I. Felner

Sonochemical synthesis and characterization of pure nanometer-sized Fe3O4 particles

Sonochemical synthesis of pure nanometer-size Fe 3 O 4 powder with particle size of ca 10 nm is reported in this article. Fe 3 O 4 can be simply synthesized by sonication of iron(II)acetate in water under an argon atmosphere. The properties of pure nanometer-size Fe 3 O 4 particles were characterized by X-ray diffraction, Mossbauer spectroscopy, transmission electron microscopy (TEM), thermogravimetric analysis (TGA) with an external magnetic field, and quantum design SQUID magnetization measurements. The prepared Fe 3 O 4 nanoparticles are superparamagnetic and its magnetization at room temperature is very low (< 1.25 emu g -1 ).

Fabrication of magnetite nanorods by ultrasound irradiation

Magnetite nanorods have been prepared by the sonication of aqueous iron(II)acetate in the presence of β-cyclodextrin. The properties of the magnetite nanorods were characterized by x-ray diffraction, Mossbauer spectroscopy, transmission electron microscopy, thermogravimetric analysis, and magnetization measurements. The as-prepared magnetite nanorods are ferromagnetic and their magnetization at room temperature is ∼78 emu/g. The particle sizes measured from transmission electron micrographs are about 48/14 nm (L/W). A mechanism for the sonochemical formation of magnetite nanorods is discussed.

Low-temperature resistivity minimum in ceramic manganites

Measurements of magnetoresistance and magnetization were carried out on ceramic samples of La0.5Pb0.5MnO3 and La0.5Pb0.5MnO3, containing 10 at. % Ag in a dispersed form. The results obtained for the resistivity at zero applied magnetic field exhibit a shallow minimum at the temperature T∼25–30 K which shifts towards lower temperatures upon applying a magnetic field and disappears at a certain field Hcr. Also the resistivity at helium temperature decreases upon applying magnetic fields. It is shown that the model of charge carriers tunneling between antiferromagnetically coupled grains may account for the results observed.

New multiple magnetic phase transitions and structures in RMn2X2, X = Si or Ge, R = rare earth

Abstract Magnetometry and dilute 57 Fe Mossbauer spectroscopy studies of RMn 2 X 2 (X = Si or Ge, R = La, Ce, Pr, Nd, Sm and Gd) at temperatures 4.2–650 K yield the following results; Fe in RMn 2 X 2 is nonmagnetic. It reveals the magnetic order in the Mn and R sublattices through transferred hyperfine fields. The compounds LaMn 2 Si 2 , LaMn 2 Ge 2 , CeMn 2 Ge 2 , PrMn 2 Ge 2 , NdMn 2 Ge 2 and SmMn 2 Ge 2 , known to be ferromagnets with T C = 300–350 K, are antiferromagnetically ordered above their corresponding T C . Their T N values extend from 385 K (SmMn 2 Ge 2 ) to 470 K (LaMn 2 Si 2 ), similar to the T N values of the antiferromagnetic heavy rare earth compounds. At the ferromagnetic-antiferromagnetic phase transition, a sharp reorientation of the Mn magnetic moments relative to the crystalline axes occurs. In SmMn 2 Ge 2 we find five magnetic phase transitions, T C (Sm) = 30 K and T C (Mn) at 105 and 345 K and T N (Mn) at 155 and 385 K. In this compound, a superposition of two six-line 57 Fe Mossbauer patterns is seen between 90 and 155 K with changing relative intensities, indicating a competition of two easy magnetization axes, with an anisotropic transferred hyperfine field at the Fe nucleus. In NdMn 2 Ge 2 we find four phase transitions, T C (Nd) = 21 K, T C (Mn) = 335 K, T N (Mn) = 415 K, and one more very sharp transition at 210 K, associated with a discontinuity in 57 Fe hyperfine interaction parameters and a sharp drop in bulk magnetization; this seems to be a transition from pure ferromagnetism to canted antiferromagnetism. The results for antiferromagnetic CeMn 2 Si 2 , PrMn 2 Si 2 and GdMn 2 Ge 2 revealed no new phenomena and are in full agreement with previous magnetization studies. In GdMn 2 Ge 2 the transferred hyperfine field at the 57 Fe nucleus is smaller at 4.2 K (below the ordering temperature of Gd) than at 90 K, proving that the transferred hyperfine field from Gd is opposite to that produced by Mn.

SYNTHESIS OF PURE AMORPHOUS FE2O3

A method for the preparation of pure amorphous Fe 2 O 3 powder with particle size of 25 nm is reported in this article. Pure amorphous Fe 2 O 3 can be simply synthesized by the sonication of neat Fe(CO) 5 or its solution in decalin under an air atmosphere. The Fe 2 O 3 nanoparticles are converted to crystalline Fe 3 O 4 nanoparticles when heated to 420 °C under vacuum or when heated to the same temperature under a nitrogen atmosphere. The crystalline Fe 3 O 4 nanoparticles were characterized by x-ray diffraction and M¨ossbauer spectroscopy. The Fe 2 O 3 amorphous nanoparticles were examined by Transmission Electron Micrography (TEM), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), and Quantum Design SQUID magnetization measurements. The magnetization of pure amorphous Fe 2 O 3 at room temperature is very low (

Magnetic properties of RSr2RuCu2O8+δ (R=Eu and Gd)

Abstract The non superconducting RSr 2 RuCu 2 O 8+ δ (R=Eu and Gd) compounds are magnetically ordered below T N =168 and 185 K, respectively. Magnetic susceptibility (ac and dc) studies indicate that the magnetic ordering is due to the Ru sublattice. The Gd sublattice in GdSr 2 RuCu 2 O 8 is antiferromagnetically ordered at 2.8 K. Irreversibility phenomena and magnetic anomalies, observed at low magnetic fields, originate from antisymmetric exchange coupling of the Dzyaloshinsky–Moria type, and from spin reorientation of the Ru moments. The magnetic behavior of this system and that of the well-known itinerant ferromagnet SrRuO 3 are compared.

