Synthesis and magnetic properties of monodisperse CoFe2O4 nanoparticles coated by SiO2

The monodisperse CoFe₂O₄ nanoparticles were synthesized by a modified chemical coprecipitation method. Coating SiO₂ on the surface of the CoFe₂O₄ nanoparticles was carried out to keep single domain particles non-interacting with cubic magnetocrystalline anisotropy. The Curie temperatures (T_c) of the monodisperse CoFe₂O₄ nanoparticles can be accurately measured because the SiO₂ shells prevented the aggregation and growth of nanoparticles at high temperature. The magnetic properties of the CoFe₂O₄@SiO₂ nanoparticles with core-shell structure in a wide temperature range (300～950?K) were investigated. It is remarkable that the coercive field (H_c) of CoFe₂O₄ nanoparticles increased from about 760?Oe to 1806?Oe after being coated with SiO₂, which increased by 137.6% compared to the uncoated samples at 300?K. The saturation magnetization (M_s) of the CoFe₂O₄@SiO₂ nanoparticles is 34.59?emu/g, which is about 52% of the naked CoFe₂O₄ nanoparticles value (66.51?emu/g) at 300?K. The hysteresis loops of the CoFe₂O₄@SiO₂ nanoparticles showed an orderly magnetic behavior at high temperature, such as the M_s, remanence magnetization (M_r) and H_c decreased as temperature increasing, being equal to zero near T_c. This is a good indication that the CoFe₂O₄@SiO₂ nanoparticles are suitable for a wide variety of technological applications at high temperature.     

Synthesis,characterization, and magnetic properties of α-MnS nanocrystals

MnS nanocrystals have been prepared by a colloidal synthesis route through the reaction of MnCl₂ and S[Si(CH₃)₃]₂ in trioctylphosphineoxide. The nanocrystals were characterized using X-ray diffraction and transmission electron microscopy. The magnetic properties were studied with SQUID magnetometry. X-ray diffraction shows that the nanocrystals are of the thermodynamically stable α-MnS (alabandite) structure. Size control was achieved by changing the concentration of the precursors. Nanocrystal sizes were measured by transmission electron microscopy, and three samples of average diameters 20, 40, and 80 nm were obtained, with narrow size distribution (σ˜9%). The zero field cooled magnetization curves for the 80-, 40-, and 20-nm samples showed a cusp at 116 K, 97 K, and 50 K respectively, all smaller than the antiferromagnetic transition temperature, T_N = 130 K, of bulk α-MnS. Below T_N the magnetization exhibits a paramagnetic behavior unlike typical antiferromagnetic materials. These results indicate that there is a mixture of paramagnetic and antiferromagnetic phases in the nanocrystals. The size dependence shows a general trend of decrease of T_N with reduced particle size, indicating size dependent magnetic ordering.     

Electronic structure and magnetic properties of Ba1-xSrxCoFe11O19 hexaferrites

We have prepared Ba_1-xSr_xCoFe₁₁O₁₉ hexaferrite nanoparticles (NPs) by using a co-precipitation method. The crystal/electronic structures and magnetic properties were then studied. Results revealed that all Ba_1-xSr_xCoFe₁₁O₁₉ NPs with particle sizes of 100–300?nm crystallized in a hexagonal structure. Both the particle shape and the unit-cell parameters are changed when Sr content (x) increases. The analysis of the electronic structure based on the Fe and Co K-edge XAS spectra proved the oxidation states of Fe and Co to be 3?+?and 2?+?, respectively, which are stable versus an x change in Ba_1-xSr_xCoFe₁₁O₁₉. Local-structural studies also revealed the average bond length between Fe and O of 1.89–1.91?Å less changed by Sr doping. Though the electronic structures of Fe and Co were unchanged, the studies about the magnetic property demonstrated a strong dependence of M_s and H_c on Sr doping. While M_s decreases from 46.1?emu/g for x?=?0–34.2?emu/g for x?=?1, H_c tends to increase from 1630?Oe for x?=?0 to ~ 2200?Oe for x?=?0.5, but slightly decreases to 2040?Oe for x?=?1. We think that the addition of the exchange interaction between Fe³⁺ and Co²⁺ ions and the changes of local-geometric structures and microstructures influenced directly M_s and H_c of NPs.     

