Field-cooled Magnetization Research Articles

Flux jumps and mechanical effects in high-temperature superconductors with multi-field coupling model

Mechanical deformation during the magnetization process of high-temperature superconductors can lead to the deterioration in their critical current. A multi-field coupling model involving four physical fields is established to investigate the role of mechanical strain in the flux jump of the superconducting bulk. The electrical, magnetic, thermal, and mechanical coupling behavior is presented and analyzed under different applied fields. The simulation results of the coupled model are compared with experimental data, which showing an agreement. The numerical results indicate that in the superconductors, mechanical deformation significantly affects the electrical, magnetic, and thermal properties owing to a reduction in the critical current density under mechanical strain. The moment and location of magnetic flux jump will change as the mechanical strain is considered. Interestingly, the occurrence of magnetic flux jumps during field-cooling magnetization leads to a decrease in stress in bulk superconductor, suggesting that the fracture may not be caused by flux jumps, which is contrary to common understanding. Moreover, the temperature changes uniformly at different locations within the superconducting bulk. In the extended study, the thermo-magnetic and mechanical properties of the bulk material under ZFCM are presented using the coupled model. The effect of mechanics on the flux jump differs between FCM and ZFCM, which is attributed to the nonlinear changes in the electric field.

Magnetisation of the Aurivillius phase with m = 4 revisited

ABSTRACT We report the successful synthesis of the Aurivillius phase Bi m+1Fe m−3Ti3O3m+3 with m = 4 via the solid-state reaction method. X-ray diffraction confirms the structure as a four-layer Aurivillius phase, and elemental analysis (EDA) shows no impurities. The magnetic properties, including temperature-dependent FC and ZFC magnetisation, were measured from helium to room temperature, aligning well with literature data. These dependences, when plotted on double logarithmic scales, show linear behaviour. However, a significant difference between simulated and experimental magnetisation profiles was observed. The simulation predicted a logistic S-shaped temperature dependence, which was not reflected in the experimental results. To address this discrepancy, we analyzed the noisy experimental data by applying the least squares local linear filter before differentiation. This analysis revealed a hidden pattern that matches the theoretical predictions, suggesting that the observed magnetisation profile results from the simultaneous contributions of two magnetic phases in the m = 4 Aurivillius multiferroic: the Aurivillius phase and an ensemble of oxygen vacancies associated with F+ defects.

A comparative study of influence of isovalent and aliovalent A-site substitutions on the structural, magnetic, and dielectric properties of Sr2CrMoO6 double perovskites

We report on a comparative study of the structural, magnetic, and dielectric and properties of isovalent and aliovalent A-site substituted Sr2-xAxCrMoO6 (A = Ca, x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.8, 1.0; A = K, La only x = 0.5) double perovskites, which were synthesized via sol-gel process. X-ray diffraction patterns and Rietveld refinements demonstrated that all the Sr2-xAxCrMoO6 powders crystallized in a cubic crystal structure with space group of Fm3‾m. SEM images revealed that the Sr2-xAxCrMoO6 powders exhibited a spherical morphology. Their chemical compositions were determined by quantitative energy dispersive X-ray spectroscopy, which were close to the nominal values. FTIR spectra analyses verified the presence of (Cr, Mo)O6 octahedra in these powders. XPS spectra validated the existence of Sr2+, K+, Ca2+, La3+, and Cr3+ ions in the powders, and oxygen existed in the forms of lattice oxygen and absorbed oxygen respectively, whereas Mo element had a mixed chemical valence state of Mo4+ and Mo6+. The Sr2CrMoO6 (SCMO) powders had a saturation magnetization (MS) of 0.016 μB/f.u. at 2 K, magnetic Curie temperature (TC) of 337 K, and irreversibility temperature (Tirr) of 329 K where the zero-field cooled (ZFC) and field-cooled (FC) magnetization (MZFC and MFC, respectively) curves became bifurcated. A spin-glass behavior was observed at temperature (TB) of 81 K. Both Tirr and TB shifted towards low temperature with increasing the external magnetic field. In addition, the MZFC and MFC curves became almost overlapped under the external field of 50 kOe, and the spin-glass behavior disappeared. The M-H data demonstrated that both isovalent and aliovalent A-site substitutions could improve the magnetic properties and B-site ordering degree (η) of the SCMO powders. However, isovalent Ca-substitution exhibited more significant enhanced effect on the magnetic properties in comparison with the aliovalent A-site substitutions either by K- or La-substitution. Among the isovalent Ca-substituted Sr2-xCaxCrMoO6 (x = 0.1–1.0) powders, Sr1.2Ca0.8CrMoO6 sample had a maximum MS value of 0.64 μB/f.u. at 2 K, over 30 times enhancement of that of the pristine SCMO powders. The Sr2-xAxCrMoO6 ceramics exhibit a strong frequency dependent dielectric behavior, which can be well interpreted by Maxwell-Wagner relaxation model. The anomalous dielectric relaxor behavior of the SCMO ceramics observed at 260 K was ascribed to the jumping of the electrons trapped in a shallower energy level created by oxygen vacancies. The dielectric constant of the isovalent Ca-substituted Sr1.5Ca0.5CrMoO6 samples was 30 times larger than that of the pristine SCMO samples. Our present results demonstrate that isovalent Ca-substitution at A-site could improve the magnetic and dielectric properties of the sol-gel derived SCMO double perovskite oxides, which have potential applications in the fields of spintronic devices and dielectric devices.

