Analysis of the Magnetic Structure of the BiFeO<Subscript>3</Subscript> Multiferroic by Mössbauer Spectroscopy

Specific features of the magnetic structure of the BiFeO₃ multiferroic have been investigated by Mössbauer spectroscopy in the temperature range of 150–450 K. The temperature dependence of the anharmonicity parameter of the cycloidal spatial spin-modulated structure has been measured, and the magnetic anisotropy constant has been estimated.     

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.     

Mössbauer study of mechanical alloying in a Mo<Subscript>80</Subscript>Fe<Subscript>20</Subscript> system

The sequence of solid state reactions upon the mechanical alloying of Mo and Fe powders with an 80: 20 atomic ratio was established by means of Mössbauer spectroscopy and X-ray diffraction. At the first stage, a nanostructure and Mo₆₃Fe₃₇ hexagonal close packed (HCP) phase are formed in Mo body-centered cubic lattice (BCC) particles. At the second stage, a body-centered cubic lattice of Mo-Fe solid solution is formed. The process is accompanied by the formation of a minor amount (about 20%) of X-ray amorphous phase.     

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.     

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.     

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 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.     

Magnetic Properties of Neutron-Irradiated RPV Steel by Mössbauer Spectroscopy

The neutron irradiated reactor pressure vessel (RPV) steels at various doses of 010¹⁸ n/cm² have been studied with Mössbauer, X-ray diffraction, and VSM. The Mössbauer data show that the value of a magnetic hyperfine field of Fe atom that exists at the martensite is 330 kOe at site 1, and 305 kOe, at site 2. At room temperature, the total absorption area of Mössbauer spectra with respect to irradiation of neutron is constant for the dose of 010¹⁶ n/cm², while over the dose of 10¹⁷ cm² the absorption area decreases rapidly. But the doublet area for the dose of 010¹⁶ n/cm² is constant, while over the dose of 10¹⁷ cm² it increases with an increase in the fluence level of neutron. The value of IS and QS at site 1,2 varied slightly with an increase in the fluence level of neutron. However, at a doublet site existing Fe³⁺ state, over the dose of 10¹⁷ cm², the values of IS and QS increase with an increase in the fluence level of neutron. Note that over the dose of 10¹⁷ n/cm² the neutron irradiated sample loses crystal structure slowly. The coercivity and remanence of the neutron irradiated samples do not change significantly. But the maximum induction decreases by 5% at 10¹⁸ n/cm², us compared with that of the as-received sample.     

Structural and Mössbauer studies of nanocrystalline Mn<Superscript>4+</Superscript>-doped Li<Subscript>0.5</Subscript>Fe<Subscript>2.5</Subscript>O<Subscript>4</Subscript> particles prepared by mechanical milling

The structure and magnetic properties of spinel-related Mn⁴⁺-doped Li_0.5Fe_2.5O₄ nanocrystalline particles of the composition Li_0.5Fe_2.25Mn_0.1875O₄, prepared by milling a pristine sample for different times, were investigated. The average crystallite and particle size, respectively, decreased form ～40 nm to ～10 nm and ～2.5 μm to ～10 nm with increasing milling time from 0 h to 70 h. Rietveld refinement of the XRD data of the non-milled sample show the Mn⁴⁺ dopant ions to substitute for Fe³⁺ at the octahedral B-sites of the spinel-related structure. The Mössbauer spectra of the milled ferrites indicate that more particles turn superparamagnetic with increasing milling time. The Mössbauer data collected at 78 K suggest that while in the non-milled sample the Mn⁴⁺ ions substitute for Fe³⁺ at the octahedral B-sites, this is reversed as milling proceeds with doped Mn⁴⁺ ions, balancing Fe³⁺ vacancies and possibly Li⁺ ions progressively migrate to the tetrahedral A-sites. This is supported by the slight increase observed in the magnetization of the milled samples relative to that of the non-milled one. The magnetic data suggest that in addition to the increasing superparamagentic component of the milled particles, thermal spin reversal and/or spin canting effects are possible at the surface layers of the nanoparticles.     

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.     