Magnetism and hyperfine interactions of 57Fe, 151Eu, 155Gd, 161Dy, 166Er and 170Yb in RM4Al8 compounds (R = rare earth or Y, M = Cr, Mn, Fe, Cu)☆

Abstract Mossbauer and magnetic susceptibility studies of sixty tetragonal RM 4 Al 8 compounds ( R = 4 f , M = 3 d element), show a wide variety of magnetic phenomena in the behaviour of 3 d transition elements. The rare earths order antiferromagnetically at temperatures below 10–30 K in all compounds. The 3 d elements, however, all behave differently. Fe in R Fe 4 Al 8 has a localized moment (effective moment of 4.4 μ B ) and orders independently of the rare earth sublattice. Mn in R Mn 4 Al 8 has also a localized moment (∼1 μ B ) but orders only when the rare earths order. Cr in R Cr 4 Al 8 has no moment of its own, but it has an induced moment (.1 μ B ) by its magnetic rare earth neighbours. Cu in R Cu 4 Al 8 is nonmagnetic. The Mossbauer studies of 151 Eu, 155 Gd, 161 Dy, 166 Er, 170 Yb and a 57 Fe probe yield all hyperfine interaction parameters including the orientation of the hyperfine field relative to the crystallographic c -axis. In addition, the studies yield the Ce, Eu and Yb valencies in the various compounds. Eu in EuFe 4 Al 8 and in EuMn 4 Al 8 and Yb in YbCr 4 Al 8 are in a mixed valent state.

Mössbauer spectroscopy of 57Fe in high Tc superconductors YbA2Fe3xCu3(1−x)O7−δ

Abstract Mossbauer spectroscopy of 57 Fe in both tetragonal and othorhombic phases of YBa 2 ( Fe x Cu 1− x ) 3 O 7− δ , with x = 0.01, 0.02 and 0.10, at temperatures 4.2 K, 75 K, 90 K, and 300 K have been performed. In all samples three major subspectra corresponding to iron in different local environments are observed. It is concluded that Fe substitutes mainly Cul. At 4.2 K, samples with x =0.01 in the “quenched” tetragonal phase exhibit magnetic hyperfine structure, due to slow spin relaxation rates, whereas in the orthorhombic superconducting phase, only samples with x =0.1 exhibit magnetic hyperfine structure, in this case probably due to spin glass magnetic order.

Crystal structure magnetic properties and hyperfine interactions in RFe4Al8 (R = rare earth) systems

Abstract X-ray, magnetic susceptibility and 57Fe, 151Eu, 155Gd and 170Yb Mossbauer studies were performed. Detailed analysis of X-ray intensities yields all ion locations and interatomic distances in the body centered tetragonal structure (space group I4/MMM). The unit cell contains two formula units. The rare earth, iron and aluminum occupy the 2(a), 8(f) and 8(i) and 8(j) crystallographic sites, respectively. The susceptibility and Mossbauer studies indicate the existence of two independent magnetic sublattices. The iron sublattice orders into an antiferromagnetic structure at about 120 K, whereas the rare earth sublattice orders (excluding those with La, Ce, Eu, Y and Lu) antiferromagnetically at about 20 K. The 57Fe, 151Eu, 155Gd and 170Yb Mossbauer studies yield, in addition to the hyperfine interaction parameters, also the direction along which the moments are aligned. In EuFe4Al8 the Eu ion is in a mixed valent state.

Influence of synthesis procedure on the YIG formation

The influence of synthesis procedure on the yttrium iron garnet (YIG; Y3Fe5O12) formation has been investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Mossbauer and magnetization measurements. The samples were prepared by coprecipitation or ceramic processing using the starting molar ratio Y2O3/Fe2O3=3:5. The fractions of Y2O3, α-Fe2O3, YFeO3 and YIG present in the samples depended on the method of materials processing and the calcination temperature. XRD of the thermally treated hydroxide coprecipitate at 1173 K showed the formation of YIG as a dominant phase, and YFeO3 and Y2O3 as associated phases, whereas upon heating at 1473 K, YIG and a small amount of YFeO3 were found. The samples produced by combining ball-milling of the starting powder and ceramic processing at 1573 K contained YIG and a smaller amount of YFeO3, as found by XRD. It was shown that high-energy ball-milling with stainless steel can be substituted by milling with agate bowl and balls, thus decreasing the contamination of the oxide system due to wear. FT-IR and 57Fe Mossbauer spectroscopic measurements were in agreement with XRD; however, the smaller amount of YFeO3 produced at 1573 K could not be detected with certainty by means of FT-IR and 57Fe Mossbauer spectroscopies. The magnetization values of end-products measured at 5 K were in agreement with their phase composition.