F. E. Spada

Magnetic anomalies in NiO nanoparticles

As first noted by Neel, antiferromagnetic nanoparticles could exhibit superparamagnetic relaxation of their spin lattices as well as permanent moments arising from uncompensated surface spins. Several samples of antiferromagnetic NiO nanoparticles with average sizes ranging from 50 to >800 A were investigated in the present study. In addition to the inverse dependence on average particle size of the susceptibility predicted by Neel, and previously reported, some unusual behavior was observed. Above the blocking temperatures (TB) of the particles, the reversible magnetization could not be fit with a Langevin function that was consistent with the physically reasonable moment representing the uncompensated spins. For the 53 A diameter particles, both zero-field-cooled (ZFC) and field-cooled (FC) loops below TB exhibit large coercive forces (several kOe) and the loops showed irreversibility up to 50 kOe. In addition, in the FC state below TB the hysteresis loops were strongly shifted. The latter behavior may ...

Anomalous properties of magnetic nanoparticles

Abstract Nanoparticles of ferrimagnetic NiFe2O4 and antiferromagnetic NiO exhibit a variety of anomalous magnetic properties. The lower coordination of surface spins is responsible in both cases for the observed behavior. This conclusion is supported by calculations of field-dependent spin distributions in these nanoparticles.

Giant magnetoresistance in heterogeneous Cu-Co and Ag-Co alloy films (invited)

Giant magnetoresistance in sputtered single films of Cu-Co and Ag-Co heterogeneous alloys is discussed. The films consist of Co-rich precipitates in a nonferromagnetic matrix. The Ag-Co films have higher DELTArho/rho and DELTArho values than the Cu-Co films, possibly due to less Co dissolved in the Ag matrix. DELTArho scales inversely with precipitate particle size, implying that Co-rich clusters less-than-or-equal-to 20 angstrom diameter may be most effective for spin dependent scattering. This trend of the data and a phenomenological model suggest that interfacial spin dependent scattering is significantly stronger than the scattering within the Co-rich particles.

Anisotropic and high magnetization rare earth transition metal compounds containing metalloids

Abstract The structural and magnetic behavior is presented for selected metalloid (B,C) containing hexagonal and tetragonal rare earth-transition metal compounds and compound series. Focus is on materials with high Fe content and resulting high magnetizations. The unusual axial ratios and features of the sheet type structures of these materials have pronounced consequences on such properties as magnetic anisotropy and magnetic hardness. Individual site anisotropy contributions are studied by temperature dependence of magnetization along easy and hard magnetic axes. As an example it is found that tetragonal Nd 2 Fe 14 B has axial anisotropy with H A = 76 kOe at 300 K but shows tendencies for a spin reorientation around 150 K. Y 2 Fe 14 B has axial anisotropy with H A = 25 kOe but does not exhibit a similar spin reorientation. This indicates that the two crystallographic Nd sites (4f and 4g) have axis and plane preference respectively, with different temperature dependencies. Axial Nd anisotropy is a consequence of the lack of Nd coordination along the z axis due to intervening thick Fe layers. Both extrinsic (fine particle) and intrinsic magnetic hardness is observed. Crystallographically disordered materials show intrinsic hardness based on domain wall pinning by local fluctuations of magnetic parameters. Strong nucleation phenomena are characteristic for ordered materials in bulk and powder form. The unusually high achievable ratios of extrinsic coercivities to anisotropy fields in the metalloid stabilized materials are related to their chemically relatively inert layer structure. This appears to lead to less corrugated surface structures and is so responsible for the characteristic domain wall nucleation processes.

X-ray diffraction and Mössbauer studies of structural changes and L10 ordering kinetics during annealing of polycrystalline Fe51Pt49 thin films

Room-temperature x-ray diffraction and Mossbauer effect techniques have been used to characterize the structural features and local atomic environments of sputtered Fe51Pt49 thin films following various isothermal treatments. Both techniques show that no significant changes occur in the chemically ordered L10 tetragonal phase after it has formed. In contrast, changes in the disordered face-centered-cubic (fcc) phase are observed prior to the transformation into the ordered tetragonal phase. Mossbauer measurements indicate the development of increasing short-range order in the disordered fcc phase with increasing annealing temperature. Asymmetries in the fcc x-ray diffraction profiles also suggest the presence of lattice distortions caused by atomic size differences commonly found in the quenched disordered fcc phase of materials that form ordered structures. Quasi-real-time kinetic measurements of the disorder→order transformation in sputtered Fe51Pt49 thin films within the temperature range 300 °C⩽T⩽400 ...

