Fe doping effect on EuTiO3: The magnetic properties and giant magnetocaloric effect

The magnetic properties and magnetocaloric effect for EuTi_1-xFe_xO₃ (x = 0.05, 0.1) compounds are investigated. When a part of Ti⁴⁺ions were substituted by Fe ions, the AFM ordering can be significantly changed to be FM. The EuTi_1-xFe_xO₃ (x = 0.05, 0.1) compounds exhibit a PM to FM transition with decreasing temperature and the Curie temperature is 6 K. Under the field changes of 1 T, and RC are valued to be 10.1 J/kg K and 50.2 J/kg for EuTi_0.95Fe_0.05O₃; 9.6 J/kg K and 47.7 J/kg for EuTi_0.9Fe_0.1O₃, without magnetic and thermal hysteresis. RC is almost twice as much as EuTiO₃ (27 J/kg) as substitution of Fe³⁺ ions for Ti⁴⁺ions, which may be attributed to the magnetic transition (AFM to FM). Therefore, the giant and large RC suggest the EuTi_1-xFe_xO₃ compounds are good materials for magnetic refrigerant.     

Magnetoelectric coupling effect of Ga0.8Fe1.2O3/Bi0.5(Na0.85K0.15)0.5TiO3 multiferroic composite films

Lead-free Ga_0.8Fe_1.2O₃/Bi_0.5(K_0.15Na_0.85)_0.5TiO₃ (GFO/BKNT) bilayer multiferroic composite films were fabricated on Pt(100)/Ti/SiO₂/Si substrates via sol-gel methods. The microstructure, domain structure, ferroelectric, piezoelectric, magnetic properties as well as magnetoelectric coupling effect were investigated for the composite films at room temperature. Well-defined interfaces between GFO and BKNT layers and clear electric domain structures are observed. A strong magentoelectric effect is obtained with magnetoelectric voltage coefficient of α_E=30.89 mV/cm Oe, which is attributed to excellent ferroelectric, piezoelectric, and magnetic properties, as well as the coupling interaction between ferromagnetic GFO and ferroelectric BKNT phases for lead-free bilayer composite films. Besides, GFO and BKNT demonstrate the similar perovskite structure with well lattice matching, which endows the outstanding coupling and fascinating magnetoelectric properties. The present work opens up the opportunity of lead-free magnetoelectric composite films for both further fundamental studies and practical device applications such as sensors, transducers and multistate memories.     

Exchange Bias in the Layered Manganese La2O3Mn2Se2 as a Function of Temperature and Cooling Magnetic Fields

We investigate the exchange bias (EB) phenomenon in the layered manganese oxychalcogenides La₂O₃Mn₂Se₂. The spin canting of the antiferromagnetic (AFM) background leads to the ferromagnetic (FM) contribution and the exchange coupling at the FM/AFM interface is responsible for the EB phenomenon. Due to the subtle equilibrium of the magnetic states, the EB field shows strong temperature and cooling field H_FC dependences. These results suggest that the EB effect in La₂O₃Mn₂Se₂ could be tuned by H_FC and temperature, which is important for designing related devices.     

Effects of Fe doping on structure,negative thermal expansion,and magnetic properties of antiperovskite Mn3GaN compounds

We synthesized antiperovskite Mn₃Ga_1−xFe_xN (0 ≤ x ≤ 0.30) compounds and investigated their negative thermal expansion (NTE) behavior, structure, and magnetic properties. A high-resolution transmission electron microscopy analysis and a selected-area electron diffraction (SAED) pattern indicate that the samples have high crystallinity with a single-phase cubic structure. Tunable NTE behavior appears below room temperature, and the NTE operation temperature range (△T) is broadened while increasing the Fe doping. Furthermore, introducing Fe in Mn₃Ga_1−xFe_xN can efficiently adjust the NTE coefficient from −232.57 × 10⁻⁶/K (x = 0) to −12.57 × 10⁻⁶/K (x = 0.20), and the corresponding △T can be broadened from △T = 22 K to 51 K. Besides, the total entropy change (ΔS_total) at the phase transitions continuously decreases from 9.2 to 4.7 J/(kg K) while increasing the Fe content from x = 0.05 to 0.10. With increasing Fe into Ga sites, the magnetic ordering varies from antiferromagnetic (AFM) to ferromagnetic (FM), the AFM to paramagnetic (PM) phase transition temperature decreases, whereas the FM to PM transition temperature increases when increasing the Fe content. The present study indicates that magnetic element doping can efficiently tune the NTE and the correlated physical features of the antiperovskite compounds.     

