Weak Ferromagnetic Ordering Research Articles

Negative Magnetization and Spin-Phonon Coupling in Faceted Nd2FeCrO6 Double Perovskite

The magnetic feature of a faceted double perovskite Nd2FeCrO6, synthesized by the solid-state reaction method, is investigated. Nd2FeCrO6 exhibits an orthorhombic structure with Fe and Cr ions in 3+ oxidation states. These ions are shifted away from the centre of FeO6/CrO6 octahedral sublattice, indicating distortion. The Isomer shift in the Mössbauer spectrum correlates with the 3+ oxidation state of Fe, and the existence of magnetic fraction leading to magnetic frustration in Nd2FeCrO6 is established. The superexchange Fe3+ -O- Fe3+ and Cr3+ -O- Cr3+ interactions favour antiferromagnetic ordering, while Fe3+ -O- Cr3+ interactions promote weak ferromagnetic ordering. The low-temperature magnetic study traces negative magnetization in FC mode for the applied external fields of 100Oe and 1000Oe, attributed to the paramagnetic effect of Nd3+ ions aligning antiparallel to the ferromagnetic component of the |Fe+Cr| sublattice internal field. Also, the negative magnetization of FC mode is found to be field-dependent. The deviation of the B2g phonon around the magnetic ordering temperature TN~250K provides evidence of spin-phonon coupling. This variation in the B2g phonon is analysed in terms of the full width at half maximum (FWHM), peak position, intensity, and height.

Effects of Ti and Sn Substitutions on Magnetic and Transport Properties of the TiFe2Sn Full Heusler Compound

The synthesis of polycrystalline TiFe2Sn samples by a route including arc melting and spark plasma sintering with Hf, Y, and In substitutions at the Ti and Sn sites is investigated. For a reduced amount of substitution, around 2 at%, the samples are single phase, while for increased amounts, secondary phases segregate. As is characteristic of these compounds, the Fe-Ti atomic disorder generates a weak ferromagnetic ordering, which is also influenced by the type of substitutional atoms and the secondary phases in the samples with a higher Hf content. The Seebeck coefficient values show an increase for Ti0.98Hf0.02Fe2Sn and for samples with an adjusted Sn content, resulting in slightly increased power factor values. These values reach a maximum for Ti0.98Hf0.02Fe2Sn at approximately 300 K and for TiFe2Sn1.05 at approximately 325 K, namely, 2.69 × 10⁻4 Wm−1K−2 and 2.52 × 10⁻4 Wm−1K−2, respectively. The thermal conductivity of all the samples with substitutions increases with respect to the pristine sample. The highest figure of merit value of 0.016 is also obtained for Ti0.98Hf0.02Fe2Sn at 325 K.

Magnetic Structure of Ba2CaCo2Si6O17 with Zigzag Spin Chains of Co2+ in High Spin‐3/2 State

The magnetic properties and crystal chemistry of the novel Ba2CaCo2Si6O17 compound exhibiting one‐dimensional zigzag chains of Co2+ ions (S = 3/2) are investigated using powder X‐ray diffraction (PXRD), scanning lectron microscopy, energy dispersive‐Xray spectroscopy, and temperature and field‐dependent magnetic measurements. The refinement of PXRD data suggests a Cmcm space group in orthorhombic symmetry for Ba2CaCo2Si6O17 compound, consisting of a CoO4 tetrahedral network including 1‐D zigzag Co2+ chains. Microstructural and elemental analysis substantiate the proper stoichiometry of the synthesized material. Magnetic measurements show paramagnetic characteristics at room temperature, followed by a transition from paramagnetic to antiferromagnetic (AFM) ordering near 17 K, superimposed with weak ferromagnetic ordering (WFM) characteristics. Magnetic hysteresis (M–H) at 5 K depicts a sigmoid curve with unsaturated magnetization even at high fields. The negative Weiss temperature value ≈−98 K substantiates the strong antiferromagnetic exchange interaction with a frustration index of ≈5.76. The effective paramagnetic moment is 3.34μB, close to the spin‐only value 3.87μB of Co2+ (S = 3/2). The competing and short‐range FM‐AFM interaction is observed because of the zigzag Co2+ chain in the compound. The onset of WFM is attributed to the zigzag magnetic chain‐induced lattice and spin distortion in Ba2CaCo2Si6O17 compound.

