Mössbauer study of 1% <Superscript>57</Superscript>Fe doped ferromagnetic insulator La<Subscript>0.825</Subscript>Ca<Subscript>0.175</Subscript>MnO<Subscript>3</Subscript>

We have employed magnetization measurements, M?ssbauer and ESR spectroscopic techniques, in order to study the ferromagnetic insulating (FMI) compound La_1-xCa_xMnO₃ (x=0.175) doped with 1% ⁵⁷Fe. We have used two samples; one prepared in air which has cation vacancies and a second in inert atmosphere, which is stoichiometric. An abrupt change of the experimental results is obtained, by all techniques, in the ferromagnetic insulating regime, in the temperature region of T_O/O//≈60 K, where an orbital rearrangement is suggested to occur. An analysis of these findings points to an orbital rearrangement transformation. Ferromagnetic resonance reveals considerable differences between stoichiometric and cation deficient samples, indicating anisotropy of the exchange interactions in the former sample with significant temperature dependence, most pronounced in the vicinity of T_O/O//     

Mössbauer investigations of Fe and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> magnetic nanoparticles for hyperthermia applications

Magnetic nanoparticles of magnetite Fe₃O₄ and Fe synthesized by physical vapor deposition with a fast highly effective method using a solar energy have been studied. Targets have been prepared from tablets pressed from Fe₃O₄ or Fe powders. Relationships between the structure of nanoparticles and their magnetic properties have been investigated in order to understand principles of the control of the parameters of magnetic nanoparticles. Mössbauer investigations have revealed that the nanoparticles synthesized from tablets of both pure iron and Fe₃O₄ consist of two phases: pure iron and iron oxides (γ-Fe₂O₃ and Fe₃O₄). The high iron oxidability suggests that the synthesized nanoparticles have a core/shell structure, where the core is pure iron and the shell is an oxidized iron layer. Magnetite nanoparticles synthesized at a pressure of 80 Torr have the best parameters for hyperthermia due to their core/shell structure and core-to-shell volume ratio.     

Mössbauer studies of stoichiometry of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>: characterization of nanoparticles for biomedical applications

The iron oxide Fe₃O₄, the mineral magnetite sometimes called ferrosoferric oxide, is notoriousy non-stoichiometric even in bulk form so its formula may be written Fe_3?δO₄. In nanoparticle form, where it has applications in medicine and information technology, it is even more susceptible to oxidation. In this paper we report synthesis and studies of superparamagnetic Fe₃O₄ nanoparticles with controlled diameters of 5.3, 10.6 and 11.9 nm. In room temperature spectra, departures from stoichiometry δ of up to 0.02 were estimated from the relative amounts of Fe ³⁺/ Fe ²⁺ and from their isomer shifts. This cannot be used for very small particles of diameter 10.6 nm and less as they are superparamagnetic at room temperature and do not show hyperfine splitting owing to fast relaxation. Such particles have promise for use in enhancing MRI signals. The magnetic spectrum is restored by the application of a relatively small magnetic field (10 kG). As the temperature is lowered the relaxation slows down and 6-line magnetic hyperfine patterns appear below a blocking temperature T_B. The values of T_B obtained are lower than those of many other researchers reported in the literature, suggesting that our particles are less affected by magnetic interactions between them. At low temperatures all the spectra are similar and closely resemble that of bulk Fe₃O₄ confirming that departures from stoichiometry are small.     

Mass and kinetic energy distribution of the species generated by laser ablation of La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript>

The mass distributions of the species generated by laser ablation from a La_0.6Ca_0.4MnO₃ target using laser irradiation wavelengths of 193 nm, 266 nm and 308 nm have been investigated with and without a synchronized gas pulse of N₂O. The kinetic energies of the species are measured using an electrostatic deflection energy analyzer, while the mass distributions of the species were analyzed with a quadrupole mass filter. In vacuum (pressure 10⁻⁷ mbar), the ablation plume consists of metal atoms and ions such as La, Ca, Mn, O, LaO, as well as multiatomic species, e.g. LaMnO⁺. The LaO⁺ diatomic species are by far the most intense diatomic species in the plume, while CaO and MnO are only detected in small amounts. The interaction of a reactive N₂O gas pulse with the ablation plume leads to an increase in plume reactivity, which is desired when thin manganite films are grown, in order to incorporate the necessary amount of oxygen into the film. The N₂O gas pulse appears to have a significant influence on the oxidation of the Mn species in the plume, and on the creation of negative ions, such as LaO⁻,O⁻ and O₂⁻.     

