Crystal and magnetic study of the disordered perovskites Ca(Mn0.5Sb0.5)O3 and Ca(Fe0.5Sb0.5)O3

We have investigated the double perovskites Ca₂MSbO₆ (M = Mn, Fe) that have been prepared by solid-state reaction (M = Fe) and wet chemistry procedures (M = Mn). The crystal and magnetic structures have been studied from X-ray (XRD) and neutron powder diffraction (NPD) data. Rietveld refinements show that the crystal structures are orthorhombic (space group Pbnm) with complete disorder of M and Sb cations, so the formula should be rewritten as Ca(M_0.5Sb_0.5)O₃. Due to this disorder no evidences of Jahn-Teller distortion can be observed in the MnO₆ octahedra of Ca(Mn_0.5Sb_0.5)O₃, in contrast with the ordered double perovskite Sr₂MnSbO₆. Ca(Fe_0.5Sb_0.5)O₃ behaves as an antiferromagnet with an ordered magnetic moment for Fe³⁺ of 1.53(4)μ_B and a propagation vector k = 0, as investigated by low-temperature NPD. The antiferromagnetic ordering is a result of the high degree of Fe/Sb anti-site disorder of the sample, which originates the spontaneous formation of Fe-rich islands, characterized by the presence of strong Fe-O-Fe antiferromagnetic couplings with enough long-range coherence to produce a magnetic contribution perceptible by NPD. By contrast, the magnetic structure of Ca(Mn_0.5Sb_0.5)O₃ cannot be observed by low-temperature NPD because the magnitude of the ordered magnetic moments is below the detection threshold for neutrons.     

Synthesis, structure and magnetic properties of Sr2Fe1−xGaxMoO6 (0 ≤ x ≤ 0.6) double perovskites

Polycrystalline Sr₂Fe_1−xGa_xMoO₆ (0 ≤ x ≤ 0.6) materials have been synthesized by solid state reaction method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature dependent NPD data shows that the compounds crystallize in the tetragonal symmetry in the space group I4/m. The anti-site (AS) defects concentration increases with Ga doping, giving rise to highly B-site disordered materials. Ga doping at the Fe-site decreases the cell volume. The evolution of bond lengths and the cation oxidation states was determined from the Rietveld refinement data. The saturation magnetization and Curie temperature decreased with the increasing Ga content in the samples. Low temperature neutron diffraction data analysis and magnetization measurements confirm the magnetic interaction as ferrimagnetic in the sample.     

Self-propagating high temperature synthesis of Ni0.35Zn0.65Fe2O4 ferrite powders

The stoichiometric Ni_0.35Zn_0.65Fe₂O₄ ferrite powders were synthesized by SHS method. In the process of SHS, the effects of the molar ratio Fe/Fe₂O₃ in the starting mixture, oxygen pressure, grain size and relative density of the raw materials on combustion temperature, combustion wave velocity, phase composition and microstructure of the combustion products were investigated. X-ray diffraction, scanning electron microscope, TEM, vibrating sample magnetometry were used to characterize the microstructure and magnetic properties of the products. The results showed that as the molar ratio Fe/Fe₂O₃ increases, the combustion temperature and combustion wave velocity increased. The same results can be observed when the oxygen pressure increased from 0.1 to 0.9 MPa. The increase of grain size and relative density of raw materials resulted in the decrease of combustion temperature and combustion wave velocity. Compared with other methods, SHS process leads to ferrite powders with improved magnetic properties.     

Influence of structural transformations on functional properties of Fe75Ni2Si8B13C2 amorphous alloy

The influence of thermal treatment on functional properties of Fe₇₅Ni₂Si₈B₁₃C₂ amorphous alloy was investigated, showing the change that resulted from thermally induced structural transformations. Thermal history of the sample was found to have a significant effect on magnetic properties. Structural transformations were identified using DSC and thermomagnetic curve and characterized using Mössbauer spectroscopy and X-ray diffraction. Further investigation of magnetic and electrical properties of the alloy showed that structural relaxation prior to crystallization affected both magnetic susceptibility and electrical resistivity of the alloy, leading to an increase in both. This was caused by a confluence of stress relieving and a decrease in number of defects and an increase in free volume in the alloy sample, not only enabling greater mobility of magnetic domain walls, but also decreasing electron density of states at the Fermi level. Annealing at temperatures below crystallization caused an increase in magnetic susceptibility of the alloy at room temperature, however, a shift in Curie temperature was not observed. The alloy also exhibits a wide supercooled liquid region before crystallization, where its functional properties remained relatively constant, exhibiting the low values of both magnetic susceptibility and electrical conductivity.     