Room temperature multiferroic properties of Ni-doped Aurivillus phase Bi5Ti3FeO15

Single-phase Aurivillius Bi₅Ti₃Fe_0.5Ni_0.5O₁₅ (BTFN) ceramics were synthesized by the solid-state reaction method. The substitution of Ni for half Fe ions does not introduce magnetic impurity phase but increases magnetic moment more than two orders. The ferroelectric and magnetic Curie temperatures are determined to be 1100 K and 726 K. The room-temperature multiferroic behavior of the BTFN ceramics were demonstrated by the ferroelectric (2P_r=8.5 μC/cm², 2E_c=74 kV/cm) and ferromagnetic (2M_r=27.86 m emu/g, 2H_c=553 Oe) measurements. The ferromagnetism can be ascribed to the aggregation of magnetic ions at the inner octahedra by Ni doping and the spin canting of magnetic-ion-based sublattices via the Dzyaloshinskii-Moriya interaction. The present work suggests the possibility of doped Bi₅Ti₃FeO₁₅ as a potential room-temperature multiferroic.     

Microwave assisted hydrothermal synthesis of (Fe,Co)3O4 nanoparticles in the presence of surfactants and effects of Co/Fe ratio on microstructure and magnetism

Microstructure and magnetic properties of nanoparticles can be tailored by optimising the synthesis procedure and changing chemical composition. In this study, a two-step procedure, i.e., coprecipitation in the presence of PEG 300 followed by microwave assisted (MW) hydrothermal synthesis, was introduced to obtain Co_xFe_3-xO₄ (x?=?0, 0.1 and 0.2) nanoparticles. It was found that with the increase of Co content, particle/crystallite size increased, with significant change of coercivity (H_c). The mixed samples of Co_xFe_3-xO₄ (x?=?0.1 and 0.2) were magnetically harder in comparison with Fe₃O₄. Тhe H_c of Fe₃O₄ was 91?Oe, while for Co_0.10Fe_2.90O₄ and Co_0.20Fe_2.80O₄, H_c was 256?Oe and 1070?Oe, respectively. Saturation magnetisation (M_s) of mixed samples also increased up to 6% compared to Fe₃O₄. A special effort was devoted to study the effects of introducing different surfactants (PEG 300, PEG 4000 or SDS) during the synthesis procedure in order to improve morphological and microstructural properties of CoFe₂O₄ nanoparticles. The influence of surfactants on physical/chemical properties of nanoparticles is discussed.     

Cryogenic magnetic properties and magnetocaloric performance in double perovskite Pr2NiMnO6 and Pr2CoMnO6 compounds

Double perovskite Pr₂NiMnO₆ (PNMO) and Pr₂CoMnO₆ (PCMO) ceramics have been synthesized and their crystal structure, microstructure, as well as cryogenic magnetic properties and magnetocaloric performance have been investigated. The crystal structure of PNMO and PCMO ceramics was determined to be monoclinic with space group of P21/n at room temperature. A ferromagnetic to paramagnetic (FM-PM) phase transition (second-order) occurred in PNMO and PCMO around 208 and 170?K, respectively. Significant reversible magnetocaloric effect (MCE) was found and the values of the maximum magnetic entropy change (-ΔS_M^max) were 3.15 and 3.91?J/kg-K under the magnetic field change (ΔH) of 0–7?T in PNMO and PCMO, respectively. The corresponding relative cooling power (RCP) values were 241.9 and 229.9?J/kg.     