Mössbauer study of the modulated magnetic structure of FeVO<Subscript>4</Subscript>

Mössbauer spectroscopy is used to study the FeVO₄ multiferroic, which undergoes two magnetic phase transitions at T _N1 ≈ 22 K and T _N2 ≈ 15 K. The first transition (T _N1) is related to transformation from a paramagnetic state into a magnetically ordered state of a spin density wave, and the second transition (T _N2) is associated with a change in the type of the spatial magnetic structure of the vanadate. The electric field gradient tensor at ⁵⁷Fe nuclei is calculated to perform a crystal-chemical identification of the partial Mössbauer spectra corresponding to various crystallographic positions of Fe³⁺ cations. The spectra measured in the range T _N2 < T < T _N1 are analyzed on the assumption about amplitude modulation of the magnetic moments of iron atoms μ_Fe. The results of model intersection of the spectra recorded at T < T _N2 point to a high degree of anharmonicity of the helicoidal magnetic structure of the vanadate and to elliptic polarization of μ_Fe. These features are characteristic of type-II multiferroics. The temperature dependences of the hyperfine interaction parameters of ⁵⁷Fe nuclei that were obtained in this work are analyzed in terms of the Weiss molecular field model on the assumption of orbital contribution to the magnetic moments of iron cations.     

Synthesis and characterization of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>–Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> magnetoelectric composites with dielectric and magnetic properties

Composite structures consisting of (001)-oriented SrTiO₃ (STO)/La_0.7Sr_0.3MnO₃ (LSMO) films of 30 nm thickness, grown on an (001) Pb(Mg_1/3Nb_2/3)TiO₃– 28 mol.% PbTiO₃ piezoelectric relaxor-ferroelectric single-crystalline wafer were investigated by means of Wide-Angle X-ray Diffraction (WAXRD) in situ under influence of a d.c. electric field with strength E up to ±18 kV/cm. The WAXRD measurements of the films and substrate reflection profiles resulted in a determination of the strain s in the films and the substrate separately. The strained state of the STO/LSMO films is effectively controlled by a huge converse piezoelectric effect of the PMN-PT substrate. The coefficients of coupling between electric-field-induced out-of-plane strain in the films and in the substrate for the composite system STO/LSMO/PMN-PT are obtained.     

Mössbauer Spectroscopy Study of the Superparamagnetism of Ultrasmall ϵ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanoparticles

The superparamagnetism of an ensemble of ?-Fe₂O₃ nanoparticles with a mean size of 3.9 nm dispersed in a xerogel SiO₂ matrix is studied by the Mössbauer spectroscopy method. It is shown that most nanoparticles at room temperature are in the superparamagnetic (unblocked) state. As the temperature decreases, the progressive blocking of the magnetic moments of the particles occurs, which is manifested in the Mössbauer spectra as the transformation of the quadrupole doublet into a Zeeman sextet. The analysis of the relative intensity of the superparamagnetic (quadrupole doublet) and magnetically split (sextets) spectral components in the range of 4–300 K provides the particle size distribution, which is in agreement with the transmission electron microscopy data. The values of the effective magnetic anisotropy constants (K_eff) are determined, and the contribution of surface anisotropy (K_S) is estimated for particles of various sizes. It is shown that the quantity K_eff is inversely proportional to the particle size, which indicates the significant contribution of the surface to the magnetic state of the ?-Fe₂O₃ nanoparticles with the size of several nanometers.     

Peculiarities of the magnetic state in the system La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>3−γ</Subscript> (0 ≤ γ ≤ 0.25)

The results of experimental investigation of the chemical phase composition, crystal structure, and magnetic properties of the manganite La_0.70Sr_0.30MnO_3?γ (0 ≤ γ ≤ 0.25) with perovskite structure depending on the concentration of oxygen vacancies are presented. It is found that the mean grain size of the stoichiometric solid solution of La_0.70Sr_0.30MnO₃ amounts approximately to 10 μm, while the grain size for anion-deficient solid solutions of La_0.70Sr_0.30MnO_3?γ is approximately 5 μm. It is found that samples with 0 ≤ γ ≤ 0.13 have a rhombohedral unit cell (with space group \(R\bar 3c\), Z = 2), while samples with γ ≥ 0.20 have a tetragonal unit cell (space group I4/mcm, Z = 2). It is proved experimentally that the magnetic phase state of the manganite La_0.70Sr_0.30MnO_3?γ changes upon a decrease in the oxygen content. It is shown that anion-deficient solid solutions of La_0.70Sr_0.30MnO_3?γ experience a number of successive magnetic phase transformations in the ground state from a ferromagnet (0 ≤ γ ≤ 0.05) to a charge-disordered antiferromagnet (γ = 0.25) via an inhomogeneous magnetic state similar to a cluster spin glass (0.13 ≤ γ ≤ 0.20). The mean size of ferromagnetic clusters (r ≈ 50 nm) in the spin glass state is estimated. It is shown that oxygen vacancies make a substantial contribution to the formation of magnetic properties of manganites. The generalized magnetic characteristics are presented in the form of concentration dependences of the spontaneous magnetic moment, coercive force, and the critical temperature of the magnetic transition. The most probable mechanism of formation of the magnetic phase state in Sr-substituted anion-deficient manganites is considered. It is assumed that in the absence of orbital ordering, a decrease in the magnetic ion coordination number leads to sign reversal in indirect superexchange interactions Mn³⁺-O-Mn³⁺.     

