Maximum Coercivity Research Articles

Flexible and ferroelectric electrospun PVDF- non-stoichiometric PZT nanogenerators (PENGs) membranes for low-frequency acoustic energy harvesting

Acoustic energy harvesting (AEH) at low-frequency has been exposed to serious problems such as low energy of input sound and lack of efficient materials to absorb the sound and convert it to electrical power. This study focused on the enhancement of the AEH performance of polyvinylidene fluoride (PVDF) membrane via the incorporation of non-stoichiometric PZT (nsPZT) nanoneedles as piezoelectric nanogenerators (PENG). The nanofibrous membranes with different amounts of PENG (x = 0, 2.5, 5, and 10 wt %) were synthesized by the electrospinning method. The nsPZT was provided via Nd3+ and Nb5+ co-doping as a novel approach. The microstructures revealed that the PENG was embedded well inside the fibers (x = 2.5), while the agglomerates were formed at higher amounts (x = 10). Also, the fiber diameters and porosity of membranes were changed. It was found that the maximum crystallization of the β-phase of PVDF has occurred at x = 2.5 (71 %). The dielectric of the PVDF membrane decreased with increasing the x factor due to the formation of further pores. In contrast, the ferromagnetic properties, piezoelectric sensitivity, and electrical conductivity were improved by adding the PENG. The maximum coercivity of 750 Oe, piezoelectric sensitivity of 1.959 V N−1, and minimum sheet resistance of 1.2 kΩ.sq−1 were achieved by x = 2.5. The noise reduction coefficient (NRC) of the PVDF membrane was enhanced drastically (53–163 %) by adding the PENG. The AEH performance was also enhanced by adding the PENG. The highest power density was achieved 0.0822 W g−1 (56.23 W m−2) by the PVDF-PENG membrane (x = 2.5) under 90 dB sound pressure at 2000 Hz frequency. These findings suggest the high potential of nanofibrous PVDF-PENG membrane for use in low-frequency AEH systems.

Structural, optical and magnetic characteristics of Mg2+- doped (Ba0.94Sr0.06)(Fe0.80Al0.20-xMgx)O3-ẟ (x = 0–0.20) oxides

Perovskite-type ABO3 oxides have attracted immense attention in recent technologies e.g., gas sensors, solar cells, fuel cells, oxygen-permeable membranes, etc. An attempt has been made here to synthesize (Ba0.94Sr0.06)(Fe0.80Al0.20-xMgx)O3-ẟ (x = 0–0.20) oxides using sol-gel method and characterized with regards to phase, Raman and magnetic properties. X-ray diffraction-cum-Rietveld refinement shows the structural transformation from cubic (Pm3m) to hexagonal (P63/mmc) via cubic (Pm3m) + rhombohedral (R3c) and increase of cubic lattice parameter ac = 4.0369–4.1296 Å with magnesium. FT-IR spectra exhibit different vibrational bands at 548, 555, 638, 667, and 692 cm−1, representing the metal-oxygen (M-O-M) bond vibrations (M - Fe, Mg, Al), deformation of MO6 octahedral, and internal changes in bond lengths. No Raman band was appeared for cubic phase with x=0–0.10. Bands between ∼ 250–450 cm−1 assign the tilting, rotation, and asymmetric bending vibrations of Fe – O, Mg – O and Al – O bonds and asymmetric bending mode of (Fe/Mg/Al)O6 octahedra. Oxygen bending vibration, anti-stretching mode and presence of oxygen vacancies lie in the bands of range ∼498–656 cm−1. Maximum coercivity (Hc ∼ 1081 Oe) and remanent magnetization (Mr ∼ 0.033 emu/g) have been observed for x = 0 and found similar variation with ‘x’. The saturation magnetization (Ms) and anisotropic constant (K1) values lie as Ms = ∼ 0.678 – 0.754 emu/g and K1 = ∼ 1.38× 104 – 1.53× 104 erg/cm3, respectively with magnesium (x). Presence of anion vacancies and high coercivity make the materials suitable for industrial applications.

