N. A. Frey

Synthesis and magnetic properties of polymer nanocomposites with embedded iron nanoparticles

Magnetic nanoparticles embedded in polymer matrices have excellent potential for electromagnetic device applications like electromagnetic interference suppression. We have synthesized polymer nanocomposites of poly(methylmethacrylate) doped with varying concentrations of iron nanoparticles (∼20 nm in size). The iron nanoparticles were produced using a microwave plasma technique and have a natural oxide surface layer for passivation. These nanocomposites were processed using melt blending technique. The polymer processing conditions were optimized to achieve good uniform dispersion of the nanoparticles in the polymer matrix. The concentration and dispersion of nanoparticles were varied in a controlled way. Surface characterization with scanning electron microscopy indicates that, to a large extent, the iron nanoparticles are embedded in the bulk; the surface mainly showed features associated with the polymer surface. Static magnetic properties such as susceptibility and M–H loops were studied using a physi...

Interparticle interactions in coupled Au-Fe3O4 nanoparticles

Complex ac susceptibility measurements are reported on composite Au–Fe3O4 nanoparticles of two different configurations—the so-called “dumbbell” and “flower” configurations. The frequency-dependent blocking temperature was fitted to two separate models in an attempt to understand the relaxation and the role of interactions present in the nanoparticle arrays. While the Neel–Arrhenius model failed to accurately describe the blocking behavior of both types of particles, the Vogel–Fulcher model was shown to fit the dumbbell particles indicating the importance of weak interparticle interactions in this system. The flower nanoparticles, however, failed to yield physical fit parameters for both models, indicating that the interactions present in these particles are not solely dipolar but likely associated with competing intraparticle interactions. Radio-frequency transverse susceptibility measurements also confirm these features.

Particle blocking and carrier fluid freezing effects on the magnetic properties of Fe3O4-based ferrofluids

We report the systematic dc and ac susceptibility studies on the particle blocking and carrier fluid freezing effects on the magnetization and relaxation processes in two different ferrofluids composed of Fe3O4 nanoparticles (mean size of ∼14 nm) suspended in hexane and dodecane, which respectively have freezing temperatures below (178 K) and above (264 K) the blocking temperature of magnetic nanoparticles (∼200 K). Experimental results reveal that these effects play a key role in the formation of glasslike peaks and magnetic anomalies in ferrofluids. Quantitative fits of the frequency dependent ac susceptibility to the Vogel–Fulcher model τ=τo exp[Ea/k(T−To)] clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics.We report the systematic dc and ac susceptibility studies on the particle blocking and carrier fluid freezing effects on the magnetization and relaxation processes in two different ferrofluids composed of Fe3O4 nanoparticles (mean size of ∼14 nm) suspended in hexane and dodecane, which respectively have freezing temperatures below (178 K) and above (264 K) the blocking temperature of magnetic nanoparticles (∼200 K). Experimental results reveal that these effects play a key role in the formation of glasslike peaks and magnetic anomalies in ferrofluids. Quantitative fits of the frequency dependent ac susceptibility to the Vogel–Fulcher model τ=τo exp[Ea/k(T−To)] clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics.

Transverse susceptibility study of the effect of varying dipolar interactions on anisotropy peaks in a three-dimensional assembly of soft ferrite nanoparticles

Collective magnetization dynamics in nanoparticle assemblies is of current interest as it forms the basis of high density storage media. It is important to understand how interparticle interactions in a three-dimensional (3D) arrangement of superparamagnetic nanoparticles would affect the overall effective magnetic anisotropy of the system. We have studied the influence of varying strengths of dipolar interaction on the static and dynamic magnetic properties of surfactant-coated monodispersed manganese zinc ferrite nanoparticles using reversible transverse susceptibility. We track the evolution of the anisotropy peaks with varying magnetic field, temperature, and interaction strength. The blocking temperature shows an increase from 28 to 32 K and the coercive field (at 10 K) shows an increase from 144 to 192 Oe as the system changes from the case of weakly interacting to strongly interacting 3D assembly of the particles.