NMR and Mössbauer study of multiferroic BiFeO<Subscript>3</Subscript>

Pulse nuclear magnetic resonance (NMR) was applied in studying the effect of ⁵⁷Fe isotope content in multiferroic BiFeO₃ on the shape of NMR spectra at 4.2 K. Strong dependences of the NMR line shape on the isotope content and transverse relaxation time were found. Consideration of these effects on NMR line shape shows that there is an undisturbed (with no anharmonicity effect) space spin-modulated structure of the cycloid type in BiFeO₃. The Mössbauer effect was also used to investigate the perovskite BiFeO₃ at 650, 295, and 87 K. Experimental spectra allowed us to obtain the distribution of hyperfine fields, which was found to be consistent with studies of the NMR line shape. The local electronic and magnetic states of the iron ion were measured.     

<Superscript>151</Superscript>Eu Mössbauer study of multiferroic Eu<Subscript>0.75</Subscript>Y<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript>

We are herewith reporting the ¹⁵¹Eu Mössbauer spectra collected on a polycrystalline powder sample of Eu_0.75Y_0.25MnO₃ from 15 K to room temperature. All the spectra consist of a single line, whose shape and related sample thickness are dependent on the temperature T. The thermal trend of the mean square displacement of Eu ion, obtained from the spectra analysis, clearly reveals a large low-temperature anharmonicity and in concomitance with the onset of the magnetic ordering consists in a linear strong decrease interrupted by two narrow wells at 29.5 K and 40 K. This behavior is interpreted in connection with the transfer of spectral weight from the 120 cm^-1 optical phonon to the electromagnonic modes. The T-trend of the central shift shows that Eu³⁺ electronic ground state in the magnetically ordered phase differs from the one in the paramagnetic state. Finally, the temperature dependence of the hyperfine field under T _N gives a contribution to interpret some controversial features regarding the phase-diagram.     

Structure and magnetic properties of La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3+δ</Subscript> compounds

The structural and magnetic properties of La_1?x Ca_xMnO_3+δ (x = 0.01–0.10) compounds with a high superstoichiometric oxygen content (0.01 < δ < 0.07) are investigated. It is demonstrated that all the studied compounds are characterized by a magnetically inhomogeneous state at temperatures below 280 K. It is found that the compound with the calcium content x = 0.05 differs substantially in structure and superstoichiometric oxygen content from the other compounds in the system under investigation. The revealed difference in properties of the compounds can be associated with the fact that a specific state or a specific phase is formed in this system at the calcium content x = 0.05.     

Mössbauer investigations of the CaMn<Subscript>7</Subscript>O<Subscript>12</Subscript> double manganite with the nuclei of <Superscript>57</Superscript>Fe probe atoms

The CaMn₇O₁₂ double perovskite-like manganite is studied using Mössbauer spectroscopy with ⁵⁷Fe impurity atoms. The hyperfine parameters of the Mössbauer spectra are found to reflect the specific local structure of this oxide. It is found that the phase transition at T ≈ 90 K is associated with the ordering of the magnetic moments of manganese cations in the octahedral sublattice. The structural phase transition R \(\overline 3 \) ? Im \(\overline 3 \) is shown to occur in the temperature interval from 380 to 450 K, where the rhombohedral (R \(\overline 3 \)) and cubic (Im \(\overline 3 \)) phases of the manganite coexist.     