Magnetocaloric properties of rare-earth element doped LCMO-Mn3O4 nanocomposites

The study reports the synthesis and magnetocaloric properties of the rare-earth element as Eu3+ doped La1.4Ca1.6Mn2O7-Mn3O4 composites prepared by the one-pot autocombustion technique. Eu3+ co-doping enhanced the Curie temperatures from 181 K for La1.4Ca1.6Mn2O7 (LCMO) to 186 K for La1.3Ca1.6Eu0.1Mn2O7 perovskites, which consequently enhanced the relative cooling power value of the compound. Moreover, these composites were also prepared in the presence of 10 wt% of Mn3O4 nanoparticles. The presence of Mn3O4 at the intervening grain boundaries between La1.4Ca1.6Mn2O7 (A) and La1.3Ca1.6Eu0.1Mn2O7 (B) phases altered the double exchange interaction between Mn3+ and Mn4+ ions. The temperature-dependent field-cooled magnetization curves showed that these nanocomposites’ interfacial magnetic interactions significantly expanded the second-order ferromagnetic-to-paramagnetic phase transition temperature. This further enhanced the magnetic entropy magnitudes |ΔSMax| up to 0.521 J kg−1 k−1 and the associated relative cooling power value to 43.67 J kg−1 under a 5T applied magnetic field. The temperature-averaged entropy change (TEC) values of composites outperformed the individual values of samples A and B between the temperature range of 80–240 K. The fundamental key of this work is to demonstrate the potentiality of enhancing the magnetic phase transition temperature and magnetocaloric effect in the framework of interfacial coupling between LCMO and the Mn3O4 phases of the nanocomposites.

Anisotropic spin-glass-like behavior in single crystal GdCo2B2

A high-quality single crystal of GdCo2B2 was successfully synthesized and subjected to a comprehensive investigation into its spin-glass characteristics. We conducted a systematic analysis of magnetization, aging effects, and alternating current (AC) susceptibility. Interestingly, a clear separation was observed between the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves under low magnetic field of 0.01 T. Furthermore, the crystal exhibited a long-time relaxation behavior at 4 K. The freezing temperature increases with frequency increasing. Additionally, we explored the response of the crystal to DC magnetization and AC susceptibility under different pressure conditions, up to 1.58 GPa. The crystal exhibits three anomalies TC = 22.0 K, T1 = 20.5 K and TN = 11.0 K under ambient pressure. An analysis of the anisotropy of the spin-glass-like behavior in the crystal was conducted, with a specific focus on the a-axis as the preferred direction of magnetization. All of the results provide evidence for the coexistence of mixed ordering states in the crystal.

