Magnetization Study in CuCr2O4 Spinel Oxide

We report magnetization and ac susceptibility as functions of the temperature and frequency for CuCr₂O₄ spinel oxide from 2 K to 300 K. Bulk CuCr₂O₄ crystallizes at room temperature in a tetragonal distorted spinel and above 865 K its structure is cubic spinel; distortion is caused by Jahn–Teller Cu²⁺ ions. The magnetization data of the polycrystalline sample indicates ferrimagnetic order below T _C =122 K. Magnetization isotherm resulted in an average magnetic moment of 0.08 μ_B/f.u. at 2 K values lowest to expected value. This discrepancy can be explained assuming a triangular configuration of spins Cr³⁺ and Cu²⁺. The ferromagnetic phase of the sample does not show glassy behavior. Its magnetic response can be explained simply from the domain wall dynamics of otherwise homogeneous ferrimagnet.     

A Simple Method for Measuring Intrinsic Blocking Temperature in Superparamagnetic Nanomaterials

Temperature-dependent magnetic flux density (B) data, clearly exhibiting a transition temperature called intrinsic blocking temperature for some metallic samples in zero field cooled-warmed (ZFC-W) curves without employing an external magnetic field, has been obtained by a simple method. The reasons of the increase and decrease in the measured B-field at low temperature in zero magnetic-field were discussed. Co, CoPt₃ and Co/Au, CoPt₃/Au core-shell nanoparticles, prepared by the reverse-micelle microemulsion method, were used as test materials. The blocking temperature was measured at a cusp of the measured magnetic field, B (produced by the sample), versus the temperature curve during warming up of the sample from a very low temperature (≤15 K) to room temperature. All the samples showed a blocking temperature at 45, 50, 40, and 42 K, respectively, for Co, CoPt₃, Co/Au, and CoPt₃/Au nanoparticles. A completely intrinsic behavior of the sample’s magnetic moment was revealed by our method since no applied external field was used, yielding a truly spontaneous magnetization behavior.     

Magnetic Properties and MCE in Heusler-Type Glass-Coated Microwires

We prepared Heusler-type Ni–Mn–Ga glass-coated microwires with total diameters, D, from 16 until 65 μm consisted of a metallic nucleus with diameters, d, between 10 and 31 μm and surrounded by the glass coating by the Taylor–Ulitovsky technique and measured the magnetocaloric effect, MCE, by the recently introduced precise method allowing the detection of a change in the temperature, ΔT, with an accuracy of below 10^?3 K. As-prepared microwires did not show ferromagnetic ordering at least near room temperature. Annealing above 773 K even for few minutes results in drastic change of magnetic properties: annealed sample show magnetization versus temperature dependence typical for ferromagnetic behavior with the Curie temperature about 315 K. Analysis of the X-ray diffraction allows us to identify presence of cubic phase in as-prepared state with the space group Fm3m and some amount of the tetragonal phase with space group I4/mmm. We measured directly the MCE, ΔT, in annealed samples. Before glass removal, we observed ΔT≈0.06 K for sample 1 and 0.08 K for sample 2. After glass removal of sample 1, ΔT increased until 0.22 K. Observed MCE is associated with magnetic (paramagnetic–ferromagnetic) and probably structural (austenite–martensite) phase transitions. Temperatures of the peak values of MCE were found to be of ～318 K and ～309 K, respectively Use of the glass-coating fabrication technique allows fabrication of composite thin wires from the brittle Ni–Mn–Ga alloy that cannot be cold-drawn to create fibers by conventional methods.     

Magnetic Measurements on Single Crystal of Double Perovskite Ca2FeMoO6

We report the magnetism properties on single crystal of Ca₂FeMoO₆ double perovskite prepared by a floating zone technique. This high quality material has been studied by X-ray powder diffraction (XRD), and magnetic measurements. The field dependence of the magnetization at 5 K, 100 K, and 300 K suggest a ferrimagnetic behavior with a saturation magnetic moment of approximately 2.1 μ_B, 2.0 μ_B, and 1.4 μ_B per formula unit, respectively. From dc susceptibility, we observed two magnetic transitions at T _C1=380 K (paramagnetic–ferrimagnetic) and T _C1=336 K (orthorhombic–monoclinic phase).     