A sonochemical method for synthesis of Fe3O4 nanoparticles and thermal stable PVA-based magnetic nanocomposite

Fe₃O₄ nanoparticles were synthesized via a simple surfactant-free sonochemical reaction. Room temperature synthesis without using inert atmosphere is the novelty of this work. The effect of different parameters on the morphology of the products was investigated. The magnetic properties of the samples were also investigated using an alternating gradient force magnetometer. Fe₃O₄ nanoparticles exhibit a ferromagnetic behavior with a saturation magnetization of 66 emu/g and a coercivity of 39 Oe at room temperature. For preparation magnetic nanocomposite, Fe₃O₄ nanoparticles were added to the polyvinyl alcohol (PVA). Nanoparticles can enhance the thermal stability and flame retardant property of the PVA matrix.     

Exchange bias driven by the structural/magnetic transition in Mn-doped SrRuO3

Magnetic properties of bulk polycrystalline Mn-doped SrRu_1−xMn_xO₃ perovskite, at doping range 0.2≤x≤0.3, were studied by both magnetization and ac-susceptibility measurements. It was found that the exchange bias (EB) effect emerges with increasing Mn doping in SrRu_1−xMn_xO₃ at x≈0.25, following the ferromagnetic (FM) to antiferromagnetic (AFM) phase transition. This transition is accompanied with the change in structure symmetry and then the EB field, H_EB, increases significantly in doping range 0.25<x≤0.3. The EB effect was verified by both cooling magnetic field, H_cool, dependence of the H_EB and training effect. A markedly nonmonotonic H_EB vs. H_cool dependence with maximum at around 40 kOe was found, resembling the behavior of phase-separated EB systems. Moreover, a clear analogy with behavior of classic EB system of Pr_1/3Ca_2/3MnO₃ was noticed, strongly suggesting that the EB effect in SrRu_1−xMn_xO₃ originates from exchange interactions at the interface of nanoscale FM clusters (size of ~1.6 nm at x=0.3) coexisting together with dominant AFM phase at the boundary of the first-order FM/AFM transition. The training effect observed is well understandable within the spin-configuration relaxation model and indicates important contribution to the EB behavior from the AFM domains rearrangement at the interface.     

Dielectric and Magnetic Properties of Sr(Fe1/2Ta1/2)O3 Complex Perovskite Ceramics

The dielectric and magnetic properties of Sr(Fe_1/2Ta_1/2)O₃ complex perovskite ceramics were systematically investigated together with the structure. The X‐ray powder diffraction analysis confirmed a B‐site disordered orthorhombic structure in space group Pbnm. Only one broadened dielectric peak with strong frequency dispersion was observed in the present ceramics, which was significantly different from that for the analogue Ba(Fe_1/2Nb_1/2)O₃ and Ba(Fe_1/2Ta_1/2)O₃. The strong dependence of sample thickness and electrode material indicated that the dielectric relaxation behavior at lower frequency was due to the interface effects. The present ceramics were spin glass state with slight ferromagnetic behavior below the Néel temperature (20 K). The co‐presence of Fe³⁺ and Fe²⁺ was confirmed by the μ_eff value.     

Controlling exchange bias in Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/FeO composite particles prepared by pulsed laser irradiation

Spherical iron oxide nanocomposite particles composed of magnetite and wustite have been successfully synthesized using a novel method of pulsed laser irradiation in ethyl acetate. Both the size and the composition of nanocomposite particles are controlled by laser irradiation condition. Through tuning the laser fluence, the Fe₃O₄/FeO phase ratio can be precisely controlled, and the magnetic properties of final products can also be regulated. This work presents a successful example of the fabrication of ferro (ferri) (FM)/antiferromagnetic (AFM) systems with high chemical stability. The results show this novel simple method as widely extendable to various FM/AFM nanocomposite systems.     