Structural and magnetic properties of LaFe0.5Cr0.5O3 studied by neutron diffraction, electron diffraction and magnetometry

The structural and magnetic properties of the perovskite type compound LaFe_0.5Cr_0.5O₃ have been studied by temperature dependent neutron powder diffraction and magnetization measurements. Rietveld refinement of the neutron diffraction data shows that the compound crystallizes in an orthorhombic perovskite structure with a random positioning of the Fe and Cr cations at the B sublattice. The magnetic structure at 10 K is a collinear antiferromagnetic one with the magnetic moment per site being equal to 2.79(4) μ_B. Magnetisation measurements confirm the overall antiferromagnetic behaviour. Moreover, it indicates a weak uncompensated magnetic moment close to the transition temperature T_N ≈ 265 K. This moment can be described by a magnetic cluster state, which remains up to 550 K. Electron diffraction patterns along with high-resolution transmission electron microscopy images reveal that the crystallites are composed by domains of different orientation, which share the same cubic perovskite sub-cell reflections.     

Effect of sintering temperature on magneto-transport properties of La0.67Ba0.33MnO3/Ba0.7Sr0.3TiO3 composites

La_0.67Ba_0.33MnO₃-20 wt.%-Ba_0.7Sr_0.3TiO₃ composites were sintered at different temperatures in order to explore the possibility of improving the magneto-transport properties of the composites. Detail studies on the magnetic and electrical transport properties for the sintered composite samples have been performed. Results show that the sintered composites have identical ferromagnetic to paramagnetic transition temperature and filamentary feature of metallic phase. When sintering temperature higher than 1300 °C, the composites show Efros-Shklovskii-like variable-range hopping in the temperature range lower than Curie temperature. For samples sintered lower than 1100 °C, a dome-like resistance peak appears at a temperature well below the Curie temperature. Magnetoresistance behavior indicates the existence of spin polarized tunneling in the low temperature range. Considering the contributions from Efros-Shklovskii-like variable-range hopping and spin polarized tunneling, the resistance peak can be well fitted.     

A study of the influence of crystallite size on the electrical and magnetic properties of CuFe2O4

An attempt has been made to clarify the fundamental assumption that the properties of materials change as the crystallite size of the material is reduced below 100 nm. CuFe₂O₄ samples of different crystallite sizes were prepared by the sol–gel and combustion methods and then analyzed by X-ray diffraction (XRD), thermal analyses (TGA/DTG) and scanning electron microscopy (SEM) techniques. The magnetic properties were studied by measuring the AC magnetic susceptibility (χ) and the Mössbauer spectroscopy. The DC electrical resistivity, dielectric constant, dielectric loss tangent, Curie temperature and hyperfine splitting of the samples change with the crystallite size. The change in the electrical properties is attributed to the formation of discrete energy levels instead of the bands. However, the magnetic parameters change due to the existence of non magnetic surface layers. The isomer shift and the hyperfine splitting show gradual increase with the increase in crystallite sizes.     

Neutron powder diffraction and magnetic study of perovskites Pb(Mn1/2Nb1/2)O3 and Pb(Mn1/4Fe1/4Nb1/2)O3

