:
   • 2700 Oe for smallest particles (650 °C)
   • 5600 Oe for colloidal particles (750 °C)
   • Up to 7700 Oe for submicron single-domain particles (900 °C)
4. Stable aqueous colloids in pH range 2–4 with electrostatic stabilization
5. Strong “jalousie effect” with transmission difference increasing with nanoplate diameter and anisotropy
6. High-frequency dynamic response up to 1000 Hz for smaller particles

These properties make the nanoparticles suitable for microfluidic stirring, mechanical impacting for cancer treatment, high-frequency light modulation, optical probing of magnetic fields, and micrometer-scale viscoelasticity sensing.

This article is based on research published in Journal of Materials Chemistry C (2024): “Hard magnetic colloidal nanoplates with tunable size for magneto-optical applications” by Jianing Chen, Jingtong Duan, Evgeny O. Anokhin, Zitian Xia, Roman D. Svetogorov, Anastasia A. Semina, Roy R. Nygaard, Artem A. Eliseev, Evgeny A. Gorbachev, and Lev A. Trusov.