Electrical transport properties driven by magnetic competition in hole-doped perovskite Pr1-xBaxMnO3 (0.25 ≤ x ≤ 0.36)

In this work, polycrystalline Pr_1-xBa_xMnO₃ (0.25 ≤ x ≤ 0.36) ceramics were synthesized, and their magnetic and electrical transport properties were systematically studied. All samples show two metal-insulator transitions (MITs) corresponding to the high temperature T_MI1 and low temperature T_MI2, respectively, besides the non-Griffith phase above the ferromagnetic (FM) transition temperature T_C. Combining the results of the transport and magnetic properties, it is found that the FM transition temperature T_C coincides with the temperature T_MI1, which is linearly related to the A-site ionic radius mismatch variance σ², indicating the enhancement of FM interactions due to the increase of the degree of B-site ordering of Mn³⁺/Mn⁴⁺ ions. The positive correlation between ferromagnetic insulators (FMI) and magnetic interactions, including the FM and short-range antiferromagnetic (AFM) interactions, is confirmed. It is suggested that the first MIT at T_MI1 is attributed to the Mn³⁺/Mn⁴⁺ double exchange interactions and the second MIT at T_MI2 is closely related to the suppression of the AFM interactions under the internal FM field induced by the Mn³⁺/Mn⁴⁺ DE interactions. This work provides not only a theoretical understanding on the origin of MIT at low temperature, but also a new way for adjusting the FMI in perovskite manganese oxide Pr_1-xBa_xMnO₃ for application.     

Hybrid nanocomposites based on superparamagnetic and ferromagnetic particles: A comparison of their magnetic and dielectric properties

Nanocomposites of magnetic nanoparticles and polymer matrices combine the properties of their components, and as such are good examples of functional nanomaterials with excellent application potential. Against this background, experimental and theoretical studies of such composites are of great interest. In this study we aim to provide insight into the static and dynamic magnetic response, as well as the dielectric response, of magnetic nanocomposites subjected to external magnetic and electric fields. We directly compare the behavior of polyurethane films doped with superparamagnetic Fe₃O₄, and blocked ferromagnetic CoFe₂O₄ nanoparticles. While a reversible, Langevin magnetization curve is observed for Fe₃O₄@PU films, hysteretic magnetic behavior is found in case of CoFe₂O₄@PU films. The hysteresis observed for CoFe₂O₄ nanoparticles can be explained by interactions at the interface between particles and polymer matrix in conjunction with its ferromagnetic nature. The results of dielectric spectroscopy experiments revealed different effects of Fe₃O₄ and CoFe₂O₄ nanoparticles on polymer dynamics.     

Magnetic Properties of Iron Phosphate Glass and Glass‐Ceramics

Magnetic properties of crystallized iron phosphate glasses and relationship between structural and magnetic properties modifications that occur during crystallization have been investigated. Iron phosphate glass exhibits the spin‐glass (SG) behavior and represents a prototype of solid with disordered spatially distributed magnetic moments. Glass of the composition 43Fe₂O₃–57P₂O₅ (wt%) was heat‐treated in air at 893, 923, and 1073 K for 24 h. The samples were studied using X‐ray diffraction, Raman spectroscopy, and dc magnetic measurements. The magnetic measurements show dominant antiferromagnetic (AF) interactions for all samples. The starting glass exhibits SG behavior, whereas magnetic behavior of samples heat‐treated at 893 and 923 K, which contain Fe₃(P₂O₇)₂ crystalline phase embedded in glass matrix, is ascribed to a mixture of superparamagnetism and SG behavior. In the sample heat‐treated at 1073 K, several peaks in the magnetization curves were observed which correspond to the various crystalline phases present in the sample: Fe₃(P₂O₇)₂, Fe₄(P₂O₇)₃ and Fe(PO₃)₃. Hysteresis loops show paramagnetic behavior at 300 K. Small curvature is present at low temperature (5 K) that can be ascribed to the AF ordering in the samples.     

