Decrease In Saturation Magnetization Research Articles

Phase Formation and Magnetic Properties of (Y1−xSmx)Co5 Melt-Spun Ribbons

Using X-ray diffraction (XRD) and a vibrating sample magnetometer (VSM), the effects of Sm substitution, wheel speed, and annealing temperature on the phase formation and magnetic properties of (Y1−xSmx)Co5 (x = 0.2, 0.3, 0.4, 0.5) melt-spun ribbons were investigated. The results indicate the following: (1) With the increase in Sm substitution, it was found that (Y1−xSmx)Co5 ribbons are entirely composed of the (Y-Sm)Co5 phase with a CaCu5-type structure. Additionally, the coercivity gradually increases, while the remanence and saturation magnetization gradually decrease. (2) As the wheel speed increases, the (Y1−xSmx)Co5 ribbons exhibit an increasing proportion of (Y-Sm)Co5 phase until reaching a speed of 40 m/s, where they are entirely composed of the (Y-Sm)Co5 phase. Magnetic measurements show that the coercivity (Hcj) and remanence (Br) of (Y0.5Sm0.5)Co5 ribbons increase gradually with increasing wheel speed, while saturation magnetization decreases. The variation in magnetic properties is mainly attributed to the formation of nucleation centers for reversed magnetic domain (2:7 and 2:17 phases); (3) (Y0.5Sm0.5)Co5 ribbons are composed of the (Y-Sm)Co5 phase and a small amount of the Sm2Co7 phase after annealing at 550 °C, 600 °C, and 650 °C. Temperature elevation promotes crystallization of the amorphous phase, resulting in a gradual decrease in coercivity, while the remanence and saturation magnetization exhibit an overall increasing trend. Through continuous optimization of the process, favorable magnetic properties were achieved under the conditions of a 0.5 Sm substitution level, a wheel speed of 40 m/s, and an annealing temperature of 550 °C, with a coercivity of 7.98 kOe, remanence of 444 kA/m, and saturation magnetization of 508 kA/m.

Structure-Related Electronic and Magnetic Properties in Ultrathin Epitaxial NixFe3-xO4 Films on MgO(001).

Off-stoichiometric NixFe3-xO4 ultrathin films (x < 2.1) with varying Ni content x and thickness 16 (±2) nm were grown on MgO(001) by reactive molecular beam epitaxy. Synchrotron-based high-resolution X-ray diffraction measurements reveal vertical compressive strain for all films, resulting from a lateral pseudomorphic adaption of the film to the substrate lattice without any strain relaxation. Complete crystallinity with smooth interfaces and surfaces is obtained independent of the Ni content x. For x < 1 an expected successive conversion from Fe3O4 to NiFe2O4 is observed, whereas local transformation into NiO structures is observed for films with Ni content x > 1. However, angle-resolved hard X-ray photoelectron spectroscopy measurements indicate homogeneous cationic distributions without strictly separated phases independent of the Ni content, while X-ray absorption spectroscopy shows that also for x > 1, not all Fe2+ cations are substituted by Ni2+ cations. The ferrimagnetic behavior, as observed by superconducting quantum interference device magnetometry, is characterized by decreasing saturation magnetization due to the formation of antiferromagnetic NiO parts.

Study of iron ion distribution in rare earth (La, Pr, Sm, Dy) doped nickel-zinc spinel ferrites through Mössbauer spectroscopy

Ni0.5Zn0.5Fe1.95RE0.05O4 (RE=La, Pr, Sm, Dy) spinel ferrites were prepared using the sol-gel method. X-ray diffraction analyses revealed that doping with rare earth ions increases the lattice parameters of the ferrites, accompanied by a decreasing trend due to lanthanide contraction. Mössbauer spectrum showed that rare earth ion doping facilitates the migration of iron ions from B sites to A sites, with the extent of migration increasing alongside the atomic number of the rare earth elements. Vibrating sample magnetometer tests indicated a general decrease in saturation magnetization following rare earth ion doping. A comprehensive investigation into the effects of different rare earth ion dopings on the properties of Ni-Zn ferrites was conducted by calculating theoretical magnetic moments and comparing them with the experimentally measured values. This comparison elucidated the mechanism behind the magnetic alterations in rare earth-doped spinel ferrites, providing theoretical support for future magnetic modulation in these materials.