Impact of annealing temperature on the structural, magnetic and dielectric properties of BaFe12-xYxO19 (x = 0.0, 0.2, 0.4, 0.6) nanocrystalline samples

This paper presents an investigation into the structural, magnetic, and dielectric characteristics of BaFe12-xYxO19 (x = 0.0, 0.2, 0.4, 0.6) nanoparticles (NPs) produced by the sol-gel method at 800, 900, 1000, 1100 °C annealing. Rietveld refinement analysis shows that the lattice parameter ratio (c/a) lies within the range of 3.93–3.96 provides the confirmation of M type hexaferrite with space group P63/mmc. The average crystallite size for BaFe12O19 is found to decrease (53.0–23.4 nm) with increasing annealing temperature while for doped BaFe12-xYxO19 it is increased from 30.7 to 96.2 nm. The magnetization found to decrease (82–17 emu/g) with Y3+ content for 800 and 900 °C annealing. However, for 1000 and 1100 °C annealing the magnetization increases and reaches to maximum 100.2 emu/g for x = 0.2 at 1100 °C annealing. The site preference of the Fe3+ ions is responsible to change the value of magnetization. The coercivity of undoped BaFe12O19 varies from 4039 to 2028 with doping content. However, maximum coercivity 4586 Oe is found for x = 0.4 at 900 °C. The dielectric function is found to vary with annealing temperature and Y content as well. The wide range of magnetic properties (soft and hard magnetic) can be used in data storage, spintronic and microwave device applications.

Inkjet Printing of Cobalt Ferrite for Hard Ferromagnetic Thick Films Manufacturing

Inkjet printing is a versatile and cheap technique for the fabrication of films, offering unique advantages in terms of scalability, precision, and customization. In recent years, there has been a growing interest in utilizing inkjet printing technology for the deposition of magnetic films with tailored properties. Cobalt ferrite (CoFe2O4) stands out due to its exceptional magnetic properties, including high coercivity, saturation magnetization, and excellent chemical stability. This paper presents a comprehensive study on the inkjet printing of cobalt ferrite magnetic films, focusing on the manufacturing process, especially on the different factors that could lead to stable multilayer depositions to achieve high thicknesses: ink solid loading, drop spacing, substrate temperature, and interlayers drying. Finally, the microstructure of the samples is investigated to identify the occurring defects after sintering between 800 and 1000 °C. The magnetic properties of the films are determined, revealing a maximum coercivity of 1.98 kOe and a magnetic saturation of 78.25 emu cm−3.

Structure and magnetic properties of SmCo4.5Cu0.3Sn0.16Ga0.04 ribbons melt-spun at 10–40 m/s

The anisotropic Nd-Fe-B magnets prepared by as-spun ribbons using hot-pressing and hot-deformation methods have been commercialized. However, the Sm-Co magnets face challenges in obtaining strong c-axis deformation textures due to the absence of a low-melting intergranular phase to facilitate grain deformation. After simultaneously adding gradient low-melting metals of Cu, Sn, and Ga to SmCo5 alloy, SmCo4.5Cu0.3Sn0.16Ga0.04 ribbons were prepared by melt-spinning at 10–40 m/s. These ribbons comprise the Sm(Co, M)5 main phase, Sn-rich Sm2(Co, M)7 grain boundary phase, and Sm117Co55.58Sn115.94-type particulate phase. Notably, the ribbons melt-spun at 30 m/s contain the highest Cu, Sn, and Ga content in the grain boundary phase, making them most suitable for producing hot-pressed and hot-deformed magnets. Additionally, the ribbons melt-spun at 20 m/s have a maximum coercivity of 26.4 kOe, while those melt-spun at 40 m/s have the highest magnetization of 61.6 emu/g.

