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1.
The microstructure and magnetic properties, i.e. the initial magnetic susceptibility, its disaccommodation, core losses and approach to ferromagnetic saturation of the bulk amorphous and partially crystallized Fe61Co10Zr2.5Hf2.5Nb2W2B20 alloy are studied. From X-ray, Mössbauer spectroscopy and electron microscopy studies we have stated that all samples in the as-quenched state are fully amorphous. However, after annealing the samples at 850 K for 30 min the crystalline α-FeCo grains embedded in the amorphous matrix are found. Moreover, from Mössbauer spectra analysis we have stated that the crystalline phase in those samples exhibits the long-range order. The alloy in the as-quenched state shows good thermal stability of the initial magnetic susceptibility. Furthermore, the intensity of the magnetic susceptibility disaccommodation in the rod is lower than in the ribbon. It is due to low quenching rate during the rod preparation which involves the reduction of free volumes. From the analysis of the isochronal disaccommodation curves, assuming the Gaussian distribution of relaxation times, we have found that activation energies of the elementary processes responsible for this phenomenon range from 1.2 to 1.4 eV. After the annealing of the samples the initial susceptibility slightly enhances and disaccommodation drastically decreases. From high-field magnetization studies we have learned that the size of structural defects depends on the quenching rate (the shape of the samples) and changes after annealing.  相似文献   

2.
Microstructure by X-ray diffraction and Mössbauer spectroscopy, and isothermal magnetic entropy changes in the bulk amorphous Fe60Co5Zr8Mo5W2B20 alloy in the as-quenched state and after annealing at 720 K for 15 min are studied. The as-cast and heat treated alloy is paramagnetic at room temperature. The quadrupole splitting distribution is unimodal after annealing indicating the more homogenous structure in comparison with that for the as-cast alloy. Curie temperature slightly increases after annealing from 265±2 K in the as-quenched state to 272±2 K and the alloy exhibits the second order magnetic phase transition. The maximum of isothermal magnetic entropy changes appears at the Curie points and is equal to 0.30 and 0.42 J/(kg·K) for the alloy in the as-quenched state and after annealing, respectively. In the paramagnetic region the material behaves as a Curie-Weiss paramagnet.  相似文献   

3.
Samples of Mn-oxide nanoparticles dispersed in an amorphous SiO2 matrix with manganese concentration 0.7 and 3 at% have been synthesized by a sol-gel method. Transmission electron microscopy analysis has shown that the samples contain agglomerates of amorphous silica particles 10-20 nm in size. In silica matrix two types of Mn-rich particles are dispersed, smaller nanoparticles with dimensions between 3 and 10 nm, and larger crystalline areas consisting of aggregates of the smaller nanoparticles. High-temperature magnetic susceptibility study reveals that dominant magnetic phase at higher temperatures is λ-MnO2. At temperatures below TC=43 K strong ferrimagnetism originating from the minor Mn3O4 phase masks the relatively weak magnetism of λ-MnO2 with antiferromagnetic interactions. Magnetic field dependence of the maximum in the zero-field-cooled magnetization for both the samples in the vicinity of 40 K, and a frequency shift of the real component of the ac magnetic susceptibility in the sample with 3 at% Mn suggest that the magnetic moments of the smaller Mn3O4 nanoparticles with dimensions below 10 nm are exposed to thermally activated blocking process just below the Curie temperature TC. Appearance of a maximum in the zero-field-cooled magnetization for both the samples below 10 K indicates possible spin glass freezing of the magnetic moments at low temperatures which might occur in the geometrically frustrated Mn sublattice of the λ-MnO2 crystal structure.  相似文献   