An insight into the structural, magnetic and dielectric properties of modified SrFe12-xPbxO19 (x = 0–0.3)

Effect of Pb2+ substitutions on the structural, magnetic and dielectric properties of SrFe12O19 in the form of SrFe12-xPbxO19 (x = 0, 0.1, 0.2 and 0.3) is carried out. All the samples crystalize in hexagonal magneto-plumbite structure with P63/mmc symmetry. An irregular but randomly distributed particles with average particle size increased from 0.791 μm to 1.131 μm due to substitutions. A characteristics Hopkinson peak is observed just below the transition temperature, may be stems from the superparamagnetic relaxations. The isothermal magnetization depicts an increase in coercivity (Hc) and decrease in saturation magnetization (Ms) due to Pb substitutions, which is further explained in the framework of site symmetry of the Fe and its multiple valence states. Mossbauer study reflects that, Fe3+/ Fe2+ present in the systems are associated with different electric environment. Furthermore, the shifting of dielectric transition by 50 °C upon substitutions is associated to dipole formation in presence of lone pair of Pb2+. A relaxor type of behavior is acquired by the system with a non-Debye type of relaxation is explained on the basis of complexity of the dipoles associated with each crystallographic sites.

A novel Fe3O4/ZnO/PANI/rGO nanohybrid material for radar wave absorbing

In this study, a novel RAM utilizing Fe3O4, ZnO, polyaniline (PANI), and reduced graphene oxide (rGO) in the form of nanohybrid is invented, namely Fe3O4/ZnO/PANI/rGO nanohybrid. In this study, the effects of the rGO fraction in Fe3O4/ZnO/PANI/rGO composition on the radar absorbing performance were investigated. Meanwhile, exploring the quality of the nanohybrid sample, such as the crystal structure, functional groups, morphology, magnetic properties, and reflection loss was also examined carefully. It shows that two crystalline phases of Fe3O4 and ZnO, as well as amorphous structures of PANI and rGO are formed independently, indicating successful nanohybrid formation. In this case, PANI functioned as a binder for Fe3O4 and ZnO, and thus, it could infiltrate the pores of rGO. Fe3O4/ZnO/PANI/rGO nanohybrid possesses superparamagnetic properties with decreasing saturation magnetization as rGO composition increases. Nevertheless, rGO enhanced the radar absorption performance with a maximum reflection loss of −8.57 dB at 11.10 GHz. In this present research, the synergy of combination between the magnetic and dielectric losses in the Fe3O4/ZnO/PANI/rGO nanohybrid is discussed together with plausible strategies to obtain highly efficient RAM.

A comparative DFT study of MgFe2O4 and MnFe2O4 spinel ferrites at various pressures to investigate the structural, mechanical, electronic, magnetic and optical properties for multifunctional applications

The structural, electronic, mechanical, optical, and magnetic properties of the MgFe2O4 and MnFe2O4 ferrites were investigated under pressures ranging from 0 to 20 GPa using the DFT-based CASTEP Code. XRD analyses indicated differences in lattice constants, volumes and d-spacing between these ferrites, with decreasing trends under applied pressure. The MgFe2O4 ferrite has lower X-ray density, but higher electronic bandgap energy compared to the MnFe2O4 ferrite, both showing semiconductor behavior. Magnetic properties show a decrease in total magnetic moment and saturation magnetization under applied pressure for both ferrites, making them potential materials for transformer and inductor cores. Mechanical properties confirm their stability, elastic behavior, ductility and ionic bond nature, suitable for electrochemical devices. The MgFe2O4 ferrite exhibits dominant absorption and optical conductivity in the visible region compared to the MnFe2O4 ferrite, making it a favorable choice for optoelectronic and photocatalytic applications.