Analysis on the microstructure and magnetic properties of MgGaFeO4 spinel compound

The single phase of the spinel ferrite MgGaFeO₄ is successfully prepared by the high-temperature solid-state reaction. The crystal structure, cation distribution, valence state, and magnetic properties are systematically investigated through various techniques in this work. From the analysis of X-ray diffraction, the face-centered cubic structure with the space group Fd3m for the single-phase sample is obtained. The magnetic hysteresis loops M(H) shows a typical magnetic hysteresis behavior at 5 K, while an obvious paramagnetic behavior can be observed at room temperature. The characteristics of magnetic properties suggest that the intrinsic ferromagnetic (FM) interaction and magnetic frustration are cooperated in the system. The specific divergence and irreversibility in the temperature-dependent magnetization curves M(T) indicate a typical spin-glass (SG) behavior, which occurring around the freezing temperature T_f = 40 K. Further analysis of the isothermal remanent magnetization (M_IRM) suggests the existence of an intensive magnetic frustration state, which leads to stable residual magnetization without decaying under a long time span. More than that, based on the corresponding fitting parameters (τ₀ = 8 × 10⁻¹² s, zv = 6.32 and T₀ = 56.2 K), the SG state in the sample is further confirmed. The following detailed analyses reveal that the competitions between antiferromagnetic (AFM) and ferromagnetic (FM) interactions definitely exist in MgGaFeO₄, which should be the response to motivate the SG behavior.     

The features of the magnetoresistance of carbon nanotubes modified with Fe

Two specimens of modified multi-wall carbon nanotubes (MWCNT) with a mass fraction of iron of 0.20 and 0.29 have been obtained by the method of metal reduction from aqueous salt solution. According to structural and phase investigations, modified MWCNT contain Fe₃C, α-Fe (ferromagnetic phase), and iron oxides FeO and Fe₂O₃ (antiferromagnetic phases with a Néel temperature of 188 K and 260 K, respectively) particles up to 8 nm in size. It is experimentally found that for MWCNT modified simultaneously by several ferromagnetic phases with different coercive forces, a giant magnetoresistive effect is observed at room temperature. For MWCNT modified with only one ferromagnetic phase, the giant magnetoresistance has not been experimentally detected. With temperature decrease, the magnetoresistance dependence on magnetic field for such modified nanotubes acquires a specific butterfly-like form, which is characteristic of the magnetic ordering of ferromagnetic and antiferromagnetic phases due to exchange anisotropy. For MWCNT modified with only one ferromagnetic phase there has been an effect of asymmetric magnetoresistance, which is related to the presence in the specimen of inhomogeneous transverse Hall voltages due to the pronounced spin-orbit interaction of conduction electrons and magnetic moments of the magnetic phase, and an anisotropic magnetoresistive effect.     

Magnetic Field‐Induced Ferroelectric Switching in Multiferroic Aurivillius Phase Thin Films at Room Temperature

Single‐phase multiferroic materials are of considerable interest for future memory and sensing applications. Thin films of Aurivillius phase Bi₇Ti₃Fe₃O₂₁ and Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ (possessing six and five perovskite units per half‐cell, respectively) have been prepared by chemical solution deposition on c‐plane sapphire. Superconducting quantum interference device magnetometry reveal Bi₇Ti₃Fe₃O₂₁ to be antiferromagnetic (T_N = 190 K) and weakly ferromagnetic below 35 K, however, Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ gives a distinct room‐temperature in‐plane ferromagnetic signature (M_s = 0.74 emu/g, μ₀H_c =7 mT). Microstructural analysis, coupled with the use of a statistical analysis of the data, allows us to conclude that ferromagnetism does not originate from second phase inclusions, with a confidence level of 99.5%. Piezoresponse force microscopy (PFM) demonstrates room‐temperature ferroelectricity in both films, whereas PFM observations on Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ show Aurivillius grains undergo ferroelectric domain polarization switching induced by an applied magnetic field. Here, we show for the first time that Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ thin films are both ferroelectric and ferromagnetic and, demonstrate magnetic field‐induced switching of ferroelectric polarization in individual Aurivillius phase grains at room temperature.     

