S. J. Stewart

Effect of ion doping on CuO magnetism

We have investigated the magnetism of 0.5 at. % Fe3+-doped CuO samples by Mossbauer spectroscopy and magnetization studies. The Curie–Weiss-type behavior observed for low-field-magnetization measurements at temperatures higher than 220 K is interpreted as an uncompensation of moments enhanced by the doping. The results are compared with 1.2 at. % Zn2+-doped CuO magnetization results and previous ones for samples doped with divalent ions. Trivalent ion doping produces an increase in the susceptibility with a peak at TN. The magnetic coupling of Fe2+ ions to the magnetic lattice is found to be about four times stronger than that of Fe3+.

Magnetic hardness features and loop shift in nanostructured CuO

Nanostructures of cupric oxide (CuO) obtained by ball milling show drastic changes in its magnetic behavior that cannot be only associated to a size effect. While sample of average size D = 29 nm presents a magnetic behavior that resembles that of bulk material with a Neel temperature of 195 K, another sample with D = 24 nm displays a departure from the magnetic features typical of bulk CuO and has magnetic hardness characteristics at low temperatures. Both samples show irreversibility above room temperature and shifts in their hysteresis loops along magnetization and field axis when field cooled in a HFC = 50 kOe to 10 K. At this temperature, an apparent exchange bias like field, “HEB”, 0.17 and 1.06 kOe were estimated for 29 and 24 nm CuO samples, respectively. Magnetic behavior differences observed in samples subjected to distinct milling times are explained as due to a proposed model for milled CuO consisting of a multilayer configuration where interfaces comprise uneven structural disorder and oxygen...

Phase transformations in Fe-doped cupric oxide

Abstract This work is a study of 57 Fe-doped CuO samples intended to obtain a compound of nominal composition (Cu 1− x Fe x )O ( x = 0.005; 0.010) through the nitrate route and solid state reactions. Afterwards, the samples were mechanically ground for different times. The specimens were characterized by Mossbauer Spectroscopy and X-ray diffraction. For samples that were not mechanically ground, the iron phases were tetragonal and cubic CuFe 2 O 4 spinel, and CuFeO 2 , delafossite, independent of the preparation method with no evidence of Fe inclusion into the CuO lattice. For mechanically ground samples, a metastable paramagnetic phase has been found.

Mössbauer study of Fe-Zn-O phases

Abstract This work is a study of the phases present in the Fe-Zn-O system with nominal compositions (Zn1−xFex)O (x=0.01, 0.05, 0.20 and 0.30). The samples obtained correspond to a mixture of ZnO, α-Fe2O3 and ZnFe2O4, in relative amounts that depend on x and on the heat treatment. For sintering temperatures above 500°C, α-Fe2O3 disappears and all the iron in the system goes into ZnFe2O4. Air-treated samples show iron only in the 3+ oxidation state while for samples treated in a reducing atmosphere, Fe3+ is reduced to Fe+2, which replaces Zn+2 in the ZnO lattice.

History-dependent magnetic properties in pure and Zn-doped cupric oxide

Abstract Magnetization curves taken in zero-field-cooling and field-cooling regimes on pure CuO and Zn-doped CuO samples demonstrate thermal irreversibility. For the Zn-doped sample, this is present above the Neel temperature ( T N ) and is reinforced below T N . Hysteresis is related to inherent frustration of the magnetic structure of CuO.

Effects of coexisting spin disorder and antiferromagnetism on the magnetic behavior of nanostructured (Fe79Mn21)1−xCux alloys

We report a magnetic study on nanostructured (Fe79Mn21)1−xCux (0.00 ≤ x ≤ 0.30) alloys using static magnetic measurements. The alloys are mainly composed by an antiferromagnetic fcc phase and a disordered region that displays a spin-glass-like behavior. The interplay between the antiferromagnetic and magnetically disordered phases establishes an exchange anisotropy that gives rise to a loop shift at temperatures below the freezing temperature of moments belonging to the disordered region. The loop shift is more noticeable as the Cu content increases, which also enhances the spin-glass-like features. Further, in the x = 0.30 alloy the alignment imposed by applied magnetic fields higher than 4 kOe prevail over the configuration determined by the frustration mechanism that characterizes the spin glass-like phase.

