Unusual magnetic hysteresis behavior of oxide spinel MnCo2O4

Magnetic hysteresis behavior of the oxide spinel MnCo₂O₄ has been studied at different temperatures below its T_c≈184 K. Normal hysteresis behavior is observed down to 130 K whereas below this temperature the initial magnetization curve, at higher magnetic fields, lies outside the main loop. No related anomaly is observed in the temperature variation of magnetization or coercivity. However, the anisotropy field overcomes the coercivity below 130 K. The unusual magnetic hysteresis behavior of MnCo₂O₄, at low temperatures, may be associated with irreversible domain wall movements due to the rearrangement of the valence electrons.     

Nanocrystallite interface effect and exchange bias phenomenology in Cr2O3 and NiO nanoparticles

Nanocrystalline Cr₂O₃ and NiO are prepared using high-energy ball milling. Average sizes of the particles obtained from Scanning Electron Microscopy and crystallite sizes obtained from X-ray diffraction are larger for Cr₂O₃ than NiO particles. At low temperature, large high-field magnetization and small coercivity lead to a weak exchange bias for Cr₂O₃, whereas small high-field magnetization and large coercivity lead to a considerable exchange bias for NiO. The training effect is observed for NiO at 4 K which could be described with a recursive formula constructed in the framework of the spin configurational relaxation model. The results suggest that the pinning mechanism at the interface between the antiferromagnetic and the weak ferromagnetic component ascribed to uncompensated spins leads to the exchange bias effect.     

Annealing effect of NiO/Co90Fe10 thin films: From bilayer to nanocomposite

Exchange-biased bilayers are widely used in the pinned layers of spintronic devices. While magnetic field annealing (MFA) was routinely engaged during the fabrication of these devices, the annealing effect of NiO/CoFe bilayers is not yet reported. In this paper, the transition from NiO/Co₉₀Fe₁₀ bilayer to nanocomposite single layer was observed through rapid thermal annealing at different temperatures under magnetic field. The as-deposited and low-temperature (<623 K) annealed samples had rock salt (NiO) and face center cubic (Co₉₀Fe₁₀) structures. On the other hand, annealing at 623 K and 673 K resulted in nanocomposite single layers composed of oxides (matrix) and alloys (precipitate), due to grain boundary oxidization and strong interdiffusion in the NiO/CoFe and CoFe/SiO₂ interfaces. The structural transition was accompanied by the reduction of grain sizes, re-ordering of crystallites, incensement of roughness, and reduction of Ni²⁺. When measured at room temperature, the bilayers exhibited soft magnetism with small room-temperature coercivity. The nanocomposite layers exhibited an enhanced coercivity due to the changes in the magnetization reversal mechanism by pinning from the oxides. At 10 K, the increased antiferromagnetic anisotropy in the NiO resulted in enhanced coercivity and exchange bias in the bilayers. The nanocomposites exhibited weaker exchange bias compared with the bilayers due to frustrated interfacial spins. This investigation on how the magnetic properties of exchange-biased bilayers are influenced by magnetic RTA provides insights into controlling the magnetization reversal properties of thin films.     

包钴型r-Fe2O3磁粉各向异性研究

包钴型γ-Fe₂O₃磁粉的矫顽力可比原γ-Fe₂O₃磁粉提高8000—32000A/m。木文研究探讨了两种包钴型γ-Fe₂O₃磁粉(包钴γ-Fe₂O₃和包钴包亚铁γ-Fe₂O₃)的单轴各向异性的起源和矫顽力增大的机制。包钴γ-Fe₂O₃磁粉矫顽力 关键词：     

Sol-gel preparation and characterization of CoFe2O4-SiO2 nanocomposites

Magnetic nanocomposites formed by cobalt ferrite particles dispersed in a silica matrix were prepared by a sol-gel process. The effects of the thermal treatment temperature and the salt concentration on the structural and magnetic properties of the composites were investigated. By controlling these parameters, CoFe₂O₄/SiO₂ nanocomposites with different crystallite size and magnetic properties were obtained. By increasing the annealing temperature and salt concentration, composites with a progressive increase in the coercive field and of the density of magnetization were produced. In particular, a nanocomposite, with a Fe/Si molar concentration of 21%, obtained by drying the gel at 150 °C and further annealing at 800 °C, has a coercivity of 2000 Oe, which is more than twice higher than the coercivity of bulk cobalt ferrite.     