High-strength superparamagnetic composite fibers

The polymer composites of magnetic nanoparticles can be possibly used in a bulk form by preserving all the novel characteristics of magnetic nanoparticles such as superparamagnetic behavior. By introducing magnetic properties of Fe₃O₄ nanoparticles into polymer fibers, novel magnetic properties combine with the advantages of composite fibers such as light-weight and ease-of-use. Using dry-jet-wet fiber spinning technology, we have successfully fabricated iron oxide/polyacrylonitrile (Fe₃O₄/PAN) composite fibers with 10 wt% nanoparticle in the polymer matrix. Composite fiber with a diameter as small as 15 μm can achieve tensile strength and tensile modulus values as high as 630 MPa and 16 GPa, respectively. Superparamagnetic properties of Fe₃O₄ nanoparticles were preserved in the composite fibers with saturation magnetization at 80 emu/g and coercivity of 165 G.     

Impact of rare earth europium (RE-Eu3+) ions substitution on microstructural,optical and magnetic properties of CoFe2−xEuxO4 nanosystems

In this study, pure cobalt ferrite (CoFe₂O₄) nanoparticles and europium doped CoFe₂O₄ (CoFe_2−xEu_xO₄; x = 0.1, 0.2, 0.3) nanoparticles were synthesized by the precipitation and hydrothermal approach. The impact of replacing trivalent iron (Fe³⁺) ions by trivalent rare earth europium (RE-Eu³⁺) ions on the microstructure, optical and magnetic properties of the produced CoFe₂O₄ nanoparticles was studied. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra exposed the consistency of a single cubic phase with the evidence of Eu₂O₃ phases for x ≥ 0.2. FTIR transmittance spectra showed that, the all investigated samples have three characteristic metal-oxygen bond vibrations corresponding to octahedral B-site (υ₁ and υ₂) and tetrahedral A-site (υ₃) around 415 cm⁻¹, 470 cm⁻¹ and 600 cm⁻¹ respectively. XRD and energy dispersive X-ray spectroscopy studies affirmed the integration of RE-Eu³⁺ ions within CoFe₂O₄ host lattice and decrease of average crystals size from 13.7 nm to 4.7 nm. Transmission electron microscopy (TEM) analysis showed the crucial role played by RE-Eu³⁺ added to CoFe₂O₄ in reducing the particle size below 5 nm in agreement with XRD analysis. High resolution-TEM (HR-TEM) analysis showed that the as-synthesized spinel ferrite, i.e., CoFe_2−xEu_xO₄, nanoparticles are single-crystalline with no visible defects. In addition, the HR-TEM results showed that pure and doped CoFe₂O₄ have well-resolved lattice fringes and their interplanar spacings matches that obtained by XRD analysis. Magnetic properties investigated by the vibrating sample magnetometer technique illustrated transformation of magnetic state from ferromagnetic to superparamagnetic at 300 K resulting in introducing RE-Eu³⁺ in CoFe₂O₄ lattice. At low temperature (~5 K) the magnetic order was ferromagnetic for both pure and doped CoFe₂O₄ samples. Substitution of Fe³⁺ ions in CoFe₂O₄ nanoparticles with RE-Eu³⁺ ions optimizes the sample nanocrystals size, cation distribution and magnetic properties for many applications.     

Spin reorientation,normal and inverse magnetocaloric effects in heavy rare-earth iron garnets

Experiments for heavy rare-earth iron garnets Ho₃Fe₅O₁₂ and Er₃Fe₅O₁₂ show a compensation effect characterized by a near zero magnetization at 134 K and 80 K, respectively. The magnetic entropy change calculated according to the Maxwell's relation is shown to be positive and negative, indicating both normal and inverse magnetocaloric effects exist in the sample. The critical temperature (134 K for Ho₃Fe₅O₁₂, 80 K for Er₃Fe₅O₁₂) of the two types of magnetocaloric effect occurs at almost the same temperature as the compensation point. However, the compensation effect is attributed to the multiple exchange interactions among the octahedral sites Fe³⁺, the tetrahedral sites Fe³⁺ and the dodecahedral sites R³⁺, while the reversal of the magnetocaloric effect is originated from the spin reorientation. The maximum magnetic entropy change of the normal magnetocaloric effect is 4.72 J kg⁻¹ K⁻¹ for Ho₃Fe₅O₁₂ at 34 K and 4.94 J kg⁻¹ K⁻¹ for Er₃Fe₅O₁₂ at 24 K, respectively. Moreover, all the positive slopes of basic Arrott plots suggest only the second-order phase transition existing in Ho₃Fe₅O₁₂ and Er₃Fe₅O₁₂.     