The structural and magnetic properties of the complex metal oxides Pb(Mn_1/2Nb_1/2)O₃ (PMNO) and Pb(Mn_1/4Fe_1/4Nb_1/2)O₃ (PMFNO), which belong to a class of disordered perovskites have been studied. The magnetic susceptibilities of PMNO showed hysteresis between field cooled and zero-field cooled conditions below the transition of 15 K, suggesting that the material has a spin-glass feature. Neutron diffraction patterns of PMNO showed no evidence of a long-range magnetic ordering at 1.5 K, which is consistent with spin-glass behavior. Rietveld refinements of neutron powder diffraction data collected at different temperatures between 1.5 and 700 K have been carried out in order to extract structural information. The crystal structure of this compound is cubic (space group Pmm) within the whole temperature interval. The Mn and Nb ions were found to be disordered over the perovskite B-sites. The main feature of this structure is the positional disorder at the Pb site, the importance of which in connection with the ferroic transitions is briefly discussed. The Pb cations show a positional disorder shifting from their high-symmetry positions along the [1 1 1] direction. The effect of Fe-doping on PMNO has been studied. The substitution of Fe at the Mn site in PMFNO results in a small changes of the magnetic properties without significant differences in the crystal structures. The factors governing the observed structural and magnetic properties of PMNO and PMFNO are discussed and compared with those of other quaternary Mn-containing perovskites. For the PbB³⁺_1/2Nb_1/2O₃ series with the isomorphous substitution B³⁺, graphs of average lattice parameters of the perovskite phase and the temperatures of ferroelectric and magnetic phase transitions as functions of the B³⁺ cation radius were constructed and are discussed. Influence of A-cation sublattice on magnetic properties is also considered.     

Memory effects in exchange coupled Fe/Co3O4 nanocomposites

Nanocomposites comprising a ferromagnet and an antiferromagnet have drawn attention recently because of their interesting physical properties and variety of technological applications. In the present work, structural, hyperfine and magnetic properties of Fe/Co₃O₄ nanocomposites prepared by a chemical route having 10–70 wt.% of Fe, have been investigated. XRD and TEM measurements confirmed polycrystalline nature of the samples having grain size in the nanometer regime. FTIR measurements show the presence of two bands of Co–O corresponding to Co²⁺ and Co³⁺. Mössbauer spectra recorded at room temperature confirm the presence of Fe in the blocked state. Presence of exchange bias at Fe–Co₃O₄ interfaces is confirmed by the magnetization measurements. Irreversibility in temperature dependent FC–ZFC measurements points to interface effect. Frequency dependent ac susceptibility measurements as well as memory effect observed in dc magnetization measurements indicate the superspin glass nature of the nanocomposites.     

Synthesis and characterization of the new layered perovskite, Na0.10(VO)0.45LaTiO4·nH2O

Na_0.10(VO)_0.45LaTiO₄·nH₂O (n ≅ 0.6) has been synthesized by an ion exchange reaction between the single-layered perovskite, NaLaTiO₄, and aqueous VOSO₄. This low temperature phase retains the structure of the parent with a slight contraction of its tetragonal unit cell. Rietveld refinement of X-ray powder diffraction data indicate that the vanadyl units are disordered within the perovskite layers. Infrared spectroscopy, electron spin resonance and magnetic susceptibility are consistent with the presence of isolated vanadyl units. Susceptibility data show Curie-Weiss behavior above 140 K.     

Synthesis and characterization of FeVO4 nanoparticles

Iron vanadate (FeVO₄) nanoparticles were synthesized by simple co-precipitation method using various surfactants such as ethylene glycol, polyethylene glycol 200 and polyethylene glycol 400 as the structure directing agents. Systematic investigations on the structural, morphological and magnetic properties of the materials have been studied. The lattice constants of the triclinic structure of FeVO₄ were calculated from the X-ray diffraction (XRD) analyses. The average grain size was estimated to be around 35 nm, which increased with increasing the calcination temperature. The stretching and bending vibrations of Fe-O were evaluated from the FT-IR spectra. Using VSM magnetometer, magnetic property was investigated through magnetic susceptibility and magnetization measurements. FeVO₄ exhibits two magnetic ordering temperatures at T ≈ 20 K and 14 K, which is due to two different chemical environments of Fe ligands such as octahedral FeO₆ and trigonal bipyramidal FeO₅ in a six-column doubly bent chain, respectively.     