Exploring induced microstructural changes in magnetically modified crude oils through nonlinear rheology and magnetometry

Adsorptive phenomena involving dispersed iron oxide superparamagnetic nanoparticles and asphaltenes in crude oil have been profiled as promising technological alternatives, particularly since these interactions can induce significant structural changes within the oil matrices, effectively inhibiting the formation of complex long-range viscoelastic structures. Furthermore, the effect of adsorbed asphaltenes on magnetic dipolar interactions among particles has been proven, showing the formation of multiple asphaltene layers that stimulate a steric repulsive barrier. Despite the discussed hindering phenomena, this research demonstrated the effectiveness of the sequence of physical processes framework to provide intra-cycle structure-rheological interpretations in large amplitude oscillatory shear of a ferrofluid-modified heavy oil, upon the application of an external magnetic field. The analysis proved that disordered nanoparticle/asphaltene aggregates are highly extended and naturally formed in the absence of magnetic forces. In contrast, in the presence of a perpendicular field applied by a controlled rate magneto-rheometer, the formation of interacting structural aggregates of several hundred nanometers was observed, analogous to magnetorheological fluids. These results were validated by adjusting a phenomenological model that effectively represented the intricate processes involved in the formation and reorientation of aggregates, based on the experimental data acquired from zero-field-cooled and field-cooled magnetization curves. This revealed a distinct blocking temperature distribution at around 274 K, which was linked to Brownian relaxation phenomena exhibited by nanoparticle aggregates. In this regard, this research provided a precise extended description of the effect of magnetic fields on the microstructural organization of complex fluids using nonlinear rheology and magnetometry.

Synthesis of cobalt-oxide nanoparticles embedded in silicon nanotubes via low-energy cobalt implantation

Silicon nanotubes (Si NTs) show a unique structure compared to other silicon nanostructures, such as porous silicon, silicon nanoparticles, and silicon nanowires. Si NTs' well-defined structure in terms of silicon shell thickness, inner diameter, and length with a high surface area makes them useful in myriad applications. In this work, we fabricate Si NTs by using ZnO nanowires as templates; the Si NTs have a shell thickness of ∼13 nm, inner diameter of ∼80 nm, and a length in the range of 1–2 μm with embedded Co3O4 nanoparticles (NPs), which are formed by annealing the cobalt ion-implanted Si NTs. The cobalt ion implantation was carried out at 50 keV with a fluence of 7.5 × 1015 ions/cm2. The morphology and composition of the cobalt-NP decorated Si NTs are characterized by using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), transmission electron microscopy (TEM), and x-ray photoemission spectroscopy (XPS). TEM, HRTEM, XPS, and Raman spectroscopy studies confirm the formation of Co3O4 nanoparticles in the NTs. The temperature-dependent zero-field-cooled (ZFC), and field-cooled (FC) magnetization for the Co implanted Si NTs before and after annealing is also measured and shows little magnetic response due to the low concentration of cobalt in the nanotubes. The fabrication process is compatible with the current semiconductor industry methods and may find potential applications in magnetic drug delivery and devices.

Signatures of nearly compensated magnetism and spin glass behavior in highly frustrated β-Mn-type Mn50Fe25+xAl25-x Heusler alloys.

The present study examines the effect of Fe/Al concentration on the structural and magnetic properties of Mn-rich Mn50Fe25+xAl25-x (x = 5, 10, 15) Heusler alloys through x-ray diffraction, temperature- and field-dependent DC magnetization, thermoremanent magnetization, magnetic memory effect, AC susceptibility measurements, and DFT calculations. The samples crystallize in a cubic β-Mn structure. The trend shows a reduction in lattice parameters (unit cell volume) with the increasing Fe proportion. These alloys exhibit strong antiferromagnetic interactions with large frustration parameters, indicating the presence of competing magnetic interactions. The DC magnetization data reveal spin glass-like features with a peak at spin glass freezing temperature (Tf). The observation of bifurcation in temperature-dependent zero-field-cooled and field-cooled magnetization curves, exponential dependence of the temperature variation of remanence and coercivity, magnetic relaxation, and magnetic memory effect below Tf support the spin-glass character of these alloys. The frequency dependence of Tf is also examined in the context of dynamic scaling laws, such as the Vogel-Fulcher law and critical slowing down model, which further supports the presence of spin glass behavior. In the theoretical DFT calculations, the electronic structure is found to be metallic and similar for both spin projections. Moreover, the antiferromagnetic arrangement of the magnetic moments, in line with the experimental observations, is stabilized by exchange interactions, resulting in an almost compensated total magnetic moment of 0.02-0.38µB/f.u. This is probably caused by the frustrated structure and non-stoichiometric compositions of Mn50Fe25+xAl25-x.

