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DOI: 10.1039/D4TC02684E (Paper) J. Mater. Chem. C, 2024, Advance Article

Hard magnetic colloidal nanoplates with tunable size for magneto-optical applications

Jianing Chena, Jingtong Duana, Evgeny O. Anokhinb, Zitian Xiab, Roman D. Svetogorovc, Anastasia A. Seminab, Roy R. Nygaarda, Artem A. Eliseev*b, Evgeny A. Gorbachev*a and Lev A. Trusov*ab
aFaculty of Materials Science, MSU-BIT University, Shenzhen 517182, China. E-mail: ev.a.gorbachev@gmail.com; trusov@inorg.chem.msu.ru
bFaculty of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russia. E-mail: artem.a.eliseev@gmail.com
cNational Research Centre “Kurchatov Institute”, Moscow 123182, Russia

Received 26th June 2024 , Accepted 20th August 2024

First published on 22nd August 2024

Abstract

We have synthesized highly anisotropic plate-like nanoparticles of aluminum-substituted strontium hexaferrite via the crystallization of 4Na2O × 9SrO × 5.5Fe2O3 × 4.5Al2O3 × 4B2O3 glass, achieving tunable sizes by adjusting the annealing temperature (650–750 °C). Particle sizes range from 39 nm × 4.5 nm to 90 nm × 7.1 nm. Aluminum substitution significantly increases the coercivity of the colloid particles up to 5600 Oe. These nanoparticles form stable aqueous colloids in the pH range of 2–4. The ferrofluids exhibit a strong “jalousie effect” of adjustable optical transmission in external magnetic fields. The transmission difference rises with increasing nanoplate diameter and anisotropy factor. The high remanence of the hexaferrite particles allows them to be manipulated by weak magnetic fields, providing high-frequency particle motion with available electromagnets. Tunable particle sizes facilitate specific applications: smaller particles offer higher relaxation frequencies and better stability, while larger particles provide superior light scattering and induced mechanical momentum. 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.

Introduction

Traditional ferrofluids are suspensions of nanosized superparamagnetic particles (mainly magnetite) in carrier liquids.1 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.2 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 SrFe12O19 or BaFe12O19.3,4 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.5–10 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.11,12 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.4,13–15 Hexaferrite ferrofluids can also be an intermediate material for creating magnetic tapes,16,17 films,18–20 and various nanocomposites and nanostructures.21–28

Stabilizing magnetically hard hexaferrite particles in the colloidal state is quite challenging. Firstly, high temperatures are usually required to form the hexaferrite phase,29,30 at which strong sintering and intensive particle growth occur. Secondly, there is a strong magnetic attraction between the particles in the colloid, so each particle must be individually stabilized sterically or electrostatically. One of the most successful approaches for producing stable aqueous colloids of hard-magnetic strontium hexaferrite nanoparticles is the crystallization of borate glasses.4 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.3,31–35 The glass-ceramic synthesis of strontium hexaferrite nanoparticles involves preparing an oxide glass containing strontium, iron, and boron oxides, followed by a controlled heat treatment to induce crystallization. This process yields well-dispersed nanoparticles embedded in a non-magnetic borate matrix free from agglomeration and sintering issues typically associated with high-temperature processing. When the matrix is dissolved in acids, the particles gain electric charge and transfer to a stable colloidal state. However, to date, the processes of hexaferrite nanoparticle formation during glass crystallization have not been studied thoroughly, so there have been no reports of controlled production of nanoparticles with the required size and shape.

Previously, we found that hexaferrite nanoparticles with a highly anisotropic plate-like shape can be obtained by crystallization of 4Na2O × 9SrO × 5.5Fe2O3 × 4.5Al2O3 × 4B2O3 glass.4 Here, we provide a detailed study of the hexaferrite particle formation during crystallization and demonstrate the fine-tuning of the particle size within the nanoscale region. Also, we discuss the magnetic properties of the particles, the stability of the hexaferrite colloids, and their static and dynamic magneto-optical properties in dependence on the particle dimensions.