Cooperative Jahn–Teller transition in Li[Li x Mn2–x ]O4: a muon-spin rotaion/relaxation (μSR) view

The magnetic nature of the spinel antiferromagnet Li[Li _x Mn_2–x ]O₄ with x = 0?0.15 has been studied with muon-spin-rotation and relaxation (μ⁺SR) spectroscopy. Both weak transverse field and zero field μ⁺SR measurements indicate that the whole sample enters into a static disordered magnetic phase below T _N for all the samples measured; T _N = 61 K for LiMn₂O₄, whereas 27–23 K for the x = 0.05?0.15 samples. It was also clarified that both the field distribution width and the field fluctuation rate show a clear change at the Jahn–Teller transition temperature (T _JT = 280 K) for LiMn₂O₄, and a short-range cooperative JT distortion appears below 280 K even for Li[Li_0.15Mn_1.85]O₄.     

Structural and Magnetic Properties of the New La2SrCo2FeO9 Triple Perovskite

In this work, we report synthesis and characterization of the new La₂SrCo₂FeO₉ triple perovskite material. The samples were produced by the solid-state reaction method. The analyses of the XRD patterns were made by Rietveld refinement through the GSAS code. The results reveal that the material crystallizes in an orthorhombic complex perovskite, space group Immm (#71) with lattice parameters a=5.4314(3) ?, b=5.4583(3) ? and c=7.7018(2) ?. SEM micrographs evidence a strongly diffused granular morphology with mean grain size of 2 μm and the EDX spectra show that the chemical composition of samples are in good agreement with the nominal values of the stoichiometric formula. ⁵⁷Fe Mössbauer spectroscopy recorded at 300 K reveals two sites in concordance with the X-ray diffraction measurements, and its valence state is +3, as determined from the isomer shift found. At 4.2 K magnetic ordering with canting of the Fe moments is found. Measurements of the magnetization as a function of temperature permitted us to determine the ferromagnetic characteristic of material with an effective magnetic moment of 9.7μ _B.     

High Field ESR Spectroscopy on GdO1−x F x FeAs

In the present work we have studied polycrystalline samples of the GdO_1?x F _x FeAs superconductor by means of high field electron spin resonance (HF-ESR) spectroscopy. A set of the samples with different levels of fluorine doping was measured in the frequency range from 10 GHz to 400 GHz in magnetic fields up to 15 T. Surprising results have been obtained on the GdO_0.85F_0.15FeAs sample with the superconducting transition temperature T _c =20 K. Gd ESR gives clear indications of the enhancement of (quasi)-static magnetic correlations which set in below ～80 K and continue to develop even in the superconducting state. This suggests an occurrence of an intimate interplay between magnetism and superconductivity in the FeAs planes which evolves in GdO_1?x F _x FeAs compound upon the fluorine doping.     

Magnetic resonance in superparamagnetic zinc ferrite

In the present work, we have synthesized zinc ferrite nanoparticles by nitrate method. Presence of almost zero value of coercivity and remanence in the hysteresis of these samples shows the superparamagnetic nature at room temperature. Electron paramagnetic resonance spectroscopy performed on these samples in the temperature range 120–300 K indicates the systematic variation of the line-shapes of the spectra with temperature. Both g-value and peak-to-peak linewidth decrease with increase in temperature. The variation of g-values and peak-to-peak linewidth with temperature has been fitted with existing models and we observed different values of activation energies of the spins for both the samples.     

La0.7Sr0.3MnO3 Nanoparticles Synthesized via the (Pechini) Polymeric Precursor Method

In this paper, we report the preparation of La_0.7Sr_0.3MnO₃ colossal magnetoresistance nanoparticles by means of the polymeric precursor method, at a temperature of 650 °C. Rietveld refinement of the X-ray powder diffraction spectra shows that the chemically-synthesized manganite is single-phase with the space group R3C. By using the peak broadening technique and Scherrer’s formula, a grain size of ～15 nm was estimated. The ferromagnetic to paramagnetic transition is sharp with a Curie temperature of T _C ～360 K. In spite of the low annealing temperature, the insulator-metal-like transition temperature (T _p ) is close to T _C . Transport measurements on the prepared samples show a magnetoresistance change of ～10 % at room temperature in a field of 2 T. This high-field MR is probably due to grain-boundary effects.     

