32 research outputs found

    Spin Disorder and Magnetic Anisotropy in Fe3O4 Nanoparticles

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    We have studied the magnetic behavior of dextran-coated magnetite (Fe3_3O4_4) nanoparticles with median particle size \left=8 nmnm. Magnetization curves and in-field M\"ossbauer spectroscopy measurements showed that the magnetic moment MSM_S of the particles was much smaller than the bulk material. However, we found no evidence of magnetic irreversibility or non-saturating behavior at high fields, usually associated to spin canting. The values of magnetic anisotropy KeffK_{eff} from different techniques indicate that surface or shape contributions are negligible. It is proposed that these particles have bulk-like ferrimagnetic structure with ordered A and B sublattices, but nearly compensated magnetic moments. The dependence of the blocking temperature with frequency and applied fields, TB(H,ω)T_B(H,\omega), suggests that the observed non-monotonic behavior is governed by the strength of interparticle interactions.Comment: 11 pages, 7 figures, 3 Table

    Large magnetic anisotropy in Ferrihydrite nanoparticles synthesized from reverse micelles

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    Six-line ferrihydrite(FH) nanoparticles have been synthesized in the core of reverse micelles, used as nanoreactors to obtain average particle sizes \approx 2 to 4 nm. The blocking temperatures TBmT_B^m extracted from magnetization data increased from 10\approx 10 to 20 K for increasing particle size. Low-temperature \MOS measurements allowed to observe the onset of differentiated contributions from particle core and surface as the particle size increases. The magnetic properties measured in the liquid state of the original emulsion showed that the \FH phase is not present in the liquid precursor, but precipitates in the micelle cores after the free water is freeze-dried. Systematic susceptibility \chi_{ac}(\emph{f},T) measurements showed the dependence of the effective magnetic anisotropy energies EaE_{a} with particle volume, and yielded an effective anisotropy value of Keff=312±10K_{eff} = 312\pm10 kJ/m3^3.Comment: 8 pages, 10 figures. Nanotechnology, v17 (Nov. 2006) In pres

    Factors associated with adherence to immunomodulator treatment in people with multiple sclerosis

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    Abstract To determine the association between factors and adherence to immunomodulator treatment in people with multiple sclerosis treated in reference centers. Cross-sectional study conducted with 188 people who used immunomodulators in three reference centers in Ceará from March to July 2012. Adherence was assessed using the Moriskscale and factors were assessed using a questionnaire addressing socioeconomic and personal characteristics, the disease, the use of immunomodulator and educational activities. The determination of the association was expressed in crude and adjusted odds ratio with a 95% confidence interval. Adherence rate was 46% and after the logistic regression model the adherence to immunomodulator treatment was positively associated with the following factors: age 18-38 years, time of diagnosis and treatment between 6 and 24 months, 0-3.5 score in the Expanded Disability Status Scale, perception of treatment benefits, intrinsic and extrinsic motivation, phone contact with the doctor and not missing the return visit. This study is important because it allowed to determine the association between factors and adherence to immunomodulator treatment in multiple sclerosis, contributing to prevention and control actions

    Interparticle interactions and surface contribution to the effective anisotropy in biocompatible iron oxide nanoparticles used for contrast agents

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    We have investigated the dynamic magnetic properties of dextran-coated magnetite ͑Fe 3 O 4 ͒ nanoparticles in the form of ͑a͒ particles suspended in a carrier liquid and ͑b͒ concentrated powder obtained from lyophilization. The blocking temperature was found to increase from T B =42͑2͒ to 52͑2͒ K ͑@ 0 H =10 mT͒ after lyophilization, showing the effects of dipolar interactions in samples with identical size distributions. The temperature dependence of the hyperfine field B hyp ͑T͒ reveals the effects of collective magnetic excitations at low temperature, and allowed us to obtain the magnetic anisotropy energy E a = 3.6ϫ 10 −21 J for noninteracting particles. The obtained values can be understood assuming only magnetocrystalline anisotropy, without any additional contributions from surface, shape, or exchange origin. Moreover, a magnetocrystalline anisotropy constant value K 1 =10 kJ/m 3 was obtained by assuming the cubic phase with easy magnetic direction ͓111͔ of the bulk material above the Verwey transition, supporting the idea that the Verwey transition is absent in nanosized particles. Accordingly, no indication of magnetic transition at T V could be observed in our measurements. From the dynamical parameters of ac susceptibility ͑f , T͒ curves, the contribution of the dipolar interactions to the total anisotropy energy barrier could be estimated to be ⍀ = 4.5ϫ 10 −21 J, larger than the single-particle value. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1853931͔ Magnetic iron oxide nanoparticles ͑MIONs͒ are routinely used as media contrast agents in clinical protocols for magnetic resonance imaging ͑MRI͒. The rationale of using MIONs is to locally increase the externally applied magnetic field, producing magnetic field heterogeneity that results in signal loss from enhanced T 2 relaxation. 1 Magnetite ͑Fe 3 O 4 ͒ and maghemite ͑␥-Fe 2 O 3 ͒ MIONs are the most frequently used materials, because of their low toxicity and high saturation magnetic moment ͑M S ϳ 80-90 emu/ g͒. To improve stability and biocompatibility the particles are coated with a polysaccharide and suspended in water-based solvents. Therefore, the efficacy of individual nanoparticles as contrast agents for MRI strongly depends on their detailed magnetic characteristics, and their interparticle interactions. The problem of magnetic interactions has showed elusive along the last years. In spite of the many physical mechanisms involved, the theoretical models have steadily evolved from the original approximation proposed by Néel 2 consisting of monodispersed and noninteracting singledomain particles, into more sophisticated models that include microscopic mechanisms such as spin disorder, surface contributions, and collective behavior for strongly correlated particles. 3-5 However, suitable experimental systems are still a key problem to be solved, because of the difficulty of synthesizing samples with different ͑controlled͒ particle size distribution and particle interactions. 6 To investigate how dipolar interactions affect the magnetic behavior of a system of single-domain particles, it would be desirable to study samples with different concentrations but identical size distribution. We have followed this approach by performing magnetic and Mössbauer measurements in a system of suspended and lyophilized magnetite nanoparticles, aimed to understand the role of interparticle interactions on the resulting magnetic anisotropy. The samples studied in this work consisted of dextrancoated magnetite nanoparticles dispersed in a water-based ferrofluid ͑Endorem™͒ for clinical applications, obtained from a commercial supplier ͑Guerbet͒. The nanoparticles were studied in two samples: ͑a͒ as supplied, i.e., suspended in a aqueous liquid carrier ͑sample E1͒ and ͑b͒, after lyophilization ͑@T = 85 K and P =10 −4 Torr, sample ES͒ to increase the particle concentration keeping the particle size distribution unaltered. The Mössbauer spectroscopy measurements were performed with a conventional constantacceleration spectrometer in transmission geometry with a ca. 50 mCi 57 Co/ Rh source between 4.2 and 296 K. When necessary, a distribution of hyperfine magnetic fields, isomer shift and quadrupole splitting have been used to fit the spectra. A foil of ␣-Fe at 296 K was used to calibrate isomer shifts and velocity scale. Magnetization and ac magnetic susceptibility measurements were performed in a commercial superconducting quantum interference device magnetometer both in zero-field-cooling and field-cooling modes, between 1.8 K Ͻ T Ͻ 250 K and under applied fields up to 7 T. The frequency dependence of both in-phase Ј͑T͒ and out-ofphase Љ͑T͒ components of the ac magnetic susceptibility a
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