Green-synthesized BiFeO3 nanoparticles for efficient photocatalytic degradation of organic dyes, antibiotic and catalytic reduction of 4-nitrophenol

Green-synthesized nanoparticles have recently emerged as promising catalysts for the removal of toxic dyes from water bodies. In this article, Bismuth ferrite nanoparticles were synthesized by a simple and low-cost method using Couroupita guianensis plant leaf extract. The particles were used as photocatalysts for degrading RhB, MO dyes, and TC antibiotics, as well as for reducing toxic 4-NP to useful 4-AP. The synthesized nanoparticles were characterized using several state-of-the-art tools. The formation of perovskite structure and the presence of biomolecules on the surface of BiFeO3 nanoparticles were confirmed by FTIR analysis. Raman spectrum revealed a rhombohedral structure of BiFeO3 nanoparticles, corroborating the XRD findings. FESEM micrograph demonstrated irregularly shaped agglomerated nanoparticles. The average hydrodynamic diameter was estimated to be 51 nm using the DLS technique. The optical energy bandgap of the bismuth ferrite was estimated using Tauc’s relation for direct bandgap semiconductors. SQUID measurements revealed that these nanoparticles exhibit a weak ferromagnetic ordering. Furthermore, these nanomaterials degraded the cationic and anionic dyes effectively because of the formation of hydroxyl radicals, confirmed by the scavenger test. The photocatalytic degradation pathway of RhB dye was investigated through LC-MS analysis, and the intermediate degradation products were identified. The prepared material also exhibited excellent catalytic reduction efficiency in a short duration of time. This study proclaims that these nanoparticles can be used as potential catalysts for the purification of water bodies.

Understanding the nature of the magnetic coupling in transition metal doped Bi2Se3

In this paper, we employ electronic structure theory to investigate the nature of the exchange coupling in transition metal doped Bi2Se3. We focus on V, Cr, Mn, and Fe, which have been under scrutiny for the realization of the quantum anomalous Hall effect. For simplicity, we model the doping process as happening inside a single layer of Bi within a single quintuple layer inside a three-formula-unit conventional hexagonal cell and consider situations of full coverage or half coverage. Due to the covalent nature of the chemical bond between transition metal atoms and selenium, this simple model is capable of describing the fundamental features of the intraplane exchange coupling, while offering a clearer analysis of the response to structural, magnetic, and electronic perturbations. In agreement with recent literature, our results confirm that the van Vleck mechanism has a very marginal contribution to the exchange coupling. Depending on the filling of the 3d shell and on the details of the electronic structure, several other mechanisms compete and cooperate to induce the magnetic order. For V, double exchange and superexchange cooperate to have the strongest ferromagnetic coupling among the investigated elements, followed by Cr where only superexchange is active. For Mn, superexchange and double exchange compete to create an extremely weak ferromagnetic order. For Fe, a strong antiferromagnetic coupling caused by the double exchange is observed to dominate over the ferromagnetic Ruderman-Kittel-Kasuya-Yosida interaction, but the high sensitivity of this competition to the doping concentration suggests that our conclusion may not hold in the dilute limit. Overall, Fe doping seems to offer the most intriguing competition of exchange mechanisms that can be tuned by adjusting the doping concentration and the details of the host, as, e.g., by replacing Se with Te or Bi with Sb. Published by the American Physical Society 2024

Flat-band and diverse quasi-fermions in Pb10(PO4)6O4

Employing a combination of first-principles calculations and low-energy effective models, we present a comprehensive investigation on the electronic structure of Pb10(PO4)6O4, which exhibits remarkable quasi-one-dimensional topological flat-band around the Fermi level. These flat bands predominantly originate from the px/py orbitals of the oxygen molecules chain at the fully-occupied 4e Wyckoff positions and thus can be well-captured by a minimal four-band tight-binding model. Furthermore, the abundant crystal symmetry inherent in Pb10(PO4)6O4 provides an ideal platform for the emergence of various quasi-fermions characterized by different dispersion, degeneracy, and dimensionality. These include a 0D four-fold degenerate Dirac fermion exhibiting quadratic dispersion, a 1D quadratic/linear nodal-line fermion along symmetric k-paths, a 1D hourglass nodal-line (HNL) fermion associated with the Dirac fermion, and a 2D symmetry-enforced nodal surface located on the kz = π plane. Moreover, when considering the weak ferromagnetic order, Pb10(PO4)6O4 transforms into a rare semi-half-metal, which is characterized by the presence of Dirac fermion and HNL fermion at the Fermi level for a single spin channel exhibiting 100 % spin polarization. Our findings reveal the rarely coexistence of flat bands, diverse topological semimetal states and ferromagnetism in Pb10(PO4)6O4, which may provide valuable insights for further exploring the intriguing interplay between superconductivity and exotic electronic states.