Experimental

Synthesis of hexaferrite particles

The glass of the initial composition 4Na2O × 9SrO × 5.5Fe2O3 × 4.5Al2O3 × 4B2O3 was prepared by a rapid melt quenching. For that purpose, the stochiometric amounts of the starting reagents (NaHCO3, SrCO3, Fe2O3, Al2O3, and H3BO3, high purity grade, Aldrich) were mixed into a 5 g batch using an agate mortar, heated with a rate of 10 °C min−1 to 700 °C and annealed for 2 h. The obtained batch was ground in the agate mortar and melted in a platinum crucible at 1250 °C using a high-temperature tube furnace (Carbolite TF1 16). After exposure for 1 h, the melt was quenched between two rotating steel rollers36 to form glassy flakes. The glass-ceramics were obtained by isothermal heat treatment of the glass at annealing temperatures (Tann) of 600–900 °C for 2 h followed by air-quenching. Then, the samples were treated with 3 wt% aqueous hydrochloric acid solution to dissolve the borate matrix and to leach the magnetic particles. The formers were separated from the solution by centrifugation, thoroughly washed with distilled water, and dried.

Preparation of hexaferrite ferrofluids

Hexaferrite ferrofluids were prepared directly during the extraction of the nanoparticles using the technique developed earlier.4 After the 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 (the colloidal stability is discussed below). Large particles and aggregates should be separated with a NdFeB magnet or by low-speed centrifugation. As a result, transparent brownish colloids were formed.

Characterization methods

Differential thermal analysis (DTA) of the glass was conducted in the air on a ZCT-B analyzer (Beijing Jingyigaoke Instruments) with a heating rate of 10 °C min−1. The Curie temperatures were determined by thermogravimetric analysis in a magnetic field of a NdFeB magnet with a heating rate of 10 °C min−1 (Mettler Toledo TGA2 instrument). The accuracy of the Curie temperatures determination is about 2 K. Powder X-ray diffraction studies (XRD) of the glass and glass-ceramics were performed using a Rigaku Smartlab SE diffractometer (CuKα radiation). XRD studies of the extracted hexaferrite particles were conducted at the synchrotron facility “KISI-Kurchatov” (X-ray structural analysis (XSA) beamline, Kurchatov Institute, Moscow, Russia) using radiation wavelengths of 0.739 Å and 0.964 Å.37 The full-profile analysis of the patterns was carried out using the Rietveld method in MAUD software (ver. 2.99).38 The instrumental parameters were obtained using the LaB6 powder standard. The line broadening study was performed using the “anisotropic no rules” model.39 The achieved parameters of a discrepancy between the experimental and calculated patterns are correspondingly lower than 0.5, 2%, and 5% for the goodness of fit, Rwp, and Rexp. Particle microstructure was examined using a scanning electron microscope (SEM) Zeiss GeminiSEM 360 and a transmission electron microscope (TEM) Jeol JEM-2100. Chemical analysis was performed by the inductively coupled plasma mass spectrometry (ICP-MS) using a PerkinElmer Avio 200 instrument. The magnetic hysteresis loops were recorded at room temperature using a Cryogenic S700 SQUID magnetometer in magnetic fields up to 50 kOe. The samples were fixed with a polymer glue to avoid moving in a magnetic field. The values of the saturation magnetization MS were obtained using the law of approach to saturation.40 The values of MS and HC were determined with estimated errors of 0.1 emu g−1 and 50 Oe, correspondingly. Dynamic light scattering (DLS) measurements were performed on an Anton Paar Litesizer 500 particle analyzer.

All magneto-optical measurements were performed in the transmission regime. DC field magneto-optics were recorded by PerkinElmer Lambda 35 spectrometer at 600 nm wavelength. A magnetic field was applied by custom-made Helmholtz coils connected to the AKIP-1125 current source. Alternating field magneto-optical measurements were carried out using the Renishaw InVia optical system equipped with a Leica DMLM microscope. The laser wavelength of 532 nm was used. Magneto-optical signal registration was performed by an electrical scheme based on a fast BPW21R (Vishay Semiconductors) photodiode connected to the Velleman PCSGU250 oscilloscope, which was also utilized as the initial AC signal source. The initial signal was amplified by Crest Audio VS1500 to produce a high magnetic field.