Adjusting the gas detection characteristics of NiFe2O4 spinel ferrite nanoparticles through the introduction of Zn doping

Nickel ferrite nanoparticles doped with zinc were synthesized at a lower reaction temperature of 180 °C. The research delved into their structural, optical, dielectric, and magnetic properties. These nanoparticles exhibited a distinct cubic spinel structure, with crystallite sizes ranging from 33 to 51 nm. FTIR analysis revealed metal oxide stretching vibrations between 400–600 nm. FESEM and HR-TEM examinations unveiled irregularly shaped nano-particles. XPS analysis disclosed the chemical states of Zn2+, Fe3+, and Ni2+ ions. Mossbauer spectra exhibited two sextet patterns related to tetrahedral (A) and octahedral (B) sites, plus a weak doublet pattern. Magnetic tests showed a decrease in saturation magnetization (Ms) but an increase in remanent magnetization (Mr) and coercivity (Hc) values upon doping.

Investigating the impact of electron beam irradiation on electrical, magnetic, and optical properties of XLPE/Co3O4 nanocomposites

Nowadays, many researchers aim to fill polymer materials with inorganic nanoparticles to enhance the polymer properties and gain the merits of the polymeric host matrix. Sol–gel synthesized Co3O4 nanoparticles are subjected to different doses of electron beam (10, 20, and 30 kGy) to study their physiochemical properties and choose the optimized nanoparticles to fill our polymeric matrix. Crosslinked polyethylene (XLPE) has been filled with 5 wt % of un-irradiated cobalt oxide nanoparticles using the melt extruder method. The structural, optical, magnetic, and electrical properties of the XLPE/Co3O4 nanocomposite before and after exposure to different doses of electron beam radiation have been characterized. The crystallite size of face-centered cubic spinel Co3O4 nanoparticles has been confirmed by XRD whereas and their unique truncated octahedral shape obviously appears in SEM micrographs. The crystallite size of Co3O4 nanoparticles has decreased from 47.5 to 31.5 nm upon irradiation at a dose of 30 kGy, and significantly decreased to 18.5 nm upon filling inside XLPE matrix. Related to the oxidation effect of the electron beam, the Co2+/Co3+ ratio on the surface of Co3O4 nanoparticles has decreased upon irradiation as verified by XPS technique. This consequently caused the partial elimination of oxygen vacancies, mainly responsible for the weak ferromagnetic behavior of Co3O4 in its nanoscale. This appears as decreased saturation magnetization as depicted by VSM. The XLPE/Co3O4 nanocomposite has also shown weak ferromagnetic behavior but the coercive field (Hc) has increased from 112.57 to 175.72 G upon filling inside XLPE matrix and decreased to 135.18 G after irradiating the nanocomposite at a dose of 30 kGy. The ionic conductivity of XLPE has increased from 0.133 × 10–7 to 2.198 × 10–3 S/cm upon filling with Co3O4 nanoparticles while a slight increase is observed upon irradiation.

An assessment of the Al50Cr21-xMn17+xCo12 (x=0, 4, 8) high-entropy alloys for magnetocaloric refrigeration application

This study investigates the magnetocaloric potential of the Al50Cr21-xMn17+xCo12 (x=0, 4, 8 at%) high-entropy alloy (HEA) series using integrated experimental and theoretical approaches. Structural analysis by X-ray diffraction and scanning electron microscopy indicate a dual phase containing B2 and body-centered cubic (BCC) structures. Magnetic characterization shows an approximately linear decrease in saturation magnetization and Curie temperature with increasing Cr content. Curie temperatures calculated by Monte Carlo simulations suggest that the measured magnetic properties originate from the B2 phase rather than the BCC phase. The enhanced magnetocaloric effect with decreasing Cr content highlights the attractiveness of HEAs in magnetocaloric applications.

Preparation and microwave absorption properties of CoFe2O4/NiCo2O4 composite powders