Identifying the Origin of Thermal Modulation of Exchange Bias in MnPS3/Fe3GeTe2 van der Waals Heterostructures.

The exchange bias phenomenon, inherent in exchange-coupled ferromagnetic and antiferromagnetic systems, has intrigued researchers for decades. Van der Waals materials, with their layered structures, offer an ideal platform for exploring exchange bias. However, effectively manipulating exchange bias in van der Waals heterostructures remains challenging. This study investigates the origin of exchange bias in MnPS3/Fe3GeTe2 van der Waals heterostructures, demonstrating a method to modulate nearly 1000% variation in magnitude through simple thermal cycling. Despite the compensated interfacial spin configuration of MnPS3, a substantial 170 mT exchange bias is observed at 5 K, one of the largest observed in van der Waals heterostructures. This significant exchange bias is linked to anomalous weak ferromagnetic ordering in MnPS3 below 40 K. The tunability of exchange bias during thermal cycling is attributed to the amorphization and changes in the van der Waals gap during field cooling. The findings highlight a robust and adjustable exchange bias in van der Waals heterostructures, presenting a straightforward method to enhance other interface-related spintronic phenomena for practical applications. Detailed interface analysis reveals atom migration between layers, forming amorphous regions on either side of the van der Waals gap, emphasizing the importance of precise interface characterization in these heterostructures.

Anomalous Hall effect in 5d/5d SrTaO3/SrIrO3 superlattices driven by ferromagnetism and spin–orbit coupling

The observation of the anomalous Hall effect (AHE) in 5d perovskite oxides has been challenging due to their lack or weak ferromagnetic order, which is necessary for breaking time-reversal symmetry. Here, we present compelling evidence of ferromagnetism and consequent AHE in a series of carefully designed and fabricated 5d/5d SrTaO3/SrIrO3 (STO/SIO) superlattices. The coexistence of Ta5+ and Ta4+ chemical states induces ferromagnetism in the STO layer, while the interfacial magnetic proximity effect further enhances it in the SIO layer, resulting in both ferromagnetism and AHE within the STO/SIO superlattice. Additionally, the strong spin–orbit coupling between Ta and Ir elements positively contributes to enhancing the AHE. This work offers an alternative approach for designing artificial materials with AHE and holds potential for advancing spintronics.

Ferrimagnetic transition, relaxor ferroelectric and optical properties in tungsten bronze Ba6MnNb9O30 ceramics

The structural, magnetic, ferroelectric, dielectric and optical properties of Ba6MnNb9O30 (BMNO) and Ba6Mn0.5Co0.5Nb9O30 (BMCNO) ceramics were investigated. The samples can be indexed by a tetragonal lattice and the space group of P4bm with the presence of a tiny amount of impurity phase Ba1.04NbO3. The sample BMNO undergoes a short-range ferrimagnetic-like transition at 43 K originating from the spin canting of the antiferromagnetic coupling between the adjacent Mn2+ and Mn3+ based sublattices via the Dzyaloshinskii-Moriya interaction, whereas the sample BMCNO exhibits a local short-range antiferromagnetic state with the presence of weak ferromagnetic ordering. Both samples show the relaxor ferroelectric nature. The sample BMCNO possesses a superior energy storage property with a total energy density of 1.58 J/cm3 and a conversion efficiency of 74.5 %, which can be mainly ascribed to the improved densification and the reduced oxygen vacancies. Two different dielectric relaxations are discussed in term of the Vogel-Fulcher law. In addition, all samples have two different absorption edges and the optical band gaps are determined to be about 2.2 eV and 2.8 eV.

Green synthesis, characterization and drug-loaded iron oxide nanoparticles derived from Nerium oleander flower extract as a nanocarrier for in vitro antibacterial efficacy

