Magnetic Interactions and Energy Barrier Enhancement in Core/Shell Bimagnetic Nanoparticles

Lavorato, Gabriel Carlos; Peddis, Davide; Lima, Enio; Troiani, Horacio Esteban; Agostinelli, E.; Fiorani, D.; Zysler, R.D.; Winkler, E.

doi:10.1021/acs.jpcc.5b04448

Cited by 45 publications

(35 citation statements)

References 66 publications

(118 reference statements)

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“…B = 1.4(1)·10 −6 K −2 was found for the bulk reference, which is very close to the reported 1.6·10 −6 K −2 for Co‐ferrite . An increase in B is expected for NPs with reduced size due to the reduction in the number of magnetic nearest neighbors . In our case, the B parameters obtained from the fits (shown in Table ) are larger for NPs with increased crystalline disorder, reflecting that an increased structural coherence enhances the thermal stability of the magnetic moment and promotes a larger magnetization at room temperature.…”

Section: Resultssupporting

confidence: 87%

Internal Structure and Magnetic Properties in Cobalt Ferrite Nanoparticles: Influence of the Synthesis Method

Lavorato

Alzamora

Contreras

et al. 2019

Part & Part Syst Charact

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The design of novel nanostructured magnetic materials requires a good understanding of the variation in the magnetic properties due to different synthesis conditions. In this work, four different procedures for fabricating Co‐ferrite nanoparticles with similar sizes between 7 and 10 nm are compared by studying their structural and magnetic properties. Non‐aqueous methods based on the thermal decomposition of metal acetylacetonates at high temperatures, either with or without surfactants, provide highly crystalline nanoparticles with large saturation magnetization values and a coherent reversal of the magnetic moment. However, variations in the density of defects and in the shape of the nanocrystals determine the distribution of switching fields and the effective magnetic anisotropy, which reaches up to ≈1 × 107 erg cm−3 for oleic acid‐capped 9 nm nanoparticles. It is shown that the saturation magnetization values for nanoparticles produced by different methods are in the range between 49 and 95 emu g−1 due to differences in the stoichiometry, in the cation occupancy, in the magnetic disorder and in the spin canting of the magnetic sub‐lattices, the latter evaluated by in‐field Mössbauer spectroscopy.

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Section: Resultssupporting

confidence: 87%

Internal Structure and Magnetic Properties in Cobalt Ferrite Nanoparticles: Influence of the Synthesis Method

Lavorato

Alzamora

Contreras

et al. 2019

Part & Part Syst Charact

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“…Using the ZFC and FC curves one can obtain the blocking temperature distribution, which is given by: f(T B ) ~ − d(χ FC − χ ZFC )/ dT 28. As observed in Fig.…”

Section: Resultsmentioning

confidence: 99%

Structural and magnetic properties of core-shell Au/Fe3O4 nanoparticles

Félix

Coaquira

Martinez

et al. 2017

Sci Rep

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We present a systematic study of core-shell Au/Fe3O4 nanoparticles produced by thermal decomposition under mild conditions. The morphology and crystal structure of the nanoparticles revealed the presence of Au core of d = (6.9 ± 1.0) nm surrounded by Fe3O4 shell with a thickness of ~3.5 nm, epitaxially grown onto the Au core surface. The Au/Fe3O4 core-shell structure was demonstrated by high angle annular dark field scanning transmission electron microscopy analysis. The magnetite shell grown on top of the Au nanoparticle displayed a thermal blocking state at temperatures below TB = 59 K and a relaxed state well above TB. Remarkably, an exchange bias effect was observed when cooling down the samples below room temperature under an external magnetic field. Moreover, the exchange bias field (HEX) started to appear at T~40 K and its value increased by decreasing the temperature. This effect has been assigned to the interaction of spins located in the magnetically disordered regions (in the inner and outer surface of the Fe3O4 shell) and spins located in the ordered region of the Fe3O4 shell.

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“…69 However, even in the uniaxial anisotropy case, the fact that magnetic single-phase and core/shell nanoparticles (particularly oxide nanoparticles) are usually not monodomains (i.e., cannot be simplified to a system of macro-spins), often exhibiting rather complex internal spin structures, 70-74 may cast some doubts over the validity of the remanence curves approach for the evaluation of dipolar interactions. [44][45][46][47][48][49][50][51][52][53][54][75][76][77] Here we investigate the interactions in γ-Fe 2 O 3 nanoparticles coated by thick SiO 2 shells (up to 62 nm) via δM plots and FORC. The δM plots show clear negative dips, apparently implying dipolar interactions, even in the extremely magnetically dilute cases.…”

Section: Introductionmentioning

confidence: 99%

Remanence Plots as a Probe of Spin Disorder in Magnetic Nanoparticles

et al. 2017

Self Cite

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Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains.Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here we demonstrate that δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the δM plots of γ-Fe 2 O 3 nanoparticles isolated by thick SiO 2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the δM plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions, as well as offer new perspectives in the use of Henkel-and δM-plots to quantify the rather elusive inhomogeneous magnetizations states in nanoparticles.Additional information on the δM and the in-field Mössbauer techniques, table with the complete results of the Mössbauer spectra fits, details of the Monte Carlo simulations, FC and ZFC magnetization curves of the VST series (Fig. S1a), Langevin scaling of M(H;T) data measured in VST45 (Fig. S1b), details on the estimate of the "magnetic size" from Langevin fits, δM plots of all the VST series and graphical analysis of the intraparticle and interparticle contributions to the dip (Fig. S2), example of hysteresis loops measured after ZFC and FC (for sample VST17, Fig. S3); X-ray diffraction patterns and lattice parameter across of the maghemite cores of different size (Fig. S4); complete results from Monte Carlo simulations showing the dependence of δM on core anisotropy (Fig. S5), surface anisotropy ( Fig. S6), exchange coupling constant ( Fig. S7) and disordered surface thickness (Fig. S8).

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Magnetic Interactions and Energy Barrier Enhancement in Core/Shell Bimagnetic Nanoparticles

Cited by 45 publications

References 66 publications

Internal Structure and Magnetic Properties in Cobalt Ferrite Nanoparticles: Influence of the Synthesis Method

Internal Structure and Magnetic Properties in Cobalt Ferrite Nanoparticles: Influence of the Synthesis Method

Structural and magnetic properties of core-shell Au/Fe3O4 nanoparticles

Remanence Plots as a Probe of Spin Disorder in Magnetic Nanoparticles

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