Druse Pyroxenes in D’Orbigny: A Mössbauer Spectroscopy Study

D’Orbigny meteorite, the largest angrite known, is peculiar in its abundance of hollow shells, abundant presence of glasses, and open druses with perfectly crystallized augites of prismatic habit. We report on the distribution of Fe2+ among the nonequivalent sites in the druse pyroxenes of D’Orbigny using Mossbauer spectroscopy measurements on 200 mg of euhedral augites (70–400 mm). Electron microprobe analyses showed augites chemically homogeneous with a slight compositional change at the outermost ∼5 μ (rim). Cation distribution over nonequivalent lattice sites in minerals can give information on its pT history. When pyroxene crystals cool slowly to temperatures below 500°C, the Fe2+ ions populate primarily the M2 position, whereas the Mg2+ ions occupy predominantly the M1 position. Crystals cooling rapidly have a more disordered Mg,Fe distribution over the M1 sites. Mossbauer spectra of our sample obtained at RT consist of an intense inner doublet due to Fe2+ at the M2 sites and a less intense outer do...

Coordination and electronic spin state of iron in Fe-doped Y2BaCuO5

Abstract Magnetic susceptibility, electron paramagnetic resonance (EPR), Mossbauer spectroscopy, and X-ray diffractometry have been used to study Y2BaCu1-xFexO5 (x = 0.05, 0.10 and 0.15). Susceptibility data show antiferromagnetic Cu-Cu interactions that are affected by Fe doping. Mossbauer and EPR parameters indicate that Fe3+ ions replace Cu at the square-pyramidal oxygen coordination sites of the structure. An electronic spin state S = 3 2 for Fe3+ is inferred. These results are compared with the electronic and spin configurations of Fe in the YBa2(Cu1-xFex)3O7-δ (1-2-3) superconductor .

Magnetic Response and Hyperfine Magnetic Fields at Fe Sites of Sr3Fe2MO9 (M = Mo, Te, W, U) Double-Perovskites

We have studied the isostructural series of double-perovskites Sr3Fe2MO9 (M = Mo, Te, W, U) by Mossbauer spectrometry and AC susceptibility measurements. The hyperfine structure of Mossbauer spectra at room temperature is attributed to the presence of high spin state Fe3+ ions sensing both static and fluctuating magnetic hyperfine fields with different relative areas that depend on M. The magnetically split signal — indistinguishable from the background in the Mo compound spectrum — increases with the Fe-site disorder in the sequence Mo < U < Te < W. The spectra at 77 K demonstrate that the W-perovskite sample is already magnetically fully ordered, while the other three cations suggest ordering temperatures that increase from Te to U to Mo. At 4.2 K all the spectra are completely magnetically split and display hyperfine fields that range from 49 up to 53 T. Coincident with the X-ray and neutron diffraction results, the hyperfine parameters are consistent with Fe atoms centered in oxygen octahedral units, coordinated to different numbers of M-centered octahedra. The AC susceptibility response is χ′max ≈ 3.5 × 10−5 emu/g.Oe for the Mo compound and increases for the W, Te and U compounds with values of χ′max 1.6 × 10−4, 3.0 × 10−4, and 9.4 × 10−3 emu/g.Oe, respectively. The out-of-phase component, χ″, could only be detected for the U compound. Its frequency dependence displays a shift that denotes a spin-glass-like state arising from the chemical disorder.

Magnetic and hyperfine interactions in systems of antiferromagnetic oxide nanoparticles with industrial applications

We report characterizations of systems of antiferromagnetic iron oxide particles of importance in catalysis where Mossbauer spectroscopy has played an essential role. We show that signals contributing to the Mossbauer spectra that arise from the surface or from non-crystalline environments still find difficulty in their assignment in spite of the large amount of research already done on hematite and similar antiferromagnetic systems.