Application of drug conjugated iron oxide hematite (α-Fe2O3) nanoparticles are of tremendous interest in biomedicine nowadays. Meanwhile, green production of iron oxide nanoparticles is gaining favour due to its sustainability, ease of usage, and biocompatibility. Therefore, this work reports on the use of hexahydrate ferric chloride and nerium oleander flower extract to synthesize nanoscaled hematite (α-Fe2O3) iron oxide particles conjugated with various drugs for antibacterial agents. Diverse morphological, physicochemical, structural, optical, and magnetic characteristics have been characterized using FESEM, EDX, XRD, UV–vis, FTIR, Raman and vibrating sample magnetometer. The synthesis of the polyshaped iron oxide nanoparticles, with average sizes ranging from 47.2 ± 20 nm, was accomplished. Furthermore, temperature-dependent variations in magnetic behavior were observed during calcination. The XRD and Raman spectra revealed hematite (α-Fe2O3) type formation of iron oxide nanoparticles. Only calcinated IO-NPs at high temperatures (700 °C) demonstrated low coercivity and residual magnetism, which revealed weak ferromagnetic ordering; other calcinated samples, including nascent ones, showed incredibly weak ferromagnetic ordering. Besides, the effectiveness of drug-encapsulated iron oxide nanoparticles against bacteria in vitro was examined. It was interesting to observe that gentamycin-coated IO-NPs tended to be more susceptible to S. aureus than E. coli bacteria, but streptomycin-conjugated IO-NPs showed the reverse trend. However, as compared to the nascent sample and the high temperature (700 °C) calcinated sample, both antibiotic-loaded IO-NPs displayed better inhibitory abilities.

Band gap tuning in samarium-modified bismuth titanate ferroelectric via iron doping for photovoltaic applications

The wide band gap of complex oxides is one of the critical challenges restricting their usage in photovoltaic cells. Therefore, examinations of their photoelectric characteristics have increasingly concentrated on materials with a low band gap. This work investigated the effects of iron doping on samarium-modified Bi4Ti3O12-based oxides (BSmT) made via a solid reaction approach to control the band gap of complex oxides. X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-Vis), and X-ray photoelectron spectroscopy (XPS) were used to study the synthesized materials’ structural, optical, and oxygen vacancies. Doping with Fe atoms significantly increased the tunability of the band gap from 3.20 eV to 1.81 eV without violating symmetry. The tuning of bandgap energy in this system was explained based on structural distortion and the formation of oxygen vacancies. This work examined the ferromagnetism and ferroelectricity ordering of the sintered BSmT:F0% and BSmT:F10% samples at room temperature to demonstrate the multiferroelectricity behavior. The findings indicate unsaturated P-E hysteresis loops driven by domain pinning caused by oxygen vacancy formation near domain boundaries. Simultaneously, significant S-type M-H hysteresis loops showed weak ferromagnetic ordering. The findings in this study are encouraging for the development of intrinsic multiferroics for photovoltaic devices for future applications.

Tailoring of structural, morphological, electrical, and magnetic properties of LaMn1-x Fe x O3 ceramics.

This study undertakes a comparative analysis of the structural, morphological, electrical, and magnetic characteristics of Fe-doped LaMnO3 ceramics. The solid-state reaction method was used to prepare Fe-doped LaMnO3 at different concentrations (0.00 ≤ x ≤ 1.00) and has been characterized using X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS), and vibrating sample magnetometry (VSM). The structural transformation from rhombohedral to orthorhombic with Fe-doping is demonstrated by Rietveld's refined XRD patterns. The positive slope in Williamsons-Hall's (W-H) plots confirms the presence of tensile strain with increasing average crystallite size. Quasi-spherical morphology of all the compositions with similar uniformity was confirmed by FESEM images. The chemical distribution of all the elements has been identified by EDS mapping images. Normal dielectric dispersion behaviour of all the samples with NTCR response is confirmed by dielectric and impedance analysis respectively. Increasing lattice volume with Fe-concentration results is increasing E a. The presence of antiferromagnetic ordering, in addition to weak ferromagnetic ordering, is indicated by the unsaturated magnetization even up to a high external field. The decrease in M S and increase in H C values due to Fe-doping reflect the influence of particle size on various magnetic parameters.

An insulating and easy magnetization-plane magnet: The DFT + U and constrained electron population study of 1 T-FeCl2

The 1 T-FeCl2, as a typical and important 2D magnetic system, has attracted much attention. However, there are some debates on its ground-state and magnetic properties. We use the first-principles calculations based on the density functional theory to study the electronic structures and magnetic properties of 1 T-FeCl2. The linear response method was used to determinate the Hubbard U value as 5.467 eV. The electrons population controlling method was adopted to evident the system was fully filled in spin-up channel and dz2 single occupied in spin-down channel insulator. Further analysis of the total energy with the second-order perturbation theory confirmed the xy-plane is the magnetization easy plane, and the z axis is the magnetization hard axis. The weak ferromagnetic order was confirmed by energy-mapping and Monte Carlo simulations. Our results clarified the unique properties of 1 T-FeCl2 and deepen the understanding of the 2D magnetism.

