Study of yield stress anomaly of Fe2MnAl single crystal by in situ TEM straining

L2₁-structured Fe₅₉Mn₁₇Al₂₄ shows a yield stress anomaly with a peak yield stress at 700?K. The aim of the work reported here was to determine the dislocation mechanisms involved in this anomalous behaviour by performing in situ straining on Fe₅₉Mn₁₇Al₂₄ single crystals in a transmission electron microscope at 300–900?K. Cross-slip of ?111? dislocations was frequently found to occur between {110} and {211} planes at all temperatures tested. At 300?K, dislocations were four-fold dissociated and the partials moved together under stress. At 700–800?K, partial dislocations with a Burgers vector of a/2?111? dominated the deformation. These partial dislocations moved independently in the ordered matrix in a jerky manner, with rapid motion between long periods of locking. X-ray diffraction measurements showed that the degree of L2₁-order slightly decreased with increasing temperature in the range 300–700?K, and dropped rapidly above 700?K. At 900?K, the material was B2-ordered. The increased yield stress at elevated temperatures is primarily attributed to the uncoupling of a/2?111? partial dislocations trailing shear-induced anti-phase boundaries.     

Frustration driven magnetic states of A-site spinels probed by <Emphasis Type="Italic">μ</Emphasis>SR

The normal A-site spinels MnAl₂O₄, FeAl₂O₄, CoAl₂O₄, as well as related mixed (Mn_0.5Fe_0.5Al₂O₄) and partially inverted (Fe_1.4Al_1.6O₄) spinels have been studied by μSR. The magnetic ions are subject to magnetic frustration by competing interactions. In all materials and at all temperatures the μSR spectra consist of two signals suggesting a bimodal distribution of the fluctuation rates of magnetic moments. A characteristic temperature T _M is found in each compound, representing either a magnetic phase transition into a long-range ordered state (MnAl₂O₄, Fe_1.4Al_1.6O₄) or the formation of a spin liquid phase (FeAl₂O₄, CoAl₂O₄, Mn_0.5Fe_0.5Al₂O₄). The magnetic ground state of MnAl₂O₄ shows coexistence of antiferromagnetic and spin liquid phases. In FeAl₂O₄ and CoAl₂O₄ long-range order is suppressed altogether, the ground state can be characterized as a fast relaxing spin liquid coexisting with a small fraction of paramagnetic spins. The partial replacement of Mn by Fe in Mn_0.5Fe_0.5Al₂O₄ prevents long-range order and leads to a spin liquid state in the low temperature limit. The partial occupancy of B-sites by magnetic ions in Fe_1.4Al_1.6O₄ strengthens the exchange coupling, allowing the formation of long-range magnetic order at a rather high temperature (~100 K). Magnetic phase diagrams are presented demonstrating that for the studied compounds the magnetic properties are determined by the degree of frustration.     

Synthesis and magnetic properties of barium–calcium hexaferrite particles prepared by sol–gel and microemulsion techniques

The preparation of W-type hexaferrite particles with the composition BaCa₂Fe₁₆O₂₇ by microemulsion and a stearic acid sol–gel method with and without surfactant has been investigated at various sintering temperatures. The structural and magnetic characteristics have been studied by X-ray diffraction (XRD), a vibrating sample magnetometer (VSM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetric (DSC) and Fourier transform infrared (FTIR) techniques. The effect of sintering temperature on the properties of BaCa₂Fe₁₆O₂₇ hexaferrites has been studied. The value of saturation magnetization (M_s) depends on types of surfactant used. The sample prepared in the presence of polyoxyethylene (20) sorbitan monooleat (Tween 80) shows low saturation magnetization (M_s=15.10 emu/g), whereas the other sample prepared in the presence of a surfactant cetyltrimethylammonium bromide (CTAB) exhibits high saturation magnetization (M_s=24.60 emu/g) compared to the normal sample.     