Synthesis of lithium doped magnesium ferrites and their vibrational and magnetic properties: Correlation of experimental and density functional theory

Recently, there has been an increased focus on investigating the possible use of ferrite nanoparticles as magnetic memory devices. In this study, we report on the structural, microstructural, Raman, density functional theory and magnetic characteristics of Mg1-xLixFe2O4 (x = 0, 0.15, 0.3, 0.45, 0.5 and 0.75). Samples were synthesized by solution combustion synthesis method. Samples were subjected to characterize X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman Spectra. XRD reveals that all synthesized samples show a single phase formation and unit cell volume, crystallite size varied with an increase in Lithium-ion concentration. SEM micrographs confirmed the porous nature increases with the rise of Li concentration. Particles are agglomeration and spherical type of nature was confirmed by TEM images. Further, Raman Spectra confirms the spinel cubic structure of MgFe2O4 nanoparticles belongs to the Fd3m space group. Despite the fact that the full unit cell has 56 atoms (Z = 8), the smallest Bravais cell consists of only 14 atoms (Z = 2). Consequently, in order to understand this effect, the electronic structure of pure and doped models was investigated through the Band Structure and Density of States (D.O.S.) profiles. First the crystalline structure of pure and Li-doped MgFe2O4 was analyzed based on the optimized lattice parameters and M − O (M = Mg, Li and Fe) bond distances. The band-gap was computed to be 2.54 eV being a direct transition between two points, in an excellent agreement with the experimental measurements. To investigate the temperature dependent magnetic properties, the M − H loops of Li doped MgFe2O4 nanoparticles traced at below and above room temperature over the applied magnetic field ±60 kOe. The M − H loops recorded at above and below room temperature demonstrated that the ferromagnetic nature of the sample. Further, the magnetic properties was investigated using temperature dependent FC and ZFC magnetization measurements. Above room temperature, the ZFC and FC curves show irreversibility and split at certain temperatures, as well as a broad maximum in ZFC curves at blocking temperature (TB). Saturation magnetization (Ms) values are higher at 15 K compared to 300 K, attributed to reduced thermal fluctuation. Our results suggest that synthesized materials are useful for room temperature magnetic memory device application.

R-z plane spatial critical current inhomogeneity-induced mechanical response of GdBCO superconducting bulks during field cooling magnetization

Benefiting from the high critical current density (J c), single-grain (RE)BCO (where RE = rare earth or Gd) bulks are capable of trapping over 17.6 T magnetic field which is crucial for the application of bulk superconductors. Nevertheless, during field cooling magnetization (FCM), the large mechanical stress induced by Lorentz forces may lead to fracture behavior in the brittle ceramic nature of (RE)BCO materials. Most previous numerical models that adopted simplified homogeneous J c had difficulty reflecting the real stress/strain situation in high temperature superconductor (HTS) bulks. Based on the proposed modified Jirsa model considering r-z plane J c inhomogeneity, we investigate the mechanical response of GdBCO bulks manufactured by top-seeded melt growth (TSMG) process. A 2D axisymmetric electromagnetic-thermal-mechanical coupled model is implemented to take into account the dependence of J c upon mechanical deformation. The simulation results show the electromagnetic-thermal-mechanical response of the r-z plane inhomogeneous J c model is lower than that obtained by the homogeneous J c model. This confirms Takahashi’s speculation (K Takahashi 2019 Supercond. Sci. Technol. 32 015007) about the mismatch between experimental data and the simulation results of homogeneous J c model, and suggests the stress levels in the bottom plane of HTS bulk are overestimated by the previous homogeneous J c model. On top of that, the overall stress level of GdBCO bulk is strongly determined by the magnitude and position of the Lorentz force load, and the stress distribution of inhomogeneous J c model is mainly concentrated in high J c regions near top surface, instead of being symmetrically distributed along the z-axis as in homogeneous J c model. The mechanical response of stainless steel reinforced GdBCO bulk was aslo simulated and analyzed. Finally, the coupling effect between the fracture strength variability caused by defects and cracks and the trapped field in GdBCO bulks with r-z plane J c inhomogeneity is further studied. This study may provide a relatively realistic mechanical response of HTS bulk during FCM, and a novel design consideration for its mechanical reinforcement.