Fe dopant controlled the ferromagnetic, structural, thermal, optical, and electrical characteristics of CdO nanoparticles

This research paper presents a study on (Fe) doped CdO nanoparticles focusing on their structural (XRD), thermal (TGA and DTA), optical (UV–vis), magnetic (VSM and ESR), and electrical characteristics. The synthesis of Cd1-xFexO with (x = 0.0, 0.01, 0.05, 0.10, 0.15, and 0.20) nanoparticles was carried out using the co– precipitation technique. Fe dopants showed a great impact on the structural parameters and the thermal stabilities of obtained nanomaterials. SEM and TEM imaging techniques are employed to investigate the morphology, size, and distribution of the Cd1-xFexO nanoparticles. UV spectroscopy was employed to better understand the electronic transitions and optical characteristics. Furthermore, magnetization measurements from ESR and VSM showed a weak ferromagnetic order with a high Fe concentration. For instance, the Ac electrical conductivity measurements were used to determine the electrical properties, dielectric losses, and charge transport mechanisms between the Fe-dopants and CdO. The structural, thermal, optical, magnetic, and electrical characteristics data are complemented to better understand for Cd1-xFexO materials for employing them in magnetic storage and sensors, photocatalysis, other future applications.

Defect mediated magnetism in Pr/TiO2- reduced graphene oxide nanocomposites

Synthesis and characterization of Pr/TiO2 nanoparticles with its reduced graphene oxide nanocomposites by sol-gel and hydrothermal reaction and its magnetic properties is presented. X ray Diffraction and Raman analysis confirms the anatase phase structure in both nanoparticles and composites with Pr doping hindering the crystallite growth. Field Emission Scanning Electron Microscope and High Resolution Transmission Electron Microscope analysis confirm unambiguously the attachment of TiO2 spherical nanoparticles onto the graphene sheets forming an intimate interfacial contact between the particle and sheets enabling electron charge transfers. Difference in surface areas and surface energies give rise to variable emission intensity of the samples. Pr/TiO2 nanocomposites exhibit an increased bandgap values than their nanoparticles. Both Photoluminescence and Electron Paramagnetic Resonance spectroscopy confirm an increase in defect structure with increase of Pr doping. Magnetization studies show weak ferromagnetic ordering for pure TiO2 nanoparticles. The soft magnetic behaviour of the synthesised materials is useful in fabricating memory based devices.

Purity and properties of spherical shaped Co3O4 nanocrystals prepared by a polyol route using a domestic microwave oven

Obtaining spherical shaped Co3O4 nanocrystals/nanoparticles (considered to be a very promising functional material) at low-cost with chemical purity, reduced crystallite size and useful physicochemical properties has been considered to be challenging. The present work (an attempt in this direction) involves the preparation of spherical shaped Co3O4 nanocrystals from cobalt acetate (dissolved in ethylene glycol) by a polyol synthesis route using a domestic microwave oven along with calcination at a suitable temperature (100/200/300/400 °C used to understand the phase evolution). X-ray diffraction (XRD, including Rietveld), infrared spectral, Raman spectral, energy dispersive X-ray absorption spectral and electron microscopic (SEM/TEM, including SAED) analyses have shown good crystallinity, chemical phase purity, spherical shape, reduced crystallite/particle size and narrow size distribution for the Co3O4 nanocrystal sample calcined at 400 °C. UV-Vis-NIR spectral analysis has shown the existence of multiple bandgap energies (1.51–1.98, 2.44–2.61 and 2.81–3.43 eV). Magnetic measurements have confirmed the antiferromagnetic nature possessing considerable magnetization below and above Neel temperature (TN), which indicates the presence of weak ferromagnetic ordering. The crystallite/particle sizes obtained for the sample calcined at 400 °C through XRD analysis, TEM analysis and using TN value (37.3 K) are 32.0, 33.6 and 31.3 nm, respectively. The method of preparation used is highly yielding, low-cost and more advantageous than many other methods used in the past.

