A. Simopoulos

Mössbauer study of La0.75Ca0.25Mn0.98Fe0.02O3 compound

A thorough study of the Mossbauer spectra (MS) of a lightly iron doped (2%) polycrystalline La0.75Ca0.25MnO3 compound has been carried out from T=4.2 K up to 300 K. The MS for T>Tc and at T=4.2 K show one component. The corresponding hyperfine parameters are compatible with S=5/2 Fe3+ in octahedral coordination. As temperature increases from 4.2 K the spectra consist of several broad sextets and doublets (especially the Tc). This first reported behavior of MS resembles that of ferromagnetic clusters.A thorough study of the M{umlt o}ssbauer spectra (MS) of a lightly iron doped (2{percent}) polycrystalline La{sub 0.75}Ca{sub 0.25}MnO{sub 3} compound has been carried out from T=4.2 K up to 300 K. The MS for T{gt}T{sub c} and at T=4.2 K show one component. The corresponding hyperfine parameters are compatible with S=5/2 Fe{sup 3+} in octahedral coordination. As temperature increases from 4.2 K the spectra consist of several broad sextets and doublets (especially the T{sub c}). This first reported behavior of MS resembles that of ferromagnetic clusters. {copyright} {ital 1997 American Institute of Physics.}

Magnetic hysteresis and Mössbauer studies in ultrafine iron particles

The magnetic hysteresis behavior of ultrafine Fe particles has been examined by SQUID magnetometry and Mossbauer spectroscopy. Ultrafine Fe particles have been prepared by vapor deposition under argon atmosphere in pressures from 1 to 6 Torr and passivated by exposure to oxygen at low pressure. Particles with size below 10 nm showed a small coercivity (∼100 Oe) at room temperature, which increased drastically upon cooling to cryogenic temperatures (1000–1500 Oe). The larger particles had the highest coercivity at room temperature (∼1000 Oe) which increased by 100% at 10 K. Mossbauer studies revealed that the Fe particles are surrounded by an Fe oxide layer composed of ultrafine Fe3O4 or γ‐Fe2O3 grains. At room temperature the oxide component gives a very broad absorption superimposed on an α‐Fe spectrum. The enhanced coercivity at room temperature may be attributed to anisotropy induced by exchange interaction between the ferromagnetic core and the ferri‐ or antiferromagnetic oxide layer.

Synthesis, thermogravimetric and 57Fe Mössbauer studies of the oxygen deficient perovskite REBaCuFeO5 + x series (RE = Y, Nd, Sm, Gd, Dy, Tm, Lu)

Abstract We present the results of thermogravimetric and 57 Fe Mossbauer studies of the oxygen deficient perovskite REBaCuFeO 5+x series with RE = Y, Nd, Sm, Gd, Dy, Tm, Lu. The presence of the removable content x of oxygen was determined by analysis of TGA measurements in a stream of Ar gas. The amount of removable oxygen was highest for the Nd compound and varied between 0.07 and 0.12 for the other compounds. By means of the Rietveld method we performed the refinement of the structure of the compounds, based on the space group P4mm, after taking into consideration that the Fe ion occupies the lb 1 ( z = 0.27) site and Cu the lb 2 ( z = 0.74) site. From the analysis of the Mossbauer spectra we found that the compounds become magnetically ordered with transition temperatures depending on the oxygen content. The values of the hyperfine parameters in all the samples indicate the ferric iron to be in a high spin state.

Origin of coercivity in (Fe,Co)-based granular films

The magnetic behavior of Co and Fe granular films was studied relative to their host matrix environment (BN, SiO2). The crystal structure of Co and Fe in the as‐deposited samples is α‐Co (hcp) and α‐Fe (bcc) respectively, with particle sizes ranging between 3 and 9 nm. The coercivities measured ranged from a few tens to a few hundreds Oe, with the higher values observed for particles embedded in the oxygen based matrix.

