V. Papaefthymiou

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.

Coordination chemistry of corrosion inhibitors of the benzotriazole type: Preparation and characterization of cobalt(II) complexes with 1-methylbenzotriazole (Mebta) and the crystal structures of [CoCl2(Mebta)2], trans-[Co(NCS)2 (Mebta)4], trans-[Co(NCS)2(MeOH)2(Mebta)2] and cis-[Co(NO3)2(Mebta)2]

Abstract An inorganic model approach to the corrosion inhibition of metals by benzotriazoles has been initiated. The preparation and characterization of monomeric cobalt(II) complexes of the types [CoX2(Mebta)2](X = Cl, Br, NCS, NO 3), (MebtaH)[CoCl3(Mebta)], (MebtaH)2[CoCl4], [Co(NCS)2(Mebta)4], [Co(NCS)2 (MeOH)2(Mebta)2] and [Co(NO3)2 L2(Mebta)2] (L = H2O, MeOH), where Mebta = 1-methylbenzotriazole, are described. Four representative complexes have been structurally characterized by single-crystal X-ray diffraction studies; [CoCl2(Mebta)2] (1), trans- [Co(NCS)2(Mebta)4] (5), trans-[Co(NCS)2 (MeOH)2(Mebta)2] (6) and cis-[Co(NO3)2(Mebta)2] (8). Mebta acts as a monodentateligand binding through N(3). The geometry about cobalt in 1 is tetrahedral, while complexes 5, 6 and 8 have octahedral stereochemistries. The new complexes were also characterized by elemental and thermogravimetric analyses, conductivity measurements, variable-temperature magnetochemistry and spectroscopic (IR, ligand field, low-temperature ESR) methods. The data are discussed in terms of the nature of bonding and known structures.

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.

Crystallographic and magnetic properties of NdFe10Mo2N0.5

Abstract NdFe 10 Mo 2 N 0.5 samples were prepared by heating fine NdFe 10 Mo 2 powders in N 2 gas. The nitrided material maintains the ThMn 12 -type structure but with a unit cell volume about 3% larger than in the original compound. Neutron diffraction experiments showed that the nitrogen atoms occupy the interstitial 2b sites and the Mo atoms the 8i sites consistent with the Mossbauer data. The Curie temperature is greatly enhanced by about 150 K. The magnetocrystalline anisotropy changed strongly from plane anisotropy in NdFe 10 Mo 2 to uniaxial anisotropy in NdFe 10 Mo 2 N 0.5 at room temperatures. The changes in the local crystallographic and electronic structure can explain the strong uniaxial anisotropy of the nitrided material.

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.

M?ssbauer and magnetization studies of Fe2BO4

Polycrystalline Fe2 BO4 prepared by solid-state reaction was investigated over a wide temperature range using Mossbauer spectroscopy and magnetization and resistivity measurements. The Mossbauer data below 270 K reveal four iron sites (two Fe2+ and two Fe3+ ) in a 1:1:1:1 ratio. Above this temperature, electron delocalization sets in between the divalent-trivalent iron ions and up to 400 K both delocalized and localized iron states are observed. The magnetization and Mossbauer data indicate that this oxoborate mixed-valence system is an L-type ferrimagnet below Tc = 155 K, with the iron atoms in each sublattice having different point symmetries and different magnetic interactions. The resistivity data show the first-order nature of the charge-ordering transition (Tco = 316 K).

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.

Surface-induced magnetism in α-Fe2O3/Ag multilayers

Abstract Anomalous ferromagnetic-like behavior, with significant magnetization and very large coercivities at low temperatures was observed in α-Fe 2 O 3 /Ag multilayers. Mossbauer studies showed the presence of two magnetic components, bulk-like and surface-like with substantially lower hyperfine field. It was found that the observed magnetic moment per particle can be accounted for by the contribution of only about 5% of the total number of surface Fe 3+ ions. This result can be explained if the magnetic moments on the surface are predominantly randomly oriented resembling a spin glass state. The negligible quadrupole interaction, observed in the Mossbauer spectra at low temperature, strongly supports the last interpretation. Large shifts in the hysteresis loops of field-cooled samples indicate strong exchange coupling between the antiferromagnetic core and the net magnetic moments on the surface.

Effect of preparation conditions on the hysteresis behavior of granular Fe-SiO2

Abstract The hysteresis behavior of Fe χ (SiO 2 ) 1- χ granular films was studied as a function of preparation conditions. The films were prepared by dc and rf magnetron sputtering over the composition range χ v = 0.1–0.9 volume fraction of Fe. The Tandem deposition method and deposition from a composite target both with and without titanium sublimation, were used to prepare the films. A typical granular structure was observed, with grain size in the range 2–20 rim. X-ray diffraction and selected area diffraction showed an α-Fe (bcc) type structure. Magnetic properties showed that by varying the deposition method and some of the sputtering parameters (sputtering rate, argon flow, film thickness and substrate temperature), it is possible to switch from a relativeIy magnetically hard sample ( H c ∼ 700 Oe) to a soft sample ( H c ∼ 20 Oe). A dramatic increase in H c has been observed at cryogenic temperatures. This result, along with the Mossbauer data, suggests a shell/core granule morphology with an Fe and/or (Fe-Si) core surrounded by Fe-Si-O.

Mössbauer studies in Fe(SiO2,BN) granular solids

Abstract Mossbauer spectroscopy has been used in Fe ( SiO 2 , BN ) granular solids in order to elucidate the large differences in their coercivity values. The Mossbauer data suggest a core/shell morphology for the Fe SiO 2 films with an Fe(Si) core surrounded by an FeSiO shell whose surface consists of Fe2+SiO particles. In Fe BN the data indicate the presence of amorphous FeB particles in a BN matrix.