Structural, Optical and Multiferroic Properties of BiFeO3 Nanoparticles Synthesized by Soft Chemical Route

BiFeO₃ nanoparticles were prepared via a soft chemical method using citric acid and tartaric acid routes followed by calcination at low temperature. Structural characterization showed remarkably different conditions for pure phase formation from both routes. The tartaric acid route was effective in obtaining pure phase BiFeO₃ nanoparticles while citric acid route required leaching in dilute nitric acid to remove impurity phases. Further optical, magnetic, and dielectric characterizations of pure phase BiFeO₃ nanoparticles obtained by tartaric acid route were done. X-ray diffraction and Raman spectroscopy confirmed the distorted rhombohedral structure of BiFeO₃ nanoparticles. The average crystallite size of BiFeO₃ nanoparticles was found to vary in the range 30–50 nm. Fourier Transformed Infrared spectra of BiFeO₃ samples calcined at different temperatures were studied in order to analyze various bond formations in the samples. UV-Visible diffuse absorption showed that BFO nanoparticles strongly absorb visible light in the wavelength region of 400–580 nm with absorption cut-off wavelength of 571 nm. The band gap of BiFeO₃ nanoparticles was found to be 2.17 eV as calculated from absorption coefficient spectra. Magnetic measurement showed saturated hysteresis loop indicating ferromagnetic behavior of BiFeO₃ nanoparticles at room temperature. Temperature dependent dielectric constant showed anomaly well below the antiferromagnetic Néel temperature indicating decrease in antiferromagnetic Néel temperature of BiFeO₃ nanoparticles.     

Spin-Reorientation and M?ssbauer Study of Orthoferrite TbFe0.75Mn0.25O3

The crystallographic and magnetic properties of TbFe_0.75Mn_0.25O₃ powder were characterized by x-ray diffraction (XRD), Mössbauer spectroscopy, and vibrating-sample magnetometry (VSM). The crystal structure was found to be orthorhombic (space group Pbnm) with lattice constants a ₀=5.317 Å, b ₀=5.604 Å, and c ₀=7.598 Å, respectively. Mössbauer spectra of TbFe_0.75Mn_0.25O₃ have been taken at various temperatures ranging from 4.2 to 550 K. For Mössbauer spectra, we have fitted the spectra to a model based on a random distribution of Fe and Mn ions on the octahedral sites. The magnetic hyperfine fields of the four pattern (B ₀,B ₁,B ₂,B ₃) at 4.2 K are found to be H _hf=553, 544, 535, and 527 kOe, respectively. Isomer shift at room temperature is 0.25?C0.26 mm/s, which means that the valence state of Fe ions is ferric (Fe³⁺). A sudden change in both the magnitude of magnetic hyperfine field and its slope between 150 and 220 K suggests that magnetic phase transition related to the spin ordering takes place abruptly. The Néel temperature was determined to be T _N=550±5 K. The inflection points arising from a spin reorientation in the temperature dependence of the magnetic moment is observed. Its spin-reorientation transition is 70 K lower than that of 250 K for pure TbFeO₃.     

Structural and Magnetic Properties of Cobalt(II) Complexes of Triphenylphosphine

This work is related to the structural and magnetic properties of the Co(PPh₃)Cl₂ metal complex for different sintering times. The structural and physical properties of the complex have been studied by using SEM, EDX, and magnetization techniques. SEM results clearly demonstrate that the grains of the complex are very well connected to each other and tightly packed. Magnetization measurements with respect to magnetic field and temperature show a paramagnetic behavior above 20 K for the all samples. Furthermore, the susceptibility (χ) as a function of temperature indicates a paramagnetic-antiferromagnetic phase transition with a negative value of the Curie–Weiss temperature, θ. In addition, increasing sintering time appears to also enhance the effective magnetic moment, μ _eff.     

Spin Coherence Time T 2 in Metallic P-doped Si at Very Low Temperature

We report NMR measurements of transverse relaxation rate 1/T ₂ of the ³¹P nuclear spins in metallic Si:P (concentration of dopant P, n=18×10¹⁸ and 56×10¹⁸ cm^?3) at temperatures between 45 mK and 5 K in a magnetic field of 7 T. Above 1.4 K, 1/T ₂ is constant independent of temperature as well as concentration and is determined by magnetic dipolar interaction between the ³¹P and ²⁹Si nuclear spins. As temperature decreases below 1.4 K, 1/T ₂ increases over the dipolar-determined value by an order of magnitude and levels off around 0.6 K. The concentration dependence of 1/T ₂ at low temperatures suggests that 1/T ₂ below 1.4 K is determined by the coupling between the ³¹P nuclear spins. We understand 1/T ₂ at low temperatures originates from the RKKY interaction. We explain the temperature dependence of 1/T ₂ between 0.6 K and 1.4 K by the motional-narrowing expression of 1/T ₂ with a temperature-dependent correlation time of the fluctuating local field due to the RKKY interaction.     