Results and discussion

Glass crystallization: hexaferrite particles morphology and chemical composition

The XRD analysis (Fig. 1a) showed that the obtained glass contained no crystalline phases. According to magnetic measurements, the glass was paramagnetic, which confirmed the absence of even very small particles of ferromagnetic phases, e.g., iron oxides or hexaferrite. Therefore, the rapid melt quenching resulted in the formation of an amorphous material. The DTA curve (Fig. 1b) revealed several thermal effects that occur when the glass is heated. The glass transition temperature is 535 °C, and then there are three exothermal effects corresponding to the glass devitrification (i.e., crystallization). The crystallization peak temperatures are 599, 673, and 751 °C. The glass begins to melt at about 860 °C.
image file: d4tc02684e-f1.tif
Fig. 1 (a) XRD patterns of the representative glass-ceramic samples. The annealing temperatures and observed phases are denoted. (b) DTA curve of the 4Na2O × 9SrO × 5.5Fe2O3 × 4.5Al2O3 × 4B2O3 glass.

The XRD patterns of the samples annealed for 2 h are shown in Fig. 1a. The samples remained amorphous up to Tann = 600 °C when the NaSr4(BO3)3 phase41 appeared in accordance with the first exothermal peak on the DTA curve. At higher annealing temperatures, accurate phase analysis becomes quite difficult due to the complex glass composition and the presence of nanosized particles. The other identified crystalline phases in the glass-ceramics are borates Al4B2O9 (ICDD PDF 79-1477) and Sr3B2O6 (ICDD PDF 31-1343) and strontium hexaferrite SrFe12O19 (ICDD PDF 84-1531). The peaks of the hexaferrite phase become visible on the XRD patterns of the glass-ceramics only above Tann = 750 °C, which means the particles have grown large enough at these temperatures. To study hexaferrite particles directly, they were extracted from the glass-ceramics by the treatment with 3 wt% HCl. The samples annealed below 650 °C dissolved almost completely. Meanwhile, from the samples annealed at Tann = 650–900 °C, a large amount of brown magnetic powders was extracted.

According to XRD, the obtained powders represented pure single-phase M-type hexaferrite, indicating the complete dissolution of the nonmagnetic phases during the acid treatment (Fig. 2). The unit cell parameters (Table 1) are reduced relative to the commonly reported ones of SrFe12O19 (a = 5.885 and c = 23.05 Å42). This indicates that iron ions are partially substituted by aluminum because the ionic radius of Al3+ (rVI = 0.535 Å) is significantly smaller than that of Fe3+ (rVI = 0.645 Å for a high-spin state).43 This is also supported by the well-studied dependencies of the cell parameters of SrFe12−xAlxO19 solid solution, which are close to Vegard's law, that is, the parameters a and c fall almost linearly with increasing aluminum content.29,44


image file: d4tc02684e-f2.tif
Fig. 2 (a)–(f) The powder XRD data and the results of the Rietveld refinement of the hexaferrite particles extracted from the glass-ceramcis obtained at different annealing temperatures: experimental patterns (yellow), theoretical patterns (red), and difference (grey). The discrepancy parameters are also presented.
Table 1 Properties of the hexaferrite particles extracted from the glass-ceramics
Tann, °C Lattice parameters XRD sizea SEM diameterb d, nm MS,c emu g−1 HC,c Oe TC, K
a, Å c, Å d, nm h, nm h/d
a Particle dimensions estimated from the full-profile analysis of X-ray diffraction patterns by the Rietveld method (d – mean crystallite diameter, h – mean crystallite thickness of the plate-like morphology).b Mean particle diameters (larger dimensions in the basal plane) obtained by approximating SEM size histograms (Fig. 3) with a lognormal distribution function.c The values of MS and HC were determined with estimated errors of 0.1 emu g−1 and 50 Oe, correspondingly.
650 5.868(2) 22.94(2) 35(2) 3.0(5) 0.086 — 15.3 2700 665
670 5.8715(4) 23.001(4) 40(2) 3.5(5) 0.088 — 38.1 3800 —
680 5.8707(4) 23.003(4) 42(2) 3.7(2) 0.088 — 39.7 4050 —
700 5.8788(3) 23.041(3) 45(2) 4.5(6) 0.100 55 46.0 4500 683
720 5.8744(2) 22.991(1) 74(2) 12(1) 0.162 — 47.5 4700 —
730 5.8721(2) 22.988(1) 92(2) 20(1) 0.217 — 48.4 5100 —
750 5.8694(1) 22.9854(5) 110(2) 37(1) 0.325 110 50.0 5900 688
800 5.86447(5) 22.9721(2) 145(5) 95(2) 0.655 175 50.0 7000 685
850 5.86489(5) 22.9762(3) 165(6) 130(4) 0.788 215 50.1 7550 685
900 5.86695(4) 22.9834(4) 195(6) 160(5) 0.821 210 51.3 7700 686