4.
Antiferromagnetic phase transition in two vanadium garnets AgCa2Co2V3O12 and AgCa2Ni2V3O12 has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature TN=6.39 K for AgCa2Co2V3O12 and 7.21 K for AgCa2Ni2V3O12, respectively. The magnetic susceptibilities exhibit broad maximum, and these TN correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χmax). The magnetic entropy changes from zero to 20 K per mol Co2+ and Ni2+ ions are 5.31 J K−1 mol-Co2+-ion−1 and 6.85 J K−1 mol-Ni2+-ion−1, indicating S=1/2 for Co2+ ion and S=1 for Ni2+ ion. The magnetic susceptibility of AgCa2Ni2V3O12 shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μeff=3.23 μB Ni2+-ion−1 and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa2Co2V3O12. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state 2E(t26e) and the first excited state is spin quartet state 4T1(t25e2) which locates extremely close to the ground state. The low spin state S=1/2 for Co2+ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.  相似文献   

5.
Magnetic susceptibility χ measurements in the range from 2 to 300 K were carried out on samples of the Cu2FeSnSe4 and Cu2MnSnSe4 compounds. It was found that Cu2FeSnSe4 was antiferromagnetic showing ideal Curie-Weiss behavior with a Néel temperature TN of about 19 K and Curie-Weiss temperature θ=−200 K, while for Cu2MnSnSe4 the behavior was spin-glass with a freezing temperature Tf of about 22 K and Curie-Weiss temperature θ=−25 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory.  相似文献   

6.
Amorphous Co2MnSi thin film was deposited using radio-frequency sputtering. The amorphous film crystallized into a single-phased L21 structure at 500 °C, which was highly disordered. The structure was meta-stable as the crystallized film decomposed upon further heating. Increasing the annealing temperature to 600 °C precipitated fcc Co together with Co2MnSi. Magnetic measurements showed that the as-deposited amorphous film was paramagnetic, exhibiting a spin glass state below 44 K. The phase transition at 500 °C produced a ferromagnetic Co2MnSi thin film whose saturation magnetic moment was considerably lower than reported values due to the disordered structure of the crystallized film.  相似文献   

7.
The paper presents measurements of magnetic permeability, magnetic after-effects, magnetostriction, DSC and XPS for the Fe80Nb6B14 amorphous alloys preliminary annealed for 1 h at temperatures ranging from 300 to 770 K. It was shown that annealing out of free volume and internal stresses causes a decrease of magnetostriction coefficient and leads to the formation of the energetically stable relaxed amorphous state. The XPS spectra show local fluctuation of boron density. This effect was attributed to the formation of small iron clusters—the characteristic feature for the relaxed amorphous phase.  相似文献   

8.
A mixture of elemental Ga and Se with the nominal composition Ga40Se60 was submitted to the Mechanical alloying technique and their structural, thermal and optical properties were followed by X-ray diffraction, differential thermal analysis, photoacoustic spectroscopy, UV-VIS‐NIR absorbance spectroscopy and Raman spectroscopy techniques. After 10 h of milling the X-ray pattern showed monoclinic Ga2Se3 phase nucleation, which is in the nanometric form, and also a minority amorphous phase. The DSC results showed exothermic reactions between 430 and 720 K attributed to amorphous-crystalline phase transition and structural relaxation of Ga2Se3 phase. Based on this a small amount of the as-milled sample was annealed at 723 K. Its XRD pattern showed evidences of grain growth, reduction of the interfacial component, as well as, disappearance of the amorphous phase. The annealing process induced thermal diffusivity increasing, while the optical band gap energy and Raman profile remained practically unchanged.  相似文献   

9.
We examined the correlation between thickness of an epitaxial VO2 phase grown on a TiO2 (0 0 1) substrate by the excimer-laser-assisted metal organic deposition (ELAMOD) process and the metal-insulator transition (MIT) property of it. The abrupt and hysteretic MIT was observed for the epitaxial films (thickness: t ≥ 6 nm), and the epitaxial film (t ≤ 4 nm) showed semiconductor behavior. When an amorphous VOx layer was prepared on the ultrathin epitaxial phase (t ≤ 4 nm) by the ELAMOD, a non-hysteretic MIT was successfully observed. The non-hysteretic MIT was found to be owing to roughened interface between the epitaxial phase and the amorphous phase, where there would be a number of structural defects.  相似文献   