In situ TEM observations of dislocation/anti-phase boundary interactions

L2₁-ordered Fe₅₉Mn₁₇Al₂₄ (in at%) single crystals were in situ strained at either 300?K or 700?K in a transmission electron microscope. At 300?K, the strain was accommodated by the glide of four-fold dissociated super-dislocations, whereas, at 700?K, the strain was accommodated by the glide of a/2?111? partials. Dislocation pile-ups occurred at a/4?111? thermal anti-phase boundaries (APBs). Screw super-dislocations frequently cross-slipped when they encountered the thermal APBs, while mixed dislocations tended to be pinned at them. This impediment is attributed to the creation of new APB segments when dislocations pass through the curvy thermal APBs.     

Chemical synthesis and magnetic properties of HCP and FCC nickel nanoparticles

In this study, we successfully synthesized single-phase hexagonal closed packed (HCP) and face-centered cubic (FCC) nickel nanoparticles via reduction of nickel nitrate hexahydrate and nickel acetate tetrahydrate, respectively, in polyethylene glycol-200. Structural information of the as-synthesized nickel nanoparticles are studied by X-ray diffraction (XRD) as a function of the molar concentration of the nickel precursor. XRD results reveal that low concentrations of nickel precursor (0.005?M and below) favor the HCP, while high concentrations favor the mixture of HCP and FCC crystal structures. Particle size of HCP structure is found in the range of ～15?nm via transmission electron microscope analysis. Vibratory sample magnetometer is employed to study its magnetic behavior and the results reveal that FCC crystalline phase shows ferromagnetic nature with high saturation magnetization (M _s?～?39.6?emu?gm^?1) as compared to metastable HCP crystalline structure (M _s?～?2?emu?gm^?1). The surfactants bonding on the surface of nickel nanoparticles are studied.     

Influence of manganese substitution and annealing temperature on the formation,microstructure and magnetic properties of Mn–Zn ferrites

The effects of annealing temperature and manganese substitution on the formation, microstructure and magnetic properties of Mn_xZn_1−xFe₂O₄ (with x varying from 0.3 to 0.9) through a solid-state method have been investigated. The correlation of the microstructure and the grain size with the magnetic properties of Mn–Zn ferrite powders was also reported. X-ray diffraction (XRD), a scanning electron microscope (SEM) and a vibrating sample magnetometer (VSM) were utilized in order to study the effect of variation of manganese substitution and its impact on crystal structure, crystalline size, microstructure and magnetic properties of the ferrite powders formed. The XRD analysis showed that pure single phases of Mn–Zn ferrites were obtained by increasing the annealing temperature to 1200–1300 °C. Increasing the annealing temperature to ?1300 °C led to abnormal grain growth with inter-granular pores and this led to a decrease in the saturation magnetization. Moreover, an increase in the Mn²⁺ ion substitution up to x=0.8 increased the lattice parameter of the formed powders due to the high ionic radii of the Mn²⁺ ion. Mn–Zn ferrites phases were formed and the positions of peaks were shifted by substituting manganese. The average crystalline size was increased by increasing the annealing temperature and decreased by increasing the substitution by manganese up to 0.8. The average crystalline size was in the range 95–137.3 nm. The saturation magnetization of the Mn–Zn-substituted ferrite powders increased continuously with an increase in the Mn concentration up to 0.8 at annealing temperatures of 1200–1300 °C. Further increase of Mn substitution up to 0.9 led to a decrease of saturation magnetization. The saturation magnetization increased from 17.3 emu/g for the Mn_0.3Zn_0.7Fe₂O₄ phase particles produced to 59.08 emu/g for Mn_0.8Mn_0.2Fe₂O₄ particles.     