Evolution of stress with L10 ordering in FePt and FeCuPt thin films

The evolution of stress in 50 nm FexPt1−x (x=0.41, 0.43, 0.49, 0.52, and 0.56) sputtered films was monitored in situ as a function of the temperature. In the as-deposited state, films have a disordered face-centered-cubic (fcc) structure with a strong (111) fiber texture and 685–978 MPa compressive stress. Below 200 °C, thermal expansion of the FexPt1−x fcc lattice produced an initial increase in residual in-plane compressive stress. Above 250 °C, a transition to a low stress state was observed, with the relaxation in the Fe0.52Pt0.48 film showing stronger temperature dependence. Comparisons of stress to a sputtered film of Fe0.35Cu0.15Pt0.5 indicate a faster transition rate to the low stress state can be achieved with the addition of Cu. In situ high temperature x-ray diffraction measurements show a decrease in (111) d spacing with an increase in temperature, and are consistent with the changes observed in the residual stress measurements. The large stress transition appears to arise from changes in the ...

Preparation and structural characterization of sputtered CoO, NiO, and Ni 0.5 Co 0.5 O thin epitaxial films

Single phase CoO, NiO, and Ni{sub 0.5}Co{sub 0.5}O epitaxial films have been prepared by reactive sputtering onto {l angle}0001{r angle} {alpha}--Al{sub 2}O{sub 3} substrates maintained at 373 K. Epitaxy was confirmed by x-ray diffraction (XRD) and high resolution electron microscopy (HREM) techniques. XRD experiments indicate that these monoxide films are cubic and contain rotation twins with the twin axis parallel to {l angle}111{r angle}. Lattice parameters for the CoO and NiO films are 0.4254{plus minus}0.0001 nm and 0.4173{plus minus}0.0006 nm, respectively, and agree with published values for the corresponding bulk oxides. The lattice parameter 0.4220{plus minus}0.0001 nm for the Ni{sub 0.5}Co{sub 0.5}O film lies between those of CoO and NiO and suggests that the mixed oxide film is compositionally homogeneous. Cross-sectional HREM images of the Ni{sub 0.5}Co{sub 0.5}O specimen show {Sigma}3(1{bar 1}2) twin boundaries perpendicular to the oxide-substrate interface. The twin regions are approximately 30 nm in size and are uniformly distributed throughout the film. The epitaxial orientation of the monoxide films with respect to the substrate can be summarized by the relationships (111) monoxide //(0001) {alpha}--Al{sub 2}O{sub 3}, (1{bar 1}0) monoxide //(1{bar 1}00) {alpha}--Al{sub 2}O{sub 3}, and (11{bar 2}) monoxide //(11{bar 2}0) {alpha}--Al{sub 2}O{sub 3}.

Thick amorphous ferromagnetic coatings via thermal spraying of spark-eroded powder

Abstract 300-μm thick amorphous ferromagnetic coatings were prepared by thermally spraying amorphous Fe75Si15B10 powder. The amorphous feedstock powder for the thermal spraying was made by the spark-erosion method. The Mossbauer, X-ray diffraction, chemical, and magnetic properties of the coatings were very similar to those of the spark-eroded powder. Consolidated magnetic amorphous materials have long been a technological goal, and are expected to provide substantial electrical energy savings.

Hc enhancement in partially reduced γ‐Fe2O3 via surface treatment with sodium polyphosphate (revisited)

The coercive force of acicular, nonstoichiometric (partially reduced) γ‐Fe2O3 particles is increased after surface treatment with solutions of sodium polyphosphate. The coercive force of 0.6×0.1 μm particles having Fe+2/Fe+3=0.134 increases from 450 to 670 Oe when treated with a 0.15 g/ml aqueous solution of (NaPO3)n≊14⋅Na2O. The magnitude of the change in the coercive force is dependent upon the relative amounts of polyphosphate and iron oxide present during treatment, but is independent of the polyphosphate solution concentration within the range 0.05–0.15 g/ml and the solution pH within the range 7–9. The enhanced coercivity values decrease slowly with time when the treated particles are exposed to air. In contrast to the partially reduced oxide, enhanced coercive forces are not observed in stoichiometric γ‐Fe2O3 after similar treatment. These results are qualitatively consistent with earlier reports that Hc can be increased in nonstoichiometric γ‐Fe2O3 via surface treatment with chainlike sodium polyp...

Structure and phase transformation of ferromagnetic shape memory alloy Ni/sub 49/Mn/sub 30/Ga/sub 21/ fine particles prepared by spark erosion

In this paper explained about the structure and martensitic transformation of Ni-Mn-Ga alloys. Temperature dependence of the magnetization curves are obtained. XRD was studied for sparked powder and annealed powder. SEM results are also discussed.