10.
TiO2 nanoparticles have been prepared by simple chemical precipitation method and annealed at different temperatures. The as-prepared TiO2 are amorphous, and they transform into anatase phase on annealing at 450 °C, and rutile phase on annealing at 900 °C. The X-ray diffraction results showed that TiO2 nanoparticles with grain size in the range of 21–24 nm for anatase phase and 69–74 nm for rutile phase have been obtained. FESEM images show the formation of TiO2 nanoparticles with small size in structure. The FTIR and Raman spectra exhibited peaks corresponding to the anatase and rutile structure phases of TiO2. Optical absorption studies reveal that the absorption edge shifts towards longer wavelength (red shift) with increase of annealing temperature.  相似文献   

11.
Polycrystalline samples of ternary rare-earth germanides R2Co3Ge5 (R=La, Ce and Pr) have been prepared and investigated by means of magnetic susceptibility, isothermal magnetization, electrical resistivity and specific heat measurements. All these compounds crystallize in orthorhombic U2Co3Si5 structure (space group Ibam). No evidence of magnetic or superconducting transition is observed in any of these compounds down to 2 K. The unit cell volume of Ce2Co3Ge5 deviates from the expected lanthanide contraction, indicating non trivalent state of Ce ions in this compound. The reduced value of effective moment (μeff≈0.95 μB) compared to that expected for trivalent Ce ions further supports valence-fluctuating nature of Ce in Ce2Co3Ge5. The observed temperature dependence of magnetic susceptibility is consistent with the ionic interconfiguration fluctuation (ICF) model. Although no sharp anomaly due to a phase transition is seen, a broad Schottky-type anomaly is observed in the magnetic part of specific heat of Pr2Co3Ge5. An analysis of Cmag data suggests a singlet ground state in Pr2Co3Ge5 separated from the singlet first excited state by 22 K and a doublet second excited state at 73 K.  相似文献   

12.
Thermo-gravimetric, differential scanning calorimetry and comprehensive 57Fe Mössbauer spectroscopy studies of amorphous and crystalline ferromagnetic glass coated (Co0.2Fe0.8)72.5Si12.5B15 micro-wires have been recorded. The Curie temperature of the amorphous phase is TC(amorp) ∼730 K. The analysis of the Mössbauer spectra reveals that below 623 K the easy axis of the magnetization is axial-along the wires, and that a tangential or/and radial orientation occurs at higher temperatures. At 770 K, in the first 4 hours the Mössbauer spectrum exhibits a pure paramagnetic doublet. Crystallization and decomposition to predominantly α-Fe(Si) and Fe2B occurs either by raising the temperature above 835 K or isothermally in time at lower temperatures. Annealing for a day at 770 K, leads to crystallization. In the crystalline material the magnetic moments have a complete random orientation. After cooling back to ambient temperature, both α-Fe(Si) and Fe2B in the glass coated wire show pure axial magnetic orientation like in the original amorphous state. The observed spin reorientations are associated with changes in the stress induced by the glass coating.  相似文献   

13.
We have investigated the magnetic and transport properties of a new ternary intermetallic compound Pr2Pd3Si5 which forms in U2Co3Si5-type orthorhombic structure (space group Ibam). At low field (0.01 T) magnetic susceptibility exhibits an abrupt increase below 7 K and peaks at 5 K, revealing a magnetic phase transition. The onset of magnetic order is also confirmed by well defined anomalies in the specific heat and electrical resistivity data. Apart from the sharp λ-type anomaly, magnetic part of specific heat also shows a broad Schottky-type hump due to crystal field effect. Magnetoresistance data as a function of temperature exhibits a pronounced peak in paramagnetic state which could be interpreted in terms of crystal field effect and short-range ferromagnetic correlations.  相似文献   