Synthesis of magneto-dielectric coatings in electron-beam produced plasma in medium vacuum

Using sequential electron-beam evaporation of high-temperature dielectric (alumina ceramic) and magnetic (iron) targets in various gas atmospheres (helium, air, and oxygen) in medium vacuum (5–8 Pa), magneto-dielectric coatings with thickness of around 2 μm were deposited from a multicomponent beam plasma at a deposition rate of 0.2–0.3 μm/min. The coating magnetic properties were explored by the ferromagnetic resonance technique, revealing that their effective magnetization depends on the type of operating gas and varied from 4.2 to 6.8 kGs (for deposition in helium) to 0.3 kGs (in oxygen), which is characteristic of oxide ferromagnetic materials and is considerably lower than the corresponding value (∼22 kGs) for thin iron films formed by vacuum arc deposition in high vacuum. X-ray structural analysis of coatings deposited in medium vacuum in helium showed that the magnetic layer has a magnetite (Fe₃O₄) structure. The alumina ceramic layer provides the dielectric properties of the magneto-dielectric coating; a relative dielectric constant of 6.0 and a conductivity of 8.4 mS/m were achieved.     

AC susceptibility and hyperfine interactions of vanadium substituted barium nanohexaferrites

Vanadium ions substituted BaFe₁₂O₁₉ nanohexaferrites, BaFe_12-xV_xO₁₉ (0.0?≤x?≤?0.1), were produced through the sol-gel auto-combustion route. The structure, morphology and the elemental compositions of various products were examined using X–ray powder diffraction, scanning electron microscopy coupled with EDX and EDS elemental mapping. These techniques confirmed the formation of the desired Ba-nanohexaferrite phases. The crystallites size was found to be 55–58?nm range for all products. The magnetic properties of BaFe_12-xV_xO₁₉ nanohexaferrites were investigated by Mossbauer spectroscopy, ZFC-FC magnetizations and AC susceptibility. The evolutions in the values of hyperfine magnetic field, isomer shift, quadrupole splitting, and line width were deduced via Mossbauer analysis. The experiments of ZFC and FC magnetizations indicated that no blocking temperature is observed in the temperature interval 2–400?K, which signals the typical ferromagnetic (FM) behavior for the produced nanohexaferrites. A super-spin glass like behavior is noticed at lower temperatures. The experiments AC susceptibility confirmed that the strength of magnetic interactions is enhanced for lower content of V³⁺ (x?=?0.02). For higher amount of V³⁺, the magnetic interactions are weakened. The obtained results are mainly accredited to the substitutions of Fe³⁺ ions by V³⁺ ions.     

Optically transparent polymer composites: A study on the influence of filler/dopant on electromagnetic interference shielding mechanism

Composite materials made of polymers and carbon-based ferromagnetic filler are attractive for electromagnetic interference shielding through a combination of reflection and microwave absorption. It is possible to enhance their shielding properties by controlling electrical conductivity, dielectric, and magnetic properties. In this work, the aforementioned properties are tailored to achieve optically transparent films with microwave absorbing properties. Nanocarbon materials, namely carbon nanotubes, graphene nanoribbons (GNR) and their ferromagnetic nanocomposites with Fe₃O₄ and cobalt in PVA-PEDOT:PSS matrix were made and tested in X-band. The highest shielding effectiveness for PVA films with nanocarbon filler was observed for 0.5 wt% GNR − Fe₃O₄ at 16.36 dB (9.7 GHz) with 79.8% transmittance.