Low temperature sintering of magnetic Ni0.5Co0.5Fe2O4 ceramics prepared from mechanochemically synthesized nanopowders

The effect of mechanochemichally synthesized nanoceramics Ni_0.5Co_0.5Fe₂O₄ (NCF) on the sintering process was studied. After 60?h of mechanochemichal treatment, the amount of formed ferrite phase reaches to about 70?vol%. From 60–100?h 10% increase in volume fraction of synthesized magnetic phase can be observed. Further increment in process time had no remarkable effect on the NCF phase formation. After 60?h, ceramic nanoparticle formation is directly reflected by TEM image and specific surface area (28?m²/g equivalent to 40?nm in diameter). The coercivity (Hc) shows a drastic diminution from 1996 to about 159?Oe by 60?h process time. Further milling treatment has no observable effect on the values of Hc. Additionally, the magnetization saturation (Ms) increases up to ~13?emu/g by 60?h mechanical milling of powders mixture. Thereafter from 60?h to 100?h, the Ms rapidly increased from 13 to 32?Oe. Finally, with continuing mechanochemichal process up to 130?h the Ms slightly diminished (~29?emu/g). Additionally, compared to synthesized powders the higher values of M_S (65?emu/g) and lower values of H_C (140?Oe) for sintered ceramic were detected. The low sintering temperature (1300?°C) for magnetic NCF sample prepared from nanoparticles may be caused by the high activity of nanoceramics.     

Synthesis,structural, electrical and magnetic properties of Bi2Fe4-xGaxO9 (0 ≤ x ≤ 1.6)

We investigate the electrical and magnetic properties of the Bi₂Fe_4-xGa_xO₉ (0?≤?x?≤?1.6) polycrystalline samples synthesized via solid-state reaction technique. Magnetic susceptibility measurements reveal that lightly doped samples (x?<?0.8) undergo successive transitions, from high-temperature paramagnetic to antiferromagnetic phase followed by a low-temperature spin-glass state while the samples with heavy doping (x?>?0.8) demonstrate paramagnetic to spin-glass (SG) transition. The variation of irreversible temperature obtained from zero field cooling (ZFC) and field cooling (FC) susceptibilities versus measured magnetic field and low-temperature magnetic hysteresis (M-H) loops support the existence of spin-glass phase. The dielectric constant (?_r) of Bi₂Fe_4-xGa_xO₉ with Ga-dilution reveals a weak-temperature sensitivity in high-temperature range (300?K?≤?T?≤?550?K), which is advantageous for high temperature capacitor applications and electronic devices.     

Cryogenic magnetic properties in the pyrochlore RE2TiMnO7 (RE = Dy and Ho) compounds

Pyrochlore RE₂TiMnO₇ (RE = Dy and Ho) compounds were synthesized using a ceramic method, and their crystal structures, microstructures, and cryogenic magnetic properties were determined. Dy₂TiMnO₇ and Ho₂TiMnO₇ compounds belong to the Fd-3m space group with a cubic pyrochlore structure and show a considerable reversible magnetocaloric effect (MCE) in the vicinity of the second-order ferromagnetic-paramagnetic (FM-PM) phase transition at the Curie temperature (T_C) of 7.6 and 7.5?K, respectively. The obtained maximal values of magnetic entropy change (-ΔS_M^max) for Dy₂TiMnO₇ and Ho₂TiMnO₇ are 12.50 and 13.95?J/kg K under a magnetic field change (ΔH) of 0–7?T, respectively. Correspondingly, the relative cooling power (RCP) values reach 530.3 and 573.2?J/kg, respectively.     

Evolution of microstructure,optical, and magnetic properties in multiferroic CuFe1-xSnxO2 (x = 0–0.05)

Evolution of the microstructure, optical, and magnetic properties have been investigated systematically in multiferroic CuFe_1-xSn_xO₂ (x?=?0–0.05) ceramics. Substitution of Sn⁴⁺ for Fe³⁺ results in expansion of CuFeO₂ lattice, and reduces the density of the material, but the metal oxidation states are unchanged. Observation of the optical properties shows that the value of the direct optical band gap (E_g) decreases with increasing Sn doping level, and that the CuFe_1-xSn_xO₂ (x?=?0–0.04) series with values >?3.1?eV. Magnetic susceptibility measurements show that Sn⁴⁺ doping decreases the Curie-Weiss temperature, i.e. weakens the strength of the antiferromagnetic interaction between high-spin Fe³⁺ ions, but does not affect the stability of the antiferromagnetic phase, and all samples undergo successive magnetic transitions at about T_N1 =?15?K and T_N2 =?11?K. However, magnetization curves show that changes occur in the magnetic interactions and both ferromagnetism and antiferromagnetism co-exist in the Sn⁴⁺-doped samples. The maximum value of the saturation magnetization of 1.8?emu·g^?1 was observed for the x?=?0.03 sample in a 2.5?kOe field. The changes in the magnetic behavior are closely related to the lattice distortion and charge compensation, which are discussed in detail in this work.     

