David M. Perry

Mesoporous Co3O4 nanostructured material synthesized by one-step soft-templating: A magnetic study

A combined magnetization and zero-field 59Co spin-echo nuclear magnetic resonance (NMR) study has been carried out on one member of a recently developed class of highly ordered mesoporous nanostructured materials, mesoporous Co3O4 (designated UCT-8, University of Connecticut, mesoporous materials). The material was synthesized using one-step soft-templating by an inverse micelles packing approach. Characterization of UCT-8 by powder x-ray diffraction and electron microscopy reveals that the mesostructure consists of random close-packed Co3O4 nanoparticles ≈ 12 nm in diameter. The N2 sorption isotherm for UCT-8, which is type IV with a type H1 hysteresis loop, yields a 134 m2/g BET surface area and a 7.7 nm BJH desorption pore diameter. The effect of heat treatment on the structure is discussed. The antiferromagnetic Co3O4 nanoparticles have a Neel temperature TN = 27 K, somewhat lower than the bulk. A fit to the Curie-Weiss law over the temperature range 75 K ≤ T ≤ 300 K yields an effective magnetic moment of μeff = 4.36 μB for the Co2+ ions, indicative of some orbital contribution, and a Curie-Weiss temperature Θ = −93.5 K, consistent with antiferromagnetic ordering. The inter-sublattice and intra-sublattice exchange constants for the Co2+ ions are J1/kB = (−)4.75 K and J2/kB = (−)0.87 K, respectively, both corresponding to antiferromagnetic coupling. The presence of uncompensated surface spins is observed below TN with shifts in the hysteresis loops, i.e., an exchange-bias effect. The 59Co NMR spectrum for UCT-8, which is attributed to Co2+ ions at the tetrahedral A sites, is asymmetrically broadened with a peak at ≈55 MHz (T = 4.2 K). Since there is cubic symmetry at the A-sites, the broadening is indicative of a magnetic field distribution due to the uncompensated surface spins. The spectrum is consistent with antiferromagnetically ordered particles that are nanometer in size and single domain.

Magnetic study of the Co-MCM-41 catalyst: Before and after reaction

A combined magnetization (both dc and ac techniques), NMR, and EPR study of the magnetic properties has been carried out on a 3 wt. % Co-loaded Co-MCM-41 catalyst, before and after reaction. Before reaction, the Co magnetization can be described by the Curie-Weiss law (50 K ≤ T ≤ 350 K) consistent with the vast majority of the Co existing in the MCM-41 structure as weakly-interacting Co2+ paramagnetic moments with an effective value μp = 4.7(2) μB. In addition, there appears to be a contribution from a small number of Co “clusters” exhibiting a magnetic transition at ≈15 K. After reaction, the Co magnetization is dominated by a magnetic contribution which saturates with a value 58% that for bulk Co, and is attributed to Co metal nanoparticles. In addition, the Co nanoparticles are characterized by a superparamagnetic blocking temperature at 10.5 K. Before reaction, Co-MCM-41 showed two Co EPR signals: (1) a strong signal at g = 1.93 and (2) a weak signal at g = 1.99. After reaction, Co59 zero-field spin-e...

Direct evidence for the source of reported magnetic behavior in “CoTe”

In order to unambiguously identify the source of magnetism reported in recent studies of the Co-Te system, two sets of high-quality, epitaxial CoTex films (thickness ≈ 300 nm) were prepared by pulse laser deposition (PLD). X-ray diffraction (XRD) shows that all of the films are epitaxial along the [001] direction and have the hexagonal NiAs structure. There is no indication of any second phase metallic Co peaks (either fcc or hcp) in the XRD patterns. The two sets of CoTex films were grown on various substrates with PLD targets having Co:Te in the atomic ratio of 50:50 and 35:65. From the measured lattice parameters c = 5.396 A for the former and c = 5.402 A for the latter, the compositions CoTe1.71 (63.1% Te) and CoTe1.76 (63.8% Te), respectively, are assigned to the principal phase. Although XRD shows no trace of metallic Co second phase, the magnetic measurements do show a ferromagnetic contribution for both sets of films with the saturation magnetization values for the CoTe1.71 films being approximate...

Nuclear magnetic resonance study of the magnetic line phases in SrCoO x (2.5 ≤ × ≤ 3)

A 59Co spin-echo nuclear magnetic resonance (NMR) study, along with complimentary magnetization and x-ray diffraction characterization, is reported for three of the four magnetic line phases which occur in the hole-doped perovskite system SrCoO x , 2.5 ≤ x ≤ 3. The magnetic line phases are the single-valence state end-member compounds SrCoO2.5 and SrCoO3, as well as the mixed-valence state compound SrCoO2.88. In this work, the Co valence was varied by changing the oxygen stoichiometry through electrochemical oxidation/reduction. Antiferromagnetic SrCoO2.5, with trivalent Co3+ (3d6), is characterized by a single strong, relatively narrow, 59Co NMR peak centered at 314 MHz. The 314 MHz peak frequency can be explained using a typical 3d hyperfine coupling constant and the Co moment value ≈3 μ B obtained from neutron diffraction measurements. Ferromagnetic SrCoO3 with tetravalent Co4+ (3d5), is characterized by a single strong, relatively narrow, 59Co NMR peak centered at 93 MHz. However, the 93 MHz peak frequency is considerably smaller than expected from the 2.1 μ B Co moment values obtained from the saturation magnetization indicating a significant orbital contribution. Ferromagnetic and mixed-valent (Co3+/Co4+) SrCoO2.88 is characterized by three broad 59Co peaks, centered at 47, 134, and 280 MHz, consistent with previous reports. Based on the NMR results from the end-member compounds, the 280 MHz peak is most likely due to Co3+ and not Co4+ as previously reported.