Kelly L. Pisane

Synthesis, structural characterization and magnetic properties of Fe/Pt core-shell nanoparticles

Structural and magnetic properties of Fe/Pt core-shell nanostructure prepared by a sequential reduction process are reported. Transmission electron microscopy shows nearly spherical particles fitting a lognormal size distribution with Do = 3.0 nm and distribution width λD = 0.31. In x-ray diffraction, Bragg lines only from the Pt shell are clearly identified with line-widths yielding crystallite size = 3.1 nm. Measurements of magnetization M vs. T (2 K–350 K) in magnetic fields up to 90 kOe show a blocking temperature TB = 13 K below which hysteresis loops are observed with coercivity HC increasing with decreasing T reaching HC = 750 Oe at 2 K. Temperature dependence of the ac susceptibilities at frequencies fm = 10 Hz–5 kHz is measured to determine the change in TB with fm using the Vogel-Fulcher law. This analysis shows the presence of significant interparticle interaction, the Neel-Brown relaxation frequency fo = 5.3 × 1010 Hz and anisotropy constant Ka = 3.6 × 106 ergs/cm3. A fit of the M vs. H data u...

Unusual enhancement of effective magnetic anisotropy with decreasing particle size in maghemite nanoparticles

In magnetic nanoparticles (NPs), the observed increase in the effective magnetic anisotropy Keff with the decrease in particle size D is often interpreted, sometimes unsuccessfully, using the equation Keff = Kb + (6KS/D), where Kb is the bulk-like anisotropy of the core spins and KS is the anisotropy of spins in the surface layer. Here, we test the validity of this relation in γ-Fe2O3 NPs for sizes D from 15 nm to 2.5 nm. The samples include oleic acid-coated NPs with D = 2.5, 3.4, 6.3, and 7.0 nm investigated here, with results on 14 other sizes taken from literature. Keff is determined from the analysis of the frequency dependence of the blocking temperature TB after considering the effects of interparticle interactions on TB. For the γ-Fe2O3 NPs with D < 5 nm, an unusual enhancement of Keff with decreasing D, well above the magnitudes predicted by the above equation, is observed. Instead the variation of Keff vs. D is best described by an extension of the above equation by including Ksh term from spins in a shell of thickness d. Based on this core-shell-surface layer model, the data are fit to the equation Keff = Kb + (6KS/D) + Ksh{[1−(2d/D)]−3−1} with Kb = 1.9 × 105 ergs/cm3, KS = 0.035 ergs/cm2, and Ksh = 1.057 × 104 ergs/cm3 as the contribution of spins in the shell of thickness d = 1.1 nm. Significance of this result is discussed.In magnetic nanoparticles (NPs), the observed increase in the effective magnetic anisotropy Keff with the decrease in particle size D is often interpreted, sometimes unsuccessfully, using the equation Keff = Kb + (6KS/D), where Kb is the bulk-like anisotropy of the core spins and KS is the anisotropy of spins in the surface layer. Here, we test the validity of this relation in γ-Fe2O3 NPs for sizes D from 15 nm to 2.5 nm. The samples include oleic acid-coated NPs with D = 2.5, 3.4, 6.3, and 7.0 nm investigated here, with results on 14 other sizes taken from literature. Keff is determined from the analysis of the frequency dependence of the blocking temperature TB after considering the effects of interparticle interactions on TB. For the γ-Fe2O3 NPs with D < 5 nm, an unusual enhancement of Keff with decreasing D, well above the magnitudes predicted by the above equation, is observed. Instead the variation of Keff vs. D is best described by an extension of the above equation by including Ksh term from spins...

Magnetic Determination of the Electronic State of Cu and Exchange Interactions in the

Using the analysis of the magnetization data in the α- and β-phases of copper phthalocyanine (CuPc) samples, the electronic state of Cu and exchange interactions are reported. After verifying the crystal structure of the powder samples using X-ray diffraction, the temperature dependence (2-250 K) of the magnetization M of both samples was measured in magnetic field H = 1 kOe and isothermally at 2 and 5 K in H up to 90 kOe. The data were analyzed first using the modified Curie-Weiss law, χ = χ₀ + C/(T + θ), showing good fit for T > 4 K and yielding θ = 2.3 K (0.2 K) for α-CuPc (β-CuPc) and spin S = 1/2 characteristic of Cu²⁺. The data were next fitted to the Bonner-Fisher model for S = 1/2 antiferromagnetic Heisenberg chain showing an excellent fit to all the M versus T data and yielding the Cu²⁺-Cu²⁺ exchange constant J/k_B = 3.4 K (0.4 K) for the α-CuPc (β-CuPc). The isothermal data of M versus H are analyzed taking exchange coupling into account. The large difference in the magnitudes of J/k_B for the two phases is discussed in terms of the differences in their crystal structures.

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The particle size (D) dependence of the effective magnetic anisotropy K<inf>eff</inf> of magnetic nanoparticles (NPs) usually shows K<inf>eff</inf> increasing with decreasing D. This dependence is often interpreted using the Eq.: K<inf>eff</inf> = K<inf>b</inf> + (6K<inf>S</inf>/D) where K<inf>b</inf> and K<inf>s</inf> are the anisotropy constants of the spins in the bulk-like core and surface layer, respectively. Here, we show that this model is inadequate to explain the observed size-dependency of K<inf>eff</inf> for smaller nanoparticles with D < 5 nm. Instead the results in NPs of maghemite (γ-Fe<inf>2</inf>O<inf>3</inf>), NiO and Ni are best described by an extension of the above model leading to the variation given by K<inf>eff</inf> = K<inf>b</inf> + (6K<inf>S</inf>/D) +K<inf>Sh</inf>{[1-(2rf/D)]⁻³ −1}, where the last term is due to the spins in a shell of thickness d with anisotropy K<inf>sh</inf>. The validation of this core-shell-surface layer (CSSL) model for three different magnetic NPs systems viz. ferrimagnetic γ-Fe<inf>2</inf>O<inf>3</inf>, ferromagnetic Ni and antiferromagnetic NiO suggests its possible applicability for all magnetic nanoparticles.