Structural,magnetic, and magnetotransport properties of LaFe0.5Ni0.5O3

The perovskite-type LaFe_0.5Ni_0.5O₃ belonging to the rhombohedral (space group R-3c) crystal structure has been synthesized for which we have identified a magnetic transition at T₁ =?8?K corresponding to the minimum observed in the derivative of temperature dependent magnetization. A bifurcation in the ZFC and FC curves is observed below T₁ that suggests a frustrated magnetic behavior. The non-zero moment above T₁ hints the possibility of the presence of a high-temperature magnetic transition in the material. The resistivity of LaFe_0.5Ni_0.5O₃ evolves as a function of temperature similar to that of a semiconductor. Mott's variable range hopping governs the conduction mechanism of the material. Presence of various anisotropy terms and inhomogeneous magnetic interactions lead to the presence of antiferromagnetic and ferromagnetic interfaces, which eventually causes a magnetic exchange bias and magnetic hysteresis in resistivity. We have also observed direction dependent magnetoresistance in the material.     

A novel bithiazole oligomer containing C60 and its ferro-complexes: syntheses and magnetic properties

A novel bithiazole oligomer (PCBT) was synthesized from C₆₀ and the diazo salt of 2,2′-diamino-4,4′-bithiazole (DABT). Its ferro-complex (PCBT-Fe²⁺) was prepared from PCBT and FeSO₄ in DMSO solution under a purified nitrogen atmosphere. The magnetic behavior of PCBT and PCBT-Fe²⁺ was measured as a function of magnetic field strength (0-60 kOe) at 5 K and as a function of temperature (5-300 K) at a magnetic field strength of 30 kOe. PCBT-Fe²⁺ complex exhibits a hysteresis cycle at 5 K, the observed coercive field (H_C) and remnant magnetization (M_r) are 690 Oe and 0.12 emu/g, respectively. The results show that PCBT is an anti-ferromagnet and its Fe²⁺-complex is a soft ferromagnet.     

The structure and magnetic properties in Dy3+ doped SmCrO3 ceramics

The Dy³⁺ doped SmCrO₃ polycrystalline ceramics are prepared by solid state method. The structure and magnetic properties are investigated. All samples show orthorhombic structure with space group Pnma. Three magnetic transitions are detected in Dy³⁺ doped SmCrO₃ samples, which arise from the Cr³⁺-Cr³⁺ interactions and the spin reorientation of Cr³⁺, respectively. Both field cooled (FC) and zero field cooled (ZFC) exchange bias (EB) effects are observed in the prepared Sm_1-xDy_xCrO₃ (x = 0 − 0.5) samples below the spin reorientation temperature (T_SR), and the EB field (H_EB) increases dramatically below T_SR. With the increase of the doping level, the H_EB is depressed. Three anomalous variations of magnetic entropy change (ΔS_M) derived from the isothermal magnetization are observed, which are consistent with the magnetic transitions. Compared with the ΔS_M after ZFC processes, the anomalous variations of ΔS_M at ~25 K almost disappear after FC processes due to the enhanced unidirectional anisotropy, and no obvious influence is observed for the other two anomalous variations after FC processes.     