Study of NiCuZn ferrite powders and films prepared by sol gel method

This paper reports that a series of NiCuZn ferrite powders and films are prepared by using sol-gel method.The effects of raw material composition and the calcinate temperature on magnetic properties of them are investigated.The NiCuZn ferrite powders are prepared by the self-propagating high-temperature synthesis method and subsequently heated at 700 C～1000 C.The results show that NiCuZn ferrite powders with single spinel phase can be formed after heat-treating at 750 C.Powders obtained from Ni 0.4 Cu 0.2 Zn 0.4 Fe 1.9 O 4 gel have better magnetic properties than those from gels with other composition.After heat-treating at 900 C for 3 h,coercivity H c and saturation magnetization M s are 9.7 Oe (1 Oe=80 A/m) and 72.4 emu/g,respectively.Different from the powders,NiCuZn films produced on Si (100) from the Ni 0.4 Cu 0.2 Zn 0.4 Fe 2 O 4 gel formed at room temperature possess high properties.When heat-treating condition is around 600 C for 6 min,samples with low H c and high M s will be obtained.The minimal H c is 16.7 Oe and M s is about 300 emu/cm 3.In comparison with the films prepared through long-time heat treating,the films prepared through short heat-treating time exhibits better soft magnetic properties.     

Magnetic ordering of Fe atoms in icosahedral Al70−x BxPd30−y Fey quasicrystals

Novel icosahedral quasicrystals, in which Fe atoms possess a magnetic moment, have been found in Al_70?x B_xPd_30?y Fe_y compounds with 5<x<10 and 10<y<20. The compounds have ferromagnetic properties, and their Curie temperature ranges between 280 and 340 K, the saturation magnetization σ _s(5 K)≈7.5 emu/g. It follows from Mössbauer spectra that only a fraction of Fe atoms (12 to 15%) are magnetically ordered at 4.2 K, and the mean saturation field 〈H _hf〉=96 kOe. The isomer shift values confirm that the atomic volume of magnetic Fe sites is larger than that of nonmagnetic Fe sites. The magnetic properties of these quasicrystals can be interpreted in terms of large magnetic clusters with a size of 185 to 290 Å. This size correspond to about 4×10⁴ “unit cells,” hence the magnetic state can be described in terms of bulk parameters. The localized magnetic moment of Fe atoms is tentatively ascribed to bonding between Fe and B, similarly to that in the amorphous Fe_～50B_～50 alloy.     

Manganese location and magnetic structure determination of SrMn2—W hexagonal ferrite by means of neutron diffraction

The Mn cation distribution in the seven sublattices of theW-type structure has been determined from high temperature neutron diffraction data of a SrMn₂Fe₁₆O₂₇ powder sample. The Mn²⁺ ions enter the tetrahedral sites of blockS with a preference for site 4e. Mn³⁺ ions in small amounts are found in the octahedral sites, substituting for Fe³⁺, while 0.3 Fe²⁺ has been found in site 6g. The sample has the formula SrMn_2.4Fe_15.6O₂₇. The crystal structure has been described in the non-centrosymmetric Pˉ62c space group, instead of the usual P6₃/mmc one. The magnetic structure, obtained from low temperature (4.2 K) data, is collinear, with the spins parallel to thec axis, according to the Gorter model. The magnetic moments of the sites give an experimental magnetization valueM _s=28.4(1.5) μ_B/f.u., in good agreement with magnetic measurements and the theoretical value.     

Synthesis of (Mg0.476Mn0.448Zn0.007)(Fe1.997Ti0.002)O4 powder and sintered ferrites by high energy ball-milling process

(Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder prepared by high energy ball-milling process were consolidated by microwave and conventional sintering processes. Phases, microstructure and magnetic properties of the ferrites prepared by different processes were investigated. The (Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder could be prepared by high energy ball-milling process of raw Fe₃O₄, MnO₂, ZnO, TiO₂ and MgO powders. Prefired and microwave sintered ferrites could achieve the maximum density (4.86 g/cm⁻³), the average grain size (15 μm) was larger than that (10 μm) prepared by prefired and conventionally sintered ferrites with pure ferrite phase, and the saturation magnetization (66.77 emu/g) was lower than that of prefired and conventionally sintered ferrites (88.25 emu/g), the remanent magnetization (0.7367 emu/g) was higher than that of prefired and conventionally sintered ferrites (0.0731 emu/g). Although the microwave sintering process could increase the density of ferrites, the saturation magnetization of ferrites was decreased and the remanent magnetization of ferrites was also increased.     