With decreasing the annealing temperature, the hexaferrite diffraction lines become considerably broadened, which shows a reduction in the particle dimensions down to the nanoscale. Moreover, the diffraction lines with (hk0) indices are noticeably narrower than those corresponding to the crystallographic planes with l-indices. This reflects a strong particle anisotropy with a smaller dimension along the crystallographic c-axis, i.e., a plate-like particle shape. The particle dimensions estimated from XRD line broadening are summarized in Table 1 (d and h correspond to the particle diameter and thickness). At the annealing temperatures Tann = 650–700 °C, the mean platelet diameter slightly rises from 35 to 45 while the ratio h/d ≈ 1/10 remains nearly constant. This temperature range matches the second peak of Tc2 on the DTA, which can be associated with the primary crystallization of the hexaferrite phase. At these low temperatures, the particle growth occurs at significant supercooling and limited diffusion due to high viscosity. Under such conditions, nucleation prevails over particle growth, which leads to the appearance of the nanoparticles. In addition, the hexaferrite crystal structure promotes preferential growth along the (001) plane (i.e., perpendicular to the c-axis), resulting in the formation of the plates. With an increase in the annealing temperature, a more intense growth of particles occurs, involving secondary grain growth by recrystallization.45 This is accompanied by an increase in the h/d ratio; that is, the particles become thicker. At Tann = 850 and 900 °C, the particle diameters were 165 and 195 nm, correspondingly, and the h/d ratio reached about 0.8. The particle size estimation from XRD line broadening is in good agreement with the scanning electron microscopy data (Fig. 3 and Table 1). The size and the thickness of the particles increase with rising annealing temperature, which is accompanied by the appearance of clearly visible crystal faces. At high annealing temperatures, the shape of the crystals can be described in general as hexagonal bipyramids truncated parallel to the basal plane. It is worth noting that the powders contain a noticeable amount of nearly uniaxial crystallites, which are not typical for hexaferrites; however, such very thick particles are commonly reported for glass crystallization at temperatures close to melting.31,32,46 Also, the hexaferrite particles in all samples should be in the magnetic single-domain state since their sizes are significantly smaller than the critical diameter of a single-domain hexaferrite particle (500 nm for the lower estimation44). Even synthesis above the melting point at Tann = 900 °C did not lead to an excessive increase in particle size.


image file: d4tc02684e-f3.tif
Fig. 3 Scanning electron microscopy images of the extracted hexaferrite powders obtained at different annealing temperatures. The corresponding size distributions are shown below.

The results of the chemical analysis by ICP-MS (Table 2) confirmed that the particles are partially substituted by aluminum. At Tann = 700 °C, the substitution degree x in SrFe12−xAlxO19 equals 0.55, while in the 750–900 °C range, it is about 0.8. The Rietveld refinement of the patterns of the samples obtained at 800–900 °C 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.44,47–49 The measured Curie temperatures (Table 1) are lower than that of pure SrFe12O19 (740 K44), which also indicates the Al substitution. At Tann above 750 °C, the Curie temperature practically does not change (considering the accuracy of the measurements of about ±2 K) because the particle compositions are very close. The Curie temperatures of the nanoparticles obtained at lower temperatures are additionally reduced, probably due to the size effects.

Table 2 Chemical analysis of the extracted hexaferrite particlesa
Tann (°C) Composition, at. ratio
Sr Fe Al
a Chemical composition is normalized to (Fe + Al) = 12 for comparison with SrFe12−xAlxO19.
700 0.95 11.45 0.55
750 1.05 11.15 0.85
800 1.05 11.25 0.75
850 1.00 11.15 0.85
900 1.05 11.20 0.80