Tuning the magnetic properties of Zr2N MXene by biaxial strain

The effects of strain on the magnetic properties of Zr₂N MXene have been investigated by the first-principles calculations. The ground state of strain-free Zr₂N MXene is intrinsically antiferromagnetic. However, the magnetic state of Zr₂N MXene tends to be ferromagnetic when the applied strain is higher than 4%. The transition of magnetic orderings from antiferromagnetism to ferromagnetism under tensile strains can be understood from the Stoner criterion. Besides, the critical temperature (T_c) is about 470 K for the strain-free Zr₂N MXene, indicating that the antiferromagnetic ordering can be robust and maintained at room temperature. The T_c of antiferromagnetic states begins to decrease once the strain is exerted. As the FM ordering is favored, however, the T_c then increases with the applied strain. Under 8% tensile strain, the T_c comes to room temperature (300 K). In addition, both the orientation of easy-axis and the magnetic anisotropy energy (MAE) of Zr₂N MXene fluctuate with the strain. At the strain of 2%, the MAE reaches the largest (203 μeV per Zr atom), mainly resulting from the spin-orbit interactions between occupied and unoccupied p_x/p_y states of Zr atoms. All these tunable and appealing properties make Zr₂N MXene desirable for spintronic applications.     

Room temperature magnetoelectric coupling study in multiferroic Bi4NdTi3Fe0.7Ni0.3O15 prepared by a multicalcination procedure

Single-phase Bi₄NdTi₃Fe_0.7Ni_0.3O₁₅ polycrystalline samples were synthesized following a multicalcination procedure. The sample exhibited multiferroic property at room temperature, which was demonstrated by the ferroelectric (2P_r=8.52 μC/cm², 2E_c=89 kV/cm at applied electric field 110 kV/cm) and magnetic (2M_r=388 m emu/g, 2H_c=689 Oe at applied magnetic field 1.04 T) hysteresis loops. More importantly, magnetoelectric coupling effect is observed from measurements of electrical properties not only under small but also under large electric signal when an external magnetic field is applied. The present results suggest a new candidate for a room temperature multiferroic material with magnetoelectric coupling effect.     

Structural transformation,dielectric and multiferroic properties of (Gd1-xBax)(Fe1-xTix)O3 ceramics by tuning composition

The structural transformation, ferro-paraelectric and magnetic ordering induced dielectric phase transitions, multiferroic properties, relaxation and conduction mechanisms of (Gd_1-xBa_x)(Fe_1-xTi_x)O₃ ceramics with different compositions (x = 0.0–0.4) have systematically been investigated. The chemical route is adopted for material synthesis. Rietveld refinement reveals the structural transformation from orthorhombic to mixed phase (coexistence of orthorhombic and tetragonal phase) for x > 0.2. The analysis of the FESEM micrographs suggests a decrease in average grain size with the increase in BT in the composition. Dielectric anomalies are explained based on ferroelectric-paraelectric phase transition of BaTiO₃, polarization induced by a spin reorientation, and antiferromagnetic ordering of GdFeO₃. The multiferroic properties of all the samples are studied from P-E, M-H, and magneto-electric plots. The improved magnetic properties in compositions x = 0.1–0.3, make them suitable candidates for spintronic devices. The maximum magneto-electric coupling coefficient of 4.77 mV/cm?Oe in the composition x = 0.3, makes it very much suitable for application in magnetoelectric coupling devices.     

Mechanosynthesis,crystal structure and magnetic characterization of M-type SrFe12O19

M-type strontium hexaferrite was prepared by mechanosynthesis using high-energy ball milling. The influence of milling parameters, hematite excess and annealing temperature on magnetic properties of SrFe₁₂O₁₉ were investigated. Commercial iron and strontium oxides were used as starting materials. It was found that mechanical milling followed by an annealing treatment at low temperature (700 °C) promotes the complete structural transformation to Sr-hexaferrite phase. For samples annealed at temperatures from 700 to 1000 °C, saturation magnetization values (M_s) are more sensitive to annealing temperature than coercivity values (H_c). The maximum M_s of 60 emu/g and H_c of 5.2 kOe were obtained in mixtures of powders milled for 5 h and subsequently annealed at 700 °C. An increase in the annealing temperature produces negligible changes in magnetic saturation and coercivity. An excess of hematite as a second phase produces a slight decrease in the saturation magnetization but leads to a significant increase in coercive field, reaching 6.6 kOe.     