Microwave-assisted Morinda citrifolia L leaves extract mediated synthesis, magnetic and antimicrobial studies of pure and calcium substituted zinc ferrite

In this study, ZnFe2O4 and calcium-substituted Zn1-xCaxFe2O4 (x = 0.2, Ca2-ZF & x = 0.4, Ca4-ZF) are synthesized using Morinda citrifolia L. leaves extract via the microwave method, and calcined at 800 °C for 4 h to enhance the crystallinity. The purity and structural features of the ferrites were analyzed using powder X-ray diffraction (PXRD) and Fourier transform infrared spectroscopy (FT-IR) techniques confirming single phase formation and functional groups. Morphology and elemental presence were examined using high-resolution scanning electron microscopy (HR-SEM) and energy dispersive X-ray analysis (EDX) that exhibit polyhedral morphology of the particles and confirm elemental composition. The bandgap of Zn1-xCaxFe2O4 determined using the Kubelka method from UV-DRS data, is found to increase as calcium concentration and is in the range 1.73–1.91 eV. Room-temperature VSM studies showed a transition from superparamagnetic behavior to soft ferromagnetic behavior upon Ca substitution. Zn0.6Ca0.4Fe2O4 (Ca4-ZF) composition exhibits maximum saturation magnetization, remnant magnetization and coercivity of 196 x10−3emu/g, 0.411 x10−3emu/g, and 0.091 Oe, respectively, making it suitable for biomedical applications such as cell separation, detection, and gene delivery support. The antibacterial and antifungal studies demonstrate better inhibition for ZF than for Cax-ZF compounds. The pure ZF shows a maximum zone of inhibition against Bacillus subtitles bacteria and Candida albicans fungus of 23.7 mm/250 μg/mL and 28.1 mm/250 μg/mL, respectively.

Microstructure optimization and coercivity enhancement of Nd-Fe-B magnets prepared via grain boundary diffusion of DyCeAl alloy

The compositional design of diffusion source plays a crucial role in improving magnetic properties of Nd-Fe-B magnet. In this work, Dy80–xCexAl20 (x=0–50, in at%) alloy was employed as the diffusion source for grain boundary diffusion. The results show that Dy-Ce co-diffusion can effectively enhance the infiltration ability of diffusion source. The maximum coercivity increment of up to 795 kA/m can be achieved when x=10 due to the deeper diffusion depth and higher Dy content in the shell, while a significant degradation of remanence is also exhibited due to the formation of a larger number of Dy-rich grains. Thus, Ce content is regulated to inhibit the deterioration of remanence. Increasing Ce content to above x=30, it is found that the formed CeFe2 phase near the surface can regulate the infiltration ability of diffusion source, reducing the area fraction of Dy-rich grain region that is detrimental to the remanence, and eventually, when x=50, the remanence is recovered to be comparable to that of as-prepared magnet, with a coercivity increment of 430 kA/m concurrently. This suggests that rationally designing the diffusion source composition enables the preparation of cost-effective magnets.

Structural and magnetic properties of M-type hexaferrite nanoparticles: A comparative analysis of BaFe12O19 and SrFe12O19 synthesized via chemical co-precipitation

In this work, a comprehensive analysis of the structural and magnetic properties of the M-type hexaferrite (BaFe12O19 and SrFe12O19) was conducted using the chemical co-precipitation approach. The analytical techniques employed include powder X-ray diffraction, Transmission Electron Microscopy, Fourier Transform Infrared Spectroscopy, and Thermogravimetric Analysis. Powder Neutron Diffraction was utilized to analyse the magnetic ordering and crystal structure. A hexagonal crystal structure with space group P63/mmc was obtained from to structural analysis. Additionally, the investigation encompassed an assessment of the maximum energy product (BH)max and magnetic properties utilizing Vibrating Sample Magnetometry across all synthesized materials. Magnetic investigations of the studied materials showed several interesting characteristics: SrM showed a maximum saturation magnetization of 49.67 emu/g and a maximum coercivity of 4874.55 Oe. Remarkably, SrFe12O19 showed twice as much saturation magnetization and remnant magnetization than BaFe12O19, suggesting more robust and effective magnetic parameters. The overarching objective of this study is to discern the optimal hexaferrite material, evaluating magnetism, coercivity, thermal stability, and energy output through a systematic exploration of crystal structure, magnetic properties, thermal behaviour, manufacturing methodologies, and microstructure. This research endeavour not only contributes crucial insights into the potential applications of both BaM and SrM but also considers their environmental implications. The study further provides valuable recommendations for selecting the most efficacious material for specific permanent magnet applications, thereby contributing to the advancement on the field of magnetic engineering. The culmination of this work is poised to facilitate informed decision-making and technological progress in the realm of hexaferrite materials with a deliberate consideration of their environmental impact.