The magnetic spinel cobalt ferrite (CoFe2O4) powders had the high microwave absorption performance at the large thicknesses on account of the low permittivity. Combining the CoFe2O4 powders with other dielectric loss materials had been suggested to improve their impedance matching and microwave absorption performance. In this work, the microwave absorption performance of the CoFe2O4 powders was enhanced by adding the various amounts of the NiCo2O4 nanoparticles as dielectric loss material. The CoFe2O4-x NiCo2O4 composite powders (x = 0, 12.5, 25, and 50 wt %) were prepared by a facile two-pot solution combustion synthesis method. Although the X-ray diffraction peaks of NiCo2O4 phase were absent, but the decrease of saturation magnetization from 61 to 30 emu/g was related to the amount of NiCo2O4 material. Furthermore, the NiCo2O4 nanoparticles were nucleated and grown on the surface of spongy CoFe2O4 powders because the synthesized CoFe2O4 powders were added to the precursor solution of NiCo2O4 material containing the nickel and cobalt nitrates as oxidants and glycine as organic fuel. The microwave absorption performance was analyzed by measuring the permittivity and permeability spectra which were applied for evaluating the reflection loss versus frequency and thickness. The CoFe2O4-25 wt % NiCo2O4 composite powders had the higher microwave absorption performance including the high reflection loss of −40 dB and broad effective bandwidth of 5.6 GHz of Ku band (12–18 GHz) at a small matching thickness of 1.9 mm. The high microwave absorption performance of the x = 25 wt % sample was attributed to its better impedance matching via a proper combination of the permittivity and permeability values.

CNTs-improved electromagnetic wave absorption performance of Sr-doped Fe3O4/CNTs nanocomposites and physical mechanism

Electromagnetic wave pollution is a serious environmental problem, and it is urgent to develop high-efficiency electromagnetic wave absorption (EMA) materials. In this paper, carbon nanotubes (CNTs) were combined with Sr-doped Fe3O4 nanoparticles to form Sr-doped Fe3O4/CNTs nanocomposites with by hydrothermal method. After the characterization of microstructure and magnetic properties, EMA properties of Sr-doped Fe3O4/CNTs nanocomposites are investigated in detail. The well-dispersed CNTs lead to the uneven average grain size of Sr-doped Fe3O4 nanoparticles and a little decrease of saturation magnetization. While CNTs greatly improve EMA performance, and SFC1 sample exhibits the minimum reflection loss value of −65.64 dB at 6.32 GHz with the thickness of 3 mm. CNTs reduces the natural resonance of magnetic loss while increasing the permittivity, conductivity, and heterogeneous interfaces. As the result, Sr-doped Fe3O4/CNTs nanocomposites exhibit not only better impedance matching but also higher electric dipole polarization relaxation loss, interfacial relaxation loss, conductivity loss, and eddy current loss. In this way, the excellent EMA performance is attributed to the good impedance matching, intense dielectric loss and enhanced eddy current loss for Sr-doped Fe3O4/CNTs nanocomposites.

Improvement in the structural, magnetic and electromagnetic behaviour of barium hexaferrites with yttrium doping for EMI shielding

Electromagnetic interference (EMI) shielding is critical for protecting electronic devices from interference generated by various sources such as power lines, radio waves, microwaves, and electronic devices. In this current study, we present a thorough investigation of the structural, magnetic, and electromagnetic properties of Y3+ doped M-type barium nano hexaferrites. Polycrystalline BaFe12−xYxO19 (x = 0.0, 0.3, 0.5, 0.7, and 0.9) hexaferrites are prepared using the citrate precursor method. The results of X-ray diffraction (XRD), Rietveld refinement and Raman spectroscopy confirmed that all samples have a single-phase, and pure hexagonal crystalline structure with space group P69/mmc. This result is consistent for all the prepared compositions. The structure and dopant materials in different chemical states of the sample are determined based on the X-ray photoelectron spectroscopy (XPS) spectrum. Field emission scanning electron microscopy (FESEM) analysis showed that the particles had a distinct hexagonal morphology. The chemical composition of the prepared hexaferrites is investigated using energy dispersive X-ray analysis (EDAX). UV spectroscopy showed an inverse relationship between the molar ratio of yttrium and the band gap. Vibrating sample magnetometer (VSM) results showed a decrease in saturation magnetization (Ms) and coercivity (Hc) with increasing fraction of yttrium ions. The shielding efficiency (SE) of the synthesized hexa materials against EMI is evaluated using a vector network analyser (VNA). In particular, effective EMI shielding was observed in the frequency spectra of 8–12 GHz (X-band region). Among the tested compositions, BaFe12–0.7Y0.7O19 with a thickness of 3 mm exhibited highest SE of 80.07 dB in the X-band region. The proposed yttrium-substituted barium M-type hexagonal ferrites exhibit significant reflection loss with enhanced absorption spectrum even when applied to an extremely thin 3 mm material, making it a possible preference as the radar-absorbed compound for EMI attenuators for weather monitoring, radar tracking, air traffic management, and gigahertz antennas. Furthermore, commercial applications including long-term magnetic media for recording, storing information, etc. are possible uses of the synthesised magnetic yttrium-doped barium M-type hexagonal ferrites.