Magnetic properties

The magnetic properties of the samples are provided in Fig. 4 and Table 1. The sample annealed at 600 °C demonstrates a reversible M(H) curve without hysteresis, which is typical for superparamagnetic particles. The saturation magnetization of 12.8 emu g−1 is close to the maghemite nanoparticles formed under similar conditions, i.e., just above the glass transition temperature, in Na2O–SrO–Fe2O3–B2O3 glasses.31 Starting from Tann = 650 °C, the samples reveal pronounced hysteresis loops with ratio MR/MS = 0.5 (MR and MS are the remanence and the saturation magnetization, respectively) characteristic for randomly oriented single-domain Stoner–Wohlfarth particles with uniaxial magnetocrystalline anisotropy.50 This coincides with the XRD results and indicates the formation of a hard-magnetic hexaferrite phase after annealing at 650 °C and above. However, the saturation magnetization at Tann = 650 °C is low and increases sharply after annealing at 670 °C. Then, the saturation magnetization increases further to Tann = 750 °C and afterward remains nearly constant. The reduced MS values at low annealing temperatures are most likely related to the size effects and possible structural defects usually characteristic of very small particles. This is especially true for small highly anisotropic hexaferrite nanoplates29,31,51 since the particle thickness only slightly exceeds one parameter of the unit cell. This is also supported by the reduced Curie temperatures (Table 1). The magnetization also decreases when aluminum ions enter the crystal lattice instead of iron in 2a and 12k sites;29,30,44 however, in this case, the difference in substitution degrees is not very large and does not correlate with changes in MS. At Tann = 750–900 °C, the extracted sub-micron particles have the magnetization of about 50 emu g−1 which is lower than reported values of fine particles of the unsubstituted strontium hexaferrite29,31,44,52 (60–70 emu g−1) and corresponds to an x value in SrFe12−xAlxO19 between 0.7 and 1.1.44 Dividing the saturation magnetization of glass ceramics by the saturation magnetization of the extracted particles, we obtain the mass fraction of the hexaferrite phase in the glass-ceramics. The samples annealed between 650 and 700 °C contained 27 wt% of hexaferrite, which is much lower than the maximum theoretical content of 38 wt%. This means that the second exothermic effect on the DTA curve, accompanied by a sharp increase in the magnetization, corresponds to a primary crystallization of hexaferrite. From 700 to 800 °C, the fraction of hexaferrite increases to 36 wt% and reaches 37 wt% at Tann = 850 and 900 °C, which indicates the secondary crystallization of hexaferrite, accompanied by recrystallization. This process may be associated with the third peak on the DTA curve.
image file: d4tc02684e-f4.tif
Fig. 4 (a) Hysteresis loops of the extracted hexaferrite powders; (b) the saturation magnetization MS of the glass-ceramics and extracted powders; (c) the coercivity HC of the glass-ceramics and extracted powders; (d) the coercivity HC of the extracted powders and microstructural parameters of the particles (diameter d, volume V and thickness-to-diameter ratio d/h).

Fig. 4c shows the dependencies of the coercivity HC of the glass ceramics and isolated powders on the annealing temperature Tann. These dependencies are very close, which indicates that the hexaferrite particles do not degrade when glass ceramics are dissolved to extract the particles. The coercivity of the particles obtained at 650 °C is 2700 Oe, and then HC gradually rises with Tann and reaches 7700 Oe at 900 °C. The increase in HC is mainly caused by an increase in particle size, and above Tann = 720 °C, the h/d ratio also rises (particles become thicker), which also contributes to an improvement in magnetic hardness due to the demagnetization form-factor44,52 (Fig. 4d). Due to aluminum substitution, the observed HC values are significantly higher compared to unsubstituted hexaferrite particles with similar morphology. For example, nanoplates with dimensions of about 40 nm × 5 nm have a coercivity of 2600 Oe31 and 3800–4500 Oe for unsubstituted and Al-substituted hexaferrite, correspondingly. Also, the highest obtained coercivity of 7700 Oe exceeds the maximum values of 6500–7000 Oe usually reported for the unsubstituted strontium hexaferrite.29,44,52 The observed increase in the coercivity is quite well studied29,30,44,52 and it is generally explained by a considerable reduction of MS due to the introduction of Al3+ ions into crystallographic sites 2a and 12k, where Fe3+ ions provide a positive contribution to the net magnetization. At the same time, the magnetocrystalline anisotropy constant K1 does not decrease so dramatically. According to the Stoner–Wolfart model,53 the coercivity of single-domain particles is expressed as HC ∝ K1 MS−1, therefore, the aluminum substitution leads to an improvement of the coercivity.