Structural and intrinsic magnetic material parameters of Pr3(Fe,Ti)29 and Pr3(Fe,Ti)29Nx

We report the study of the structural and the intrinsic magnetic properties of the Pr member of the newly discovered class of R3(Fe,Ti)29 compounds and its nitride. The X-ray powder diffraction pattern of the alloy is indexed in monoclinic symmetry with lattice parameters a = 10.647(1) A, b = 8.6014(7) A, c = 9.755(1) Aand β = 96.92(1)° and the structure is described in the A2/m space group. Atomic positions and bond lengths are given. Nitrogenation results in a lattice expansion of 6.6% corresponding to ∼ 4N atoms per formula unit. The Curie temperature is 392(5) K, and the saturation magnetization, the anisotropy field and the average hyperfine field at room temperature are 135.4 A m2/kg, 3.9 and 20.3 T, respectively. A magnetic phase transition is observed at ∼ 160 K. After nitrogenation the Curie temperature increases to 721(5) K, and the saturation magnetization to 174.8 A m2/kg, the anisotropy field 7.2 T and the average hyperfine field 30.1 T at room temperature. Mossbauer spectroscopy, X-ray powder diffraction and magnetization measurements on magnetically oriented powder samples provide evidence of the presence of an easy-cone-type magnetocrystalline anisotropy for both the parent and nitrided compounds in the temperature range 85–300 K. The cone angles calculated from the fitted Mossbauer spectra are 34° for the parent compound and 36° for the nitrided compound.

Size effects on the magnetic properties of fine Fe-Cr particles

Abstract The magnetic and structural properties of Fe 1003− x Cr x ultrafine particles with x = 5–20 have been studied as a function of particle size. Particles with a size in the range of 80–360 A were prepared by gas evaporation under argon atmosphere. The particles with smaller diameter had a high coercivity at low temperatures and showed a stronger temperature dependence of coercivity. The x = 20 particles with a size 80 A had a coercivity about 2100 Oe at 10 K with a superparamagnetic blocking temperature about 150 K. Mossbauer spectra showed the presence of Fe-Cr, α-Fe and Fe-oxide components in the bigger particles, and Fe-Cr and Fe-oxides in the smaller particles. The coercivity at low temperatures increased with decreasing particle size and this was attributed to the higher percentage of Fe-oxide on the surface of the smaller particles. This interpretation was further supported by the temperature dependence of coercivity of Fe–Cr particles sandwiched between two Ag films.

Structure and magnetic anisotropy of Fe/Pt multilayers

A series of Fe/Pt multilayers prepared by magnetron sputtering have been studied by XRD, conversion electron Mossbauer spectroscopy (CEMS) and SQUID magnetometry. The samples with individual Fe thickness of 3 A show orientation of the magnetic moment close to perpendicular to the film plane and Curie temperatures near 350 K. Interdiffusion at the interface is manifested in a distribution of hyperfine fields.

Defect clusters in Fe1−xO and their ferrimagnetic properties

Ferrimagnetic properties, large saturation magnetization (Ms) and low temperature coercivity (Hc) was observed in nonstoichiometric Fe1−xO films. This unusual behavior was attributed to the existence of spinel type defect clusters coherently embedded in the FeO matrix. A model, which explains the Fe1−xO properties and predicts a linear dependence between Ms and the Fe1−xO lattice parameter a with the Fe3O4 data falling on the same line, is proposed. The experimental Ms vs a curve is in excellent agreement with the theoretical prediction, which indicates the existence of a fundamental relation between FeO, Fe1−xO, and Fe3O4.

Site occupancy of Fe in the oxygen-saturated YSr2Cu3−xFexOy compound for x=0.25 up to 1

Abstract We have studied the crystal structure, the site occupancy of iron at the Cu(1), Cu(2) positions and the magnetic properties of the YSr 2 Cu 3− x Fe x O y system ( x =0.25 up to 1) by using X-ray diffraction, Mossbauer spectroscopy and magnetic-susceptibility data. The system is superconducting with T c =40, 30 and 20 K for x =0.25, 0.30 and 0.35, respectively, while for x ⩾0.4 no superconductivity is observed down to T =4.2 K. The structure is tetragonal for all x and the refinement was made assuming the space group P4/mmm. The Mossbauer data reveal three iron sites one of which is assigned to the Cu(2) planes and the other two to the Cu(1) chains. Their occupancy varies with x . The above data are discussed in comparison with the YBa 2 Cu 3− x Fe x O y system.

Structural and Mössbauer studies in REBaCuFeO5+x compounds

Abstract We have studied the non superconducting phase REBaCuFeO 5+x (where RE=rare earth). with neutron, x-ray powder diffraction and Mossbauer spectroscopy.