Co-doped In2O3 thin films: Room temperature ferromagnets

Laser-ablated Co-doped In₂O₃ thin films were fabricated under various growth conditions on R-cut Al₂O₃ and MgO substrates. All Co:In₂O₃ films are well-crystallized, single phase, and room temperature ferromagnetic. Co atoms were well substituted for In atoms, and their distribution is greatly uniform over the whole thickness of the films. Films grown at 550 °C showed the largest magnetic moment of about 0.5 μ_B/Co, while films grown at higher temperatures have magnetic moments of one order smaller. The observed ferromagnetism above room temperature in Co:In₂O₃ thin films has confirmed that doping few percent of magnetic elements such as Co into In₂O₃ could result in a promising magnetic material.     

Effect of crystallinity of Co layer on perpendicular exchange bias in Au-capped ultrathin Co film on Cr2O3(0 0 0 1) thin film

The effect of the crystalline quality of ultrathin Co films on perpendicular exchange bias (PEB) has been investigated using a Au/Co/Au/α-Cr₂O₃ thin film grown on a Ag-buffered Si(1 1 1) substrate. Our investigation is based on the effect of the Au spacer layer on the crystalline quality of the Co layer and the resultant changes in PEB. An α-Cr₂O₃(0 0 0 1)layer is fabricated by the thermal oxidization of a Cr(1 1 0) thin film. The structural properties of the α-Cr₂O₃(0 0 0 1) layer including the cross-sectional structure, lattice parameters, and valence state have been investigated. The fabricated α-Cr₂O₃(0 0 0 1) layer contains twin domains and has slightly smaller lattice parametersthan those of bulk-Cr₂O₃. The valence state of the Cr₂O₃(0 0 0 1) layer is similar to that of bulk Cr₂O₃. The ultrathin Co film directly grown on the α-Cr₂O₃(0 0 0 1) deposited by an e-beam evaporator is polycrystalline. The insertion of a Au spacer layer with a thickness below 0.5 nm improves the crystalline quality of Co, probably resulting in hcp-Co(0 0 0 1). Perpendicular magnetic anisotropy (PMA) appears below the Néel temperature of Cr₂O₃ for all the investigated films. Although the PMA appears independently of the crystallinequality of Co, PEB is affected by the crystalline quality of Co. For the polycrystalline Co film, PEB is low, however, a high PEB is observed for the Co films whose in-plane atom arrangement is identical to that of Cr³⁺ in Cr₂O₃(0 0 0 1). The results are qualitatively discussed on the basis of the direct exchange coupling between Cr and Co at the interface as the dominant coupling mechanism.     

Magnetic behavior of nanocrystalline CoFe2O4

Magnetic nanoparticles of CoFe₂O₄ have been synthesized under an applied magnetic field through a co-precipitation method followed by thermal treatments at different temperatures, producing nanoparticles of varying size. The magnetic behavior of these nanoparticles was investigated. As-grown nanoparticles demonstrate superparamagnetism above the blocking temperature, which is dependent on the particle size. One of the nanoparticles demonstrated a constricted magnetic hysteresis loop with no or small coercivity and remanence at low magnetic field. However, the loop opens up at high magnetic field. This magnetic behavior is attributed to the preferred Co ions and vacancies arrangements when the CoFe₂O₄ nanoparticles were synthesized under an applied magnetic field. Furthermore, this magnetic property is strongly dependent on the high temperature heat treatments that produce Co ions and vacancies disorder.     

Spin-glass-like behaviour of the mixed spinel systems MnxZn1-xCr2O4 and Mn0.75Mg0.25(Cr1-yVy)2O4

NMR and susceptibility measurements have been made on a randomly mixed insulating ferrimagnet and antiferromagnet, Mn_xZn_1-xCr₂O₄. The thermoremanence and the induced unidirectional anisotropy were observed for concentrations lower than x = 0.80, after field cooling. The compound Mn_0.75Mg_0.25Cr₂O₄ shows similar behaviour. When the latter is doped with V³⁺ at the B sites, its magnetic anisotropy increases strongly, but the change in the unidirectional anisotropy is smooth.     

