Mössbauer studies and magnetic properties of spinel lead ferrite

In the present communication, we have reported the synthesis of nanocrystalline lead ferrite (PbFe₂O₄) by citrate mediated autocombustion method. X-ray diffraction pattern reveals the single phase formation in cubic (spinel) structure. The particle size and the surface morphology of the samples are characterized by TEM and SEM analysis. Magnetic studies are carried out using vibrating sample magnetometer (VSM) shows a very high coercive field for the material. Mössbauer studies were performed to investigate the local symmetry i.e. Fe is in octahedral/tetrahedral site and the charge states of Fe ions.     

Studies of AC electrical conductivity and initial magnetic permeability of rare-earth-substituted Li–Co ferrites

In the present work, the AC electrical conductivity and initial magnetic permeability were investigated for some Rare-earth-substituted spinel ferrites. These ferrites are of composition Li_0.5−0.5xCo_xFe_2.4−0.5xR_0.1O₄ (where x=0.0$x=0.0$, 0.5, and 1; R=Y, Yb, Eu, Ho and Gd). They were prepared by standard ceramic techniques. With respect to AC electrical conductivity, measurements show dispersion with frequency at low temperatures. This dispersion obeys the universal power law. The frequency exponent of the power law decreases with both Co ion content and temperature. This indicates that the classical barrier hopping mechanism is the predominant one in these samples. On the other hand, the behavior of the initial magnetic permeability with temperature exhibits multidomain structure only for the samples with x=0.0$x=0.0$, and single domain structure otherwise.     

Enhanced hyperthermia performance in hard-soft magnetic mixed Zn0.5CoxFe2.5−xO4/SiO2 composite magnetic nanoparticles

High quality Zn0.5CoxFe2.5?xO4(x=0,0.05,0.1,0.15)serial magnetic nanoparticles with single cubic structures were prepared by the modified thermal decomposition method,and Zn0.5CoxFe2.5?xO4/SiO2 composite magnetic nanoparticles were prepared by surface modification of SiO2.The magnetic anisotropy of the sample increases with the increase of the doping amount of Co2+.When the doping amount is 0.1,the sample shows the transition from superparamagnetism to ferrimagnetism at room temperature.In the Zn0.5CoxFe2.5?xO4/SiO2 serial samples,the maximum value of specific loss power(SLP)with 1974 W/gmetal can also be found at doping amount of x=0.1.The composite nanoparticles are expected to be an excellent candidate for clinical magnetic hyperthermia.     

Electron spin resonance and cation distribution studies of the Co0.6Zn0.4MnxFe2−xO4 ferrite system

The ESR spectra of the ferrite system Co_0.6Zn_0.4Mn_xFe_2−xO₄ (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) were obtained at room temperature. The experimental values of the magnetic moment (μ_exp) were estimated from the ESR spectra and the cation distribution was consequently established from the values of μ_exp. The systematic decrease in ESR line width observed in our present study was attributed to the decrease of Fe²⁺ concentration with increasing Mn content. The resonance field decreases and reaches a minimum at high values of Mn content whereas the magnetic moment reaches a maximum at these values. The IR spectra were recorded in the range 200–1200 cm⁻¹. The bands at 569 (ν₁) and 389 cm⁻¹ were assigned to the tetrahedral and octahedral complexes, respectively. The band at 441 cm⁻¹ is due to the Mn–O bond vibration. The theoretical lattice parameter was calculated and was found to be larger than the experimental one a_exp due to the presence of Mn⁴⁺ ions.     

Neutron diffraction studies of the diluted spinel ferrite ZnxMg0.75−xCu0.25Fe2O4

Neutron diffraction studies have been performed in the temperature range 1050 K⩾T⩾10 K on the spinel system Zn_xMg_0.75−xCu_0.25Fe₂O₄ (x=0.0, 0.25, 0.5 and 0.75) prepared in the ceramic sintering method. Cation distributions in all the four compositions have been determined from the analysis of neutron data. A perturbed ferrimagnetic order has been observed in the compositions with x values lying in the low to intermediate range, where apparently, randomly canted spins and small fluctuating clusters are superimposed on the ferrimagnetic long range order. A complete breakdown of the ferrimagnetic stability occurs at x=0.75 as indicated by the absence of magnetic long range order. A huge diffuse scattering signal appears at the low-Q region broadening the (1 1 1) diffraction line in the low temperature neutron patterns of this composition indicating the formation of large magnetic clusters. A cluster spin glass state is suggestive for this composition.     