Magnetism and cluster glass dynamics in geometrically frustrated LuFe2O4

We report on the magnetic properties of high quality LuFe2O4 single crystals grown by the floating zone method. dc and ac susceptibility measurements and analysis reveal a ferrimagnetic transition at ∼240 K followed by a re-entrant cluster glass transition below 225 K, with an additional magnetic transition around 170 K. Strong frequency dependence of the real (χ′) and imaginary (χ″) parts of the ac susceptibility observed at both these temperatures indicate glassy behavior and we quantitatively fit the data to a cluster glass model, τ=τo(Tf/Tg−1)−zv. Our studies show that these multiple transitions are consistent with the picture of ferrimagnetic clusters in the iron oxide planes with triangular lattice configuration favoring spin frustration and glass dynamics.

Growth and characterization of sputtered BSTO∕BaM multilayers

Multilayers of Ba0.5Sr0.5TiO3 (BSTO) and BaFe12O19 (BaM), with tunable permeability and permittivity are attractive systems for radio frequency and microwave applications. We have grown multilayers of BSTO and BaM using magnetron sputtering on Al2O3 substrates. Film growth conditions such as sputtering parameters were optimized to obtain high quality multilayers. X-ray diffraction established that both BSTO and BaM were formed and cross-sectional SEM studies showed sharp interfaces between BSTO and BaM layers. Magnetization showed a large coercivity (∼2000Oe) consistent with the hexaferrite component. The hysteresis loops also revealed the distinct influence of magnetocrystalline and shape anisotropies at different temperatures.

Giant Magnetoimpedance for Biosensing in Drug Delivery

Iron oxide (Fe3O4) non‐specific superparamagnetic nanoparticles of 30 nm size are introduced into human embryonic kidney (HEK‐293) cells by intracellular uptake. The nanoparticles are magnetised by two superimposed magnetic fields, an externally applied DC field and an AC field generated by the high‐frequency current flowing through Co64.5Fe2.5Cr3Si15B15 amorphous ribbons. The resulted fringe fields from the nanoparticles are detected via the magnetoimpedance change in the ribbons covered and uncovered by thin gold layer. The gold covering is considered an improvement due to its biocompatibility and because it avoids the biocorrosion process on the ribbon. The MI responses in both cases are clearly dependent on the presence of the magnetic nanoparticles inside the cells and on the value of the external field.

Magnetic Anisotropy and Magnetocaloric Effect (MCE) in NiFe 2 O 4 Nanoparticles

Magnetic refrigeration based on the magnetocaloric effect (MCE) has been proposed as an attractive alternative to gas compression technology. Some calculations suggest that MCE can be obtained with higher efficiencies than compressor driven refrigeration. We examine the MCE in a system of nickel ferrite nanoparticles with size range of 6 to 15 nm. A peak in the MCE at 55 K is observed that increases with higher magnetic fields. This gives this system a relatively high peak entropy change compared to other ferrite systems. A sensitive radio-frequency (RF) transverse susceptibility measurement has also been used to study the magnetic anisotropy. We show that the MCE peak is not associated with the blocking temperature and is likely a field-driven surface spin reorientation which also has a signature in the transverse susceptibility. Work supported by NSF-CTS-0408933 and ARO- W911NF-05-1-0354

Exchange Bias in CrO2/Cr2O3 Bilayer Thin Films

We have investigated the exchange bias in CVD grown epitaxial CrO2/Cr2O3 bilayer thin films using hysteresis loops and resonant RF transverse susceptibility. M-H loops indicated an enhanced coercivity without appreciable loop shift and the transverse susceptibility in CrO2/Cr2O3 bilayers revealed features associated with both the ferromagnetic and antiferromagnetic phases. In addition, TS yielded large anisotropy constant (Keff) values depending on the fraction of Cr2O3 present. The large anisotropy fields observed cannot be accounted for by the variable thickness of CrO2 alone and are indicative of possible exchange coupling between CrO2 and Cr2O3 phases that significantly affects the effective magnetic anisotropy.