Exchange bias in thin-film (Co/Pt)3/Cr2O3 multilayers

We have fabricated exchange-biased Co/Pt layers ((0.3 nm/1.5 nm)×3) on (0 0 1)-oriented Cr₂O₃ thin films. The multilayered films showed extremely smooth surfaces and interfaces with root mean square roughness of ≈0.3 nm for 10 μm×10 μm area. The Cr₂O₃ films display sufficient insulation with a relative low leakage current (1.17×10⁻² A/cm² at 380 MV/m) at room temperature which allowed us to apply electric field as high as 77 MV/m. We find that the sign of the exchange bias and the shape of the hysteresis loops of the out-of-plane magnetized Co/Pt layers can be delicately controlled by adjusting the magnetic field cooling process through the Néel temperature of Cr₂O₃. No clear evidence of the effect of electric field and the electric field cooling was detected on the exchange bias for fields as high as 77 MV/m. We place the upper bound of the shift in exchange bias field due to electric field cooling to be 5 Oe at 250 K.     

包钴型γ-Fe2O3磁粉矫顽力的研究

包钴型γ-Fe₂O₃磁粉分为包钴γ-Fe₂O₃(简记为Co-γ-Fe₂O₃)和包钴包亚铁γ-Fe₂O₃(简记为CoFe-γ-Fe₂O₃)两种,它们的矫顽力可比γ-Fe₂O₃磁粉的提高100至400Oe左右,本工作对这两种磁粉矫顽力增大的原因作了探讨,认为它们矫顽力增大的机制不同:CO-γ-Fe₂O₃矫顽力增大是由于表面包覆一层Co(OH)₂使表面各向异性增大,而CoFe-γ-Fe₂O₃则是由于表面包覆的是钴铁氧体,γ-Fe₂O₃与钴铁氧体之间发生耦合作用,使矫顽力增大。     

Investigation of exchange bias in 0.1MFe2O4/0.9BiFeO3 (M=Co, Cu, Ni) nanocomposite

The 0.1MFe₂O₄/0.9BiFeO₃ (M=Co, Cu, Ni) nanocomposite samples were synthesized by the sol-gel method. Phase composition analysis was carried out, which showed that these bulk samples were composed of a ferrimagnetic MFe₂O₄ (M=Co, Cu, Ni) and a ferroelectric antiferromagnet (FEAF) BiFeO₃ phases, respectively. The magnetic properties of all the samples were investigated by measuring their magnetization as a function of temperature and magnetic field. These results indicated that the magnetic hysteresis loops of 0.1CuFe₂O₄/0.9BiFeO₃ sample sintered in air atmosphere at 550 °C for 3 h exhibited a negative shift and an enhanced coercivity at low temperature ascribed to strong exchange coupling between the BiFeO₃ and CuFe₂O₄ grains. However, there were no magnetic hysteresis loops in both the 0.1CoFe₂O₄/0.9BiFeO₃ sample and the 0.1NiFe₂O₄/0.9BiFeO₃ sample. In view of these results, we tend to think the CuFe₂O₄/BiFeO₃ nanocomposite system may be a useful multifunctional material.     

In situ synthesis and characterization of LiNi0.5La0.08Fe1.92O4-polyaniline core-shell nanocomposites

Core-shell-structured LiNi_0.5La_0.08Fe_1.92O₄-polyaniline (PANI) nanocomposites with magnetic behavior were synthesized by in situ polymerization of aniline in the presence of LiNi_0.5La_0.08Fe_1.92O₄ nanoparticles. The structure, morphology and magnetic properties of samples were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), UV-vis absorption, transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) technique. The results of spectroanalysis indicated that there was interaction between PANI chains and ferrite particles. TEM study showed that LiNi_0.5La_0.08Fe_1.92O₄-PANI nanocomposites presented a core-shell structure with a magnetic core of 30-50 nm diameter and an amorphous shell of 10-20 nm thickness. The nanocomposites under applied magnetic field exhibited the hysteresis loops of the ferromagnetic nature. The saturation magnetization and coercivity of nanocomposites decreased with decreasing content of LiNi_0.5La_0.08Fe_1.92O₄. The polymerization mechanism and bonding interaction in the nanocomposites have been discussed.     