Synthesis,structural, electrical and magnetic studies of La0.5Ca0.45−xSrxBa0.05MnO3

Half doped mixed valence manganite system La_0.5Ca_0.45?xSr_xBa_0.05MnO₃ (with x=0.1, 0.2, and 0.3) synthesized through a low temperature nitrate route is systematically investigated in this paper. The electronic transport and magnetic properties are analyzed and compared apart from the study of unit cell structure and composition. The system is found to crystallize only in orthorhombic structure (Pnma) and the electronic phase transitions are observed to be of second order. The charge and orbital ordering have been observed to coexist with ferromagnetism in x=0.1 compound. Application of small polaron and variable range hopping models to resistivity data of the system corresponding to high temperature range shows increasing mobility of e_g electrons with x, with the later model describing the electronic transport very closely than the former. The temperature dependent magnetization of the compounds shows monotonic increase of paramagnetic to ferromagnetic transition (T_C) with x. Ferromagnetism is exhibited for the complete temperature range down from respective T_C in contrast to antiferromagnetism usually exhibited by half-doped compounds in the low temperature range. The plots of magnetization versus magnetic field reveal a transition from soft to hard magnetic character for all the compounds as the temperature is lowered.     

Effects of A-site disorder on magnetic,electrical and thermal properties of La0.5−xLnxCa0.5−ySryMnO3manganites

The magnetic, electrical and thermal properties in the La_0.5?xLn_xCa_0.5?ySr_yMnO₃ (Ln=Pr, Nd, Sm) bulk system were investigated. Detailed dc magnetization and linear ac susceptibility measurements reveal that the samples first undergo phase transition from paramagnetic to ferromagnetic phase and then to an antiferromagnetic phase upon further cooling. It is found that both the Curie and Neel temperatures decrease systematically with increasing A-site disorder in these manganites. The electrical resistivity exhibits semiconducting behavior throughout the temperature range investigated and the electronic conduction mechanism can be conveniently described within the framework of the variable range hopping model above T=150 K. The Seebeck coefficient (S) in the magnetically ordered regime infers that the complicated temperature dependence of S is an indication of electron–magnon scattering. Specific heat measurements depict a broadened hump in the vicinity of T_C, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The temperature dependence of thermal conductivity, κ(T), reveals a positive dκ/dT in the paramagnetic region, which may be related to the local anharmonic lattice distortions associated with small polarons.     

Magnetic properties of the mixed spinel Co1+xSnxFe2−2xO4

The structural and magnetic properties of the mixed spinel Co_1+xSn_xFe_2?2xO₄ system for 0.1≤x≤0.5 have been studied by means of X‐ray diffraction, magnetization, a.c. susceptibility and Mössbauer effect measurements. X‐ray intensity calculations indicate that Sn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Co²⁺ ions substitute for A‐site Fe³⁺ ions. The lattice constants are determined and the applicability of Vegard's law has been tested. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at tetrahedral (A) and octahedral (B) sites for x≤0.4. The Mössbauer intensity data show that Sn possesses a preference for the B‐site of the spinel. As expected, the hyperfine field and Curie temperature determined from a.c. susceptibility decreases with increasing Sn content. The variation of the saturation magnetic moment per formula unit measured at 77 and 300 K with Sn content is satisfactorily explained on the basis of Néel's collinear spin ordering model for x=0.1–0.4.     