Effects of Mo doping on the phase transformation behaviors and the magnetic properties of rare-earth-free MnBi alloy

The high-performance rare-earth-free low-temperature phase Mn–Bi-based permanent magnet alloys (LTP-MnBi) have gained widespread attention, but it still presents a challenge. Here, the LTP-MnBi alloy with Mo element doping was prepared by using arc-melting and subsequent high-vacuum annealing. The crystalline structure, phase transformation behavior, and magnetic properties were systematically investigated. The result showed that Mo element doping significantly improved the stability of the LTP in MnBi alloy. The investigation revealed that the disparity in atomic radius between Mo and Mn, Bi elements played a pivotal role in the formation and stability of the LTP-MnBi. With the increase of Mo content in the LTP-MnBi alloy, the coercivity value remarkably increased from 8.1 kOe to 11.7 kOe with a decrease of saturation magnetization from 71.6 emu/g to 56.5 emu/g, and the maximum energy product gradually reduced from 10.6 MGOe to 7.9 MGOe. The underlying mechanism revealed that magnetic properties were closely related to the fraction and crystal structure of LTP-MnBi. Therefore, magnetic properties of Mo-doped MnBi alloy make them an attractive candidate among the rare-earth-free permanent magnets.

High-performance Fe–Si soft magnetic composites with controllable silicate/nano-Fe composite coating

The design of magnetic insulation coating structure has always been a challenge for high-performance soft magnetic composites (SMCs). In this work, we prepared Fe–Si SMCs with silicate/nano-Fe composite coating successfully by in-situ oxidation method combined with spark plasma sintering (SPS). The formation mechanism of the composite coating and its effect on the electro-magnetic properties of Fe–Si SMCs were investigated. The results showed that a uniform Fe2O3 coating can be obtained by reactions between Fe and H2O/O2 during in-situ oxidation process, and became thicker with the increased oxidation time. After sintering, the oxide coating was transformed into a composite coating composed of Fe2SiO4 with excellent insulation and nano-Fe with high ferromagnetism, which resulted from the interfacial reaction between Fe2O3 coating and Fe–Si core. The increased oxidation time led to the gradually thicker composite coating, and resulted in a linear decrease in saturation magnetization, indicating good controllability of the coating. However, excessive oxidation time led to the increased eddy current loss as well as the core loss due to the weakened resistivity. Thus, the Fe–Si SMCs exhibited high saturation magnetic induction (1.66T) and very low core loss (643.9 kW/m3 at 0.1 T/50 kHz) especially when the oxidation time was 1 h.

Tuning the gas sensing properties of spinel ferrite NiFe2O4 nanoparticles by Cu doping

Copper substituted nickel ferrite nanoparticles (NiFe2O4) nanoparticles were prepared using hydrothermal technique at lower reaction temperature (175 °C). The structural, optical, dielectric, and magnetic properties were analyzed. The prepared nanopowders exhibited single phase-cubic spinel structure with a mean crystallite size in a narrow range 30–41 nm. Fourier Transform Infrared Spectroscopy (FTIR) analysis revealed the presence of stretching vibration of metal oxide in the range 400–600 nm. Field Emission Scanning Electron Microscopy (FESEM) and High Resolution-Transmission Electron Microscopy (HR-TEM) observations showed non-uniform cuboidal shaped nano-sized particles. The chemical and oxidation state of the constituents were revealed by X-ray photoelectron spectroscopy (XPS) (Cu2+, Fe3+ and Ni2+). Mössbauer spectra manifested two sextet patterns attributed to tetrahedral (A) and octahedral (B) sites with a doublet pattern of low intensity. Magnetic measurement revealed the decrease of saturation magnetization (Ms) value and increase of remanence magnetization (Mr) and coercivity (Hc) values while doping. A higher specific capacitance was obtained for x = 0.5 (1417 F/g) with a pseudocapacitive behavior. At room temperature, Liquefied Petroleum Gas (LPG) showed high selective response (1300 s for 250 ppm) compared to other tested gases.

