Salah A. Makhlouf

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.

Humidity sensing properties of NiO/Al2O3 nanocomposite materials

Abstract Humidity sensing properties measured by impedance spectroscopy (IS) and DC conductivity for a group of high surface area nanostructured 5–30% w/w NiO/Al 2 O 3 composite materials are reported. The nanocomposites were prepared via a sol–gel method, which involved mixing of the respective sol–gel precursors and the subsequent drying and calcination at 873 K for 3 h. Accordingly, nanosized NiO particles dispersed in γ-Al 2 O 3 matrix were formed. Conductivity changes amount to six orders of magnitudes were observed in response to 5–90% relative humidity (RH) change in the measuring chamber. Results indicate that humidity sensing was increased with the increase of NiO content in the composite. Moreover, conductance kinetics and conduction mechanism are discussed in terms of surface texture and nanostructured morphology of the composite materials that facilitate ionic transport mechanism.

Structural and magnetic properties of non-equilibrium b.c.c. nickel prepared by leaching of mechanically alloyed Ni35Al65

Abstract Mechanically alloyed nickel aluminide of the composition Ni 35 Al 65 retains its b.c.c. (B2) structure after being treated with 20% KOH solution by which at least five-sixths of the Al atoms are removed. X-ray diffraction, transmission electron microscopy, high pressure differential thermal analysis and magnetization measurements were carried out for both as-milled and leached alloys. The b.c.c. nickel is a strong paramagnet and not stable above 470 K: it transforms to the ferromagnetic f.c.c. nickel at high temperatures.

Electron spin resonance study of NiO antiferromagnetic nanoparticles

The electron spin resonance (ESR) spectra of antiferromagnetic nanoparticle NiO specimens have been investigated as a function of temperature at x-band (microwave) frequencies. Below the nominal Neel temperature, the x-band resonances arising from the bulk antiferromagnets, including NiO particles with diameters greater than 100 A, all vanish due to the emergence of large molecular exchange fields. The ESR resonance signals of 60 A antiferromagnetic nanoparticles, however, persist to the lowest temperatures. These nanoparticle resonance lines shift to lower fields rapidly as the temperature is decreased, while the lineshapes broaden and distort.

Giant magnetoresistance of Fe-cluster-dispersed Ag films

Abstract We report on giant magnetoresistance (GMR) of Fe-cluster-dispersed Ag films. Composite thin films with a thickness of about 2000 A and the compositional ratios Fe/Ag = 20/80, 33/67 and 45/55 have been produced by an ionized cluster beam (ICB) technique. Measurements of X-ray diffraction, transmission electron microscope, magnetization and magnetoresistance display the following salient features: (1) GMR is observed without any heat treatment of the samples, (2) GMR of about 25% is obtained for Fe/Ag = 33/67, and (3) MR is not saturated even in a magnetic field of 140 kOe for these specimens, although the magnetizations are saturated in a magnetic field of about 3 kOe.

Structural and magnetic properties of nanocrystalline b.c.c. cobalt particles obtained by leaching of mechanically alloyed CoAl

Abstract Nanocrystalline b.c.c. cobalt powder has been prepared using a combination technique of mechanical alloying and leaching aluminum in basic solution. The structure and magnetic properties of the as-milled and the leached alloys were examined by X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and magnetization measurements. Even after removing most of the aluminum atoms by leaching, the specimen topotactically keeps the b.c.c. structure. The b.c.c. cobalt is ferromagnetic is stable below 430 K and transforms into the ferromagnetic f.c.c. cobalt at higher temperatures. The magnetic moment of the cobalt atom is smaller in the b.c.c. phase than in the f.c.c. phase.

Dielectric behavior and ac conductivity study of NiO∕Al2O3 nanocomposites in humid atmosphere

Humidity sensing characteristics of NiO∕Al2O3 nanocomposites, prepared by sol-gel method, are studied by impedance spectroscopy. Modeling of the obtained impedance spectra with an appropriate equivalent circuit enables us to separate the electrical responses of the tightly bound chemisorbed water molecules on the grain surfaces and the loosely associated physisorbed water layers. Dependence of the dielectric properties and ac conductivity of the nanocomposites on relative humidity (RH) were studied as a function of the frequency of the applied ac signal in the frequency range of 0.1–105Hz. The electrical relaxation behavior of the investigated materials is presented in the conductivity formalism, where the conductivity spectra at different RHs are analyzed by the Almond-West formalism [D. P. Almond et al., Solid State Ionics 8, 159 (1983)]. The dc conductivity and the hopping rate of charge carriers, determined from this analysis, show similar dependences on RH, indicating that the concentration of mobile...

Mossbauer Study on Nonequilibrium Disordered Fe-Al Alloys Produced by DC Sputtering

Mossbauer spectra of Fe 1- x Al x alloys produced by facing target type DC sputtering have been measured for x <0.75 at room temperature and 4.2 K . In the bcc alloys, hyperfine field distributions obtained by analyzing these spectra can be divided into three components characterized by three broad maxima. A nearest neighbor environment model, assuming random distribution of atoms, was successfully applied to explain the magnetic behavior of bcc alloys indicating that the sputtered FeAl alloys are in disordered state. The lattice constants of the bcc Fe-Al alloys were discussed in connection with the magnetic moment.

Formation of nominal Ni amorphous phase by mechanical milling and leaching

Abstract A nominal Ni amorphous phase was obtained by leaching Al atoms from mechanically alloyed Ni 40 Al 60 amorphous alloy. X-ray diffraction, transmission electron microscopy and magnetization measurements were utilized to characterize the as-milled and leached specimens. The Ni amorphous phase is a strong paramagnet and transforms to a ferromagnetic f.c.c. phase above 500 K.