Hexaferrite ferrofluids: particle morphology and colloidal stability. During the extraction of hexaferrite particles from glass-ceramics obtained at 650–750 °C, the formation of stable colloids is possible (see Experimental section for details). TEM images of the colloidal particles are given in Fig. 5, and the particle dimensions are summarized in Table 3. The particles represent a plate-like shape with a mean h/d ratio of about 0.1. Most of the particles obtained at 650–720 °C turn into a colloidal state, and with an increase in the annealing temperature, a noticeable amount of sediment appears. This can be illustrated by the particle size distribution (Fig. 5). For example, for the sample with Tann = 700 °C, the distributions for colloidal and all extracted particles are almost the same. In contrast, for the sample annealed at 750 °C, the distribution is cut off on the right, indicating the maximum diameter of the particles that can be suspended in the colloid (about 200 nm). This leads to a slight decrease in the coercive force from 5900 Oe for all extracted particles (Table 1) to 5600 Oe for colloidal ones, while for the rest of the samples, these values coincide.
image file: d4tc02684e-f5.tif
Fig. 5 Transmission electron microscopy images of the colloidal hexaferrite particles obtained at different annealing temperatures. The corresponding size distributions are shown below in red. The blue histograms refer to all particles extracted from glass ceramics.
Table 3 Properties of colloidal hexaferrite particles
Tann (°C) TEM sizea A/A0b
d, nm h, nm h/d H‖k H⊥k
a Mean particle dimensions (d – diameter, h – thickness of the plate-like particle) obtained by approximating TEM size histograms (Fig. 5) with a lognormal distribution function.b Relative absorbance of the colloids at 600 nm in a magnetic field H = 100 Oe directed parallel and normal to the light propagation vector k.
650 39 4.5 0.115 1.09 0.86
670 43 4.8 0.112 1.10 0.82
680 47 5.1 0.109 1.15 0.77
700 48 5.4 0.113 1.15 0.74
720 73 6.7 0.092 1.23 0.63
730 79 6.5 0.082 1.25 0.60
750 90 7.1 0.079 1.26 0.59

The particles in the colloids are electrostatically stabilized.4 According to zeta-potential measurements (Fig. 6), the surface of the particles is positively charged in an acidic solution and negatively charged in an alkaline one due to the adsorption of H+ and OH− ions, respectively. The isoelectric point at which the particle surface is discharged corresponds to pH about 7. In a neutral solution, particles aggregate rapidly and irreversibly due to strong magnetic attraction, so they can no longer be redispersed, unlike many conventional non-magnetic nanoparticles. In a highly acidic environment, as well as at a high concentration of ions in a solution (for example, such conditions are created when glass-ceramics are dissolved during the colloids preparation), the particle surface charge is screened which leads to the agglomeration. However, such agglomeration is reversible, since the ionic shells prevent close particle contact. Therefore, when the pH is increased and excess ions are removed, the particles can form stable colloids. It is also important to note that at a pH below 2, the particles begin to dissolve, so they should not be kept for a long time in such conditions. For sample obtained at Tann = 700 °C, in the pH range from 2 to 4, the zeta-potential values are higher than +30 mV, corresponding to a good stabilization by a surface charge54 (Fig. 6). DLS measurements confirm that the hydrodynamic diameter in this range is almost constant, while outside, it increases dramatically, accompanied by flocculation of the colloid and further particle sedimentation. At pH = 2–4, the colloid remains unchanged for long periods (at least for a few weeks). For smaller particles (Tann = 650 °C), the stability range expands to pH = 1–5; for the largest ones (Tann = 750 °C), it shrinks to pH = 2.8–3.5. The particle size also affects the maximum concentration of colloids. It decreases from about 300 mg L−1 to 150 mg L−1 for Tann = 650 and 750 °C, respectively.