Improved magnetic properties of Fe substituted La2CoMnO6 for spintronic applications

A detailed study of magnetic behaviour of double perovskites was carried out by developing La2Co1-xFexMnO6; (x = 0.0, 0.2, 0.5, 0.8 and 1.0) via sol–gel technique. All samples showed single magnetic transition except pure LCMO (x = 0.0) and LCFMO2 (x = 0.5). Temperature dependent inverse magnetic susceptibility (χ-1) was fitted in accordance with Curie-Weiss law. μeff (effective magnetic moment) was determined for all the developed samples. Pure LCMO showed a downward divergence from Curie-Weiss law below 230 K but above transition temperature which indicates the characteristics of Griffiths phase. Saturation magnetization (Ms) increased initially and then decreased, whereas coercive field (Hc) was found to be monotonous decreased with increase in Fe content. LCFMO1 (x = 0.2) was noticed to have maximum Ms (38.87 emu/g) which indicated that the magnetic properties of pure LCMO were improved with Fe substitution, but for lower substitution only. Magnetization vs. field (M−H) curves were not well saturated even at a magnetic field of 60 kOe, therefore, values of Ms for all the samples were estimated using law of approach to saturation. With increase in Fe content, an increased anti-site disorder among Mn and Co strengthened antiferromagnetic (AFM) super exchange interactions and weakened ferromagnetism (FM). Therefore, substitution of Fe on Co-site was responsible for systematic decrease in saturation magnetization when × exceeded 0.2.

Structural, magnetic, and metal-carboxylate interactions investigations in alginate-encapsulated MFe2O4 (M = Co, Ni, Zn)

Magnetic ferrites nanoparticles (MFNs) MFe2O4 (M = Co, Ni, Zn) were synthesized via coprecipitation method and then encapsulated by Alginate polymer through cross-linking in aqueous calcium chloride solution. The obtained products were characterized by XRD, SEM, FTIR spectroscopy, TGA, and VSM. X-ray diffraction results indicate a decrease in diffractions picks intensities after encapsulating the MFNs with Alginate. Furthermore, cation distribution among interstitial sites was determined and it indicates an average inversion degree of 0.017, 0.142, and 0.8 for CoFe2O4, NiFe2O4, and ZnFe2O4 respectively, these values are in good agreement with the Rietveld refinement of X-ray diffraction patterns. The interaction of MFNs surface and the carboxylic group of alginates was investigated in detail for the first time, FTIR spectroscopy and VSM were used for this purpose. Therefore, from the wavenumber separation Δ values, between the frequencies of the characteristic peaks for asymmetric and COO− stretching vibration and symmetric COO− stretching vibration, we found 185.16 for Na-Alginate, 177.45 cm−1 for Alg@CoFe and Alg@Ni, while Alg@Zn have a wavenumber separation of 192.88 cm−1, which is characteristic to bidentate chelating coordination. The study of magnetic properties reveals a decrease of saturation magnetization after the encapsulation of nanoparticles, from 80.86, 47.17, and 4.36 emu/g for CoFe2O4, NiFe2O4 and ZnFe2O4 respectively to reach 18.43, 13.53, and 0.69 for their corresponding beads. This decrease is explained by the interaction between COO− and the surface of the nanoparticles.

Effect of Gd substitution on structure, optical and magnetic properties, and heating efficiency of Fe3O4 nanoparticles for magnetic hyperthermia applications

This work presents the synthesis of Fe3-xGdxO4 nanoparticles and the investigation of the impact of Gd content (x) on structural characteristics of nanoparticles, including lattice parameters, crystallite size, and X-ray density. The high-resolution images of Transmission Electron Microscopy reveal that the synthesized Fe3-xGdxO4 nanoparticles have a narrow distribution in size statistics between 9 and 12.5 nm. Interestingly, analyzing different prepared samples reveals that both the saturation magnetization and magnetic anisotropy decrease simultaneously with the increase of band gap while remaining independent of the specific absorption rate (SAR) value. The SAR is greater than that of pure Fe3O4 nanoparticles. In a specific case of Fe2.75Gd0.25O4 sample where the optimal value of x is 0.25, the highest SAR value obtained is 237.4 W/g. Besides, the experimental results demonstrate an optimal doping content with the highest SAR value of Fe2.75Gd0.25O4 nanoparticles along with the theoretical explanation by analyzing the Linear Theory Response framework based on the relationship between magnetic anisotropy and diameter with SAR value. The theoretical and experimental evidence fully demonstrate that doping is an effective approach for optimizing as-synthesized NPs to enable efficient hyperthermia applications.