14.
Thin films of Ti-B-N with different N contents were deposited on Si(1 0 0) at room temperature by reactive unbalanced close-field dc-magnetron sputtering using three Ti targets and one TiB2 target in an Ar-N2 gas mixture. The effect of N content on bonding structure, microstructure, phase configuration, surface roughness and mechanical properties have been investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), cross-sectional scanning electron microscopy (SEM), plan-view and cross-sectional high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and microindentation measurements. It was found that the N content significantly affected phase segregation and microstructure. The nitrogen-free TiB0.65 films showed an amorphous compound consisting of Ti and TiB2 (Ti-TiB2). After adding about 28 at.% N, Ti was preferentially bonded to N to form TiN, accompanying with formation of small amounts of TiB and BN bonds. At this stage they combined TiB2 to form a two-phase nanocomposite with microstructures comprising of nanocrystalline (nc-) TiN phase in nitrogen-containing amorphous (a-) TiB2 matrix. Addition of more N promoted formation of BN bonding at cost of TiB2, which resulted in formation of nanocomposite nc-TiN/a-(TiB2, BN) thin films. A small grain less than 8 nm in size was found at low N content, and the grain size increased with increasing N content. A low microhardness value of about 20 GPa was obtained in the amorphous Ti-TiB2 compound, and a maximum microhardness value of about 50 GPa was achieved in nc-TiN/a-TiB2. A decrease of microhardness took place after formation of BN (i.e. amorphous matrix composed by both TiB2 and BN) with further increasing N content, and a hardness value of about 35 GPa was followed at high N contents. The surface roughness strongly depended on the phase configuration. The higher the mole fraction of nanocrystalline TiN phase, the rougher the surface became.  相似文献   

15.
Nanocomposite made of 10 wt% of Co2.4Al0.6O4 particles dispersed in an amorphous SiO2 matrix has been synthesized by a sol-gel method. X-ray diffraction, transmission electron microscopy and magnetic measurements have been used to characterize the properties of nanocomposite. Most of the particles are well crystallized and have an average diameter below 100 nm. Smaller particles with size below 10 nm have also been observed. A large value of the effective magnetic moment per Co2+ ion of 5.08 μB and negative and the low Curie-Weiss paramagnetic temperature Θ∼−6 K, obtained from the high-temperature susceptibility data, indicate a possible mixing of Co2+ and Co3+ ions between tetrahedral and octahedral sites of the spinel crystal lattice. The measurements of static and dynamic magnetic susceptibilities have shown that Co2.4Al0.6O4 particles in SiO2 matrix display a spin glass behavior at low temperatures.  相似文献   

16.
Because the optically induced second harmonic generation (SHG) is prevented by symmetry in a centrosymmetric material, one needs to form noncentrosymmetric processes in order to observe the SHG. However, one of the efficient ways to enhance the noncentrosymmetricity of a material is to dope it with an appropriate impurity and amount. We grow Cu-doped CdI2 layered nanocrystal structures from the mixture of CdI2 and CuI using the standard Bridgman-Stockbarger method and investigate the nano-confined effects by studying the second-order optical effect via the measurements of SHG. The second-order susceptibility for the nanocrystals is calculated and the values at liquid helium temperature range from 0.38 to 0.83 pm V−1 for the thicknesses of 10-0.8 nm respectively. The size dependence demonstrates the nano-sized quantum-confined effect with a clear increase in the SHG with decreasing the thickness of the nanocrystal or crystal temperature. Since the local electron-phonon anharmonicity is described by third-order rank tensors in disordered systems, the SHG is very similar to that one introduced for the third-order optical susceptibility. It has been confirmed by observing the large photoluminescent yield of the pure crystals. The Raman scattering spectra taken for thin nanocrystals confirm the phonon modes originating from interlayer phonons crucially responsible for the observed effects. The obtained results show that the Cu-doped CdI2 layered nanocrystals are promising materials for applications in optoelectronic nano-devices.  相似文献   

17.
Organic-inorganic composite SiO2-Al2O3 films have been prepared by sol-gel using methacryloxypropyl trimethoxysilane and aluminum sec-butoxide as the precursors. By introduction of organic groups into the inorganic backbone, the smooth and crack-free films could be readily achieved by a one-step dip-coating process, with the thickness up to 4.6 μm after being post-baked at 200 °C for 2 h. The films presented in an amorphous phase with an acceptable chemical homogeneity. Owing to the formation of chelate rings, the gel films showed a strong photosensitivity to ultraviolet light at 325 nm. The uniform fine patterns of SiO2-Al2O3 thick films could be well defined by ultraviolet light imprinting simply using a mask. These performances of SiO2-Al2O3 films indicate the potential for integrated optical systems.  相似文献   