Cobalt Ferrite Nanorods Synthesized with a Facile "Green" Method in a Magnetic Field.

We report a new facile method for the synthesis of prolate cobalt ferrite nanoparticles without additional stabilizers, which involves a co-precipitation reaction of Fe3+ and Co2+ ions in a static magnetic field. The magnetic field is demonstrated to be a key factor for the 1D growth of cobalt ferrite nanocrystals in the synthesis. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy are applied to characterize the morphology and structure of the obtained nanoparticles. According to TEM, they represent nanorods with a mean length of 25 nm and a diameter of 3.4 nm that have a monocrystalline structure with characteristic plane spacing of 2.9 Å. XRD and Raman spectroscopy confirm the spinel CoFe2O4 structure of the nanorods. After aging, the synthesized nanorods exhibit maximum saturation magnetization and coercivity equal to 30 emu/g and 0.3 kOe, respectively. Thus, the suggested method is a simple and "green" way to prepare CoFe2O4 nanorods with high aspect ratios and pronounced magnetic properties, which are important for various practical applications, including biomedicine, energy storage, and the preparation of anisotropic magnetic nanocomposites.

Observation of Ferromagnetism in Dilute Magnetic Halide Perovskite Semiconductors.

Dilute magnetic semiconductors (DMSs) have attracted much attention because of their potential use in spintronic devices. Here, we demonstrate the observation of robust ferromagnetism in a solution-processable halide perovskite semiconductor with dilute magnetic ions. By codoping of magnetic (Fe2+) and aliovalent (Bi3+) metal ions into CH3NH3PbCl3 (MAPbCl3) perovskite, ferromagnetism with well-saturated magnetic hysteresis loops and a maximum coercivity field of 1280 Oe was observed below 12 K. The ferromagnetic resonance measurements revealed that the incorporation of aliovalent ions modulates the carrier concentration and plays an essential role in realizing the ferromagnetism in dilute magnetic halide perovskites. Magnetic ions are proposed to interact through itinerant charge carriers to achieve ferromagnetic coupling. Our work provides a new avenue for the development of solution-processable magnetic semiconductors.

Study on the relationship between structure and magnetic properties of τ-phase MnAl prepared by cryo-milling

The unique characteristics, such as high magnetic moment, high Curie temperature, large magnetic crystalline anisotropy, and low cost, make the τ-phase MnAl a promising candidate as the market alternatives which could fill the gap between the rare earth magnets and ferrite magnets. Because at low temperatures the metal will become more brittle, the cryomilling technology may allow the τ-phase MnAl to be broken quickly and introduce few crystal defects, and then obtain better permanent magnetic properties. In this study, the cryomilling technology was used to grind the τ-phase MnAl, and the structural and magnetic properties of the obtained powder sample were investigated. It was found that compared to room-temperature ball milling, the morphology of cryomilled powders is granular and the powder agglomeration is dramatically suppressed at a low temperature, and as a result the larger particles tend to be broken down into smaller ones rather than being agglomerated to larger thin layers. The grain size D reduced continuously from 145 nm to 36 nm when the sample was milled for 120 minutes. The coercivity increased with increasing the cryo-milling time, while the saturation magnetization decreased. The maximum coercivity of up to 4.9 kOe was obtained by cryo-milling for 100 minutes. By the neutron diffraction analysis, it was confirmed that the decrease of the saturation magnetization with increasing milling time is mainly due to the migration of Mn atoms from 1a site to the 1d site and the decrease of Mn atomic magnetic moment.

Magnetic Properties and Microstructure of an Isothermal Annealed and Mechanically Milled Nanocrystalline Ag−Mn Alloy

The alloy 50Ag−50Mn (wt.%) was prepared using ball milling. After 50 h of milling, the samples were isothermally annealed for 1 h at a time at three different temperatures ranging from 350∘C to 550∘C. By analyzing the X-ray diffraction data, Rietveld refinement structure analysis was used to measure the lattice strain and crystallite size. The introduction of lattice strain and the decrease in crystallite size as a function of milling time may both contribute to the increase in coercivity. The analysis of superconducting quantum interference devices demonstrates the soft ferromagnetic behavior of as-milled and annealed Ag−Mn alloy. After being annealed at 350∘C for 1 h, the 50Ag−50Mn alloy’s maximum coercivity was discovered to be 253 Oe.