The influence of oxygen vacancies on the magnetic state of La<Subscript>0.50</Subscript>D<Subscript>0.50</Subscript>MnO<Subscript>3−γ</Subscript> (D=Da,Sr) manganites

The crystal structure and magnetic and electric transport properties of polycrystalline La_0.50D_0.50MnO_3?γmanganites (D=Ca, Sr) were studied experimentally depending on the concentration of oxygen vacancies. The La_0.50Sr_0.50MnO_3?γ system of anion-deficient compositions was found to be stable and possess a perovskite structure only up to the γ=0.25 concentration of oxygen vacancies, whereas, for the La_0.50Ca_0.50MnO_3?γ system, we were able to obtain samples with the concentrations of oxygen vacancies up to γ=0.50. The stoichiometric La_0.50D_0.50MnO₃ (D=Ca, Sr) compositions had O-orthorhombic (Ca) and tetragonal (Sr) unit cells. The unit cell of the anion-deficient La_0.50Sr_0.50MnO_3?γ manganites also became O-orthorhombic when the concentration of oxygen vacancies increased γτ;0.16). Oxygen deficiency in La_0.50Sr_0.50MnO_3?γ first caused the transition from the antiferromagnetic to the ferromagnetic state γ～0.06) and then to the spin glass state γ～0.16). Supposedly, the oxygen vacancies in the reduced La_0.50Sr_0.50MnO_{3? γ} samples with γ≥0.16 were disordered. The special feature of the La_0.50Ca_0.50MnO_3?γ manganites was a nonuniform distribution of oxygen vacancies in the La_0.50Ca_0.50MnO_2.75 phase. In the La_0.50Ca_0.50MnO_2.50 phase, the type of oxygen vacancy ordering corresponded to that in Sr₂Fe₂O₅, which led to antiferromagnetic ordering. The specific electric resistance of the La_0.50D_0.50MnO_3?γ anion-deficient samples increased with increasing oxygen deficiency. The magnetoresistance of all samples gradually increased as a result of the transition to the magnetically ordered state. Supposedly, the La_0.50Ca_0.50MnO_3?γ manganites in the range of oxygen vacancy concentrations 0.09≤γ≤0.50 had a mixed state and contained microdomains with different types of magnetic ordering. The experimentally observed properties can be interpreted based on the model of phase layering and the model of superexchange magnetic ordering.     

Resistance-switching properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> thin films with Ag–Al alloy top electrodes

A novel Ag–Al alloy electrode has been prepared on the La_0.67Ca_0.33MnO₃ (LCMO) film grown by pulsed laser deposition, with the aim to improve its resistance-switching properties. Nonlinear, asymmetric, and hysteretic current–voltage characteristics and reversible polarity-dependent switching properties are achieved in the Ag–Al alloy/LCMO/Pt structure. Detailed current–voltage characteristics analysis indicates that the resistance-switching behavior can be well explained by the mechanism of trap-controlled space charge limited conduction at the Ag–Al alloy/LCMO interface. The LCMO film with an Ag–Al alloy top electrode exhibits much better resistance-switching properties than that with an Al top electrode, including the shorter switching time and more stable switching process, demonstrating that the Ag–Al alloy electrode is a promising electrode materials of manganite films for resistance random access memory applications.     

Domain state–dependent magnetic formation of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles analyzed via magnetic resonance

Magnetic properties, arising from surface exchange and interparticle interactions of the Fe₃O₄ (magnetite) nanoparticles, were investigated in the temperature range of 5–300 and 120–300 K using vibrating sample magnetometer technique and electron spin resonance spectroscopy, respectively. The research was based on to figure out the origin of intraparticle interactions and the change of interparticle interactions in wide size range Fe₃O₄ nanoparticles. The analyses were done for samples having almost same particle size distributions. The average particle sizes were changed in between 30 ± 2 and 34 ± 2 nm. The observed magnetization values were demonstrated the mixture of single-domain size particles, exhibiting both single-domain (SD) and superparamagnetic (SPM) states. The symmetry of resonance curves changed according to the ratio of SD and SPM-stated particles in mixture under located temperature. The changes of anisotropy up to domain state were understood by freezing magnetic moment in glycerol matrix from room temperature to 120 K under 5-kG field. The shift of H _R values to higher magnetic fields and the more symmetric resonance spectrum proved the effect of anisotropy and interparticle interactions fields on magnetic behave. In addition, the origin of intra-interaction was exposed from Fe³⁺ centers and exchange coupling in between Fe²⁺, Fe³⁺, and O⁻, and Fe³⁺ centers found from g factor (g).     