The magnetoelectric perovskite Sr3Fe2TeO9: An insight from magnetic measurements and neutron powder diffraction

A study of the crystallographic and magnetic structures of the double perovskite Sr₃Fe₂TeO₉ has been carried out on a polycrystalline sample using neutron powder diffraction (NPD) data between 10 and 650 K. An analysis of the NPD patterns at room temperature has shown that this compound crystallises in the tetragonal space group I4/m with a = 5.5614(7) Å and c = 7.867(1) Å and has a partially ordered arrangement of Fe and Te at the B-sites. The compound undergoes an I4/m → Fm-3m improper ferroelectric phase transition near 460 K. A low-temperature ferrimagnetic ordering (below T_N = 260 K) has been followed from the magnetisation measurements and sequential NPD data analysis. In good agreement with magnetic measurements the ferrimagnetic structure with very weak magnetisation is defined by the propagation vector k = (0, 0, 0). In addition to the obtained experimental results on magnetic and electric properties some aspects of magnetoelectricity in this perovskite are also discussed and compared with those of another quaternary oxide Sr₃Fe₂B⁶⁺O₉.     

Characterization and magnetic properties of Sm- and Gd-substituted CoFe2O4 nanoparticles prepared by forced hydrolysis in polyol

Pure nanoparticles of the CoFe_2−xRE_xO₄ (RE = Gd, Sm; x = 0.0, 0.1) system have been prepared by forced hydrolysis in polyol. The insertion of Sm³⁺ and Gd³⁺ cations into the cobalt ferrite structure has been investigated. X-ray micro-analysis (EDX) shows that the RE contents are close to the nominal ones. X-ray diffraction (XRD) evidences a cell size increase with slight distortions in the spinel-like lattice indicating the entrance of RE³⁺ ions. Micro-Raman spectroscopy confirms the cubic inverse-spinel structure and rules out the existence of impurities like hematite. Magnetic measurements (SQUID) show important differences in the magnetic properties of the unsubstituted and substituted particles. All the particles are superparamagnetic at room temperature and ferrimagnetic at low temperature. However, their main magnetic characteristics appear to be directly dependent on the RE content.     

Hydrothermal fabrication of various morphological α-Fe2O3 nanoparticles modified by surfactants

Two kinds of various morphological α-Fe₂O₃ nanoparticles modified by anionic surfactant (sodium dodecylsulfonate, SDS) and cationic surfactant (hexadecyipyridinium chloride, HPC), respectively, have been synthesized via hydrothermal method, using simple inorganic salt (NH₄)₃Fe(C₂O₄)₃ and alkali NaOH as starting precursors. Meanwhile, α-Fe₂O₃ nanoparticles without surfactant are also fabricated under the same conditions for comparison. The resultant products were characterized by means of Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron micrograph (TEM) combined with electron diffraction (ED) and magnetization measurements. It is interesting that the obtained α-Fe₂O₃ nanoparticles without surfactant are polyhedral with average particle size of 90 ± 35 nm; while the obtained α-Fe₂O₃ nanoparticles modified by SDS are ellipsoidal with mean particle size of major axis: ca. 420 nm; minor axis: ca. 205 nm and those modified by HPC are spherical with mean particle size of ca. 185 nm observed from TEM. In addition, magnetic hysteresis measurements reveal that the α-Fe₂O₃ nanoparticles modified by two surfactants show enhancement in coercivity (H_c) and the remanent magnetization (M_r) compared with those of the obtained α-Fe₂O₃ nanoparticles without surfactant at room temperature. The experimental results suggest that the surfactants not only significantly influence the size and shape of the particles, but also their magnetic properties.     

New type of incommensurate magnetic ordering in Mn3TeO6

The complex metal oxide Mn₃TeO₆ exhibits a corundum related structure and has been prepared both in forms of single crystals by chemical transport reactions and of polycrystalline powders by a solid state reaction route. The crystal structure and magnetic properties have been investigated using a combination of X-ray and neutron powder diffraction, electron microscopy, calorimetric and magnetic measurements. At room temperature this compound adopts a trigonal structure, space group with a = 8.8679(1) Å, c = 10.6727(2) Å. A long-range magnetically ordered state is identified below 23 K. An unexpected feature of this magnetic structure is several types of Mn-chains. Under the action of the incommensurate magnetic propagation vector k = [0, 0, 0.4302(1)] the unique Mn site is split into two magnetically different orbits. One orbit forms a perfect helix with the spiral axis along the c-axis while the other orbit has a sine wave character along the c-axis.     