Study of magnetic and magnetocaloric properties of calcium doped La0.97−xCaxHo0.03MnO3 compound

This study reports the magnetic and magnetocaloric properties of different concentrations of alkaline earth metal, such as Ca-doped La0.97−xCaxHo0.03MnO3 (x = 0.3, 0.33, and 0.37) composites, which were synthesized via autocombustion technique. The second-order paramagnetic-to-ferromagnetic phase transition temperature appeared at the temperature-dependent field-cooled magnetization curve. The result shows an increase in the Curie temperature of the compound with Ca2+ doping. In addition, increasing the Ca2+ doping concentration further increased the change in magnetic entropy, − ∆SM, up to 0.206 J kg−1 K−1, resulting in a higher RCP value up to 30 J kg−1 at x = 0.37. This work’s key aspect is demonstrating the potentiality of enhancing the magnetocaloric effect in the framework via the spin coupling mechanism of Ca2+-doped rare-earth perovskite compounds.Graphical abstract

Magnetic effect and chemical distribution study of LCMNO3 perovskite by photoelectron spectroscopy

The crystal structure, x-ray photoemission spectroscopy, magnetic characteristics, and magnetocaloric performance of perovskite La0.55Ca0.45Mn0.8Nb0.2O3 (LCMNO) ceramic powder were analyzed. The orthorhombic structure with a Pnma space group was found as the main phase in the as-received compound after the XRD pattern refinement. The mixed-valence states of manganese were detected by x-ray photoemission spectroscopy (XPS). The analysis of the La3d line revealed the complex nature of the lanthanum states. The evaluation of Zero-Field-Cooled and Field-Cooled magnetization performance provided conclusive evidence for the existence of the antiferromagnetic Mn3O4 phase at a temperature of TN = 42 K and the coexistence of CO- AFM and FM domains in LCMNO. Despite the observed significant (δTFWHM) value due to relatively low magnetic entropy change (ΔSM), the relative cooling power (RCP) is relatively moderate. By adding Nb4+, the changes in Mn valences and their effect on the magnetic cooling effect are lessened.

Magnetic studies of ultrafine CoFe2O4 nanoparticles with different molecular surface coatings.