Microstructure and magnetic characteristics of nanocrystalline Ni0.5Zn0.5 ferrite synthesized by a spraying-coprecipitation method

Nanocrystalline Ni0.5Zn0.5 ferrite with average grain sizes ranging from 10 to 100 nm is prepared by using a spraying-coprecipitation method. The results indicate that the nanocrystalline Ni0.5Zn0.5 ferrite is ferromagnetic without the superparamagnetic phenomenon observed at room temperature. Specific saturation magnetization of nanocrystalline Nio.sZno.5 ferrite increases from 40.2 to 75.6 emu/g as grain size increases from 11 to 94nm. Coercivity of nanocrystalline Ni0.5Zn0.5 ferrite increases monotonically when d 〈 62 nm.The relationship between the coercivity and the mean grain size is well fitted into a relation Hc - d^3. A theoretically evaluated value of the critical grain size is 141nm larger than the experimental value 62nm for nanocrystalline Ni0.5Zn0.5 ferrite. The magnetic behaviour of nanocrystalline Ni0.5Zn0.5 ferrite may be explained by using the random anisotropy theory.     

Synchrotron X-ray diffraction studies on the phase transitions in the spinel LixMn3−xO4 intercalation compounds

Detailed investigations have been undertaken of the lithium for manganese substitution effect on the LiMn₂O₄, in the system Li_xMn_3−xO₄, for 0.95≤x≤1.05, that is for the nearly stoichiometric lithium content. Synchrotron X-ray measurements have been performed in the temperature range 10–300 K. The diffraction experiments were carried out at the DESY-HASYLAB high-resolution powder diffractometer (beamline B2), equipped with a closed-cycle He-cryostat. Very small changes in the lithium content influence clearly the low-temperature crystal structure of Li_xMn_3−xO₄, spinels and the nature of phase transitions. It was found that for x=0.95 the sample remains tetragonal in the whole 10–300 K temperature range. The stoichiometric LiMn₂O₄ transforms from cubic to orthorhombic at about 280 K. For x=1.0125 the temperature of phase transition from cubic to orthorhombic decreases down to about 260 K, whereas for x=1.025 the transformation goes from cubic to tetragonal phase, at the temperature 220 K. No phase transition has been observed for the cubic sample with x=1.0375. These results partly explain the divergences in recent reports on the low-temperature structure and phase transformations of lithium manganese oxides.     

Photoacoustic spectra of Fe3+ and Cr3+ in lithium chromo-ferrites,Li0.5Fe2.5−xCrxO4 (0≤x≤2.5)

Photoacoustic spectra of polycrystalline Li_0.5Fe_2.5−xCr_xO₄ (x=0, 0.5, 1.25, 2.0 and 2.5) and LiFeO₂ have been investigated at 300K in the wavelength range 200–2500 nm. The spectra of x=0 composition arises from octahedral and tetrahedral Fe³⁺ ions in the spinel lattice. The Crsubstituted Li-ferrite phases exhibit broad maxima in the range 1400–1600nm and these are attributed to tetrahedral Cr³⁺ present in these phases besides the usual octahedral Cr³⁺ ions. The results are discussed in the light of recent work on tetrahedral Cr³⁺. The results of ferrimagnetic resonance spectra of Li ferrites and their Cr-substituents are also reported and correlated with photoacoustic spectra.     

Crystallographic and optical properties of Li1−xTa1−xEu2xO3 solid solutions

Abstract

The effects of europium substitution on crystalline solubility, structural changes and optical properties of Li_1?xTa_1?xEu_2xO₃ solid solutions are reported. The solid solutions exist over a very limited range of europium concentrations (x ≤ 0.02). They were characterized by X-ray powder diffraction and density measurements. The liquid nitrogen fluorescence and excitation spectra of Eu³⁺ in the solid solutions were measured in the frequency range 12,200–45,450 cm^?1. The emission mainly arises from the ⁵ D ₀ level to Stark components of the five lowest ⁷ F _j (j = 0, 1, 2, 3, 4) states. It was inferred from the analyses of the optical and density data that the Eu³⁺ ions may lie on both Li and Ta sites, but not on the intrinsic vacant lattice sites.     