Preparation of cobalt-zinc ferrite (Co0.8Zn0.2Fe2O4) nanopowder via combustion method and investigation of its magnetic properties

Cobalt-zinc ferrite (Co_0.8Zn_0.2Fe₂O₄) was prepared by combustion method, using cobalt, zinc and iron nitrates. The crystallinity of the as-burnt powder was developed by annealing at 700 °C. Crystalline phase was investigated by XRD. Using Williamson-Hall method, the average crystallite sizes for nanoparticles were determined to be about 27 nm before and 37 nm after annealing, and residual stresses for annealed particles were omitted. The morphology of the annealed sample was investigated by TEM and the mean particle size was determined to be about 30 nm. The final stoichiometry of the sample after annealing showed good agreement with the initial stoichiometry using atomic absorption spectrometry. Magnetic properties of the annealed sample such as saturation magnetization, remanence magnetization, and coercivity measured at room temperature were 70 emu/g, 14 emu/g, and 270 Oe, respectively. The Curie temperature of the sample was determined to be 350 °C using AC-susceptibility technique.     

Synthesis of single-phase Sr3Co2Fe24O41 Z-type ferrite by polymerizable complex method

Synthesis of single-phase Sr₃Co₂Fe₂₄O₄₁ Z-type (Sr₃Co₂Z) ferrite was realized by adopting the polymerizable complex method. Crystal structure of samples has been investigated by powder X-ray diffraction (XRD). Single-phase Sr₃Co₂Z ferrite was obtained by heating at 1473 K for 5 h in air. Magnetic properties were discussed by measurements of M-H curves with vibrating sample magnetometer (VSM). Sr₃Co₂Z ferrite prepared by polymerizable complex method showed typical M-H curve of soft ferrite, with a saturation magnetization of 21.5μ_B/formula unit (50.5 emu/g) and a coercive force of 0.014 T at room temperature.     

The crystal and magnetic structure of Ti-substituted LaCrO3

The crystal and magnetic structures of LaCrO₃ and La(Cr_0.90Ti_0.10)O₃ have been investigated between 5 and 350 K by means of neutron powder diffraction and DC magnetic measurements. Both compounds are characterized by an antiferromagnetic G_x-type ordering at low temperature. Structural features suggest the occurrence of Ti in the tetra-valent state. Despite the mixed valence induced by Ti-substitution leading to the occurrence of the Jahn-Teller species Cr²⁺, no evidence for long range ferromagnetism can be detected. On account of a miscibility gap, a higher degree of Ti-substitution at the Cr site cannot be achieved; as a consequence the solid state solubility of Ti in LaCrO₃ at 1573 K has been ascertained.     

Nb NMR and Fe EPR study of local magnetic properties of magnetoelectric Pb(Fe1/2Nb1/2)O3

The ⁹³Nb NMR and Fe³⁺ EPR spectra of a magnetoelectric Pb(Fe_1/2Nb_1/2)O₃ (PFN) single crystal as well as of PFN ceramics and nanopowder have been measured. The relation between the chemical disorder and the local magnetic structure has been determined. The data show the occurrence of an antiferromagnetic (AFM) phase transition at 145 and 135 K in both single crystal and ceramic samples, respectively. The AFM phase transition is however essentially smeared out in the nanopowder. Two different Nb sites exist which have different local magnetic fields which are essentialy distributed in value. These two Nb sites correspond to different types of “chemical” order: Fe rich, Nb poor and Fe poor, Nb rich regions. The temperature dependences of the sublattice magnetizations and the values of the superhyperfine fields at the Nb ions are obtained. NMR data suggest that a spin-glass like state of PFN below T = 20 K arises from the Fe poor, Nb rich regions.     

Synthesis of Mg[Ti2]O4 by spark plasma sintering

We have successfully prepared magnetic frustrated compound Mg[Ti₂]O₄ by a novel technique of spark plasma sintering. X-ray diffraction confirms that the sample is single phase and the spinel parameter a = 8.504(7) Å at the room temperature. The magnetic susceptibility shows that an abnormal drop is exhibited at about 254 K, indicating a magnetic single formation, and that a paramagnetic susceptibility increase which obeys the Curie Law appears below about 150 K, revealing that the number of Ti³⁺ ions which don't take part in spin dimer is about 2.6%. The resistivity shows a jump at the temperature of 254 K, in agreement with the transition temperature of measurement of the susceptibility.