Surface functionalized ultrafine CoFe2O4 nanoparticles (NPs), with mean diameter ∼5 nm, were investigated by means of DC magnetization and AC susceptibility over the temperature range of 4-400 K. All NPs present the same CoFe2O4 core, with different molecular surface coatings, increasing gradually the number of carbon atoms in the coating layer: glycine (C2H5NO2), alanine (C3H7NO2), aminobutanoic acid (C4H9NO2), aminohexanoic acid (C6H13NO2), and aminododecanoic acid (C12H25NO2). Samples were intentionally fabricated in order to modulate the core-core magnetic dipolar interaction, as the thickness of the coating layer increases with the number of carbon atoms in the coating molecule. The magnetic data of the uncoated CoFe2O4 NPs were also collected for comparison. All investigated CoFe2O4 NPs (coated and uncoated) are in a magnetically blocked state at room temperature as evidenced by ZFC/FC measurements and the presence of hysteresis with ∼700 Oe coercivity. Low temperature magnetization scans show slightly constricted hysteresis loops with coercivity decreasing systematically with a decreasing number of carbon atoms in the coating molecule, possibly resulting from differences in magnetic dipole coupling between NPs. Large thermomagnetic irreversibility, slow monotonic increase in the FC magnetization and non-saturation of the magnetization give evidence for the cluster glass (CG) nature in the CoFe2O4 NPs. The out of phase part (χ'') of AC susceptibility for all samples shows a clear frequency dependent hump which was analyzed to distinguish superparamagnetic (SPM), cluster glass (CG) and spin glass (SG) behavior by using Néel-Arrhenius, Vogel-Fulcher, and power law fittings. These analyses rule out the SPM state and suggest the presence of significant inter-cluster dipolar interaction, giving rise to CG cooperative freezing in the high-temperature region. In the low-temperature range, however, the disordered spins on the nanoparticle's surface play an important role in the formation of the SG-like state, as evidenced by Arrott plots and temperature dependency of dM/dH in the initial magnetization curves. In summary, the magnetic measurements showed that undercooling the system evolves from a SPM state of weakly interacting spin clusters, through the CG state induced by strong dipolar interaction, to the SG state resulting from the frustration of the disordered surface spins.

Comparison of physical properties of ferrofluids based on mineral transformer oil and bio-degradable gas-to-liquid oil

Ferrofluids based on mineral transformer oil have been intensively studied over two decades owing to their outstanding thermal and electrical insulating properties. However, current environmental demands on transformer operation give priority to the use of biodegradable transformer oils. Clearly, the differences in physical properties of biodegradable oils and those refined from crude oil determine the physical properties of ferrofluids based on the two types of oils. In this study, ferrofluids with various concentrations of iron oxide nanoparticles have been prepared on mineral transformer oil (M−oil) and biodegradable transformer oil based on gas-to-liquid technology (S-oil). The ferrofluids were subjected to experimental investigation of magnetization, AC magnetic susceptibility, thermal conductivity, viscosity and dielectric response. Based on zero-field-cooled and field-cooled magnetization curves, a lower temperature of a specific magnetization maximum associated with the melting of the oil (phase transition) was found for M−samples than for S-samples. The effect of carrier liquid on the onset of extrinsic superparamagnetism in addition to the intrinsic superparamagnetism is observed. Spectra of AC magnetic susceptibility of both types of ferrofluids exhibit quasi constant behaviour with amoderate decrease at high frequencies of amagnetic field. The viscosity of M−samples is slightly higher than that of S-samples. On the other hand, M−samples exhibit lower thermal conductivity than S-samples. In both cases, the thermal conductivity linearly decreases with temperature. Dielectric spectroscopy has revealed remarkable dielectric dispersion below 100 Hz in both ferrofluid types. The relaxation has been ascribed to interfacial polarization, which can be considered as a serious drawback from an electrical engineering application point of view. The measured physical properties of the ferrofluids are analyzed in regard to the density and viscosity of the base oils.

Unveiling the crystal and magnetic texture of iron oxide nanoflowers.

Iron oxide nanoflowers (IONF) are densely packed multi-core aggregates known for their high saturation magnetization and initial susceptibility, as well as low remanence and coercive field. This study reports on how the local magnetic texture originating at the crystalline correlations among the cores determines the special magnetic properties of individual IONF over a wide size range from 40 to 400 nm. Regardless of this significant size variation in the aggregates, all samples exhibit a consistent crystalline correlation that extends well beyond the IONF cores. Furthermore, a nearly zero remnant magnetization, together with the presence of a persistently blocked state, and almost temperature-independent field-cooled magnetization, support the existence of a 3D magnetic texture throughout the IONF. This is confirmed by magnetic transmission X-ray microscopy images of tens of individual IONF, showing, in all cases, a nearly demagnetized state caused by the vorticity of the magnetic texture. Micromagnetic simulations agree well with these experimental findings, showing that the interplay between the inter-core direct exchange coupling and the demagnetizing field is responsible for the highly vortex-like spin configuration that stabilizes at low magnetic fields and appears to have partial topological protection. Overall, this comprehensive study provides valuable insights into the impact of crystalline texture on the magnetic properties of IONF over a wide size range, offering a deeper understanding of their potential applications in fields such as biomedicine and water remediation.