18.
Second-order optical susceptibilities were established in the optically poled erbium doped tellurite glasses near the melting temperature. The non-linear optical susceptibility was formed by bicolor coherent optical treatment performed by two coherent laser beams originated from 50 ps Nd-YAG laser (λ = 1.32 μm) exciting the high pressure hydrogen laser cell emitting at 1907 nm. The non-centrosymmetric grating of the medium was created by coherent superposition of the fundamental laser illumination at 1907 nm and the doubled frequency one at 953.5 nm. The maximally all-optically poled SHG occurs for 2% doped Er2O3 (in weighting units) TeO2-GeO2-PbO glass. It was found that the photoinduced SHG demonstrates a saturation during the photo-treatment of 9-10 min using the two beams polarized at angle about 45° between them. During the coherent bicolor optical treatment it was achieved the value of second-order susceptibility up to 3.6 pm/V at 1907 nm. The optimal ratio between the fundamental beam with power density about 1.1 GW/cm2 and writing doubled frequency seeding beam about 0.015 GW/cm2 corresponds to the maximal of photoinduced SHG. For glasses with lower concentration of Er2O3, the relaxation of the second-order optical susceptibility is substantially longer and achieves SHG value that corresponds to 80% of the maximal ones. It is necessary to emphasize that efficient optically-poled grating exists only within the narrow temperature range near the glassing temperature. Possible physical mechanisms of the phenomenon observed are discussed. Generally the used glasses possess better parameters than early investigated germinate glasses.  相似文献   

19.
In this work the Mn5Si3 and Mn5SiB2 phases were produced via arc melting and heat treatment at 1000 °C for 50 h under argon. A detailed microstructure characterization indicated the formation of single-phase Mn5Si3 and near single-phase Mn5SiB2 microstructures. The magnetic behavior of the Mn5Si3 phase was investigated and the results are in agreement with previous data from the literature, which indicates the existence of two anti-ferromagnetic structures for temperatures below 98 K. The Mn5SiB2 phase shows a ferromagnetic behavior presenting a saturation magnetization Ms of about 5.35×105 A/m (0.67 T) at room temperature and an estimated Curie temperature between 470 and 490 K. In addition, AC susceptibility data indicates no evidence of any other magnetic ordering in 4-300 K temperature range. The magnetization values are smaller than that calculated using the magnetic moment from previous literature NMR results. This result suggests a probable ferrimagnetic arrangement of the Mn moments.  相似文献   

20.
Thermal stability and crystallization of the Fe81B12Si4C2 alloy were investigated in the temperature range 25-700 °C by the XRD and Mössbauer analysis. It was shown that on heating the as-prepared amorphous Fe81B12Si4C2 alloy undergoes thermal stabilization through a series of structural transformations involving the process of stress-relieving (temperature range 200-400 °C), followed by a loss of ferromagnetic properties (Curie temperature at 420 °C) and finally crystallization (temperature range 450-530 °C). The process of crystallization begins by formation of two crystal phases: Fe3B and subsequently Fe2B, as well as a solid solution α-Fe(Si). With increase in annealing temperature, the completely crystallized alloy involved only two phases, Fe2B and solid solution α-Fe(Si).XRD patterns established a difference in phase composition and size of the formed crystallites during crystallization depending on the side (fishy or shiny) of the ribbon. The first nuclei of the phase α-Fe3Si were found on the shiny side by XRD after heat treatment even at 200 °C but the same phase on the fishy side of ribbon was noticed after heat treatment at 450 °C. The largest difference between the contact and free surface was found for the Fe2B phase crystallized by heating at 700 °C, showing the largest size of crystallites of about 130 nm at 700 °C on the free (shiny) surface.  相似文献   

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