image file: d4tc02684e-f6.tif
Fig. 6 Zeta-potential (left) and hydrodynamic diameter (right) of the hexaferrite colloidal particles vs. pH for the sample obtained at 700 °C.
Hexaferrite ferrofluids: static and dynamic magneto-optical properties. 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 (for example, plate-like), this leads to a remarkable and unique phenomenon – the magnetic field-dependent optical transmission of the colloids,4,14 or “jalousie effect”. When the nanoplates are positioned perpendicular to the light beam, the optical transmission of the colloid is minimal (closed state). When the light is directed along the plates, the transmission is maximum (open state). For a typical colloid with a particle concentration of 150 mg L−1, the transmittance T in the closed and open states is about 15 and 30%, respectively. To compare the optical properties of different colloids, regardless of their concentration, it is convenient to use optical absorption rather than transmission since it has a linear dependence on concentration. Thus, the comparison can be made in terms of relative absorbance variation A/A0, where A is the absorbance in the applied magnetic field and A0 – at zero fields (Fig. 7a).
image file: d4tc02684e-f7.tif
Fig. 7 Static and dynamic magneto-optical characteristics of colloids based on hexaferrite nanoplates. (a) Relative absorbance vs. an external magnetic field; (b) relative absorbance vs. HF nanoplate diameter in a magnetic field of 100 Oe; (c) oscillations of the transmitted light signal under an AC magnetic field (amplitude of 5 Oe and frequency of 100 Hz); (d) complex magneto-optical response vs. AC magnetic field frequency (amplitude of 5 Oe) for HF particles with the mean diameter of 90 and 48 nm, obtained at 740 and 700 °C, respectively.

The origin of the magneto-optical effect for diluted colloids was previously described for magnetic nanorods55 and nanoplates.14 It lies in the difference between light absorbance alongside principal axes of ellipsoidal particles:

 
image file: d4tc02684e-t1.tif(1)
where k is a wavenumber, l is a wave path through the solution, N is the number of particles per unit volume, V is a particle volume, and Im〈α〉 is the imaginary part of complex average polarizability α. Polarizabilities alongside the principal axes of a single particle are defined by the depolarization factor:56
 
image file: d4tc02684e-t2.tif(2)
where ε and εm are complex dielectric constants for particles and solvent at selected wavelength and Ni is depolarization factor along i-th particle principal axis. For oblate ellipsoids depolarization factors were extracted analytically57 as:
 
image file: d4tc02684e-t3.tif(3)
 
image file: d4tc02684e-t4.tif(4)
where m = d/h is an anisotropy factor. However, basic solution does not include particle orientation function. To include it in current model more complex solution was developed.55 Firstly, it requires definition of the average particle orientation value in the applied magnetic field:
 
image file: d4tc02684e-t5.tif(5)
where θ denotes angle between particle magnetic moment and external field, μ is a particle magnetic moment and H is applied field strength. Using eqn (5) polarizabilities for parallel and perpendicular orientation of incident light beam and magnetic field were extracted as:
 
image file: d4tc02684e-t6.tif(6)
 
image file: d4tc02684e-t7.tif(7)
These two values can be inserted into eqn (1) to obtain corresponding field dependencies. Moreover, in the case of proposed relative absorbance coordinates, all terms related to solution concentration are neglected and resulting variation is obtained by direct division by a0, which for disordered particles is defined as a0 = 1/3·(ah + 2αd).

Due to the high intrinsic magnetic moment and high shape anisotropy of the particles, the magneto-optical response of hexaferrite colloids is very sensitive to the magnetic field (Fig. 7a). 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. The effect reaches its maximum in the field of about 100 Oe when all the particles line up in the same direction. According to TEM (Table 3), the mean diameters of the obtained colloidal particles vary from 39 to 90 nm, and the h/d ratio simultaneously reduces from 0.12 to 0.08. This leads to a natural increase in the magneto-optical response of the colloids and the differences between the open and closed states (Fig. 7b and Table 3). However, even the smallest particles produce an easily detectable effect.