Effect of Cr substitution on nickel spinel ferrite’s surface morphology, structure, antibacterial activity and magnetic properties

This study reports the sol–gel auto-combustion (SGACT) synthesis of Cr-substituted NiFe2O4 nanoferrites with a composition, of CrxNi(1−x)Fe2O4 (where x = 0.000, 0.025, 0.050, 0.100, 0.150, 0.200), and it examines the effects of chromium on spinel ferrite’s structure and properties. The doping of Cr at the nickel site is first reported in the present study. The computed crystallite size “t” from the XRD results was found in the range of 34.2–40.4 nm, whereas, FESEM plots show larger-sized grains due to the aggregation of a large number of crystallites for the produced nanoferrites. Maximum saturation magnetization of 42.44 emu/g was attained by the Cr0.050Ni0.950Fe2O4 (x = 0.050) specimen, whereas, the Cr0.200Ni0.800Fe2O4 (x = 0.200) specimen attains the maximum coercivity in the current study. All these excellent values of magnetic parameters make our SGACT-produced nanoferrites useful for magnetic recording media applications. In S. aureus, Cr0.200Ni0.800Fe2O4 nanoparticles showed the highest zone of inhibition (ZOI) whereas, Cr0.050Ni0.950Fe2O4 nanoparticles showed the least ZOI. On the other hand, in E. coli, Cr0.850Ni0.150Fe2O4, Cr0.200Ni0.800Fe2O4 and Cr0.050Ni0.950Fe2O4 nanoparticles showed good ZOI. This makes our developed nanoferrites beneficial for biological applications as antibacterial agents.

Effects of intrinsic and extrinsic multi-phased lattice structure and chemical heterogeneity on the control of ferrimagnetic properties in Co rich spinel ferrite

The present work reports the structural and magnetic properties in spinel ferrite composition around Co2.25Fe0.75O4 as the function of self-composite structure (ratio of two spinel phase components), coexisting impurity phase of hematite, and non-stoichiometric chemical composition. The neutron diffraction patterns have been used to determine lattice structure and magnetic moment. The neutron depolarization experiments have indicated spin reorientation at lower temperature for the single phased sample. The magnetization measurements confirmed the blocking of magnetic domains, possible magnetic spin compensation and magnetic exchange bias effect. The sample coexisting with small hematite phase has shown the maximum coercivity (∼14.682 kOe), remanent magnetization (∼1.4657 μB) and saturation magnetization (∼2.027 μB) with small exchange bias shift (- 71 Oe) at 5 K. The single phased sample exhibited the lowest magnetic moment (∼0.3775 μB) with maximum exchange bias shift (- 543 Oe) and the highest cooling-field induced enhancement of coercivity (3691 Oe). The experiments confirmed a large scope of tuning the ferrimagnetic parameters by controlling the phase fraction and interfacial phases in Co rich spinel ferrite.

Silver and nickel modified cobalt-zinc nanostructured ferrites for potential applications

In this analysis, silver and nickel modified cobalt-zinc nanostructured ferrites, with chemical compositions of Co0.5Zn0.5AgxNiyFe2-x-yO4 (x = 0.0, 0.01, 0.02, 0.03; y = 0.0, 0.02, 0.03, 0.04) were prepared employing sol–gel auto-combustion (SGAC). All samples were inspected for elementary, structural, microstructural, and magnetic traits. The Fd3m space group geometry with pure spinel phase for the produced nanoferrites was shown by Rietveld’s refined X-ray diffraction patterns. Using the Scherrer formula, X-ray diffraction indicated that samples attain a crystallite size (t) of 38-63 (± 0.01) nm. The field emission scanning electron microscopy revealed that grain growth was not uniform but rather agglomerated, of varying shapes and sizes. The vibrational stretching within the metal-oxygen at interstitial sites was confirmed by Fourier-transform infrared spectroscopy, which clearly indicates the creation of Co-Zn spinel nanoferrites. Furthermore, in all the produced samples, five active Raman vibrational modes (Eg, 3T2g, A1g) are present, and all of these are related to the cubic spinel structure. A vibrating sample magnetometer is utilized to examine the magnetic traits of produced magnetic samples, displaying soft magnetic behavior. The Co0.5Zn0.5AgxNiyFe2-x-yO4 (x = 0.00; y = 0.00) sample attains the maximum saturation magnetization (Ms = 64.94 (± 0.001) emu g−1), whereas the maximum coercivity (Hc = 217.33 ± 0.001 Oe) was attained by the Co0.5Zn0.5AgxNiyFe2-x-yO4 (x = 0.03; y = 0.04) sample, respectively. Therefore, due to the magnetic softness and excellent values of magnetic parameters of the integrated samples, it is possible to use them for potential applications such as recording media, switching, multi-layer chip indicators (MLCIs), and power applications.