Structure and energy storage properties of Ti vacancies charge compensated Re 2O3-doped SrTiO3 (Re = Pr, Nd, Gd) ceramics

By introducing Ti vacancies in advance for charge compensation, single cubic perveskite rare-earth doped strontium titanate ceramics with the formula Re _0.02Sr_0.98Ti_0.995O₃ (Re–STO, Re = Pr, Nd, Gd) were prepared via solid-state reaction method. All Re–STO ceramics sintered at 1,410 °C in air for 3 h showed a similar dense microstructure with the grain size lower than 10 μm. The room temperature dielectric constant ε _r of Re–STO ceramics was higher than 3,000 (@ 1 kHz) and changed <8 % within the applied bias electric field. Especially, the room temperature dielectric loss tanδ of Re–STO ceramics was lower than 0.02 (@ 1 kHz), and the average breakdown strength E _b surpassed 14 kV/mm, demonstrating that Re–STO ceramics could be very promising for high-voltage capacitor applications. The temperature stability of the ε _r of Re–STO ceramics was evaluated in a temperature range of ?50 to 200 °C. Meanwhile, the energy storage density of Re–STO ceramics was investigated as a function of the applied bias electric field.     

Microwave-assisted synthesis and magnetic studies of cobalt oxide nanoparticles

An efficient microwave-assisted route has been used to synthesize nanoparticles of cobalt oxide. The particles were well characterized by transmission electron microscopy (TEM) which showed that the average diameter of the particles is around 6 nm. X-ray diffraction (XRD) studies further confirmed the formation of the spinel Co₃O₄. Purity of the products was detected by Fourier transform infrared spectroscopy (FTIR) combined with thermal gravimetric analysis (TG/DTG). The magnetic measurements revealed a small hysteresis loop at room temperature indicating a weak ferromagnetic nature of the synthesized Co₃O₄ nanoparticles. The magnetic moment of the particles was measured to be 4.27 μ_eff.     

Heat capacity, thermopower and magnetoresistance effects in multiferroic La0.5Bi0.5Mn0.5Fe0.5O3

We have carried out systematic investigations in perovskite multiferroic La_0.5Bi_0.5Mn_0.5Fe_0.5O₃ by means of X-ray diffraction, magnetisation, electrical resistivity, thermoelectric and heat capacity measurements. The magnetic behaviour of this composition is rather complex, though the magnetisation curve seems to be like a weak ferromagnetic material. However, there is no clear evidence of λ-anomaly in the heat capacity data down to 2 K, yet this behaviour corroborate the trends of semiconducting silicon below room temperature. The sensitivity of magnetic behaviour to the iron-manganese ratio is also demonstrated. In presence of an external field of 7 T, it exhibits a magnetoresistance of ?5 % at 130 K. The thermoelectric value increases linearly with decreasing temperature, and at room temperature the value is +85 μV/K, which is associated with the p-type polaronic conductivity.     

Study of Influence of Annealing Time on Some Physical Properties of ZnO:Cu Nanorods Grown by a Simple Chemical Bath Deposition Method

A simple chemical bath deposition method was used to prepare high density ZnO nanorods on ZnO seeded Si substrates. Upon the nanorods growth, Cu-doping was achieved by diffusion process at 500 ^°C for different times in vacuum ambient. The structural, optical, and magnetic properties of the obtained ZnO:Cu nanorods were then examined. XRD analysis showed that undoped and ZnO:Cu samples were highly c-axis oriented with a hexagonal wurtzite structure. SEM analysis indicated that ZnO:Cu nanorods had diameters of ～200 nm and lengths of ～1.5 μm. X-ray photoemission spectroscopy demonstrated that Cu was successfully doped into ZnO in a divalent state. Photoluminescence results showed that Cu-doping caused a decrease in the green band intensity of nanorods compared to undoped ZnO. Room temperature magnetic measurements showed that pure ZnO nanorods exhibited ferromagnetism that might be ascribed to defect-induced d⁰ ferromagnetism. All the ZnO:Cu nanorods also showed the room temperature ferromagnetism that was attributed to the bound magnetic polarons (BMPs).     