Textured M-type barium hexaferrite Ba(ZnSn)xFe12−2xO19 with c-axis anisotropy and high squareness ratio

A series of oriented M-type barium hexaferrites (BaM) of composition Ba(ZnSn)_xFe_12?2×O₁₉ (where x?=?0, 0.2, 0.4, and 0.6) was fabricated. Pure BaM phase formation for all samples was confirmed by the X-ray diffraction patterns. The scanning electron microscopy images revealed that the grains in the oriented BaM had realigned along the hexagonal crystallographic c-axis, which was in good agreement with the results of magnetic hysteresis loops. Moreover, by treating under appropriate sintering conditions, oriented BaM with a high squareness ratio (M_r/M_s =?0.81) was obtained. Owing to the Zn–Sn substitution, the magnetocrystalline anisotropy field (H_k) decreased dramatically while maintaining the c-axis anisotropy, except for x?=?0.6. A narrow ferromagnetic resonance linewidth ΔH of 667?Oe was measured when x?=?0.4. The oriented BaM are potential candidates for the design of self-biased microwave devices such as circulators and isolators operated at low frequencies.     

Temperature-induced strain mediated magnetization changes in NiFe2O4/BaTiO3 heterostructure

Herein, we report the temperature-induced magnetization changes of NiFe₂O₄ thin film, which is coated over a ferroelectric BaTiO₃ ceramic substrate. The solid-state reaction method was adopted for the preparation of ferroelectric BaTiO₃ (BT) substrate, whereas NiFe₂O₄ (NFO) film was deposited by spin-coating method. Rietveld refinement revealed that BT substrate has a tetragonal (P4mm) crystal system along with a minor orthorhombic phase (Amm2) at room temperature. The GIXRD analysis confirms the phase purity of NFO/BT heterostructure. Polarization hysteresis with respect to electric field (P-E loop) and the temperature-dependent dielectric measurement of BT substrate demonstrate its typical ferroelectric and phase transition behavior, respectively. Magnetization hysteresis loops were recorded for the NFO/BT heterostructure at 150, 240 and 300?K. A significant increase in the remnant magnetization (M_R) and coercive field (H_C) of NFO film are noticed while cooling the heterostructure below 300?K. Variation in the magnetization of NFO film corresponds to the change in the structural phase transition (Amm2 at 240?K and R3c at 150?K) of BT while cooling below RT. The interfacial strain mediated coupling is the primary mechanism attributed to the temperature-induced changes in the magnetization of NFO/BT heterostructure.     

Improved coercivity of W- and M-type composite hexaferrites using two different synthesis routes

Two series of W- and M-type composite hexaferrites were synthetized using the conventional ceramic method in two different synthesis routes. In the first synthesis route, W- and M-type composite hexaferrites were formed by reaction in the pre-sintering stage. In the second synthesis route, W- and M-type composite hexaferrites were obtained by physically mixing W- and M-type hexaferrites in the second ball milling stage. The influence of M-type phase on the structure and magnetic properties of W-type hexaferrites synthesized in both synthesis routes are investigated in detail. For both synthesis routes, X-ray diffraction (XRD) patterns of all samples indicated crystallization of W- and M-type hexaferrites phase and scanning electron microscopy (SEM) measurements revealed hexagonal platelet-like grains. In the pre-sintering stage, the saturation magnetization (4πM_s) of all samples ranged between 3.30 kG and 4.25 kG. The remanence ratio (M_r/M_s) changed from 0.77 to 0.88 with the increase of pre-sintering temperature. The coercivity (H_c) of all samples ranged between 456 Oe and 1846 Oe. The ferromagnetic resonance (FMR) linewidth (ΔH) ranged between 466 Oe and 748 Oe. In the second ball milling stage, 4πM_s and M_r/M_s maintained a relatively stable value of ~4.5 kG and ~0.89, respectively. H_c increased from 519 Oe to 620 Oe with the increase of M-type hexaferrites. ΔH ranged between 317 Oe and 573 Oe. We note that in both synthesis routes only the composite hexaferrites synthesized in the pre-sintering stage have a significantly improved coercivity from 456 Oe to 1846 Oe. This enhancement in coercivity is attributed mainly to the effect of exchange coupling among ferrite grains. In addition, the ΔH decreases with the addition of M-type hexaferrites.