Sol–gel auto-combustion synthesis,structural and enhanced magnetic properties of Ni substituted nanocrystalline Mg–Zn spinel ferrite

Nanocrystalline arrays of Ni²⁺ substituted Mg–Zn spinel ferrite having a generic formula Mg_0.7−xNi_xZn_0.3Fe₂O₄ (x=0.0, 0.2, 0.4 and 0.6) were successfully synthesized by sol–gel auto-combustion technique. The fuel used in the synthesis process was citric acid and the metal nitrate-to-citric acid ratio was taken as 1:3. The phase, crystal structure and morphology of Mg–Ni–Zn ferrites were investigated by X-ray diffraction, scanning electron microscopy, and Fourier transformer infrared spectroscopy techniques. The lattice constant, crystallite size, porosity and cation distribution were determined from the X-ray diffraction data method. The FTIR spectroscopy is used to deduce the structural investigation and redistribution of cations between octahedral and tetrahedral sites of Mg–Ni–Zn spinel structured material. Morphological investigation suggests the formation of grain growth as the Ni²⁺ content x increases. The saturation magnetization and magneton number were determined from hysteresis loop technique. The saturation magnetization increases with increasing Ni²⁺ concentration ‘x’ in Mg–Zn ferrite.     

Structural,electrical and magnetic properties of Zr–Mg cobalt ferrite

Spinel cobalt ferrite, CoFe_2−xM_xO₄ has been synthesized by substitution of the combination of metallic elements M=Zr–Mg by the microemulsion method using polyethylene glycol as a surfactant. Powder X-ray diffraction analysis reveals that the substitution results in shrinkage of the unit cell of cobalt ferrite due to higher binding energy of the synthesized samples. The energy-dispersive X-ray fluorescence analysis confirms the stoichiometric ratios of the elements present. The thermogravimetric analysis shows that the minimum temperature required for the synthesis of these substituted compounds is 700 °C. A two-point probe method was employed for the measurement of the electrical resistivity in a temperature range of 293±5 to 673±5 K. It appears that there is a decrease in the number of Fe²⁺/Fe³⁺ pairs at the octahedral sites due to the substitution and corresponding migration of some of the Fe³⁺ ions to tetrahedral sites, consequently increasing the resistivity and the activation energy of hopping of electron at the octahedral sites. The susceptibility data also suggest migration of Fe³⁺ to tetrahedral site in the initial stage, which results in an increase in A–B interactions leading to large increase in the blocking temperature (T_B) as observed in samples having dopant content x=0.1.     

Cation distribution and entropic disordering in mixed spinel ferrite Ga0.4Fe0.6NiCr1−yAlyO4

Mössbauer measurements (4.2T298°K) indicate the presence of entropic disordering in (Ga _0.4 ³⁺ Fe _0.6 ³⁺ Al _p ³⁺ )[Ni²⁺Cr _1–y ³⁺ Al _y-p ³⁺ Fe _p ³⁺ ]O₄ and a linear increase in p with y between 0(y=0) and 0.11(y=1). Entropic spins are located only on A-sites indicating relatively large A-sublattice frustration. Weak A-B coupling could explain this separate sublattice behaviour. Magneton number, alongwith Mössbauer p values, shows B-site canting which increases with y revealing weakness of A-B interaction. Present system can be called a frustrated ferrimagnet or entropie ferrimagnet where ferrimagnetic and frustrated (entropie) spins are coexisting.     

Structural,electrical, magnetic,elastic, and internal friction studies of Nd1−xCaxMnO3 (x=0.2, 0.33, 0.4, and 0.5) manganites

A series of Nd_1?xCa_xMnO₃ (x=0.2, 0.33, 0.4, and 0.5) manganites was prepared by sol–gel route by sintering at 1300 °C, mainly to understand the correlation between electron, spin, and phonon couplings. The internal friction and longitudinal modulus along with electrical and magnetic properties have been measured. All the samples are found to exhibit anomalies at T_C, T_N, and T_CO transition temperatures. The anomalies in longitudinal modulus and the internal friction peak at T_CO are attributed to Jahn–Teller effect. A strong correlation between the temperature dependent elastic, anelastic, resistivity, and ac susceptibility properties has been observed and an effort has been made to explain the observed anomalous behavior by a qualitative model.     

Mössbauer studies of the charge transfer process in the system (Li0.5Fe0.5)1−xFexFe2O4

Conclusion The formation of heterovalent pairs seems to be proved for the compounds rich in Fe²⁺ ions (x0.5) and the electron-phonon coupling seems to change with decreasing concentration of Fe²⁺ ions.Nevertheless the pairs model is not necessarily an unique interpretation of the data for x0.5 /1/ and the broadening of the Fe^m+ ions spectrum could be worked considering electronic and atomic disorders.