Investigation of quasicrystalline Al<Subscript>62.5</Subscript>Cu<Subscript>25</Subscript>Fe<Subscript>12.5</Subscript> and crystalline β-Al<Subscript>50</Subscript>Cu<Subscript>33</Subscript>Fe<Subscript>17</Subscript> alloys by X-ray photoelectron spectroscopy

The electronic spectra of the valence band and core levels of the surface of polygrain alloys with the icosahedral structure and the β-(CsCl)-type solid solution of Al₅₀Cu₃₃Fe₁₇ were investigated by X-ray photoelectron spectroscopy (XPS). The obtained XPS spectra of the Al_62.5Cu₂₅Fe_12.5 alloy, in comparison with those of the crystalline Al₅₀Cu₃₃Fe₁₇ alloy demonstrate narrowing and a decrease in asymmetry of the Fe2p core level and a decrease in the electron state density N(E _F ) near the Fermi level, features expected for the poorly conducting icosahedral phase. The XPS data are compared with the estimates of N(E _F ) based on the low-temperature specific heat measurements.     

Magnetic property and Mössbauer analysis of SrSn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Bold">x</Emphasis></Subscript>O<Subscript>3</Subscript> prepared by a sol-gel method

The dilute magnetic properties of SrSn_1?xFe_xO₃ (x = 0.01 ? 0.15) prepared by sol-gel and thermal decomposition methods were investigated by ⁵⁷Fe Mössbauer spectrometry, magnetometry, and X-ray diffractometry. It was found that SrSnO₃ doped with 2–8 % Fe show weak ferromagnetism although only paramagnetic doublets are observed in ⁵⁷Fe Mössbauer spectra at room temperature (RT), whereas SrSnO₃ doped with 10–15 % Fe show relatively strong ferromagnetism, and the sextets are additionally observed in the ⁵⁷Fe Mössbauer spectra at RT. The weak ferromagnetism by doping 2–8 % Fe is considered to be caused by the induced magnetic defects, and the ferromagnetism by doping 10–15 % Fe are considered mainly due to the magnetic coupling between dilute Fe ³⁺ partially substituted at Sn ⁴⁺ sites in the orthorhombic structure of SrSnO_3?δ accompanying the oxygen deficiencies. It is further remarkable that poor crystalline 8 % Fe doped SrSnO_3?δ obtained by annealing at 600 ^°C shows relatively high saturation magnetization and low coercivity.     

Mössbauer study of the magnetocaloric compound AlFe<Subscript>2</Subscript><Emphasis Type="Bold">B</Emphasis><Subscript>2</Subscript>

Mössbauer spectroscopy in the ferromagnetic AlFe₂ B ₂ reveals T_c=299 K and shows good agreement with magnetic measurements. The crystals are plate-shaped. The flakes are found from X-ray diffraction to be in the crystallographic ac-plane in the orthorhombic system. The axes of the principle electric field gradient tensor are, by symmetry, colinear with the crystal a-, b- and c-axes. By using information about the quadrupole splitting and line asymmetry in the paramagnetic regime together with the quadrupole shift of the resonance lines in the ferromagnetic regime the magnetic hyperfine field direction is found to be in the ab-plane having an angle =40^° to the b-axis.     

<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.     

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.     

A structural,magnetostrictive and Mössbauer study of Tb<Subscript>0.3</Subscript>Dy<Subscript>0.7</Subscript>(Fe<Subscript>0.9</Subscript><Emphasis Type="Italic">T</Emphasis><Subscript>0.1</Subscript>)<Subscript>1.95</Subscript> alloys

The effect of IIIA metal and transition metalT substitution for Fe on crystal structure, magnetostriction and spontaneous magnetostriction, anisotropy and spin reorientation of a series of polycrystalline Tb_0.3 Dy_0.7 (Fe_0.9 T _0.1)_1.95 (T=Mn, Fe, Co, B, Al, Ga) alloys at room temperature were investigated systematically. It was found that the primary phase of the Tb_0.3 Dy_0.7 (Fe_0.9 T _0.1)_1.95 alloys is the MgCu₂-type cubic Laves phase structure for different substitution. The magnetostrictionλ _s decrases greatly for the substitution of IIIA metal, B, Al and Ga, but is saturated more easily for Al and Ga substitution, showing that the Al and Ga substitution is beneficial to a decrease in the magnetocrystalline anisotropy of Tb_0.3 Dy_0.7 (Fe_0.9 T _0.1)_1.95 alloys. However, the substitution of transition metal Mn and Co decreases slightly the magnetostrictionλ _s . It was also found that the effect of different substitutions on the spontaneous magnetostrictionλ ₁₁₁ is distinct. The analysis of the Mössbauer spectra indicates that the easy magnetization direction in the {110} plane deviates slightly from the main axis of symmetry for Al and Ga substitution, namely spin reorientation, but it does not change evidently for B, Mn and Co substitution.     