Synthesis and magnetic properties of hard/soft SrFe12O19/Ni0.7Zn0.3Fe2O4 nanocomposite magnets

Magnetic nanocomposite SrFe₁₂O₁₉/Ni_0.7Zn_0.3Fe₂O₄ powders with different weight fractions of the Ni_0.7Zn_0.3Fe₂O₄ soft ferrite were synthesized by a combination of the sol–gel self-propagation and glyoxilate precursor methods. The results of magnetic measurements revealed the higher Mr/Ms ratio for the nanocomposites than that for the single phase SrFe₁₂O₁₉ which proves the existence of the intergrain exchange coupling between hard and soft magnetic phases with the exchange spring behavior. The highest Mr/Ms ratio of 0.63 was obtained in the composite consisting of 80 wt% of SrFe₁₂O₁₉ and 20 wt% Ni_0.7Zn_0.3Fe₂O₄. The microstructural studies of this sample exhibited the average dimensions of hard and soft phases about 20 nm and 15 nm, respectively which are small enough for strong exchange coupling according to the theoretical studies. The variations of the reduced remanence (Mr/Ms) with increasing the weight fraction of the soft phase could be also explained by the role of the exchange and dipolar interactions in tuning the magnetic properties of the nanocomposites.     

Fabrication and magnetic properties of Ni0.50.5Zn0.5Fe2O4 nanofibres by electrospinning

NiZn ferrite/polyvinylpyrrolidone composite fibres were prepared by sol–gel assisted electrospinning.Ni0.5Zn0.5Fe2O4 nanofibres with a pure cubic spinel structure were obtained subsequently by calcination of the composite fibres at high temperatures.This paper investigates the thermal decomposition process,structures and morphologies of the electrospun composite fibres and the calcined Ni0.5Zn0.5Fe2O4 nanofibres at different temperatures by thermogravimetric and differential thermal analysis,x-ray diffraction,Fourier transform infrared spectroscopy and field emission scanning electron microscopy.The magnetic behaviour of the resultant nanofibres was studied by a vibrating sample magnetometer.It is found that the grain sizes of the nanofibres increase significantly and the nanofibre morphology gradually transforms from a porous structure to a necklace-like nanostructure with the increase of calcination temperature.The Ni0.5Zn0.5Fe2O4 nanofibres obtained at 1000 C for 2 h are characterized by a necklace-like morphology and diameters of 100–200 nm.The saturation magnetization of the random Ni0.5Zn0.5Fe2O4 nanofibres increases from 46.5 to 90.2 emu/g when the calcination temperature increases from 450 to 1000 C.The coercivity reaches a maximum value of 11.0 kA/m at a calcination temperature of 600 C.Due to the shape anisotropy,the aligned Ni0.5Zn0.5Fe2O4 nanofibres exhibit an obvious magnetic anisotropy and the ease magnetizing direction is parallel to the nanofibre axis.     

Enhancement of magnetic anisotropy in mechanically attrited Cr2O3 nanoparticles

Cr₂O₃ nanoparticles of sizes from 24 to 12 nm were synthesized by mechanical grinding. Magnetic hysteresis loops were observed in the temperature range 5-300 K. Zero-field magnetization measurements showed two peaks, at low temperature in the range 36-52 K and at high temperature in the range 255-290 K. They were found to shift to higher temperatures as the particle size was reduced. This was ascribed due to the enhancement of the effective anisotropy constant with a decrease in particle size. The exchange bias was found to increase as the particle size became smaller. This is believed to arise due to an increase in uncompensated spins as a result of large surface area created.     