Microstructural and magnetic properties of BiFeO3/Bi2Fe4O9 binary nanoparticles with a bimodal size distribution

Binary nanoparticles composed of BiFeO3 and Bi2Fe4O9 have attracted considerable interest due to their magnetic properties and exchange bias effect. In this paper, two samples of well-crystallized BiFeO3/Bi2Fe4O9 binary nanoparticles with varying compositions, containing approximately 3 % and 32 % of the Bi2Fe4O9 crystalline phase, were investigated to understand the impact of microstructural variations on the magnetic behavior of the samples. Both BiFeO3 and Bi2Fe4O9 phases of binary nanoparticles were synthesized in the same synthesis process by calcining amorphous precursors obtained after solvent evaporation from aqueous solutions containing iron and bismuth nitrates and tartaric acid. The identification and quantification of crystalline phases of the produced samples were performed using the X-ray diffraction technique and multiphase Rietveld refinement, respectively. The structure and microstructures of the synthesized BiFeO3/Bi2Fe4O9 nanoparticles were carried out using transmission electron microscopy techniques. The atomic composition of the samples was investigated by energy-dispersive X-ray. The magnetic properties were studied by the DC magnetization technique. Transmission electron microscopy analyses showed that both samples exhibit a bimodal particle size distribution, with average particle sizes of ∼ 116 nm and ∼ 6.5 nm for the ∼ 3 % Bi2Fe4O9 sample. In contrast, for the ∼ 32 % Bi2Fe4O9 sample the average particle sizes were ∼ 108 nm and ∼ 7 nm, respectively. Energy-dispersive X-ray analyses revealed a Fe-rich impurity phase in both samples. Temperature-dependent zero-field-cooled and field-cooled magnetization curves present a large separation between them below 300 K and zero-field-cooled curves are consistent with a broad (bimodal) size distribution of the nanoparticles for both samples. Magnetic hysteresis loops measured at 5 K after the samples are cooled from 400 K in a zero magnetic field show weak ferromagnetism for the ∼ 3 % Bi2Fe4O9 sample, whereas the ∼ 32 % Bi2Fe4O9 sample presents an improved ferromagnetism. An exchange bias-type effect was observed in both samples. The results suggest that the increase of ultrasmall Bi2Fe4O9 nanoparticles leads to an improvement in the coercivity and remanence of BiFeO3/Bi2Fe4O9 binary nanoparticles.

Magnetic properties of exchanged-coupled nanostructured core–shell particles of CoO@MnFe[formula omitted]O[formula omitted

In this work the structural and magnetic properties of nanoparticles of bimagnetic core–shell CoO@MnFe2O4 were reported. The structure and the morphology of the nanoparticles were characterized by X-ray powder diffraction and by transmission electron microscopy. Hysteresis loops were recorded by varying the applied magnetic field (H) in the range ± 85 kOe for several temperature values (T). The T-dependence of the zero-field- and the field-cooling magnetization were investigated from room temperature until 5 K for H = 100 Oe. The coercivity was found to be enhanced to about 2.34 kOe at 5 K while shiftings in the hysteresis loops due to the exchange-bias effect were also observed reaching 472 Oe at 5 K. The blocking temperature for this core–shell nanoparticle system was also found to be ≈ 103 K. The enhancement in the coercivity and the exchange-bias effect are discussed within a model based on the exchange-coupling of the magnetic moments at the core–shell interface.