Correlation of the chemical composition, phase content, structural characteristics and magnetic properties of the Bi-substituted M-type hexaferrites

Bi-substituted M-type hexaferrites, BaFe12-xBixO19 (0.1 ≤ x ≤ 1.2), or Bi-BaM, were produced by the solid-state reactions. The correlation between the phase content, chemical composition, crystal structure features, and peculiarities of the magnetic properties of Bi-BaM was established using XRD (X-ray diffraction), SEM (scanning electron microscopy), and VSM (vibrational sample magnetometry). XRD phase analysis made it possible to establish the limit of substitution of Fe3+ ions by Bi3+ ions. It was shown that at a low substitution level (x ≤ 0.3), no impurity phases were detected, and the samples are characterized by a single-phase state with the space group (SG) P63/mmc. As the degree of substitution (x ≥ 0.6) increases, the formation of impurity phases was observed, which can be explained by the difficulties of ion diffusion in the process of solid-phase synthesis as well as the formation of defects in the magnetoplumbite structure due to the large ionic radius of Bi3+. As impurity phases in the studied compositions (x ≥ 0.6) the following were noted: BiFeO3 (SG: Pnma); BiO2 (SG: Fm-3m); BaBi2O6 (SG: R-3); and Ba0.5Bi1.5O2.16 (SG: Im-3m). The content of the main phase (SG: P63/mmc) decreases from 95.11 to 88.27 vol% with an increase in x from 0.6 to 1.2, respectively. Analysis of SEM images showed the growth of particles up to 10 μm, depending on the concentration of bismuth oxide during hexaferrite synthase. The Bi-BaM magnetic characteristics were examined using VSM in the range of 3 T at 300 K. Due to the magnetic structure's frustration, with increased x a decrease in saturation magnetization (Ms) was found. There were two concentration diapasons with different speeds of Ms decrease. In the first diapason, the main contribution belong to the magnetic structure frustration in the frame of the main phase (P63/mmc) due to the long-range Fe-O-Fe exchange interaction weakening (under Bi substitution). In the second diapason, the main contribution belong to the impurity phase formation and decrease of the main magnetic phase concentration in samples.

Effect of rapid solidification and post-processing on microstructure, magnetic and structural transition temperatures and magnetic properties in Ni50Mn29Ga21 magnetic shape-memory alloy

This study investigated the effect of rapid solidification and post-processing on microstructure, composition and magneto-structural properties of Ni50Mn29Ga21 (at.%) magnetic shape memory alloy (MSMA). Samples with varying cooling rates (1.1 ∙ 102 to 3.2 ∙ 103K/s) were prepared using arc melting for a comparative study in as-solidified condition and after two different heat treatments. A notable change from columnar-dendritic to cellular-dendritic solidification and a decrease in solidification length scale was observed at more rapid solidification. All samples showed Mn enrichment and corresponding Ga depletion at the sub-grain boundaries. The fine-scale solidification structure, chemical heterogeneity and lack of ordering resulted in broad martensitic transformations. At more rapid solidification, a decrease in saturation magnetization and abnormal slope changes in the hysteresis loop were observed. Heat-treatment resulted in grain growth, significant reduction in transformation breadth, and increase in magnetization saturation. Rapidly solidified samples with an estimated characteristic cooling rate of 1.1 ∙ 103K/s most closely resembled microstructures and magneto-structural properties from laser directed energy deposition (L-DED) processing. The results of this study help to understand cooling rate effects in L-DED additive manufactured Ni-Mn-Ga and provide post-processing routes that will result in microstructures and properties that resemble those of functional MSMA.