Microstructure and magnetic properties of Co82-(x+y)Zr13V5BxSiy melt-spun ribbons

The combined addition of Boron (B) and Silicon (Si) to the rapidly quenched Zr-Co ribbons on improvement of hard magnetic properties was investigated in Co82-(x+y)Zr13V5BxSiy alloy compositions which were annealed at various temperatures. The ribbons mainly contain rhombohedral Co5Zr phase and small amount of Zr2Co11, fcc-Co and cubic Zr6Co23 phases. The addition of small amounts of Boron (B) reduces the grain size effectively of the magnetic phases, and thus significantly increases coercivity. The introduction of Silicon (Si) into Zr-Co alloys contributes to an increase in the coercivity but results in slight decreases in the magnetization. It has been observed that with increase in annealing temperature beyond 650 °C, the amount of the soft magnetic phases (Co23Zr6 and fcc-Co) have increased, while the amount of hard magnetic Co5Zr phase is decreased. It has been observed that with increase in annealing temperature beyond 650 °C, the amount of the soft magnetic phases (Co23Zr6 and fcc-Co) have increased. High coercivity values were observed in all the ribbons annealed at 650 °C. Among all the ribbons prepared, the maximum coercivity of 6.14 kOe was observed in Co79Zr13V5B1Si2 ribbons annealed at 650 °C due to the formation of a high volume fraction of hard magnetic phase (Co5Zr) and homogeneous finer grains throughout the matrix. The achieved high BHmax value of 6.14 MGOe was measured for same composition annealed at 600 °C. Microstructural analysis have shown fine grains with average grain size of 140–180 nm in Co79Zr13V5B1Si2 ribbons annealed at 650 °C.

Selective laser melting of permalloy magnetic core with complex geometry

The additive manufacturing (AM) technology is theoretically feasible in fabricating components with desired geometry; however, the practice in the AM of the magnetic functional component with complex geometry is still rare. Here, we report the manufacturing of magnetic cores with complex geometry from Fe-50 %Ni permalloy powders using selective laser melting (SLM), and the mechanical and magnetic properties as well as the microstructure with annealing are also investigated. The grain size of samples is from 50 to 100 μm, which exceeds the diameter of the powder. The SLM-manufactured components indicate moderate soft magnetic properties, their maximum magnetization, initial permeability, and coercivity are ∼ 151 emu/g, 310, and 6.9 Oe, respectively; and more impressively, an extremely high strength of ∼ 1060 MPa are also shown through sand-blasting and annealing at 1300 °C. The combined excellent mechanical and soft magnetic properties make it more practical and durable in the application of complex magnetic functional components manufactured by SLM.

Tuning the microstructure, magnetostatic and magnetodynamic properties of highly Al-substituted M-type Sr/Ca hexaferrites prepared by citrate-nitrate auto-combustion method