Microwave absorption and hyperthermia properties of titanium dioxide–nickel zinc copper ferrite nanocomposite

Ni0.5Zn0.4Cu0.1Fe2O4 ferrite nanoparticles (NZCUFO) were prepared using a conventional co-precipitation method. The NZCUFO nanoparticles were then combined with titanium dioxide matrix to make the nanocomposite of [TiO2]0.2[Ni0.5Zn0.4Cu0.1Fe2O4]0.8 (TNZCUFO). The formation of the appropriate crystallographic phase was confirmed by analyzing the X-ray diffraction pattern of the samples. Investigations extracted from the X-ray fluorescence spectroscopy, high-resolution transmission electron microscopy, and Raman spectroscopy imply the successful association of ferrite nanoparticles into the TiO2 matrix. At 300 K, the saturation magnetization of NZCUFO and TNZCUFO reaches 34.6 and 23.4 emu/g, respectively. Mössbauer spectra recorded at 77 and 300 K indicate the presence of superparamagnetic behavior. These results were in agreement with the data obtained from magnetic hysteresis loop, zero field cooled, and field-cooled magnetization measurements. Both the samples were exposed to alternating current inductive heating and the extracted results successfully satisfy the condition of hyperthermia therapy for cancer treatment. The electromagnetic wave absorption capabilities of the samples were measured in the microwave region (8–18 GHz). The microwave reflection loss reaches −13.6 dB at 11.6 GHz and −29.7 dB at 11 GHz for the 2 mm thick samples of NZCUFO and TNZCUFO, respectively. The balanced matching of magnetic and dielectric loss, high magnetization, and specific absorption rate of the samples would be extremely useful for diverse soft magnetic applications.

Evidence for suppression of collective magnetism in Fe-Ag granular multilayers

Evidence for the suppression of collective magnetic behavior of dipolarly interacting Fe nanoparticles is found in Fe-Ag granular multilayers. Interaction of Fe particles separated by an Ag layer is studied as a function of the nominal thickness of the Ag layer. The surprisingly increasing interaction with increasing Ag-layer thickness, verified by memory-effect measurements, is explained by the formation of pinholes in the Ag layer at small Ag thicknesses, allowing direct ferromagnetic coupling between the Fe particles. This coupling may hinder the frustration of superspins favored by dipolar interactions. At larger Ag thicknesses, the Ag layer is continuous without pinholes and frustration leads to the appearance of the superspin-glass state. The effect of increasing interactions correlates well with the growing deviation at low temperatures of the measured field-cooled (FC) magnetization from the interaction-free FC curve calculated by a model based on the relaxation of two-level systems. Similar phenomenon is reported in a recently published paper (Sánchez et al., Small2022, 18, 2106762) where a dense nanoparticle system is studied. Here the collective magnetic behavior of the particles due to dipolar interactions is suppressed when the anisotropy energy of the individual particles exceeds a certain threshold.

Investigation of magnetic properties and colossal magnetoresistance in nanocrystalline doped manganite

Here in, we report structural, magnetic, and magneto-transport properties of nanocrystalline La0.6Ag0.2Bi0.2MnO3 prepared using citrate sol–gel method. By using scanning and transmission electron microscopy measurements, the morphology and particle size of the sample have been confirmed. The Mn2p x-ray photoelectron spectroscopy spectra revealed the nanoparticles contained the coexistence of Mn3+ and Mn4+ ions with Mn3+/Mn4+ ratio of 2:1. Field-cooled and zero field-cooled magnetization protocols with temperature span of 5 K–300 K, confirm the paramagnetic (PM) to ferromagnetic (FM) phase transition at critical temperature, ∼ 146 K. The complete investigation of isothermal magnetization (), Arrott plots, and magnetocaloric effect as well as quantitative analysis of second-order phase transition has been studied. The criticality at the PM-FM transition was examined for the sample, and the obtained critical exponents were verified for their reliability through the utilization of the scaling hypothesis and Kouvel-Fisher plot. We observed a large magnetic entropy change (∼7 J-Kg−1K−1) at upon 5 T magnetic field strength. The renormalized magnetic entropy change plots are found to collapse onto a single curve, thus verifying the universality of the sample. Above the metal–insulator transitions the electrical resistivity shows a small polaron hopping conduction mechanism, however, at low temperatures scattering mechanism dominates and the whole range was explained by the universal percolation model. The colossal value of negative MR is found to be 88% at 168 K under an applied field strength of 2 T. As a result of our experimental data, we can grasp the intuitive understanding of magnetic as well as transport properties in Bi-doped manganite systems potential for magnetic sensors and spintronics applications.