Mössbauer and magnetic studies of the phase state of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/La<Subscript>0.9</Subscript>Ca<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> composites

The formation of an intermediate phase in SrFe₁₂O₁₉/La_0.9Ca_0.1MnO₃ composites was demonstrated for the first time using only Mössbauer spectroscopy. The SrFe₁₂O₁₉/La_0.9Ca_0.1MnO₃ composite was prepared by the two-stage (sol–gel and hydrothermal) synthesis with varying initial conditions. The X-ray diffraction studies showed that the composite consisted of two phases: well-formed structures of manganite La_0.9Ca_0.1MnO₃ and hexagonal ferrite SrFe₁₂O₁₉. It was found that nanocrystalline La_0.9Ca_0.1MnO₃ particles with size d ? 150 nm formed in the composites at the surface of plate-like SrFe₁₂O₁₉ crystallites. The Mössbauer studies showed that the composite contained additional (intermediate) phase La_0.9Ca_0.1Mn(Fe)O₃ that formed at the interface between SrFe12O19 and La_0.9Ca_0.1MnO₃ phases. The intermediate phase concentration increased with the molar content of La_0.9Ca_0.1MnO₃; in this case, the fraction of the surface of SrFe₁₂O₁₉ crystallites coated with La_0.9Ca_0.1MnO₃ increased, which led to the increase in the total area of the interface surface and the intermediate phase concentration.     

Mössbauer study of SnO<Subscript>2</Subscript> powders doped with dilute <Superscript>57</Superscript>Fe prepared by a sol-gel method

SnO₂ powders, doped with various ⁵⁷Fe contents were prepared by a sol-gel method, and annealed finally at 500 °C and 650 °C. These samples were characterized by Mössbauer spectroscopy, vibrating sample magnetometer (VSM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) to investigate the relationship of magnetic properties, grain sizes, annealing temperatures and Mössbauer parameters. The particle sizes of SnO₂ powders reduced to less than 100 nm with the increase of Fe contents up to 5%. Rutile SnO₂ was the only phase obtained for all samples. Room temperature Mössbauer spectra suggest the presence of two different paramagnetic iron sites for all samples and one magnetically relaxed species for those samples with the lowest iron concentrations. The magnetization increased with the Fe content, but was reduced for the samples annealed at 650 °C perhaps due to a segregation of α-Fe₂O₃ doped with tin.     

Sm valence states and magnetic properties in SmBe<Subscript>13</Subscript> and SmTi<Subscript>2</Subscript>Al<Subscript>20</Subscript> investigated by Sm synchrotron-radiation-based Mössbauer spectroscopy

Synchrotron-Radiation-based \(^{\mathrm {149}}\)Sm Mössbauer spectroscopy was applied to Sm intermetallics, SmBe₁₃ and SmTi₂Al₂₀. Temperature dependence of the Mössbauer parameters in SmBe₁₃ indicate the Sm valence state is purely trivalent. SmBe₁₃ also showed second-order Doppler shift in synchrotron-radiation-based \(^{\mathrm {149}}\)Sm Mössbauer spectroscopy. The Mössbauer parameters obtained in SmTi₂Al₂₀ suggest that the Sm valence is fluctuating and the magnitude of the magnetic moment is reduced by hybridization between 4f and conduction electrons and/or effect of crystal electric field.     

The Influence of the Annealing Temperature on the Properties of Sn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> Powders Evidenced by EMR Spectroscopy

Electron magnetic resonance (EMR) investigations of Sn_{1− x} Fe _x O₂ (x = 0.07) powders annealed at different temperatures are reported. EMR spectra show the presence of both the isolated paramagnetic Fe³⁺ ions incorporated into the SnO₂ lattice and a ferromagnetically ordered component. By increasing the annealing temperature, the EMR spectrum attributed to the ferromagnetically coupled Fe³⁺ ions diminishes probably due to the migration of Fe ions and the formation of the second phase. The presence of the second phase was evidenced by X-ray diffraction measurements.