Synthesis of tunable sized capped magnetic iron oxide nanoparticles highly soluble in organic solvents

Surface modification of magnetic nanoparticles by organic surfactants is known to provide them with solubility in organic solvents (ferrofluids), which undoubtedly is an important property in several applications and studies. In this report, the main interest is focused on structural, magnetic and adsorption properties of iron oxide nanoparticles that are derived under water/toluene biphase conditions in the presence of oleic acid or oleylamine as the capping agents. The surfactants provide them with excellent stability and solubility in organic solvents like toluene or chloroform. Furthermore, by adding the appropriate surfactant or altering the temperature of the aqueous phase at the initial stage of the reaction we achieve a size control of the nanoparticles within the range 6–18 nm. The presence of capping agents or high reaction temperatures favours the formation of smaller nanoparticles. The adsorption of the surfactants (chemisorption) was identified with FT-IR spectroscopy, while Mössbauer studies have been performed to representative samples in order to identify the presence of either γ-Fe₂O₃ or Fe₃O₄, depending on the reaction temperature. Finally, the magnetic properties of representative samples have been studied at 5 K and room temperature.     

Magnetization reversal and tunable exchange bias behavior in Mn-substituted NiCr<Subscript>2</Subscript>O<Subscript>4</Subscript>

Single phase samples of Ni(Cr_1?xMn _x )₂O₄ (x = 0–0.50) were synthesized by using sol–gel route. Investigation of structural, magnetic, exchange bias and magnetization reversal properties was carried out in the bulk samples of Ni(Cr_1?xMn _x )₂O₄. Rietveld refinement of the X-ray diffraction patterns recorded at room temperature reveals the tetragonal structure for x = 0 sample with I4₁/amd space group and cubic structure for x ≥ 0.05 samples with \( {\text{Fd}\bar{3}\text{m}} \) space group. Magnetization measurements show that all samples exhibit ferrimagnetic behavior, and the transition temperature (T_C) is found to increase from 73 K for x = 0 to 138 K for x = 0.50. Mn substitution induces magnetization reversal behavior especially for 30 at% of Mn in NiCr₂O₄ system with a magnetic compensation temperature of 45 K. This magnetization reversal is explained in terms of different site occupation of Mn ions and the different temperature dependence of the magnetic moments of different sublattices. Study of exchange bias behavior in x = 0.10 and 0.30 samples reveals that they exhibit negative and tunable positive and negative exchange bias behavior, respectively. The magnitudes of maximum exchange bias field of these samples are found to be 640 and 5306 Oe, respectively. Exchange bias in x = 0.10 sample originates from the anisotropic exchange interaction between the ferrimagnetic and the antiferromagnetic components of magnetic moment. The tunable exchange bias behavior in x = 0.30 sample is explained in terms of change in domination of one sublattice moment over the other as the temperature is varied.     

Coexistence of Superconductivity and Ferromagnetic Phases in YBa2Cu3O7−δ Nanoparticles

High-temperature superconductor YBa₂Cu₃O_7?δ (YBCO) nanopowders were synthesized by the citrate-gel route, which is a modification of the sol-gel method. The fine powders were calcinated at 860 and 900 °C. They were of small size, in the range of 30–35 nm. X-ray diffraction (XRD) patterns verified production of the orthorhombic superconducting phase in all samples. Measuring the magnetic properties of these nanoparticles at room temperature, via a vibrating sample magnetometer (VSM), indicated ferromagnetism behavior in the YBa₂Cu₃O_7?δ nanoparticles. As the size of the nanoparticles decreased, the magnetic saturation of all samples increased. The development of the ferromagnetism effect was attributed to the presence of surface oxygen vacancies that lead to electron redistribution on the different ions at the surface. Thus, in an innovative work, the produced samples were annealed at 700 °C for 5 h under 0.8–0.9 bar of air atmosphere. The results showed that a small increase in the nanoparticle size provided a dramatic increase of magnetic saturation in all samples. Thus, we can say that the annealing process at vacuum improves the ferromagnetic properties of YBCO nanoparticles.