Impact of sonochemical synthesis condition on the structural and physical properties of MnFe2O4 spinel ferrite nanoparticles

Herein, we report sonochemical synthesis of MnFe₂O₄ spinel ferrite nanoparticles using UZ SONOPULS HD 2070 Ultrasonic homogenizer (frequency: 20 kHz and power: 70 W). The sonication time and percentage amplitude of ultrasonic power input cause appreciable changes in the structural, cation distribution and physical properties of MnFe₂O₄ nanoparticles. The average crystallite size of synthesized MnFe₂O₄ nanoparticles was increased with increase of sonication time and percentage amplitude of ultrasonic power input. The occupational formula by X-ray photoelectron spectroscopy for prepared spinel ferrite nanoparticles was (Mn_0.29Fe_0.42)[Mn_0.71Fe_1.58]O₄ and (Mn_0.28Fe_0.54) [Mn_0.72Fe_1.46]O₄ at sonication time 20 min and 80 min, respectively. The value of the saturation magnetization was increased from 1.9 emu/g to 52.5 emu/g with increase of sonication time 20 min to 80 min at constant 50% amplitude of ultrasonic power input, whereas, it was increased from 30.2 emu/g to 59.4 emu/g with increase of the percentage amplitude of ultrasonic power input at constant sonication time 60 min. The highest value of dielectric constant (ε′) was 499 at 1 kHz for nanoparticles at sonication time 20 min, whereas, ac conductivity was 368 × 10⁻⁹ S/cm at 1 kHz for spinel ferrite nanoparticles at sonication time 20 min. The demonstrated controllable physical characteristics over sonication time and percentage amplitude of ultrasonic power input are a key step to design spinel ferrite material of desired properties for specific application. The investigation of microwave operating frequency suggest that these prepared spinel ferrite nanoparticles are potential candidate for fabrication of devices at high frequency applications.     

Magnetic State of Quasiordered Fe–Al Alloys Doped with Ga and B: Magnetic Phase Separation and Spin Order

Results of structural, magnetic, and Mössbauer studies of quasi ordered alloys Fe₆₅Al_{35 ? x}M _x (M _x = Ga, B; x = 0, 5 at %) are presented. The magnetic state of examined structurally–single-phase alloys at low temperatures is interpreted from the viewpoint of magnetic phase separation. An explanation is proposed for the observed behavior of magnetic characteristics of Fe₆₅Al₃₅ and Fe₆₅Al₃₀Ga₅ in the framework of the model of two magnetic phases, a ferromagnetic-type one and a spin density wave. The boron-doped alloy Fe₆₅Al₃₀B₅ is shown to demonstrate behavior that is typical of materials with the ferromagnetic type of ordering.     

Controlling the composition and the magnetic properties of hexagonal Co2Z ferrite powders synthesized using two different methods

Polycrystalline Co₂Z hexaferrite (Ba₃Co₂Fe₂₄O₄₁) powders have been prepared via two wet chemical routes: sol gel auto-combustion (SGA) and co-precipitation (CP) methods. The effects of synthesis conditions on the crystal structure, crystallite size, morphology, and magnetic properties were systematically studied. The results revealed that single Co₂Z hexaferrite phase was obtained at relatively low temperature 1250 ^°C for 5 h via the SGA method whereas it was formed at 1300 ^°C for 6 h using the CP pathway. The microstructures of the pure powders appeared as a hexagonal platelet-like structure. The particle size of the pure Co₂Z powders by CP method was higher than as obtained by the SGA method. The soft characteristic loops were obtained for single Co₂Z hexaferrite phase synthesized using both routes. High saturation magnetization (M _s =53.7 emu/g) was achieved for the Co₂Z powder synthesized via the CP method compared with the formed single phase synthesized via the SGA method (M _s =47.0 emu/g). Good dielectric and dynamic magnetic properties were obtained for the Co₂Z powder synthesized via the SGA method in comparison with the sample synthesized via CP method. The results obtained are discussed on the basis of electromagnetic theory.     