Another distinctive feature of hexaferrite colloids is the high-speed switching between closed and open states compared to other magnetic colloidal systems.13,58,59 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 (Fig. 7c). The frequency of the magneto-optical response doubles because the integral orientation of the particles is the same for the antiparallel directions of the magnetic field of the same magnitude. The doubling can be removed by applying an additional biasing DC field.13 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. This lag leads to a phase shift of the magneto-optical response relative to the reference field. The particle dynamics can be illustrated by measuring the complex magneto-optical response Φ = Φ′ + Φ′′, where the real part corresponds to in-phase component and the imaginary one reflects the tangential component of the signal (Fig. 7d). As long as the particles rotate freely (usually at low frequencies), the real part is maximal, and the imaginary part is close to zero. When there is a lag and a phase shift, Φ′ begins to fall, and Φ′′ begins to grow reaching the maximum at a frequency attributed to the Brown relaxation.60,61 At higher frequencies, both components are reduced, indicating that the particles cannot follow the magnetic field. As can be seen from Fig. 7d, the frequency dependence of the magneto-optical response for smaller particles is shifted to a higher frequency range, e.g., Φ′′ peaks at 600 Hz and 1000 Hz for 90 nm × 7.1 nm and 48 nm × 5.4 nm particles, correspondingly. Thus, small particles are better suited for high-frequency applications, while a stronger magneto-optical response of large particles is necessary for greater sensitivity (e.g., in low fields).

Hexaferrite ferrofluids: prospects. The proposed hexaferrite colloids have unique advantages, making them prospective for emerging applications. Due to the high remanence of hexaferrite nanoparticles, the system can be adjusted by weak magnetic fields with a magnitude of just a few Oersted. This opens the opportunity to effectively utilize small and simple electromagnets to generate high-frequency particle rotation in liquids. Moreover, the demonstrated tuning of particle size by the glass-ceramic method enables obtaining specific sizes for certain applications. The small particles possess higher frequencies of relaxation, better stability against agglomeration, higher mobility, and penetration ability, whereas the large ones demonstrate better light scattering and develop higher mechanical momentum. Thus, the magnetically induced rotational motion of hexaferrite nanoplates can be utilized for micro-scale stirring (e.g., in microfluidics),3,62,63 mechanical impact transmission (e.g., for the treatment of cancer cells),11,12,64 high-frequency light modulation, and optical probing of small magnetic fields,13–15 and also micrometer-scale viscoelasticity sensing (by detecting the AC phase lag of the optical response).65

Conclusions

In summary, we have demonstrated, for the first time, the production of highly anisotropic plate-like nanoparticles of Al-substituted strontium hexaferrite with a tunable size by the crystallization of 4Na2O × 9SrO × 5.5Fe2O3 × 4.5Al2O3 × 4B2O3 glass. With an increase in the annealing temperature in the 650–750 °C range, the size of the nanoparticles increases from 39 nm × 4.5 nm to 90 nm × 7.1 nm. The partial substitution by aluminum leads to a significantly higher coercive force compared to unsubstituted hexaferrite. So, for the smallest particles, HC equals 2700 Oe; for the largest ones, it reaches 5600 Oe. At higher temperatures, submicron single-domain particles with coercive force up to 7700 Oe are formed.

The nanoparticles form stable aqueous colloids in the pH range of about 2–4. Due to the plate-like shape and large permanent magnetic moment of the particles, the colloids demonstrate a strong dependence of optical transmission on the applied magnetic field strength and direction, or the so-called “jalousie effect”. The difference in transmission of the colloids in the closed and open states rises with an increase in the diameter of the nanoplates and the anisotropy factor h/d. However, small colloidal particles exhibit a wider range of high-frequency dynamics.

Author contributions

J. C., J. D., E. O. A., A. A. S.: synthesis and optimization, DTA and TGA; J. C., J. D., E. O. A.: data analysis; E. O. A., L. A. T.: magnetic measurements; Z. X.: DLS measurements; R. D. S.: synchrotron XRD experiment; A. A. S., A. A. E.: magneto-optical measurements; R. R. N.: chemical analysis, data analysis; E. A. G., L. A. T.: conceptualization, management and coordination, XRD analysis; A. A. E., E. A. G., L. A. T.: manuscript preparation, discussion and review. J. C. and J. D. contributed equally. All authors have read and approved the decisive version of the manuscript.

Data availability

Further details of the crystal structure investigations may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-mail: E-mail: crysdata@fizkarlsruhe.de; , https://www.fizkarlsruhe.de/requestfordepositeddata.html) on quoting the depository number CSD 2097573–2097581. Data for this article, including XRD, TEM, SEM, DTA, magnetic and magneto-optics data are available at OSF at https://osf.io/ed9qf/?view_only=ff30b4f894ca42429ea20256a1562310.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

A. A. E. acknowledges the Russian Science Foundation (Grant No. 23-73-10045) for support.

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