Coexistence of relaxor ferroelectricity and magnetism in multi-element substituted Aurivillius phases Pb1-2xBi1.5+2xNd0.5Nb2-xMnxO9

We synthesized the novel double-layer Aurivillius phases Pb1-2xBi1.5+2xNd0.5Nb2-xMnxO9 using the fluxes-salt approach and systematically studied its crystal structure, morphology, dielectric, ferroelectric, magnetic and optical properties. The single-phase samples for x = 0, 0.1, and 0.3 were identified by X-ray diffraction (XRD). The refinement results demonstrated that all samples adopt an A21am orthorhombic structure and the cell volume becomes smaller with increasing x values. The perovskite B-site is occupied by Nb5+/Mn3+ ions, suggesting the local distributions of Mn3+ ions as Mn–O–Mn and Mn–O–Nb bonds based on the splitting of IR spectra. The grain morphology was observed as an anisotropic plate-like that became larger with increasing x. The broader dielectric peak with a strong frequency dependence reflected the relaxor-ferroelectric behavior and the higher Tc signifies an increased structural distortion. The non-linear fashion with open ferroelectric hysteresis loops demonstrated that ferroelectric ordering occurs at room temperature. The weak ferromagnetic ordering also existed with the introduction of Mn3+ ions. Further investigation into potential multiferroic properties in Aurivillius materials will be greatly aided by these findings.

Intrinsic giant magnetoresistance due to exchange-bias-type effects at the surface of single-crystalline NiS2 nanoflakes.

The coexistence of different properties in the same material often results in exciting physical effects. At low temperatures, the pyrite transition-metal disulphide NiS2 hosts both antiferromagnetic and weak ferromagnetic orders, along with surface metallicity dominating its electronic transport. The interplay between such a complex magnetic structure and surface-dominated conduction in NiS2, however, is still not understood. A possible reason for this limited understanding is that NiS2 has been available primarily in bulk single-crystal form, which makes it difficult to perform studies combining magnetometry and transport measurements with high spatial resolution. Here, NiS2 nanoflakes are produced via mechanical cleaving and exfoliation of NiS2 single crystals and their properties are studied on a local (micron-size) scale. Strongly field-asymmetric magnetotransport features are found at low temperatures, which resemble those of more complex magnetic thin film heterostructures. Using nitrogen vacancy magnetometry, these magnetotransport features are related to exchange-bias-type effects between ferromagnetic and antiferromagnetic regions forming near step edges at the nanoflake surface. Nanoflakes with bigger steps exhibit giant magnetoresistance, which suggests a strong influence of magnetic spin textures at the NiS2 surface on its electronic transport. These findings pave the way for the application of NiS2 nanoflakes in van der Waals heterostructures for low-temperature spintronics and superconducting spintronics.

Aliovalent Calcium Substitution: An Effective Way to Enhance the Magnetoelectric Coupling Properties of Multiferroic BiFeO3

Kinetics of magnetoelectric coupling in bismuth ferrite (BiFeO3) with the substitution of aliovalent calcium ion is investigated. The resulting structural, dielectric, magnetic, and magnetoelectric (ME) coupling properties are explained in terms of chemical pressure originating from the substituent ion. Substitution of calcium finds to reduce the oxygen vacancies. Two important substitution concentrations find to be critical in ferroelectric (FE) and ME coupling properties. At 10 at% calcium, highest remanent polarization is obtained and maximum ME coupling coefficient is observed at 20 at%, which suggests calcium concentration between 10% and 20% can be optimized to obtain good electrical and ME coupling properties of BiFeO3. Maximum ME coupling coefficient at calcium concentration 20 at% is close to numerical calculations reported in the literature. Magnetic properties are also favorably modified by the substitution; a coupled antiferromagnetic and weak ferromagnetic ordering is obtained. Also, a variation in blocking temperature and spin relaxation is observed with respect to the substituent ion concentration.

Antiferromagnet-Ferromagnet Transition in Fe1-xCuxNbO4.

Iron niobates, pure and substituted with copper (Fe1-xCuxNbO4 with x = 0-0.15), were prepared by the solid-state method and characterized by X-ray diffraction, Raman spectroscopy, and magnetic measurements. The results of the structural characterizations revealed the high solubility of Cu ions in the structure and better structural stability compared to the pure sample. The analysis of the magnetic properties showed that the antiferromagnetic-ferromagnetic transition was caused by the insertion of Cu2+ ions into the FeNbO4 structure. The pure FeNbO4 structure presented an antiferromagnetic ordering state, with a Néel temperature of approximately 36.81K. The increase in substitution promoted a change in the magnetic ordering, with the state passing to a weak ferromagnetic order with a transition temperature (Tc) higher than the ambient temperature. The origin of the ferromagnetic ordering could be attributed to the increase in super-exchange interactions between Fe/Cu ions in the Cu2+-O-Fe3+ chains and the formation of bound magnetic polarons in the oxygen vacancies.