Synthesis of α-Fe2O3 nanobelts and nanoflakes by thermal oxidation and study to their magnetic properties

α-Fe₂O₃ nanobelts and nanoflakes have been successfully synthesized by oxidation of iron-coated ITO glass in air. The X-ray diffraction, Raman spectrum and scanning electron microscopy are carried out to characterize the nanobelts and nanoflakes. The formation mechanism has been presented. Significantly, the magnetic investigations show that the magnetic properties are strongly shape-dependent. The magnetization measurements of belt-like and flake-like α-Fe₂O₃ in perpendicular exhibit ferromagnetic feature with the coercivity (H_c) and saturation magnetization (M_s) of 334.5 Oe and 1.35 emu/g, 239.5 Oe and 0.12 emu/g, respectively. For the parallel, belt-like and flake-like α-Fe₂O₃ also exhibit ferromagnetic feature with the H_c and M_s of 205.5 Oe and 1.44 emu/g, 159.6 Oe and 0.15 emu/g, respectively.     

α- and β-Mn precipitates in the spinodal Fe30Ni20Mn25Al25 alloy

Precipitates in the spinodal alloy Fe₃₀Ni₂₀Mn₂₅Al₂₅, produced by drop-casting followed by hot extrusion, have been investigated using transmission electron microscopy. The as-extruded microstructure consisted of alternating, coherent body-centred cubic and B2 (ordered body-centred cubic) rods aligned along ?100? directions with a wavelength of ～60?nm. Upon annealing at 550°C, precipitates were formed having two distinct morphologies: relatively spherical precipitates with the α-Mn crystal structure and more elongated precipitates with the β-Mn crystal structure. Both precipitates existed simultaneously after annealing at this temperature for up to at least 99?h. The compositions of the precipitates and the orientation relationships between the precipitates and the spinodal matrix were determined.     

Study of the magnetic and structural properties of Mn-, Fe-, and Co-doped ZnO powder

A study of the magnetic and structural properties of Zn_1−xM_xO powder (where x=0 or 0.01, and M=Mn, Fe or Co) produced by the proteic sol–gel process was undertaken. The sample crystal structure was analyzed by XRD and magnetic measurements were carried out in a SQUID magnetometer. Of the XRD analysis, all samples had hexagonal wurtzite crystal structure with P63mc space group, and no secondary phase was observed. It is observed of the M(H) measures at 2 K, that the Co- and Mn-doped ZnO displayed saturation magnetizations (M_s) of approximately 2 and 3.2 emu/g, respectively, and no remanence (M_r) was observed, indicating a superparamagnetic behavior in these samples. However, the Fe-doped sample showed a ferromagnetic behavior with M_s∼0.34 emu/g, M_r∼0.05 emu/g, and coercivity (H_c)∼1090 Oe. Already at room temperature, the M(H) measurements reveal a purely paramagnetic behavior for Mn- and Fe-doped ZnO, indicating that the Curie temperature (T_c) is below 300 K. However, a weak superparamagnetic behavior was observed in the Co-doped sample, indicating that T_c>300 K.     

Magnetic properties of nanostructured MnZn ferrite

Mn_0.5Zn_0.5Fe₂O₄ nanoparticles (10-30 nm) have been prepared via mechanochemical processing, using a mixture of two single-phase ferrites, MnFe₂O₄ and ZnFe₂O₄. SQUID measurements (field-cooled magnetization curves and hysteresis loops) were performed to follow the mechanically induced evolution of the MnFe₂O₄/ZnFe₂O₄ mixture submitted to the high-energy milling process. The resulting single MnZn nanoferrite phase was characterized by SQUID (M-H curve), Faraday balance (M-T curve) and transmission electron microscopy. The magnetic characteristics of the mechanosynthesized material were compared with those of bulk Mn_0.5Zn_0.5Fe₂O₄. It was found that the saturation magnetization of nanostructured Mn_0.5Zn_0.5Fe₂O₄ (87.2 emu/g) is lower than that of the bulk Mn_0.5Zn_0.5Fe₂O₄, but, the Néel temperature of the sample (583 K) is higher than that of the bulk Mn_0.5Zn_0.5Fe₂O₄.     