Silane treatment of Fe3O4 and its effect on the magnetic and wear properties of Fe3O4/epoxy nanocomposites

In this study, the effect of silane treatment of Fe₃O₄ on the magnetic and wear properties of Fe₃O₄/epoxy nanocomposites was investigated. Fe₃O₄ nanopowders were prepared by coprecipitation of iron(II) chloride tetrahydrate with iron(III) chloride hexahydrate, and the surfaces of Fe₃O₄ were modified with 3-aminopropyltriethoxysilane. The magnetic properties of the powders were measured on unmodified and surface-modified Fe₃O₄/epoxy nanocomposites using SQUID magnetometer. Wear tests were performed on unmodified and surface-modified Fe₃O₄/epoxy nanocomposites under the same conditions (sliding speed: 0.18 m/s, load: 20 N).The results showed that the saturation magnetization (M_s) of surface-modified Fe₃O₄/epoxy nanocomposites was approximately 110% greater than that of unmodified Fe₃O₄/epoxy nanocomposites. This showed that the specific wear rate of surface-modified Fe₃O₄/epoxy nanocomposites was lower than that of unmodified Fe₃O₄/epoxy nanocomposites. The decrease in wear rate and the increase in magnetic properties of surface-modified Fe₃O₄/epoxy nanocomposites occurred due to the improved dispersion of Fe₃O₄ into the epoxy matrix.     

Magnetoelectric characterization of xNi0.75Co0.25Fe2O4-(1−x)BiFeO3 nanocomposites

Magnetoelectric (ME) nanocomposites containing Ni_0.75Co_0.25Fe₂O₄-BiFeO₃ phases were prepared by citrate sol-gel process. X-ray diffraction (XRD) analysis showed phase formation of xNi_0.75Co_0.25Fe₂O₄-(1−x)BiFeO₃ (x=0.1, 0.2, 0.3 and 0.4) composites on heating at 700 °C. Transmission electron microscopy revealed the formation of powders of nano order size and the crystal size was found to vary from 30 to 85 nm. Dispersion in dielectric constant (ε) and dielectric loss (tan δ) in the low-frequency range have been observed. It is seen that nanocomposites exhibit strong magnetic properties and a large ME effect. On increasing Ni_0.75Co_0.25Fe₂O₄ contents in the nanocomposites, the saturation magnetization (M_S) and coercivity (H_C) increased after annealing at 700 °C. The large ME output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The large value of ME output can be attributed to small grain size of ferrite phase of nanocomposite being prepared by citrate precursor process.     

Magneto-optical study of slanted Co nanowires embedded in CeO2/SrTiO3(0 0 1)

Sequential pulsed laser deposition of CoO and CeO₂ at 650 °C under vacuum leads to the formation of a slanted Co nanowires assembly embedded in CeO₂/SrTiO₃(0 0 1) epilayers. High temperature magneto-optical Faraday measurements were performed, which revealed a Faraday ellipticity of 1.3° at a wavelength of 450 nm for 300 nm thick samples and which allowed to access the magnetic properties. From the analysis of the coercivity dependence on temperature, it is shown that the magnetic anisotropy of the slanted Co nanowires is dominated by shape anisotropy and that their magnetization reversal is localized.     

Synthesis of Cr2O3/TNTs nanocomposite and its photocatalytic hydrogen generation under visible light irradiation

A novel Cr₂O₃/TNTs nanocomposite was prepared by loaded suitable amount of amorphous Cr₂O₃ on titanate nanotubes (TNTs) via hydrothermal reaction and impregnation process. XRD, SEM and TEM results demonstrated that the amorphous Cr₂O₃ nanoparticles were homogeneously dispersed on the surface of TNTs. The diffuse reflectance UV–visible absorption spectra exhibited that the spectral response of TNTs was extended to visible light region by coupled with Cr₂O₃. The 2.5Cr₂O₃/TNTs nanocomposite showed the highest activity of hydrogen generation by photocatalytic water-splitting under visible light irradiation (λ > 400 nm). The high activity of H₂ evolution for Cr₂O₃/TNTs nanocomposites was associated with the donor level in the forbidden band of TNTs semiconductor provided by dopant Cr³⁺ and a probably photocatalytic mechanism was proposed.