K. Parvin

Effect of annealing on the structure and magnetic properties of graphite encapsulated nickel and cobalt nanocrystals

We report the structure and magnetic studies of carbon coated nanocrystals of nickel and cobalt synthesized in a special low carbon to metal ratio arc chamber. Powder x-ray diffraction profiles show peaks associated with single phase of fcc nickel or cobalt and major peaks of graphite with no evidence of carbides or solid solutions of carbon in the metal. Measured lattice spacing of crystalline particles and that of graphite coating from high-resolution transmission electron microscope images also confirm such findings. Magnetization measurements as a function of temperature in the range 20–900 °C give a Curie temperature equal to that of bulk metal within the experimental error. Upon heating and recooling of the particles a larger magnetization as high as 57% of bulk Co and 53% of bulk Ni was measured. Also M–H hysteresis loop of the particles have been measured at room temperature after annealing in the temperature range 20–650 °C for Ni, and 20–900 °C for Co. The dependence of room temperature saturati...

Synthesis and magnetic properties of monodisperse Fe3O4 nanoparticles

We report the high temperature reaction of iron acetylacetonate in phenyl ether in the presence of oleic acid and oleylamine that was used to synthesize monodisperse Fe3O4 nanoparticles. X-ray diffraction profile and high-angle annular dark-field images give evidence of self-assembled arrays with nanoparticle size of 4 nm. Magnetization versus temperature in the temperature range 2.5–160 K was measured in zero-field-cooled and field-cooled experiments and a blocking temperature Tb=20 K was obtained. Above Tb the nanoparticles show superparamagnetic behavior and the magnetization versus field for various temperature follows the Langevin function. M-H curves below Tb indicate the ferromagnetic behavior with Hc=60–400 Oe for temperature T=2.5–18.5 K.

Positive holes in magnesium oxide - Correlation between magnetic, electric, and dielectric anomalies

Magnetic susceptibility measurements of high purity MgO single crystals (<50-wt. ppm transition metals) by means of a vibrating-sample magnetometer shows an anomaly at 800 K. At the same temperature the electric conductivity increases anomalously, the static dielectric constant epsilon increases from 9 to approximately 150, a pronounced positive surface charge appears, and Fe2+ in the MgO matrix oxidizes to Fe3+. The data are consistent with O2(2-) (peroxy) defects, representing self-trapped, spin-paired positive holes at Mg2+ vacancy sites. Diamagnetic at low temperatures, the holes start to decouple their spins > 600 K, probably forming at first V0 centers (two O- at an Mg2+ vacancy), then V- centers (single O- at an Mg2+ vacancy), and releasing mobile O- states. These O- represent itinerant charge carriers on acceptor levels near the O 2p-dominated valence band and conduct by O- /O2- valency fluctuations. The O- concentration is of the order of 8 X 10(19) cm-3.

Magnetic properties of a mixture of two nanosized Co-S powders produced by hydrothermal reduction

In this paper, we report the synthesis and morphology of mixture of nanosized of CoS and Co/sub 9/S/sub 8/, and discuss their ferromagnetic and superparamagnetic behavior with a blocking temperature of about 10 K.

Nuclear spin-lattice relaxation and pair breaking in superconducting (LaGd)Al2

Abstract Measured zero-field nuclear spin-lattice relaxation rates in the superconducting magnetic alloy ( La Gd)Al2 have been analyzed in terms of direct impurity and conduction-electron (Korringa) contributions. The temperature dependence of the Korringa rate exhibits the effect of impurity pair breaking near the superconducting transition.

Dissociation of O2−2 defects into paramagnetic O− in wide band‐gap insulators: A magnetic susceptibility study of magnesium oxide

The magnetic susceptibility of single‐crystal MgO has been measured in the temperature range 300–1000 K, using a Faraday balance. The high‐purity crystal (<100 ppm transition metals), grown from the melt in a H2O‐containing atmosphere, was found to be paramagnetic due to the presence of defects on the O2− sublattice. The defects derive from OH− introduced into the MgO matrix by the dissolution of traces of H2O during crystal growth. The OH− converts into O2−2 and H2. Each O2−2 represents two coupled, spin‐paired O− states. The observed strongly temperature‐dependent paramagnetism can be described by three contributions that overlay the intrinsic diamagnetism of MgO and arise from (i) the low level of transition‐metal impurities, (ii) O− generated by O2−2 dissociation, and (iii) O− states trapped by quenching from high temperatures from previous experiments.

The magnetic properties of annealed graphite-coated Ni and Co nanocrystals

Magnetic studies of graphite-coated nanocrystals of nickel and cobalt (5-50 nm) synthesized in a special fullerene are chamber are reported. Magnetization measurements as a function of temperature in the range 20-900/spl deg/C indicate the ferromagnetic nature of the nanocrystals. Upon heating and recooling of the nanocrystals a larger magnetization was measured, The dependence of room temperature M/sub s/, M/sub r/, and H/sub c/ of the nanocrystals on annealing temperature is reported. The data are consistent with increase in size of single domain particles, or transition of the nanocrystals from single domain to multi-domain due to particle size growth in the annealing process.

Mössbauer spectroscopy and magnetic properties of copper-304 stainless steel multilayer films

The magnetic behavior of multilayer films of Cu-304 stainless steel (304SS) with equal layer thickness fabricated by magnetron sputtering at ambient temperature is reported. For small layer thickness (t=0.5–1.0 nm), the films show epitaxial fcc growth of 304SS on fcc copper, and multilayers are nonmagnetic due to nonmagnetic nature of fcc 304SS. For larger layer thickness (t=1.0–50.0 nm), 304SS grows epitaxially as fcc near the copper interface accompanied by a bcc 304SS growth away from the interface. As a result, two regimes are observed in 1.0–50.0 nm thickness range. Medium layer thickness (t=1.0–1.35 nm) multilayers show weak ferromagnetism characterized by small magnetization due to comparable thickness of ferromagnetic bcc 304SS and nonmagnetic fcc 304SS. A strong ferromagnetic behavior is observed for thick samples (t=1.35–50.0 nm) due to 304SS layer being primarily ferromagnetic bcc phase with very small percentage of fcc near the interface. Mossbauer effect spectroscopy and magnetometry were uti...

Nuclear spin relaxation and quasiparticle excitations in superconducting aluminum-3d alloys

Aluminum-27 nuclear spin-lattice relaxation times T1 have been measured in the normal and superconducting states of dilute alloys of Mn, Cr, and V in aluminum. These measurements prove the spectrum of thermally excited quasiparticle states in the superconductor and its modification by nonmagnetic resonant impurities. Theoretical studies of such systems indicate that in the Hartree-Fock (HF) approximation the BCS character of the host is basically retained in the alloy, with a reduced pairing interaction and a small additional broadening of quasiparticle states at the gap edge. Spin-fluctuation improvements on the HF approximation renormalize the parameters involved, but do not change the basic picture. Our T1 data indicate gap edge broadening an order of magnitude greater than predicted. The discrepancy is nearly as large for V and Cr impurities as for Mn, which makes it unlikely that localized spin fluctuations play an important role in the broadening. Recent speculation on the effect of dislocation pinning by resonant impurities may be relevant to these results.