Magnetic characterization of dual phase FeZrB soft magnetic alloy

The magnetic properties and the annealing process of Fe₇₈Zr₇B₁₅ amorphous ribbons are investigated by X-ray diffraction (XRD), differential scanning calorimetry, and vibrating sample magnetometer. The fully amorphous structure of the as-quenched ribbons is confirmed by the XRD pattern. The Curie temperature and the saturation magnetization M_s of the ribbons are 305 °C and 124.3 emu/g, respectively. Annealing at 550 °C can result in an increase in M_s with annealing time due to the increasing crystallized volume fraction of α-Fe phase. The optimized annealing process is established at 550 °C for 20-30 min with maximum M_s of 146.6 emu/g. The morphology of the ribbons annealed at 550 °C is observed by scanning electron microscopy, showing that nanocrystalline α-Fe grains are dispersed in an amorphous matrix.     

Mn2+doping-induced-effects on commensuration and incommensuration of potassium zinc chloride crystal

Undoped and Mn²⁺-doped with different concentrations of potassium zinc chloride (KZC) crystals were grown from aqueous solutions by slow evaporation. The dielectric constant (ε), dielectric losses (tan?δ) and ac conductivity (σ_ac) of the crystals in the ferroelectric-commensurate, incommensurate and normal phases have been measured as a function of frequency, in the range 1–100?kHz, and temperature, in the range 300–580?K. Virgin samples were subjected to measurements of the frequency dependence at selected temperatures and measurements of the temperature dependence was then followed using the same samples. The increase of ε with T could be due to a combination of conductivity, structural variations and discommensuration (DC) formation and pinning as well. The increase of tan?δ with temperature was attributed to relaxation loss in addition to conduction loss, which increases more rapidly with temperature. The ac conductivity (σ_ac) and tan?δ along the polar axis of KZC increased significantly with increasing Mn²⁺ content while ε decreased. σ_ac changed with frequency according to a power law of the form σ_ac?=?f? ^s where 0.15<s<1.27. A linear decrease of ε and tan?δ with increasing the frequency was also found. The obtained results were treated by considering the effect of Mn²⁺-doping on stripples nucleation, DC evolution/annihilation, DC-lattice formation and DCs pinning by the crystal lattice and/or structural defects for virgin and thermally treated samples.     

Synthesis of iron oxide nanoparticles under oxidizing environment and their stabilization in aqueous and non-aqueous media

Synthesis of magnetite (Fe₃O₄) nanoparticles under oxidizing environment by precipitation from aqueous media is not straightforward because Fe²⁺ gets oxidized to Fe³⁺ and thus the ratio of Fe³⁺:Fe²⁺=2:1 is not maintained during the precipitation. A molar ratio of Fe³⁺:Fe²⁺ smaller than 2:1 has been used by many to compensate for the oxidation of Fe²⁺ during the preparation. In this work, we have prepared iron oxide nanoparticles in air environment by the precipitation technique using initial molar ratios Fe³⁺:Fe²⁺?2:1. The phases of the resulting powders have been determined by several techniques. It is found that the particles consist mainly of maghemite with little or no magnetite phase. The particles have been suspended in non-aqueous and aqueous media by coating the particles with a single layer and a bilayer of oleic acid, respectively. The particle sizes, morphology and the magnetic properties of the particles and the ferrofulids prepared from these particles are reported. The average particle sizes obtained from the TEM micrographs are 14, 10 and 9 nm for the water, kerosene and dodecane-based ferrofluids, respectively, indicating a better dispersion in the non-aqueous media. The specific saturation magnetization (σ_s) value of the oleic-acid-coated particles (∼53 emu/g) is found to be lower than that for the uncoated particles (∼63 emu/g). Magnetization σ_s of the dodecane-based ferrofluid is found to be 10.1 emu/g for a volume fraction of particles ?=0.019. Zero coercivity and zero remanance on the magnetization curves indicate that the particles are superparamagnetic (SPM) in nature.