Mössbauer studies of spatial spin-modulated structure and hyperfine interactions in multiferroic Bi<Superscript>57</Superscript>Fe<Subscript>0.10</Subscript>Fe<Subscript>0.85</Subscript>Cr<Subscript>0.05</Subscript>O<Subscript>3</Subscript>

Results of Mössbauer investigations on ⁵⁷Fe nuclei in multiferroic material Bi⁵⁷Fe_0.10Fe_0.85Cr_0.05O₃ in the temperature range from 5.2 to 300 K are presented. Bulk rhombohedral samples were obtained by solidstate synthesis at high pressure. Mössbauer spectra were analyzed using the model of spatial incommensurate spin-modulated structure of the cycloidal type. Information on the influence of substituting Cr cations for Fe cations on hyperfine spectral parameters was obtained: the shift and quadrupolar shift of a Mössbauer line, and isotropic and anisotropic contributions into the hyperfine magnetic field. The anharmonicity parameter m of the spatial spin-modulated structure increases almost 1.7 times at 5.2 K when BiFeO₃ is doped with chromium. The data on m were used for calculation of the uniaxial magnetic anisotropy constants and their temperature dependences for pure and chromium-doped BiFeO₃.     

Structural and magnetoelectric properties of MFe<Subscript>2</Subscript>O<Subscript>4</Subscript>–PZT (M = Ni,Co) and La<Subscript>x</Subscript>(Ca,Sr)<Subscript>1-x</Subscript>MnO<Subscript>3</Subscript>–PZT multilayer composites

Thick-film layered magnetoelectric composites consisting of ferromagnetic and ferroelectric phases have been synthesized with nickel ferrite (NFO), cobalt ferrite, La_0.7Sr_0.3MnO₃ (LSMO), or La_0.7Ca_0.3MnO₃ (LCMO) and lead zirconate titanate (PZT). Structural, magnetic, and ferromagnetic resonance characterization shows evidence for defect-free ferrites, but deterioration of manganite parameters. The resistivity and dielectric constants are smaller than expected values. The magnetoelectric effect (ME) is stronger in ferrite–PZT than in manganite–PZT. The ME voltage coefficient _E at room temperature is the highest in NFO–PZT and the smallest for LCMO–PZT. The transverse ME effect is an order of magnitude stronger than the longitudinal effect. The magnitude of _E correlates well with magnetic permeability for the ferrites. PACS 75.80.+q; 75.70.Gg; 75.60.-d     

XRD,impedance, and Mössbauer spectroscopy study of the Li<Subscript>3</Subscript>Fe<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> + Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> composite for Li ion batteries

The synthesis procedure of the Li₃Fe₂(PO₄)₃?+?Fe₂O₃ composite is presented. The monoclinic (A type) and hematite phases were detected by X-ray diffraction after the synthesis of the composite. The structural α–β (at a temperature of 460 K) and β–γ (at a temperature of 523 K) phase transitions in the composite were indicated by the anomalies of the electrical conductivity, dielectric permittivity, and changes of activation energies of conductivity. Two phase transitions have been detected in the Li₃Fe₂(PO₄)₃?+?Fe₂O₃ composite by ⁵⁷Fe Mössbauer spectroscopy: the phase transition in Li₃Fe₂(PO₄)₃ from the paramagnetic to antiferromagnetic phase at temperature T _N?=?29.5 K and the Morin phase transition in Fe₂O₃ at temperature T _M?=?235 K.     

Preparation of dense La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>Ga<Subscript>0.8</Subscript>Mg<Subscript>0.2</Subscript>O<Subscript>3-δ</Subscript> with high ionic conductivity by solid-state synthesis

The results of a systematic investigation on the effects of processing steps, via solid-state reactions, on structural phase characteristics and ionic conductivity of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ solid electrolyte are reported. The main purpose of this work is to establish an optimized route for obtaining good densification and high ionic conductivity of this solid electrolyte. Processing routes with three successive calcinations at 1250 °C followed by attrition milling (R1), and with two sequences of calcination at 1350 °C with intermediary attrition milling (R2) give rise to near full density at 1450 °C sintering temperature. The rate of grain growth is fast when the relative density reaches 95%. Elemental mapping reveals uniform distribution of the constituents in the matrix along with La₄Ga₂O₉, LaSrGa₃O₇ and sub-micrometer MgO grains at grain boundaries. The ionic conductivity of grains remains unchanged with the processing route and sintering profile. The blocking effect of charge carriers at grain boundaries decreases with increasing the dwell temperature.     

Mössbauer and X-ray diffraction study of structural phase transitions in LaMnO<Subscript>3 + δ</Subscript>

The influence of the conditions of thermal treatment on the formation and mutual transitions of structural modifications of the LaMnO_{3 + δ} compound doped with ⁵⁷Fe (4%) have been investigated by Mössbauer spectroscopy and X-ray diffraction analysis. It is found that heat treatment in vacuum and in air ensures reversible structural transformations of two orthorhombic and one rhombohedral phases (space groups Pnma and \(R\bar 3c\), respectively). Changes in the configuration of the oxygen environment of cations at reversible phase transitions are considered.