Double substitution of strontium hexaferrite by calcium and aluminum leads to a tremendous rise of hard magnetic properties, such as coercivity and natural ferromagnetic resonance frequency (NFMR). However, the properties are also inextricably linked to the material microstructure (especially, particle size), to the solid solution inhomogeneity as well as aluminum ions distribution among iron sites in crystal structure. In this work, we obtained M-type hexaferrite particles Sr1-x/12Cax/12Fe12-xAlxO19 (x = 4–6) via a facile citrate-nitrate auto-combustion method and studied the influence of the annealing temperature in a broad range on the microstructure, features of crystal structure and hard magnetic properties. At low annealing temperatures (900–1000°С) hexaferrite nanoparticles with 90% of nominal Al content and a wide chemical distribution are formed. Next, with an increase in the annealing temperature the distribution significantly narrows, chemical composition becomes close to the nominal one and particles size transfer firstly to submicron, then to micron range. The aluminum distribution over iron sites is independent distinctly on the annealing temperature. For all the compositions single domain particles with the maximum coercivity values between 22.8 and 36 kOe are obtained at 1200 °C. At 900–1000 °C the samples demonstrate coercivities up to 25 kOe, while above 1300 °C, the crystallites begin to pass into a polydomain state with a reduced coercivity. The hexaferrites with narrow chemical distribution reveal resonance absorption in sub-terahertz band. The highest NFMR frequency of 270 GHz was observed for x = 5.5 sample annealed at 1400 °C.

Optimization of Magnetic Cobalt Ferrite Nanoparticles for Magnetic Heating Applications in Biomedical Technology.

Using magnetic nanoparticles for extracorporeal magnetic heating applications in bio-medical technology allows higher external field amplitudes and thereby the utilization of particles with higher coercivities (HC). In this study, we report the synthesis and characterization of high coercivity cobalt ferrite nanoparticles following a wet co-precipitation method. Particles are characterized with magnetometry, X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy (TEM) and calorimetric measurements for the determination of their specific absorption rate (SAR). In the first series, CoxFe3-xO4 particles were synthesized with x = 1 and a structured variation of synthesis conditions, including those of the used atmosphere (O2 or N2). In the second series, particles with x = 0 to 1 were synthesized to study the influence of the cobalt fraction on the resulting magnetic and structural properties. Crystallite sizes of the resulting particles ranged between 10 and 18 nm, while maximum coercivities at room temperatures of 60 kA/m for synthesis with O2 and 37 kA/m for N2 were reached. Magnetization values at room temperature and 2 T (MRT,2T) up to 60 Am2/kg under N2 for x = 1 can be achieved. Synthesis parameters that lead to the formation of an additional phase when they exceed specific thresholds have been identified. Based on XRD findings, the direct correlation between high-field magnetization, the fraction of this antiferromagnetic byphase and the estimated transition temperature of this byphase, extracted from the Mössbauer spectroscopy series, we were able to attribute this contribution to akageneite. When varying the cobalt fraction x, a non-monotonous correlation of HC and x was found, with a linear increase of HC up to x = 0.8 and a decrease for x > 0.8, while magnetometry and in-field Mössbauer experiments demonstrated a moderate degree of spin canting for all x, yielding high magnetization. SAR values up to 480 W/g (@290 kHz, 69 mT) were measured for immobilized particles with x = 0.3, whit the external field amplitude being the limiting factor due to the high coercivities of our particles.

One-Dimensional Mn5Si3 Nanorods: Fabrication, Microstructure, and Magnetic Properties via a Novel Casting-Extraction Route.

This study presents a simple and innovative approach for producing one-dimensional Mn5Si3 nanorods through a casting-extraction process. In this technique, the Mn5Si3 nanorods were synthesized by reacting Mn and Si during brass solidification and extracted by electrochemical etching of the brass matrix. The effect of the cooling rate during casting on the nanorods' dimension, morphology, and magnetic properties was investigated. The results demonstrate that the prepared high-purity Mn5Si3 nanorods had a single-crystal D88 structure and exhibited ferromagnetism at room temperature. The morphology of the nanorods was an elongated hexagonal prism, and their preferred growth was along the [0001] crystal direction. Increasing the cooling rate from 5 K/s to 50 K/s lead to a decrease in the dimension of the nanorods but an increase in their ferromagnetism. At the optimal cooling rate of 50 K/s, the nanorods had a diameter and length range of approximately 560 nm and 2~11 μm, respectively, with a highest saturation magnetization of 7.5 emu/g, and a maximum coercivity of 120 Oe. These properties make the fabricated Mn5Si3 nanorods potentially useful for magnetic storage applications, and this study also provides a new perspective on the preparation of one-dimensional nanomaterials.